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Professor Peter Ralph


As the Executive Director of The Plant Functional Biology and Climate Change Cluster (C3) I have the privilege of leading a dynamic, multidisciplinary team dedicated to improving our predictions about the impacts of climate change.

Over the past 20 years my research has examined seagrasses, freshwater macrophytes, macroalgae and terrestrial plants from the scale of whole organisms to cellular and biochemical processes.

Due to the expertise that I have developed in the measurement of photosynthetic processes in seagrasses I am a member of the Blue Carbon International Scientific Working Group, formed under the auspices of the International Union for the Conservation of Nature, United Nations Environmental Program and the Intergovernmental Oceanographic Commission. In addition I am also an invited member of the Integrated Marine Observing System National Bio-optical Working Group.

Within C3 I am also the Team Leader for the Aquatic Processes Group (APG): a team of both highly experienced and outstanding early career marine ecologists, biologists and oceanographers. The APG has developed major strengths in assessing the impact of human-induced change, as well as climate change, on ecologically important aquatic plant systems.

Within the APG I oversee specialist teams that support research into

UTS is the administering body for the newly established CSIRO Marine and Coastal Carbon Biogeochemistry Cluster of which I am the Co- Leader. This brings together the expertise of CSIRO and the research capabilities of eight Australian universities in Australia's largest ever coastal blue carbon accounting, mapping and measurement study.


Professional Activities
Review staff for Marine Ecology-Progress Series

Co-convener of ARC network meeting BEAM January 2004

Co-convened conference: International Workshop on Marine Applications of Chlorophyll a Fluorescence.
University of Sydney, 4-8 February, 2000.

Co-convened conference: International Workshop on Marine Applications of Chlorophyll a Fluorescence.
University of Sydney, 4-8 April, 1998.

Co-convener of ASPAB
University of Sydney, 13-15 February 2006

Recent Media Releases

Employment History
Current Position Professor
2001-2003 University Reader
1999-2002 Senior Lecturer
1997-1999 Lecturer and Program Director of Coastal Resource Management, UTS
1997-1998 Associate Lecturer, UTS

Image of Peter Ralph
Director, Plant Functional Biology & Climate Change
Professor, School of the Environment
Associate Member, Centre for Technology in Water and Wastewater
Director, Plant Functional Biology & Climate Change
Core Member, Plant Functional Biology & Climate Change
BAppSc (NSWIT), MAIBiol (UTS), PhD (UTS)
+61 2 9514 4070

Research Interests

Aquatic Processes Group

Can supervise: Yes

  • Marine Primary Producers
  • Coastal and Marine Ecosystems
  • Tropical Reef Ecosystems
  • GIS and Resource Assessments
  • Australian Biota
  • Biocomputing
  • Mapping and Remote Sensing
  • Biology and Ecology for Engineers


Ralph, P.J., Wilhelm, C., Lavaud, J., Jakob, T., Petrou, K. & Kranz, S.A. 2011, 'Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation' in Suggett, D.J., Borowitzka, M.A. & Prasil, O. (eds), Chlorophyll a Fluorescence in Aquatic Sciences : Methods and Applications, Springer, United Kingdom, pp. 75-89.
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This chapter investigates the use of chlorophyll a fluorescence to better understand changes in the regulation of photosynthetic electron transport. It describes the different electron pathways utilised by photosynthetic organisms, including pathways used in photosynthesis as well as alternative electron cycling (AEC). The major photoprotective processes are described, in particular, non-photochemical quenching (NPQ) and its three components, energy-dependent quenching (qE), state-transition quenching (qT), and photoinhibition (qI). Fluorescence and NPQ responses to light stress are compared across a higher plant, diatom and cyanobacteria. Photosynthesis is a complex interaction of complementary processes making the identification and isolation of a particular photosynthetic pathway or process inherently difficult. Therefore, we describe the use of chemicals which allow for the differentiation of mechanistic photosynthetic processes, such as electron transport pathways, CO2 fixation and the use of trans-thylakoid proton gradients, which can be effectively understood and quantified using chlorophyll fluorescence detection techniques.
Warner, M.E., Lesser, M.P. & Ralph, P.J. 2011, 'Chlorophyll fluorescence in reef building corals' in Suggett, D.J., Borowitzka, M.A. & Prasil, O. (eds), Chlorophyll a Fluorescence in Aquatic Sciences : Methods and Applications, Springer, UK, pp. 209-221.
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The ecological success of reef-building corals throughout the tropics is due in large part to the endosymbiotic dinoflagellates that reside within the gastrodermal cells of these cnidarian hosts. These algae, belonging to the genus Symbiodinium, are often referred to by the common term of zooxanthellae. This mutualism between Symbiodinium spp. and tropical and sub-tropical coral species has been a key component to the evolutionary persistence of reef-building corals since the Triassic (Stanely 2003). The importance of these algae in the long-term success of reef-building corals cannot be over emphasized, as they can contribute a significant portion of photosynthetically derived carbon to the host via translocation. The coral metabolizes this carbon, thereby meeting up to 90 percent or more of the animals daily metabolic demand from the byproducts of photosynthesis by the symbionts (Muscatine 1990).
Ralph, P.J., Tomasko, D., Moore, K., Seddon, S. & Macinnis-Ng, C.M.O. 2006, 'Human impacts on seagrasses: Eutrophication, sedimentation, and contamination' in Seagrasses: Biology, Ecology and Conservation, Springer Netherlands, pp. 567-593.
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Growth of human populations along coastal environments, as well as poor water management practices have resulted in the complete loss of seagrass meadows (Kemp et al., 1983; Larkum and West, 1990; Short and Wyllie-Echeverria, 1996; Peters et al., 1997). For example, the catastrophic loss of seagrasses clearly illustrated in Fig. 1 is linked to coastal development and associated reduction in water quality. 2006/2007 Springer. All Rights Reserved.
Larkum, A.W.D., Drew, E.A. & Ralph, P.J. 2006, 'Photosynthesis and metabolism in seagrasses at the cellular level' in Seagrasses: Biology, Ecology and Conservation, Springer Netherlands, pp. 323-345.
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Seagrasses are marine angiosperms from the Families Potamagetonaceae and Hydrocharitaceae (Chapter 1). These are by origin land plants that developed both aerial photosynthesis and aerial flowering and then returned to a successful fully submerged marine habitat, from where their algal forbears derived. Following the evolution of land plants there have been few developments in terms of photosynthesis: the only outstanding developments being adaptations to arid conditions, such as the development of C4 photosynthesis in semi-arid plants and Crassulacean Acid Metabolism (CAM) in plants from desert regions (Bowes et al. 2002). 2006/2007 Springer. All Rights Reserved.
Beer, S., Bjork, M., Gademann, R. & Ralph, P.J. 2001, 'Measurements of Photosynthetic Rates in Seagrasses' in Short, F.T. & Coles, R.G. (eds), Global Seagrass Research Methods, Elsevier Science, Netherlands, pp. 183-198.
Ralph, P.J., Brown, K., Wearing, S.L. & Nudd, B. 1998, 'GIS - A tool for integrating tourism and mariculture in Wallis Lakes' in Wearing Stephen (ed), Planning Recreation and Tourism with Australian Communities, HM Leisure Planning Pty Ltd, Melbourne, pp. 95-102.


Howes, J.M., Stuart, B.H. & Ralph, P.J. 2013, 'Assessing metabolic variation of endosymbiotic and cultured Symbiodinium microadriaticum using synchrotron imaging FTIR spectroscopy', 7th International Workshop on Infrared Microscopy and Spectroscopy with Accelerator-Based Sources Abstracts.
Chartrand, K.M., Rasheed, M., Petrou, K. & Ralph, P.J. 2012, 'Establishing tropical seagrass light requirements in a dynamic port environment', Proceedings of the 12th International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012, ReefBase, Australia.
Tropical seagrasses inhabit naturally turbid waters with dynamic light environments and variable water quality in coastal waters adjacent to the Great Barrier Reef. Large tidal fluxes amplify the magnitude of these conditions with extreme high and low light over relatively short time scales (i.e. hours). Large port developments in the region have the potential to confound the complex relationships between seagrass physiology and this dynamic light field with the onset of dredging and their associated turbid plumes. Understanding the capacity for seagrasses to respond to changes in the quantity and quality of the light environment will allow for prediction of how seagrass species and populations will tolerate changes in light attenuation that may occur during dredging. We present a strategy for determining seasonal-specific light requirements for an intertidal tropical seagrass community in a port environment. Locally relevant light requirements are established by describing the relationships among photosynthetic inputs and losses, tidal exposure, shifts in spectral light quality, seasonality and the capacity to utilise below ground carbon reserves. The outcomes of the study provide guidelines for a mitigation strategy that is focused on maintaining critical windows of light to support seagrass growth and the longer term survival of these productive coastal ecosystems.
Sinutok, S., Hill, R., Doblin, M.A. & Ralph, P.J. 2010, 'Rising ocean temperature and ocean acidification will reduce productivity and calcification in Halimeda sp. and benthic foraminifera from the Great Barrier Reef', Euro ISRS Symposium 2010, Grafisch Service Center, Wageningen, The Netherlands, pp. 191-191.
Schrameyer, V., Kraemer, W., Hill, R., Doblin, M.A., Kai, B. & Ralph, P.J. 2010, 'Nutritional status of hard and soft corals influences photosynthesis capacity of Symbiodinium sp. and vitality of the holobiont', Euro ISRS Symposium 2010, Grafisch Service Center, Wageningen, The Netherlands, pp. 108-108.
Hill, R., Brown, C., DeZeeuw, K., Campbell, D. & Ralph, P.J. 2010, 'Increased rate of D1 repair in coral symbionts during bleaching is insufficient to counter accelerated photoinactivation', Euro ISRS Symposium 2010, Grafisch Service Center, Wageningen, The Netherlands, pp. 87-87.
Petrou, K., Doblin, M.A., Hassler, C.S. & Ralph, P.J. 2009, 'Multiple stressors on the sea ice diatom Fragilariopsis cylindrus - photophysiological impacts of seasonal freezing and melting of sea ice'.
Doblin, M.A., Ralph, P.J., Oubelkheir, K., Hassler, C.S., Suthers, I. & Thompson, P. 2009, 'Using IMOS to bridge the gap between direct measurements of marine primary production and models'.
Sinutok, S., Hill, R. & Ralph, P.J. 2009, 'The effect of light intensity on photosynthetic efficiency and calcification in three reef building species of Halimeda'.
Calcareous green algae from the genus Halimeda are widely distributed in tropical and subtropical marine environments. Halimeda is important as a carbonate sediment producer, as a source of food for herbivores, as a phototroph, and as a provider of shelter and nursery ground for invertebrates on coral reefs. Halimeda cylindracea, H. macroloba, and H. opuntia collected from the intertidal zone at Heron Island reef in the southern Great Barrier Reef of Australia were maintained in flow-through seawater tanks under three different irradiances (50, 400 and 900 mol photon m-2 s-1) for a 4 day period. Measures of photosynthetic efficiency were investigated using Pulse Amplitude Modulated (PAM) fluorometry and chlorophyll concentrations (a and b) were determined after 4 days on an apical lobe of the thallus. Growth and calcification were examined by staining the thallus with Alizarin Red-S solution. Pigmentation, growth, and calcification were found to be independent of irradiance, remaining constant over time between treatments, except for chlorophyll a in H. cylindracea which was higher in low light than in higher light treatments. The results indicate that each species responded differently under the high light treatment at midday, with H. cylindracea and H. macroloba showing a reduction in photochemical quenching (Y(II)), indicating downregulation in photosynthetic efficiency. This was mirrored by an increase in non-regulated energy dissipation yield of PSII (Y(NO)), suggesting that photoinhibition was occurring. In comparison to this species, H. opuntia showed no changes in Y(II) and Y(NO) while H. macroloba showed higher Y(II) and lower Y(NO) only in the morning and the evening. Non-photochemical quenching yield (Y(NPQ)) was low in all treatments and in all three species which may indicate a low potential for photoprotection mechanisms in these algae. However, further investigation is required which run over an extended period of time and between seasons.
Wilson, K.G. & Ralph, P.J. 2008, 'A comparison of the effects of Tapis crude oil and dispersed crude oil on subtidal Zostera capricorni', International Oil Spill Conference - IOSC 2008, Proceedings, pp. 859-864.
The toxic effects of Tapis crude oil and dispersed Tapis crude oil on subtidal Zostera capricorni were studied. Limited photosynthetic impact was detected in Z. capricorni exposed to the water soluble fraction of the non-dispersed Tapis crude oil treatments. No significant photosynthetic impact was evident in the dispersed Tapis crude oil treatment even though the total petroleum hydrocarbon (TPH) concentration in these treatments was higher than in the non-dispersed Tapis crude oil treatments. Plants from both treatments recovered following replenishment from the surrounding sea water. A substantial reduction of the TPH within the mesoscosms over the 10 hr exposure period was apparent and would likely suggest a rapid loss of the toxic mixture to the sediments rather than assimilation by the sea grass. This is an abstract of a paper presented at the International Oil Spill Conference 2008 Proceedings (Savannah, GA 5/4-8/2008).
Petrou, K., Shelly, K., Hassler, C.S., Schoemann, V., Doblin, M.A. & Ralph, P.J. 2007, 'Ocean productivity in a changing world: Iron-limitation of Southern Ocean phytoplankton and implication for Antarctic meltwater productivity'.
Macinnis-Ng, C.M.O. & Ralph, P.J. 2002, 'Towards a more ecologically relevant assessment of the impact of heavy metals on the photosynthesis of the seagrass, Zostera capricorni', Marine Pollution Bulletin, pp. 100-106.
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This in situ study used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of copper, cadmium, lead and zinc on the seagrass Zostera capricorni. Custom-made portable in situ exposure (PIE) chambers were developed so seagrasses could be dosed within the meadow. Z. capricorni was exposed to 0.1 and 1 mg l-1 of metal solutions for 10 h. During this time and for the subsequent four-day recovery period, the effective quantum yield of photosystem II (PS II) (?F/Fm?) was measured. While the results were variable, copper and zinc exposed samples had a depressed ?F/Fm? during the exposure period. Samples exposed to zinc recovered to pre-exposure levels but those exposed to copper did not. Cadmium and lead did not impact on the chlorophyll a fluorescence and the chlorophyll pigment data supported these findings. This study presents an innovative new application of chlorophyll a fluorescence stress assessment. 2002 Elsevier Science Ltd. All rights reserved.

Journal articles

Macreadie, P.I., Schliep, M.T., Rasheed, M.A., Chartrand, K.M. & Ralph, P.J. 2014, 'Molecular indicators of chronic seagrass stress: A new era in the management of seagrass ecosystems?', Ecological Indicators, vol. 38, pp. 279-281.
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Petrou, K., Trimborn, S., Rost, B., Ralph, P.J. & Hassler, C.S. 2014, 'The impact of iron limitation on the physiology of the Antarctic diatom Chaetoceros simplex', Marine Biology, vol. 161, no. 4, pp. 925-937.
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Iron availability strongly governs the growth of Southern Ocean phytoplankton. To investigate how iron limitation affects photosynthesis as well as the uptake of carbon and iron in the Antarctic diatom Chaetoceros simplex, a combination of chlorophyll a fluorescence measurements and radiotracer incubations in the presence and absence of chemical inhibitors was conducted. Iron limitation in C. simplex led to a decline in growth rates, photochemical efficiency and structural changes in photosystem II (PSII), including a reorganisation of photosynthetic units in PSII and an increase in size of the functional absorption cross section of PSII. Iron-limited cells further exhibited a reduced plastoquinone pool and decreased photosynthetic electron transport rate, while non-photochemical quenching and relative xanthophyll pigment content were strongly increased, suggesting a photoprotective response. Additionally, iron limitation resulted in a strong decline in carbon fixation and thus the particulate organic carbon quotas. Inhibitor studies demonstrated that, independent of the iron supply, carbon fixation was dependent on internal, but not on extracellular carbonic anhydrase activity. Orthovanadate more strongly inhibited iron uptake in iron-limited cells, indicating that P-type ATPase transporters are involved in iron uptake. The stronger reduction in iron uptake by ascorbate in iron-limited cells suggests that the re-oxidation of iron is required before it can be taken up and further supports the presence of a high-affinity iron transport pathway. The measured changes to photosystem architecture and shifts in carbon and iron uptake strategies in C. simplex as a result of iron limitation provide evidence for a complex interaction of these processes to balance the iron requirements for photosynthesis and carbon demand for sustained growth in iron-limited waters. 2014 The Author(s).
Sinutok, S., Hill, R., Khl, M., Doblin, M.A. & Ralph, P.J. 2014, 'Ocean acidification and warming alter photosynthesis and calcification of the symbiont-bearing foraminifera Marginopora vertebralis', Marine Biology, vol. 161, no. 9, pp. 2143-2154.
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The impact of elevated CO2 and temperature on photosynthesis and calcification in the symbiont-bearing benthic foraminifer Marginopora vertebralis was studied. Individual specimens of M. vertebralis were collected from Heron Island on the southern Great Barrier Reef (Australia). They were maintained for 5 weeks at different temperatures (28, 32 C) and pCO2 (400, 1,000 ?atm) levels spanning a range of current and future climate-change scenarios. The photosynthetic capacity of M. vertebralis was measured with O2 microsensors and a pulse-amplitude-modulated chlorophyll (Chl) fluorometer, in combination with estimates of Chl a and Chl c 2 concentrations and calcification rates. After 5 weeks, control specimens remained unaltered for all parameters. Chlorophyll a concentrations significantly decreased in the specimens at 1,000 ?atm CO2 for both temperatures, while no change in Chl c 2 concentration was observed. Photoinhibition was observed under elevated CO2 and temperature, with a 70-80 % decrease in the maximum quantum yield of PSII. There was no net O2 production at elevated temperatures in both CO2 treatments as compared to the control temperature, supporting that temperature has more impact on photosynthesis and O2 flux than changes in ambient CO2. Photosynthetic pigment loss and a decrease in photochemical efficiency are thus likely to occur with increased temperature. The elevated CO2 and high temperature treatment also lead to a reduction in calcification rate (from +0.1 to >-0.1 % day-1). Thus, both calcification and photosynthesis of the major sediment-producing foraminifer M. vertebralis appears highly vulnerable to elevated temperature and ocean acidification scenarios predicted in climate-change models. 2014 Springer-Verlag Berlin Heidelberg.
Hassler, C.S., Ridgway, K.R., Bowie, A.R., Butler, E.C.V., Clementson, L.A., Doblin, M.A., Davies, D.M., Law, C., Ralph, P.J., Van Der Merwe, P., Watson, R. & Ellwood, M.J. 2014, 'Primary productivity induced by iron and nitrogen in the Tasman Sea: An overview of the PINTS expedition', Marine and Freshwater Research, vol. 65, no. 6, pp. 517-537.
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The Tasman Sea and the adjacent subantarctic zone (SAZ) are economically important regions, where the parameters controlling the phytoplankton community composition and carbon fixation are not yet fully resolved. Contrasting nutrient distributions, as well as phytoplankton biomass, biodiversity and productivity were observed between the North Tasman Sea and the SAZ. In situ photosynthetic efficiency (FV/FM), dissolved and particulate nutrients, iron biological uptake, and nitrogen and carbon fixation were used to determine the factor-limiting phytoplankton growth and productivity in the North Tasman Sea and the SAZ. Highly productive cyanobacteria dominated the North Tasman Sea. High atmospheric nitrogen fixation and low nitrate dissolved concentrations indicated that non-diazotroph phytoplankton are nitrogen limited. Deck-board incubations also suggested that, at depth, iron could limit eukaryotes, but not cyanobacteria in that region. In the SAZ, the phytoplankton community was dominated by a bloom of haptophytes. The low productivity in the SAZ was mainly explained by light limitation, but nitrogen, silicic acid as well as iron were all depleted to the extent that they could become co-limiting. This study illustrates the challenge associated with identification of the limiting nutrient, as it varied between phytoplankton groups, depths and sites. CSIRO 2014.
Tamburic, B., Guruprasad, S., Radford, D.T., Szabo, M., Lilley, R.M., Larkum, A.W.D., Franklin, J.B., Kramer, D.M., Blackburn, S.I., Raven, J.A., Schliep, M. & Ralph, P.J. 2014, 'The Effect of Diel Temperature and Light Cycles on the Growth of Nannochloropsis oculata in a Photobioreactor Matrix', PLOS ONE, vol. 9, no. 1.
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Macreadie, P.I., Baird, M.E., Trevathan-Tackett, S.M., Larkum, A.W.D. & Ralph, P.J. 2014, 'Quantifying and modelling the carbon sequestration capacity of seagrass meadows - A critical assessment', Marine Pollution Bulletin, vol. 83, no. 2, pp. 430-439.
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Seagrasses are among the planet's most effective natural ecosystems for sequestering (capturing and storing) carbon (C); but if degraded, they could leak stored C into the atmosphere and accelerate global warming. Quantifying and modelling the C sequestration capacity is therefore critical for successfully managing seagrass ecosystems to maintain their substantial abatement potential. At present, there is no mechanism to support carbon financing linked to seagrass. For seagrasses to be recognised by the IPCC and the voluntary C market, standard stock assessment methodologies and inventories of seagrass C stocks are required. Developing accurate C budgets for seagrass meadows is indeed complex; we discuss these complexities, and, in addition, we review techniques and methodologies that will aid development of C budgets. We also consider a simple process-based data assimilation model for predicting how seagrasses will respond to future change, accompanied by a practical list of research priorities. 2013 Elsevier Ltd.
Jeans, J., Szab, M., Campbell, D.A., Larkum, A.W.D., Ralph, P.J. & Hill, R. 2014, 'Thermal bleaching induced changes in photosystem II function not reflected by changes in photosystem II protein content of Stylophora pistillata', Coral Reefs, vol. 33, no. 1, pp. 131-139.
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Scleractinian corals exist in a symbiosis with marine dinoflagellates of the genus Symbiodinium that is easily disrupted by changes in the external environment. Increasing seawater temperatures cause loss of pigments and expulsion of the symbionts from the host in a process known as coral bleaching; though, the exact mechanism and trigger of this process has yet to be elucidated. We exposed nubbins of the coral Stylophora pistillata to bleaching temperatures over a period of 14 daylight hours. Fifty-nine percent of the symbiont population was expelled over the course of this short-term treatment. Maximum quantum yield (F V/F M) of photosystem (PS) II for the in hospite symbiont population did not change significantly over the treatment period, but there was a significant decline in the quantity of PSII core proteins (PsbA and PsbD) at the onset of the experimental increase in temperature. F V/F M from populations of expelled symbionts dropped sharply over the first 6 h of temperature treatment, and then toward the end of the experiment, it increased to an F V/F M value similar to that of the in hospite population. This suggests that the symbionts were likely damaged prior to expulsion from the host, and the most damaged symbionts were expelled earlier in the bleaching. The quantity of PSII core proteins, PsbA and PsbD, per cell was significantly higher in the expelled symbionts than in the remaining in hospite population over 6-10 h of temperature treatment. We attribute this to a buildup of inactive PSII reaction centers, likely caused by a breakdown in the PSII repair cycle. Thus, thermal bleaching of the coral S. pistillata induces changes in PSII content that do not follow the pattern that would be expected based on the results of PSII function. 2013 Springer-Verlag Berlin Heidelberg.
Wangpraseurt, D., Larkum, A.W.D., Franklin, J., Szabo, M., Ralph, P.J. & Kuhl, M. 2014, 'Lateral light transfer ensures efficient resource distribution in symbiont-bearing corals', Journal of Experimental Biology, vol. 217, no. 4, pp. 489-498.
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Coral tissue optics has received very little attention in the past, although the interaction between tissue and light is central to our basic understanding of coral physiology. Here we used fibre-optic and electrochemical microsensors along with variable chlorophyll fluorescence imaging to directly measure lateral light propagation within living coral tissues. Our results show that corals can transfer light laterally within their tissues to a distance of ?2cm. Such light transport stimulates O2 evolution and photosystem II operating efficiency in areas >0.5-1 cm away from direct illumination. Light is scattered strongly in both coral tissue and skeleton, leading to photon trapping and lateral redistribution within the tissue. Lateral light transfer in coral tissue is a new mechanism by which light is redistributed over the coral colony and we argue that tissue optical properties are one of the key factors in explaining the high photosynthetic efficiency of corals. 2014. Published by The Company of Biologists Ltd.
Jeans, J., Szabo, M., Campbell, D.A., Larkum, A., Ralph, P.J. & Hill, R. 2014, 'Thermal bleaching induced changes in photosystem II function not reflected by changes in photosystem II protein content of Stylophora pistillata', Coral Reefs, vol. 33, pp. 131-139.
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Brodersen, K.E., Lichtenberg, M., Ralph, P.J., Kuhl, M. & Wangpraseurt, D. 2014, 'Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals', Journal of the Royal Society Interface, vol. 11, p. 20130997.
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Wangpraseurt, D., Polerecky, L., Larkum, A.W.D., Ralph, P.J., Nielsen, D.A., Pernice, M. & Khl, M. 2014, 'The in situ light microenvironment of corals', Limnology and Oceanography, vol. 59, no. 3, pp. 917-926.
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We used a novel diver-operated microsensor system to collect in situ spectrally resolved light fields on corals with a micrometer spatial resolution. The light microenvironment differed between polyp and coenosarc tissues with scalar irradiance (400-700 nm) over polyp tissue, attenuating between 5.1- and 7.8-fold from top to base of small hemispherical coral colonies, whereas attenuation was at most 1.5-fold for coenosarc tissue. Fluctuations in ambient solar irradiance induced changes in light and oxygen microenvironments, which were more pronounced and faster in coenosarc compared with polyp tissue. Backscattered light from the surrounding benthos contributed > 20% of total scalar irradiance at the coral tissue surface and enhanced symbiont photosynthesis and the local O2 concentration, indicating an important role of benthos optics for coral ecophysiology. Light fields on corals are species and tissue specific and exhibit pronounced variation on scales from micrometers to decimeters. Consequently, the distribution, genetic diversity, and physiology of coral symbionts must be coupled with the measurements of their actual light microenvironment to achieve a more comprehensive understanding of coral ecophysiology. 2014, by the Association for the Sciences of Limnology and Oceanography, Inc.
Brodersen, K.E., Lichtenberg, M., Ralph, P.J., Kuhl, M. & Wangpraseurt, D. 2014, 'Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals', Journal of the Royal Society Interface, vol. 1, no. 93.
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The light field on coral reefs varies in intensity and spectral composition, and is the key regulating factor for phototrophic reef organisms, for example scleractinian corals harbouring microalgal symbionts. However, the actual efficiency of light utilization in corals and the mechanisms affecting the radiative energy budget of corals are underexplored. We present the first balanced light energy budget for a symbiont-bearing coral based on a fine-scale study of the microenvironmental photobiology of the massive coral Montastrea curta. The majority (more than 96%) of the absorbed light energy was dissipated as heat, whereas the proportion of the absorbed light energy used in photosynthesis was approximately 4.0% under an irradiance of 640 mol photons m-2 s-1. With increasing irradiance, the proportion of heat dissipation increased at the expense of photosynthesis. Despite such low energy efficiency, we found a high photosynthetic efficiency of the microalgal symbionts showing high gross photosynthesis rates and quantum efficiencies (QEs) of approximately 0.1 O2 photon-1 approaching theoretical limits under moderate irradiance levels. Corals thus appear as highly efficient light collectors with optical properties enabling light distribution over the corallite/tissue microstructural canopy that enables a high photosynthetic QE of their photosynthetic microalgae in hospite.
Wangpraseurt, D., Polerecky, L., Larkum, A., Ralph, P.J., Nielsen, D.A., Pernice, M. & Kuhl, M. 2014, 'The in situ light microenvironment of corals', Limnology and Oceanography, vol. 59, no. 3, pp. 917-926.
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We used a novel diver-operated microsensor system to collect in situ spectrally resolved light fields on corals with a micrometer spatial resolution. The light microenvironment differed between polyp and coenosarc tissues with scalar irradiance (400700 nm) over polyp tissue, attenuating between 5.1- and 7.8-fold from top to base of small hemispherical coral colonies, whereas attenuation was at most 1.5-fold for coenosarc tissue. Fluctuations in ambient solar irradiance induced changes in light and oxygen microenvironments, which were more pronounced and faster in coenosarc compared with polyp tissue. Backscattered light from the surrounding benthos contributed . 20% of total scalar irradiance at the coral tissue surface and enhanced symbiont photosynthesis and the local O2 concentration, indicating an important role of benthos optics for coral ecophysiology. Light fields on corals are species and tissue specific and exhibit pronounced variation on scales from micrometers to decimeters. Consequently, the distribution, genetic diversity, and physiology of coral symbionts must be coupled with the measurements of their actual light microenvironment to achieve a more comprehensive understanding of coral ecophysiology.
Wangpraseurt, D., Larkum, A., Franklin, J., Szabo, M., Ralph, P.J. & Kuhl, M. 2014, 'Lateral light transfer ensures efficient resource distribution in symbiont-bearing corals', Journal of Experimental Biology, vol. 217, pp. 489-498.
View/Download from: Publisher's site
Coral tissue optics has received very little attention in the past, although the interaction between tissue and light is central to our basic understanding of coral physiology. Here we used fibre-optic and electrochemical microsensors along with variable chlorophyll fluorescence imaging to directly measure lateral light propagation within living coral tissues. Our results show that corals can transfer light laterally within their tissues to a distance of ~2 cm. Such light transport stimulates O2 evolution and photosystem II operating efficiency in areas >0.51 cm away from direct illumination. Light is scattered strongly in both coral tissue and skeleton, leading to photon trapping and lateral redistribution within the tissue. Lateral light transfer in coral tissue is a new mechanism by which light is redistributed over the coral colony and we argue that tissue optical properties are one of the key factors in explaining the high photosynthetic efficiency of corals.
Trevathan-Tackett, S.M., Macreadie, P.I., Ralph, P.J. & Seymour, J.R. 2014, 'Detachment and flow cytometric quantification of seagrass-associated bacteria', Journal of Microbiological Methods, vol. 102, pp. 23-25.
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A new protocol was developed to detach bacteria from seagrass tissue and subsequently enumerate cells using flow cytometry (FCM). A method involving addition of the surfactant Tween 80 and vortexing resulted in maximum detachment efficiency of seagrass attached bacteria, providing a robust protocol for precisely enumerating seagrass-associated bacteria with FCM. Using this approach we detected cell concentrations between 2.0 105 and 8.0 106 cells mg- 1 DW tissue.
Tout, J., Jeffries, T.C., Webster, N.S., Stocker, R., Ralph, P.J. & Seymour, J.R. 2014, 'Variability in Microbial Community Composition and Function Between Different Niches Within a Coral Reef', Microbial Ecology, pp. 1-13.
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To explore how microbial community composition and function varies within a coral reef ecosystem, we performed metagenomic sequencing of seawater from four niches across Heron Island Reef, within the Great Barrier Reef. Metagenomes were sequenced from seawater samples associated with (1) the surface of the coral species Acropora palifera, (2) the surface of the coral species Acropora aspera, (3) the sandy substrate within the reef lagoon and (4) open water, outside of the reef crest. Microbial composition and metabolic function differed substantially between the four niches. The taxonomic profile showed a clear shift from an oligotroph-dominated community (e.g. SAR11, Prochlorococcus, Synechococcus) in the open water and sandy substrate niches, to a community characterised by an increased frequency of copiotrophic bacteria (e.g. Vibrio, Pseudoalteromonas, Alteromonas) in the coral seawater niches. The metabolic potential of the four microbial assemblages also displayed significant differences, with the open water and sandy substrate niches dominated by genes associated with core house-keeping processes such as amino acid, carbohydrate and protein metabolism as well as DNA and RNA synthesis and metabolism. In contrast, the coral surface seawater metagenomes had an enhanced frequency of genes associated with dynamic processes including motility and chemotaxis, regulation and cell signalling. These findings demonstrate that the composition and function of microbial communities are highly variable between niches within coral reef ecosystems and that coral reefs host heterogeneous microbial communities that are likely shaped by habitat structure, presence of animal hosts and local biogeochemical conditions. 2014 Springer Science+Business Media New York.
Kanazawa, A., Blanchard, G.J., Szab, M., Ralph, P.J. & Kramer, D.M. 2014, 'The site of regulation of light capture in Symbiodinium: Does the peridinin-chlorophyll a-protein detach to regulate light capture?', Biochimica et Biophysica Acta - Bioenergetics, vol. 1837, no. 8, pp. 1227-1234.
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Dinoflagellates from the genus Symbiodinium form symbiotic associations with cnidarians including corals and anemones. The photosynthetic apparatuses of these dinoflagellates possess a unique photosynthetic antenna system incorporating the peridinin-chlorophyll a-protein (PCP). It has been proposed that the appearance of a PCP-specific 77 K fluorescence emission band around 672-675 nm indicates that high light treatment results in PCP dissociation from intrinsic membrane antenna complexes, blocking excitation transfer to the intrinsic membrane-bound antenna complexes, chlorophyll a-chlorophyll c 2-peridinin-protein-complex (acpPC) and associated photosystems (Reynolds et al., 2008 Proc Natl Acad Sci USA 105:13674-13678).We have tested this model using time-resolved fluorescence decay kinetics in conjunction with global fitting to compare the time-evolution of the PCP spectral bands before and after high light exposure. Our results show that no long-lived PCP fluorescence emission components appear either before or after high light treatment, indicating that the efficiency of excitation transfer from PCP to membrane antenna systems remains efficient and rapid even after exposure to high light. The apparent increased relative emission at around 675 nm was, instead, caused by strong preferential exciton quenching of the membrane antenna complexes associated with acpPC and reaction centers. This strong non-photochemical quenching (NPQ) is consistent with the activation of xanthophyll-Associated quenching mechanisms and the generally-observed avoidance in nature of long-lived photoexcited states that can lead to oxidative damage. The acpPC component appears to be the most strongly quenched under high light exposure suggesting that it houses the photoprotective exciton quencher. 2014 Elsevier B.V.
Hill, R., Szabo, M., Rehman, A.U., Vass, I., Ralph, P.J. & Larkum, A.W.D. 2014, 'Inhibition of photosynthetic CO2 fixation in the coral Pocillopora damicornis and its relationship to thermal bleaching', JOURNAL OF EXPERIMENTAL BIOLOGY, vol. 217, no. 12, pp. 2150-2162.
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Petrou, K., Trimborn, S., Khl, M. & Ralph, P.J. 2014, 'Desiccation stress in two intertidal beachrock biofilms', Marine Biology.
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Chlorophyll a fluorescence was used to look at the effect of desiccation on the photophysiology in two beachrock microbial biofilms from the intertidal rock platform of Heron Island, Australia. The photophysiological response to desiccation differed between the beachrock microbial communities. The black biofilm from the upper shoreline, dominated by Calothrix sp., showed a response typical of desiccation-tolerant cyanobacteria, where photosynthesis closed down during air exposure with a rapid and complete recovery upon rehydration. In contrast, the pink biofilm from the mid-intertidal zone, dominated by Blennothrix sp., showed no distinct response to desiccation stress and instead maintained reduced photosynthesis throughout drying and re-wetting cycles. Spatial differences in photosynthetic activity within the black biofilm were evident with a faster recovery rate of photosynthesis in the surface cyanobacteria than in the deeper layers of the biofilm. There was no variation with depth in the pink biofilm. The photophysiological differences in desiccation responses between the beachrock biofilms exemplify the ecological niche specialisation of these complex microbial communities, where the functional differences help to explain their vertical distribution on the intertidal shoreline. 2014 Springer-Verlag Berlin Heidelberg.
Szab, M., Parker, K., Guruprasad, S., Kuzhiumparambil, U., Lilley, R.M., Tamburic, B., Schliep, M., Larkum, A.W.D., Schreiber, U., Raven, J.A. & Ralph, P.J. 2014, 'Photosynthetic acclimation of Nannochloropsis oculata investigated by multi-wavelength chlorophyll fluorescence analysis', Bioresource Technology, vol. 167, pp. 521-529.
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Multi-wavelength chlorophyll fluorescence analysis was utilised to examine the photosynthetic efficiency of the biofuel-producing alga Nannochloropsis oculata, grown under two light regimes; low (LL) and high (HL) irradiance levels. Wavelength dependency was evident in the functional absorption cross-section of Photosystem II (?II(?)), absolute electron transfer rates (ETR(II)), and non-photochemical quenching (NPQ) of chlorophyll fluorescence in both HL and LL cells. While ?II(?) was not significantly different between the two growth conditions, HL cells upregulated ETR(II) 1.6-1.8-fold compared to LL cells, most significantly in the wavelength range of 440-540nm. This indicates preferential utilisation of blue-green light, a highly relevant spectral region for visible light in algal pond conditions. Under these conditions, the HL cells accumulated saturated fatty acids, whereas polyunsaturated fatty acids were more abundant in LL cells. This knowledge is of importance for the use of N. oculata for fatty acid production in the biofuel industry. 2014 Elsevier Ltd.
Raven, J.A. & Ralph, P.J. 2014, 'Enhanced biofuel production using optimality, pathway modification and waste minimization', Journal of Applied Phycology.
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In response to their environment, algae in the wild may use an approximation to optimality of resource allocation in cellular structures, photosynthetic pigments, enzymes, transporters in membranes and RNAs and in their genetic material. However, under controlled conditions, when algae are grown for biofuel (lipid) production for example, some of these processes can be altered to increase the efficiency of photosynthesis and therefore, lipid yield. This suggests that there is scope for selecting mutations and for genetic engineering at various levels in the photosynthetic apparatus with the aim of increasing efficiency of photon use and the rate of transformation of resources per unit biomass to improve biofuel yields. More specifically, the wavelength range covered by photosynthetic pigments and photochemical reaction centres could be increased, the number of protons transported from the thylakoid lumen to the stroma per unit ATP synthesised by the ATP synthetase could be decreased, the fluctuating light effect could be utilised and photosynthetic pathways changed, e.g. replacing part or all of the current machinery for autotrophic fixation of inorganic carbon. There are also possibilities for decreasing carbon loss by decreasing 'wasteful' aspects of dark respiration and of dissolved organic carbon loss. Provided that the environmental fluctuations to which algal growth conditions are constrained, there are possibilities for decreasing the resource costs of protection from ROS, and by down-regulating photoprotective mechanisms, as well as limiting the capacity to repair processes related to photoinhibition. Decreased protein turnover is also a potential energetic saving. These interventions apply to individual processes; however, this may not be immediately incorporated into the optimal allocation of resources by the alga, and further intervention using a system biology approach may be required. 2014 Springer Science+Business Media Dordrecht.
Tamburic, B., Szab, M., Tran, N.A.T., Larkum, A.W.D., Suggett, D.J. & Ralph, P.J. 2014, 'Action spectra of oxygen production and chlorophyll a fluorescence in the green microalga Nannochloropsis oculata', Bioresource Technology, vol. 169, pp. 320-327.
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The first complete action spectrum of oxygen evolution and chlorophyll a fluorescence was measured for the biofuel candidate alga Nannochloropsis oculata. A novel analytical procedure was used to generate a representative and reproducible action spectrum for microalgal cultures. The action spectrum was measured at 14 discrete wavelengths across the visible spectrum, at an equivalent photon flux density of 60?molphotonsm-2s-1. Blue light (~414nm) was absorbed more efficiently and directed to photosystem II more effectively than red light (~679nm) at light intensities below the photosaturation limit. Conversion of absorbed photons into photosynthetic oxygen evolution was maximised at 625nm; however, this maximum is unstable since neighbouring wavelengths (646nm) resulted in the lowest photosystem II operating efficiency. Identifying the wavelength-dependence of photosynthesis has clear implications to optimising growth efficiency and hence important economic implications to the algal biofuels and bioproducts industries. 2014 Elsevier Ltd.
Szab, M., Wangpraseurt, D., Tamburic, B., Larkum, A.W.D., Schreiber, U., Suggett, D.J., Khl, M. & Ralph, P.J. 2014, 'Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis', Plant Physiology and Biochemistry, vol. 83, pp. 159-167.
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Pulse Amplitude Modulation (PAM) fluorometry has been widely used to estimate the relative photosynthetic efficiency of corals. However, both the optical properties of intact corals as well as past technical constrains to PAM fluorometers have prevented calculations of the electron turnover rate of PSII. We used a new Multi-colour PAM (MC-PAM) in parallel with light microsensors to determine for the first time the wavelength-specific effective absorption cross-section of PSII photochemistry, ?II(?), and thus PAM-based absolute electron transport rates of the coral photosymbiont Symbiodinium both in culture and in hospite in the coral Pocillopora damicornis. In both cases, ?II of Symbiodinium was highest in the blue spectral region and showed a progressive decrease towards red wavelengths. Absolute values for ?II at 440nm were up to 1.5-times higher in culture than in hospite. Scalar irradiance within the living coral tissue was reduced by 20% in the blue when compared to the incident downwelling irradiance. Absolute electron transport rates of P.damicornis at 440nm revealed a maximum PSII turnover rate of ca. 250 electrons PSII-1 s-1, consistent with one PSII turnover for every 4 photons absorbed by PSII; this likely reflects the limiting steps in electron transfer between PSII and PSI. Our results show that optical properties of the coral host strongly affect light use efficiency of Symbiodinium. Therefore, relative electron transport rates do not reflect the productivity rates (or indeed how the photosynthesis-light response is parameterised). Here we provide a non-invasive approach to estimate absolute electron transport rates in corals. 2014 Elsevier Masson SAS.
Gustafsson, M.S.M., Baird, M.E. & Ralph, P.J. 2014, 'Modeling photoinhibition-driven bleaching in Scleractinian coral as a function of light, temperature, and heterotrophy', Limnology and Oceanography, vol. 59, no. 2, pp. 603-622.
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It has been proposed that corals with symbiotic algae (Symbiodinium) bleach under thermal stress due to temperature-dependent inactivation of the Rubisco protein that impairs CO2 uptake, causing a backlog of electrons that result in the formation of damaging Reactive Oxygen Species. We present a numerical model of this mechanism of photoinhibition for symbiotic algae residing within coral tissue. The resulting rate of bleaching depended on temperature, light intensity, and the rate of heterotrophic feeding. The model was validated using three independently published experimental data sets. The model was capable of capturing both the diurnal change in the state of the photosystem, as well as changes in the symbiont population and the coral host caused by different temperature, light, and feeding treatments. Elevated temperatures and light led to a degradation of the photosystem and the expulsion of symbiont cells. If the coral fed heterotrophically, this degradation of the photosynthetic apparatus was reduced, but still a clear decrease in maximum quantum yield (Fv: Fm) and cell numbers was observed when the coral was exposed to elevated temperature. The reduction in chlorophyll content of cells at elevated temperatures and light was compared with the observational bleaching index Degree Heating Days (DHD). As quantified by DHD, the model was found to bleach under similar thermal stress regimes as field studies, except under elevated heterotrophic feeding conditions, which resulted in reduced severity of bleaching over a 90 d period. 2014, by the Association for the Sciences of Limnology and Oceanography, Inc.
Zhang, D., Glasby, T.M., Ralph, P.J. & Gribben, P.E. 2014, 'Mechanisms influencing the spread of a native marine alga', PLoS ONE, vol. 9, no. 4.
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Like invasive macrophytes, some native macrophytes are spreading rapidly with consequences for community structure. There is evidence that the native alga Caulerpa filiformis is spreading along intertidal rocky shores in New South Wales, Australia, seemingly at the expense of native Sargassum spp. We experimentally investigated the role physical disturbance plays in the spread of C. filiformis and its possible consequences for Sargassum spp. Cleared patches within beds of C. filiformis (Caulerpa habitat) or Sargassum spp. (Sargassum habitat) at multiple sites showed that C. filiformis had significantly higher recruitment (via propagules) into its own habitat. The recruitment of Sargassum spp. to Caulerpa habitat was rare, possibly due in part to sediment accretion within Caulerpa habitat. Diversity of newly recruited epibiotic assemblages within Caulerpa habitat was significantly less than in Sargassum habitat. In addition, more C. filiformis than Sargassum spp. recruited to Sargassum habitat at some sites. On common boundaries between these two macroalgae, the vegetative growth of adjacent C. filiformis into cleared patches was significantly higher than for adjacent Sargassum spp. In both experiments, results were largely independent of the size of disturbance (clearing). Lastly, we used PAM fluorometry to show that the photosynthetic condition of Sargassum spp. fronds adjacent to C. filiformis was generally suppressed relative to those distant from C. filiformis. Thus, physical disturbance, combined with invasive traits (e.g. high levels of recruitment and vegetative growth) most likely facilitate the spread of C. filiformis, with the ramifications being lower epibiotic diversity and possibly reduced photosynthetic condition of co-occurring native macrophytes. 2014 Zhang et al.
Brodersen, K.E., Lichtenberg, M., Ralph, P.J., Khl, M. & Wangpraseurt, D. 2014, 'Radiative energy budget reveals high photosynthetic efficiency in symbiont-bearing corals', Journal of the Royal Society Interface, vol. 11, no. 93.
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The light field on coral reefs varies in intensity and spectral composition, and is the key regulating factor for phototrophic reef organisms, for example scleractinian corals harbouring microalgal symbionts. However, the actual efficiency of light utilization in corals and the mechanisms affecting the radiative energy budget of corals are underexplored. We present the first balanced light energy budget for a symbiont-bearing coral based on a fine-scale study of the microenvironmental photobiology of the massive coral Montastrea curta. The majority (more than 96%) of the absorbed light energy was dissipated as heat, whereas the proportion of the absorbed light energy used in photosynthesis was approximately 4.0% under an irradiance of 640 ?mol photons m-2 s-1. With increasing irradiance, the proportion of heat dissipation increased at the expense of photosynthesis. Despite such low energy efficiency, we found a high photosynthetic efficiency of the microalgal symbionts showing high gross photosynthesis rates and quantum efficiencies (QEs) of approximately 0.1 O2 photon-1 approaching theoretical limits under moderate irradiance levels. Corals thus appear as highly efficient light collectors with optical properties enabling light distribution over the corallite/tissue microstructural canopy that enables a high photosynthetic QE of their photosynthetic microalgae in hospite. 2014 The Author(s) Published by the Royal Society. All rights reserved.
Robinson, C., Suggett, D.J., Cherukuru, N., Ralph, P.J. & Doblin, M.A. 2014, 'Performance of Fast Repetition Rate fluorometry based estimates of primary productivity in coastal waters', Journal of Marine Systems, vol. 139, pp. 299-310.
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Capturing the variability of primary productivity in highly dynamic coastal ecosystems remains a major challenge to marine scientists. To test the suitability of Fast Repetition Rate fluorometry (FRRf) for rapid assessment of primary productivity in estuarine and coastal locations, we conducted a series of paired analyses estimating 14C carbon fixation and primary productivity from electron transport rates with a Fast Repetition Rate fluorometer MkII, from waters on the Australian east coast. Samples were collected from two locations with contrasting optical properties and we compared the relative magnitude of photosynthetic traits, such as the maximum rate of photosynthesis (Pmax), light utilisation efficiency (?) and minimum saturating irradiance (EK) estimated using both methods. In the case of FRRf, we applied recent algorithm developments that enabled electron transport rates to be determined free from the need for assumed constants, as in most previous studies. Differences in the concentration and relative proportion of optically active substances at the two locations were evident in the contrasting attenuation of PAR (400-700nm), blue (431nm), green (531nm) and red (669nm) wavelengths. FRRF-derived estimates of photosynthetic parameters were positively correlated with independent estimates of 14C carbon fixation (Pmax: n=19, R2=0.66; ?: n=21, R2=0.77; EK: n=19, R2=0.45; all p<0.05), however primary productivity was frequently underestimated by the FRRf method. Up to 81% of the variation in the relationship between FRRf and 14C estimates was explained by the presence of pico-cyanobacteria and chlorophyll-a biomass, and the proportion of photoprotective pigments, that appeared to be linked to turbidity. We discuss the potential importance of cyanobacteria in influencing the underestimations of FRRf productivity and steps to overcome this potential limitation.
Wangpraseurt, D., Tamburic, B., Szab, M.N., Suggett, D., Ralph, P.J. & K Hl, M. 2014, 'Spectral effects on Symbiodinium photobiology studied with a programmable light engine', PLoS ONE, vol. 9, no. 11.
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The spectral light field of Symbiodinium within the tissue of the coral animal host can deviate strongly from the ambient light field on a coral reef and that of artificial light sources used in lab studies on coral photobiology. Here, we used a novel approach involving light microsensor measurements and a programmable light engine to reconstruct the spectral light field that Symbiodinium is exposed to inside the coral host and the light field of a conventional halogen lamp in a comparative study of Symbiodinium photobiology. We found that extracellular gross photosynthetic O 2 evolution was unchanged under different spectral illumination, while the more red-weighted halogen lamp spectrum decreased PSII electron transport rates and there was a trend towards increased light-enhanced dark respiration rates under excess irradiance. The approach provided here allows for reconstructing and comparing intra-tissue coral light fields and other complex spectral compositions of incident irradiance. This novel combination of sensor technologies provides a framework to studying the influence of macro- and microscale optics on Symbiodinium photobiology with unprecedented spectral resolution.
Schrameyer, V., Wangpraseurt, D., Hill, R., Khl, M., Larkum, A.W.D. & Ralph, P.J. 2014, 'Light respiratory processes and gross photosynthesis in two scleractinian corals', PLoS ONE, vol. 9, no. 10.
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The light dependency of respiratory activity of two scleractinian corals was examined using O2 microsensors and CO2 exchange measurements. Light respiration increased strongly but asymptotically with elevated irradiance in both species. Light respiration in Pocillopora damicornis was higher than in Pavona decussata under low irradiance, indicating species-specific differences in light-dependent metabolic processes. Overall, the coral P. decussata exhibited higher CO2 uptake rates than P. damicornis over the experimental irradiance range. P. decussata also harboured twice as many algal symbionts and higher total protein biomass compared to P. damicornis, possibly resulting in self-shading of the symbionts and/or changes in host tissue specific light distribution. Differences in light respiration and CO2 availability could be due to host-specific characteristics that modulate the symbiont microenvironment, its photosynthesis, and hence the overall performance of the coral holobiont.
Tout, J., Jeffries, T.C., Webster, N.S., Stocker, R., Ralph, P.J. & Seymour, J.R. 2014, 'Variability in microbial community composition and function between different niches within a coral reef.', Microbial ecology, vol. 67, no. 3, pp. 540-552.
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To explore how microbial community composition and function varies within a coral reef ecosystem, we performed metagenomic sequencing of seawater from four niches across Heron Island Reef, within the Great Barrier Reef. Metagenomes were sequenced from seawater samples associated with (1) the surface of the coral species Acropora palifera, (2) the surface of the coral species Acropora aspera, (3) the sandy substrate within the reef lagoon and (4) open water, outside of the reef crest. Microbial composition and metabolic function differed substantially between the four niches. The taxonomic profile showed a clear shift from an oligotroph-dominated community (e.g. SAR11, Prochlorococcus, Synechococcus) in the open water and sandy substrate niches, to a community characterised by an increased frequency of copiotrophic bacteria (e.g. Vibrio, Pseudoalteromonas, Alteromonas) in the coral seawater niches. The metabolic potential of the four microbial assemblages also displayed significant differences, with the open water and sandy substrate niches dominated by genes associated with core house-keeping processes such as amino acid, carbohydrate and protein metabolism as well as DNA and RNA synthesis and metabolism. In contrast, the coral surface seawater metagenomes had an enhanced frequency of genes associated with dynamic processes including motility and chemotaxis, regulation and cell signalling. These findings demonstrate that the composition and function of microbial communities are highly variable between niches within coral reef ecosystems and that coral reefs host heterogeneous microbial communities that are likely shaped by habitat structure, presence of animal hosts and local biogeochemical conditions.
Gardner, S.G., Nielsen, D.A., Petrou, K., Larkum, A.W.D. & Ralph, P.J. 2014, 'Characterisation of coral explants: a model organism for cnidariandinoflagellate studies', Coral Reefs.
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Coral cell cultures made from reef-building scleractinian corals have the potential to aid in the pursuit of understanding of the cnidariandinoflagellate symbiosis. Various methods have previously been described for the production of cell cultures in vitro with a range of success and longevity. In this study, viable tissue spheroids containing host tissue and symbionts (coral explants) were grown from the tissues of Fungia granulosa. The cultured explants remained viable for over 2 months and showed morphological similarities in tissue structure and internal microenvironment to reef-building scleractinian corals. The photophysiology of the explants (1 week old) closely matched that of the parent coral F. granulosa. This study provides the first empirical basis for supporting the use of coral explants as laboratory models for studying coral symbioses. In particular, it highlights how these small, self-sustaining, skeleton-free models can be useful for a number of molecular, genetic and physiological analyses necessary for investigating hostsymbiont interactions at the microscale.
Brodersen, K.E., Nielsen, D.A., Ralph, P.J. & Khl, M. 2014, 'A split flow chamber with artificial sediment to examine the below-ground microenvironment of aquatic macrophytes', Marine Biology, vol. 161, no. 12, pp. 2921-2930.
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We present a new experimental set-up enabling fine-scale examination of how changing environmental conditions affect the below-ground biogeochemical microenvironment of aquatic macrophytes. By means of microsensor and planar optode technology, the influence of plant-mediated radial O2 release on the below-ground chemical microenvironment of Zostera muelleri and Halophila ovalis was determined in high spatio-temporal resolution. The seagrass specimens were cultured in a new split flow chamber with artificial sediment made of a deoxygenated seawateragar solution with added sulphide. Microelectrode measurements revealed radial O2 release from the rootshoot junction of both Z. muelleri and H. ovalis during both light stimulation and darkness, resulting in a rapid decrease in H2S concentration, and a significant drop in pH was observed within the plant-derived oxic microzone of Z. muelleri. No radial O2 release was detectable from the below-ground tissue of Z. muelleri during conditions of combined water-column hypoxia and darkness, leaving the plants more susceptible to sulphide invasion. The spatial O2 heterogeneity within the immediate rhizosphere of Z. muelleri was furthermore determined in two dimensions by means of planar optodes. O2 images revealed a decrease in the spatial extent of the plant-derived oxic microzone surrounding the below-ground tissue during darkness, supporting the microelectrode measurements. This new experimental approach can be applied to all rooted aquatic plants, as it allows for direct visual assessment of the below-ground tissue surface during microprofiling, while enabling modification of the above-ground environmental conditions.
Gustafsson, M.S.M., Baird, M.E. & Ralph, P.J. 2013, 'The interchangeability of autotrophic and heterotrophic nitrogen sources in Scleractinian coral symbiotic relationships: A numerical study', Ecological Modelling, vol. 250, pp. 183-194.
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The success of corals in tropical oligotrophic waters depends largely on their symbiotic relationship with the dinoflagellate algae residing in their tissues. Understanding the dynamics of this symbiosis is essential to predict how corals respond to environmental stressors, such as changes in nutrients availability, water temperatures and irradiance. This study presents a numerical model of the symbiotic relationship between a heterotrophic coral (cnidarian) host and autotrophic symbiotic dinoflagellates, including the major metabolic and physical functions of the system, under non-bleaching conditions. The coral acquires nitrogen (N) through two processes, uptake of dissolved inorganic nitrogen (VDINH) and heterotrophic feeding (ZN). Numerical experiments were used to highlight the importance of these different sources of N for coral survival and growth. The model was analyzed for four external nutrient supply scenarios, using combinations of two VDINH rates (high and low) and two ZN rates (high and low), and for a range of light levels. The model outputs showed the importance of the algae symbionts to the coral host as a source of both N and C when the feeding rate was limited, with heterotrophic feeding providing only 14% of the N needed to sustain the host biomass for the low ZN+high VDINH scenario. In contrast, with no light or low light, conditions under which the symbiont population dies, the host was able to survive if ZN was high. Living inside the host the symbiont population thrived as long as there was enough light, as well as, DIN and DIC in the host tissues, independent of whether N was supplied as ZN or VDINH. Translocation and recycling of nutrient were two of the most important features of this model, emphasizing why it is essential to resolve host and symbiont in a coral model. The model highlights that the interchangeability of N sources, and the ability to exchange and recycle nutrients in the host-symbiont system, is the key to coral survival in nutrient poor environments. 2012 Elsevier B.V.
Sinutok, S., Hill, R., Doblin, M.A. & Ralph, P.J. 2013, 'Diurnal photosynthetic response of the motile symbiotic benthic foraminiferan marginopora vertebralis', Marine Ecology Progress Series, vol. 478, pp. 127-138.
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Movement of the symbiont-bearing foraminiferan Marginopora vertebralis and photo physiological response to diurnal fluctuations in irradiance were investigated in field and laboratory experiments. The abundance of M. vertebralis from both light-exposed and sheltered habitats was determined 5 times during the day, from pre-dawn to post-dusk. M. vertebralis abundance was significantly higher in sheltered compared to exposed habitats at midday under high irradiance, and this movement enabled the algal symbionts to avoid excessive photoinhibition. The diurnal changes in photosynthetic efficiency were not consistent with the typical midday solar maximum downregulation of photosystem II observed in other photoautotrophs and was likely due to the negatively phototactic capacity of the foraminifera. To confirm the light-dependent movement of foraminifera, individuals in exposed and sheltered habitats were exposed to the photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) in the laboratory. The lack of movement in DCMU-exposed specimens confirmed light-dependent movement and subsequent disruption of signalling between the host foraminiferan and the algal symbionts. Analysis of chlorophyll and xanthophyll pigments, as well as symbiont density, indicated that under high irradiance, foraminiferal symbionts have the capacity to reduce light stress by activating photoprotective mechanisms. The negatively phototactic behaviour prevented chlorophyll degradation, symbiont loss and bleaching, suggesting that it is the primary mechanism for controlling light exposure in these foraminifera. This behaviour provides a competitive advantage over other sessile organisms in avoiding photoinhibition and bleaching by moving away from over-saturating irradiance, towards less damaging light fields. Inter-Research 2013.
Baird, M.E., Ralph, P.J., Rizwi, F., Wild-Allen, K. & Steven, A.D.L. 2013, 'A dynamic model of the cellular carbon to chlorophyll ratio applied to a batch culture and a continental shelf ecosystem', Limnology and Oceanography, vol. 58, no. 4, pp. 1215-1226.
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A novel parameterization of the dynamical relationship between cellular carbon (C) and chlorophyll (Chl) is developed using a Chl synthesis term that includes the physiological status of the cell and the effect of packaging of pigments within cells. The geometric derivation highlights the non-linear relationship between Chl content and absorption due to the package effect. When parameterized for a generic 3 ?m radius phytoplankton cell, the model reproduces the magnitude and daily variations of C: Chl and C: nitrogen ratios of the diatom Skeletonema costatum in published laboratory experiments. The parameterization is then applied in a three-dimensional biogeochemical model containing three phytoplankton classes in the coastal waters off southeast Tasmania, Australia, which demonstrates the behavior of the dynamic Chl parameterization over a range of light-and nutrient-limiting environments for phytoplankton of different sizes and growth rates. The model produces C: Chl ratios of ~12-20 (weight: weight) and ~60-80 for phytoplankton communities dominated by fast-growing small and fast-growing large cells, respectively, close to the ratios of 17 and 76 observed at two sampling stations during periods with diatom-and flagellate-dominated communities. Throughout the simulation, community C: Chl ratios generally vary between 12 and 200, which is similar to the range observed globally. In the new parameterization, C: Chl ratios are most influenced by the package effect for light-limited, slow-growing large microalgae, with physiological processes becoming important for smaller, nutrient-limited, fast-growing microalgae. 2013, by the Association for the Sciences of Limnology and Oceanography, Inc.
Petrou, K., Jimenez-Denness, I., Chartrand, K., McCormack, C., Rasheed, M. & Ralph, P.J. 2013, 'Seasonal heterogeneity in the photophysiological response to air exposure in two tropical intertidal seagrass species', Marine Ecology Progress Series, vol. 482, pp. 93-106.
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Photosynthesis, chlorophyll a fluorescence, leaf bio-optical properties and pigments were measured in 2 tropical intertidal seagrass species, Zostera muelleri ssp. capricorni and Halophila ovalis before, during and after air-exposure over a tidal cycle. Data were collected across 4 seasons (October and January-growing seasons; May and July-senescent seasons) to determine seasonal dynamics in physiological responses to air exposure. Both species showed clear light-dependent responses with a decline in photosynthetic efficiency and increased photoprotection during periods of combined maximum daily irradiance and air exposure for all seasons. In Z. muelleri ssp. capricorni there was a negative correlation between air-exposed effective quan - tum yield and light intensity, suggesting exposure was driving this decline. Conversely, sensitivity (decline in effective quantum yield of photosystem II) to increased irradiance dominated the response in H. ovalis, with no change in the magnitude of this response between air-exposed and submerged blades. The response to air exposure observed in Z. muelleri ssp. capricorni showed seasonal variation, with a greater decline in photosynthesis during the spring (October). Tidal exposure did not provide intertidal seagrasses a 'window' of photosynthetic respite (increase in photosynthesis) from high natural or anthropogenic turbidity. However, the periods immediately prior to and after exposure were important for providing an optimum period for net photosynthetic gain. Inter-Research 2013.
Krmer, W.E., Schrameyer, V., Hill, R., Ralph, P.J. & Bischof, K. 2013, 'PSII activity and pigment dynamics of Symbiodinium in two Indo-Pacific corals exposed to short-term high-light stress', Marine Biology, vol. 160, no. 3, pp. 563-577.
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This study examined the capacity for photoprotection and repair of photo-inactivated photosystem II in the same Symbiodinium clade associated with two coexisting coral species during high-light stress in order to test for the modulation of the symbiont's photobiological response by the coral host. After 4 days exposure to in situ irradiance, symbionts of the bleaching-sensitive Pocillopora damicornis showed rapid synthesis of photoprotective pigments (by 44 %) and strongly enhanced rates of xanthophyll cycling (by 446 %) while being insufficient to prevent photoinhibition (sustained loss in Fv/Fm at night) and loss of symbionts after 4 days. By contrast, Pavona decussata showed no significant changes in Fv/Fm, symbiont density or xanthophyll cycling. Given the association with the same Symbiodinium clade in both coral species, our findings suggest that symbionts in the two species examined may experience different in hospite light conditions as a result of different biometric properties of the coral host. 2012 Springer-Verlag Berlin Heidelberg.
Clark, J.S., Poore, A.G.B., Ralph, P.J. & Doblin, M.A. 2013, 'Potential for adaptation in response to thermal stress in an intertidal macroalga', Journal of Phycology, vol. 49, no. 4, pp. 630-639.
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Understanding responses of marine algae to changing ocean temperatures requires knowledge of the impacts of elevated temperatures and the likelihood of adaptation to thermal stress. The potential for rapid evolution of thermal tolerance is dependent on the levels of heritable genetic variation in response to thermal stress within a population. Here, we use a quantitative genetic breeding design to establish whether there is a heritable variation in thermal sensitivity in two populations of a habitat-forming intertidal macroalga, Hormosira banksii (Turner) Descaisne. Gametes from multiple parents were mixed and growth and photosynthetic performance were measured in the resulting embryos, which were incubated under control and elevated temperature (20C and 28C). Embryo growth was reduced at 28C, but significant interactions between male genotype and temperature in one population indicated the presence of genetic variation in thermal sensitivity. Selection for more tolerant genotypes thus has the ability to result in the evolution of increased thermal tolerance. Furthermore, genetic correlations between embryos grown in the two temperatures were positive, indicating that those genotypes that performed well in elevated temperature also performed well in control temperature. Chlorophyll a fluorescence measurements showed a marked decrease in maximum quantum yield of photosystem II (PSII) under elevated temperature. There was an increase in the proportion of energy directed to photoinhibition (nonregulated nonphotochemical quenching) and a concomitant decrease in energy used to drive photochemistry and xanthophyll cycling (regulated nonphotochemical quenching). However, PSII performance between genotypes was similar, suggesting that thermal sensitivity is related to processes other than photosynthesis. 2013 Phycological Society of America.
Hong, Y., Burford, M.A., Ralph, P.J., Udy, J.W. & Doblin, M.A. 2013, 'The cyanobacterium cylindrospermopsis raciborskii is facilitated by copepod selective grazing', Harmful Algae, vol. 29, pp. 14-21.
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Blooms of the toxin-producing cyanobacterium Cylindrospermopsis raciborskii occur in tropical and subtropical lakes during spring-summer but the mechanisms behind bloom formation are unclear. This study tests the hypothesis that C. raciborskii accumulations in freshwater systems are facilitated by selective copepod grazing. Prey selection was examined in a series of experiments with C. raciborskii and the green alga, Chlamydomonas reinhardtii, as well as within natural phytoplankton assemblages. Clearance rates of the copepod Boeckella sp. on a C. raciborskii diet were 2-4 times lower than that of a common cladoceran Ceriodaphnia sp. when both grazers had prey choice. More C. raciborskii was cleared by Boeckella sp. when in mixed natural phytoplankton assemblages, but the clearance rate declined when nutrient replete C. reinhardtii was added, demonstrating that when alternate "high quality" algae were present, so did C. raciborskii consumption. The clearance rates of Boeckella sp. on two toxic C. raciborskii strains were significantly lower than on a non-toxic strain, and on C. raciborskii with low cellular P content. When we tested the grazing preference of a copepod dominated mixed zooplankton community on C. raciborskii during the early bloom period, clearance rates were relatively low (0.05-0.20mlindividual-1h-1), and decreased significantly as the proportion of C. raciborskii increased above 5%. These results suggest that C. raciborskii persistence could be promoted by copepods preferentially grazing on other algae, with significant loss of top-down control as C. raciborskii abundance increases. 2013 Elsevier B.V.
York, P.H., Gruber, R.K., Hill, R., Ralph, P.J., Booth, D.J. & Macreadie, P.I. 2013, 'Physiological and Morphological Responses of the Temperate Seagrass Zostera muelleri to Multiple Stressors: Investigating the Interactive Effects of Light and Temperature', PLoS ONE, vol. 8, no. 10.
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Understanding how multiple environmental stressors interact to affect seagrass health (measured as morphological and physiological responses) is important for responding to global declines in seagrass populations. We investigated the interactive effects of temperature stress (24, 27, 30 and 32C) and shading stress (75, 50, 25 and 0% shade treatments) on the seagrass Zostera muelleri over a 3-month period in laboratory mesocosms. Z. muelleri is widely distributed throughout the temperate and tropical waters of south and east coasts of Australia, and is regarded as a regionally significant species. Optimal growth was observed at 27C, whereas rapid loss of living shoots and leaf mass occurred at 32C. We found no difference in the concentration of photosynthetic pigments among temperature treatments by the end of the experiment; however, up-regulation of photoprotective pigments was observed at 30C. Greater levels of shade resulting in high photochemical efficiencies, while elevated irradiance suppressed effective quantum yield (?F/FM'). Chlorophyll fluorescence fast induction curves (FIC) revealed that the J step amplitude was significantly higher in the 0% shade treatment after 8 weeks, indicating a closure of PSII reaction centres, which likely contributed to the decline in ?F/FM' and photoinhibition under higher irradiance. Effective quantum yield of PSII (?F/FM') declined steadily in 32C treatments, indicating thermal damage. Higher temperatures (30C) resulted in reduced above-ground biomass ratio and smaller leaves, while reduced light led to a reduction in leaf and shoot density, above-ground biomass ratio, shoot biomass and an increase in leaf senescence. Surprisingly, light and temperature had few interactive effects on seagrass health, even though these two stressors had strong effects on seagrass health when tested in isolation. In summary, these results demonstrate that populations of Z. muelleri in south-eastern Australia are sensitive to small chronic temperature increases and light decreases that are predicted under future climate change scenarios. 2013 York et al.
Macreadie, P.I., Allen, K., Kelaher, B.P., Ralph, P.J. & Skilbeck, C.G. 2012, 'Paleoreconstruction of estuarine sediments reveal human-induced weakening of coastal carbon sinks', Global Change Biology, vol. 18, no. 3, pp. 891-901.
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Human activities in coastal areas frequently cause loss of benthic macrophytes (e.g. seagrasses) and concomitant increases in microalgal production through eutrophication. Whether such changes translate into shifts in the composition of sediment detritus is largely unknown, yet such changes could impact the role these ecosystems play in sequestrating CO 2. We reconstructed the sedimentary records of cores taken from two sites within Botany Bay, Sydney - the site of European settlement of Australia - to look for human-induced changes in dominant sources of detritus in this estuary. Cores covered a period from the present day back to the middle Holocene (~6000 years) according to 210Pb profiles and radiocarbon ( 14C) dating. Depositional histories at both sites could not be characterized by a linear sedimentation rate; sedimentation rates in the last 30-50 years were considerably higher than during the rest of the Holocene. C : N ratios declined and began to exhibit a microalgal source signature from around the time of European settlement, which could be explained by increased nutrient flows into the Bay caused by anthropogenic activity. Analysis of stable isotopic ratios of 12C/ 13C showed that the relative contribution of seagrass and C 3 terrestrial plants (mangroves, saltmarsh) to detritus declined around the time of rapid industrial expansion (~1950s), coinciding with an increase in the contribution of microalgal sources. We conclude that the relative contribution of microalgae to detritus has increased within Botany Bay, and that this shift is the sign of increased industrialization and concomitant eutrophication. Given the lower carbon burial efficiencies of microalgae (~0.1%) relative to seagrasses and C 3 terrestrial plants (up to 10%), such changes represent a substantial weakening of the carbon sink potential of Botany Bay - this occurrence is likely to be common to human-impacted estuaries, and has consequences for the role these systems play in helping to mitigate climate change. 2011 Blackwell Publishing Ltd.
Petrou, K., Kranz, S.A., Doblin, M.A. & Ralph, P.J. 2012, 'Photophysiological responses of fragilariopsis cylindrus (bacillariophyceae) to nitrogen depletion at two temperatures', Journal of Phycology, vol. 48, no. 1, pp. 127-136.
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The photosynthetic efficiency and photoprotective capacity of the sea-ice diatom, Fragilariopsis cylindrus (Grunow) W. Krieg., grown in a matrix of nitrogen repletion and depletion at two different temperatures (-1C and +6C) was investigated. Temperature showed no significant effect on photosynthetic efficiency or photoprotection in F. cylindrus. Cultures under nitrogen depletion showed enhanced photoprotective capacity with an increase in nonphotochemical quenching (NPQ) when compared with nitrogen-replete cultures. This phenomenon was achieved at no apparent cost to the photosynthetic efficiency of PSII (F V/F M). Nitrogen depletion yielded a partially reduced electron transport chain in which maximum fluorescence (F M) could only be obtained by adding 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). reoxidation curves showed the presence of Q B nonreducing PSII centers under nitrogen depletion. Fast induction curves (FICs) and electron transport rates (ETRs) revealed slowing of the electrons transferred from the primary (Q A) to the secondary (Q B) quinone electron acceptors of PSII. The data presented show that nitrogen depletion in F. cylindrus leads to the formation of Q B nonreducing PSII centers within the photosystem. On a physiological level, the formation of Q B nonreducing PSII centers in F. cylindrus provides the cell with protection against photoinhibition by facilitating the rapid induction of NPQ. This strategy provides an important ecological advantage, especially during the Antarctic spring, maintaining photosynthetic efficiency under high light and nutrient-limiting conditions. 2011 Phycological Society of America.
Buxton, L., Takahashi, S., Hill, R. & Ralph, P.J. 2012, 'Variability in the primary site of photosynthetic damage in symbiodinium sp. (dinophyceae) exposed to thermal stress', Journal of Phycology, vol. 48, no. 1, pp. 117-126.
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Exposure to elevated temperature is known to cause photosynthetic inhibition in the coral symbiont Symbiodinium sp. Through the use of the artificial electron acceptor, methyl viologen, this study identified how reduced photosynthetic capacity occurs as a result of inhibition up- and/or downstream of ferredoxin in Symbiodinium sp. in hospite and in culture. Heterogeneity between coral species and symbiont clades was identified in the thermal sensitivity of photosynthesis in the symbionts of the scleractinian corals Stylophora pistillata and Pocillopora damicornis, as well as among Symbiodinium cultures of clades A, B, and C. The in hospite symbionts of S. pistillata and the cultured clade C Symbiodinium both exhibited similar patterns in that their primary site of thermal inhibition occurred downstream of ferredoxin at 32C. In contrast, the primary site of thermal inhibition occurred upstream of ferredoxin in clades A and B at 32C, while at 34C, all samples showed combined up- and downstream inhibition. Although clade C is common to both P. damicornis and S. pistillata, the manner of thermal inhibition was not consistent when observed in hospite. Results showed that there is heterogeneity in the primal site of thermal damage in Symbiodinium among coral species and symbiont clades. 2011 Phycological Society of America.
Jimenez, I.M., Larkum, A.W.D., Ralph, P.J. & Khl, M. 2012, 'In situ thermal dynamics of shallow water corals is affected by tidal patterns and irradiance', Marine Biology, vol. 159, no. 8, pp. 1773-1782.
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We studied the diel variation of in situ coral temperature, irradiance and photosynthetic performance of hemispherical colonies of Poriteslobata and branching colonies of Poritescylindrica during different bulk water temperature and tidal scenarios on the shallow reef flat of Heron Island, Great Barrier Reef, Australia. Our study presents in situ evidence that coral tissue surface temperatures can exceed that of the surrounding water under environmental conditions typically occurring during low tide in shallow reef or lagoon environments. Such heating may be a regular occurrence on shallow reef flats, triggered by the combined effects of high irradiance and low water flow characteristic of low Spring tides. At these times, solar heating of corals coincides with times of maximum water temperature and high irradiance, where the slow flow and consequent thick boundary layers impede heat exchange between corals and the surrounding water. Despite similar light-absorbing properties, the heating effect was more pronounced for the hemispherical P. lobata than for the branching P. cylindrica. This is consistent with previous laboratory experiments showing the evidence of interspecific variation in coral thermal environment and may result from morphologically influenced variation in convective heat transfer and/or thermal properties of the skeleton. Maximum coral surface warming did not coincide with maximum irradiance, but with maximum water temperature, well into the low-tide period with extremely low water flow in the partially drained reef flat, just prior to flushing by the rising tide. The timing of low tide thus influences the thermal exposure and photophysiological performance of corals, and the timing of tidally driven coral surface warming could potentially have different physiological impacts in the morning or in the afternoon. 2012 Springer-Verlag.
Wilson, K.G. & Ralph, P.J. 2012, 'Laboratory testing protocol for the impact of dispersed petrochemicals on seagrass', Marine Pollution Bulletin, vol. 64, no. 11, pp. 2421-2427.
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To improve the effectiveness of oil spill mitigation, we developed a rapid, logistically simple protocol to detect petrochemical stress on seagrass. Sections of leaf blades from Zostera muelleri subsp. capricorni were exposed to the water accommodated fraction (WAF) of non-dispersed and dispersed Tapis crude oil and fuel oil (IFO-380) for 5. h. Photosynthetic health was monitored by assessing changes in effective quantum yield of photosystem II (?F/Fm') and chlorophyll a pigment concentrations. Loss of total petroleum hydrocarbons (TPH) was measured using an oil-in-water fluorometer, whilst GC-MS analyses quantified the hydrocarbon components within each treatment. Few significant differences were detected in the chlorophyll a pigment analyses; however, ?F/Fm' appeared sensitive to petrochemical exposure. Dispersing both types of oil resulted in a substantial increase in the TPH of the WAF and was generally correlated with a greater physiological impact to the seagrass health, compared with the oil alone. 2012 Elsevier Ltd.
Wangpraseurt, D., Larkum, A.W.D., Ralph, P.J. & Kuehl, M. 2012, 'Light gradients and optical microniches in coral tissues', FRONTIERS IN MICROBIOLOGY, vol. 3.
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Jimenez, I.M., Larkum, A.W.D., Ralph, P.J. & Khl, M. 2012, 'Thermal effects of tissue optics in symbiont-bearing reef-building corals', Limnology and Oceanography, vol. 57, no. 6, pp. 1816-1825.
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Reflectance spectroscopy and microscale temperature measurements were used to investigate links between optical and thermal properties of corals. Coral tissue heating showed a species-specific linear correlation to the absorptance of incident irradiance. Heat budgets estimated from absorptance and thermal boundary layer measurements indicated differences in the relative contribution of convection and conduction to heat loss in Porites lobata and Stylophora pistillata, and a higher heat conduction into the skeleton of the thin-tissued branching S. pistillata as compared to the massive thick-tissued P. lobata. Decreasing absorptance associated with bleaching resulted in decreased surface warming of coral tissue. Action spectra of coral tissue heating showed elevated efficiency of heating at wavelengths corresponding to absorption maxima of major zooxanthellae photopigments. Generally, energy-rich radiation (< 500 nm) showed the highest heating efficiency. Species specific relationships between coral tissue heating and absorptance can be strongly affected by differences in the thermal properties of the skeleton and/or tissue arrangement within the skeletal matrix, indicating a yet unresolved potential for coral shape, size, and tissue thickness to affect heat dissipation and especially the conduction of heat into the coral skeleton. 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.
Hill, R., Larkum, A.W.D., Pril, O., Kramer, D.M., Szab, M., Kumar, V. & Ralph, P.J. 2012, 'Light-induced dissociation of antenna complexes in the symbionts of scleractinian corals correlates with sensitivity to coral bleaching', Coral Reefs, vol. 31, no. 4, pp. 963-975.
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Elevated temperatures in combination with moderate to high irradiance are known to cause bleaching events in scleractinian corals, characterised by damage to photosystem II (PSII). Photoprotective mechanisms of the symbiont can reduce the excitation pressure impinging upon PSII. In the bleaching sensitive species, Acropora millepora and Pocillopora damicornis, high light alone induced photoprotection through the xanthophyll cycle, increased content of the antioxidant carotenoid, ?-carotene, as well as the dissociation of the light-harvesting chlorophyll complexes. The evidence is compatible with either the membrane-bound chlorophyll a-chlorophyll c2-peridinin-protein (acpPC) complex or the peripheral peridinin-chlorophyll-protein complex, or both, disconnecting from PSII under high light. The acpPC complex potentially showed a state transition response with redistribution towards photosystem I to reduce PSII over-excitation. This apparent acpPC dissociation/reassociation was promoted by the addition of the xanthophyll cycle inhibitor, dithiothreitol, under high irradiance. Exposure to thermal stress as well as high light promoted xanthophyll de-epoxidation and increased ?-carotene content, although it did not influence light-harvesting chlorophyll complex (LHC) dissociation, indicating light, rather than temperature, controls LHC dissociation. Photoinhibition was avoided in the bleaching tolerant species, Pavona decussata, suggesting xanthophyll cycling along with LHC dissociation may have been sufficient to prevent photodamage to PSII. Symbionts of P. decussata also displayed the greatest detachment of antenna complexes, while the more thermally sensitive species, Pocillopora damicornis and A. millepora, showed less LHC dissociation, suggesting antenna movement influences bleaching susceptibility. 2012 Springer-Verlag.
Gilbert, J.A., Hill, R., Doblin, M.A. & Ralph, P.J. 2012, 'Microbial consortia increase thermal tolerance of corals', Marine Biology, vol. 159, no. 8, pp. 1763-1771.
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This study examined the response of a coral holobiont to thermal stress when the bacterial community was treated with antibiotics. Colonies of Pocillopora damicornis were exposed to broad and narrow-spectrum antibiotics targeting coral-associated ? and ?-Proteobacteria. Corals were gradually heated from the control temperature of 26 to 31 C, and measurements were made of host, zooxanthellar and microbial condition. Antibiotics artificially reduced the abundance and activity of bacteria, but had minimal effect on zooxanthellae photosynthetic efficiency or host tissue protein content. Heated corals without antibiotics showed significant declines in F V/F M, typical of thermal stress. However, heated corals treated with antibiotics showed severe tissue loss in addition to a decline in F V/F M. This study demonstrated that a disruption to the microbial consortium diminished the resilience of the holobiont. Corals exposed to antibiotics under control temperature did not bleach, suggesting that temperature may be an important factor influencing the activity, diversity and ecological function of the holobiont bacterial community. 2012 Springer-Verlag.
Sinutok, S., Hill, R., Doblin, M.A., Khl, M. & Ralph, P.J. 2012, 'Microenvironmental changes support evidence of photosynthesis and calcification inhibition in Halimeda under ocean acidification and warming', Coral Reefs, vol. 31, no. 4, pp. 1201-1213.
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The effects of elevated CO2 and temperature on photosynthesis and calcification of two important calcifying reef algae (Halimedamacroloba and Halimeda cylindracea) were investigated with O2 microsensors and chlorophyll a fluorometry through a combination of two pCO2 (400 and 1,200 ?atm) and two temperature treatments (28 and 32 C) equivalent to the present and predicted conditions during the 2100 austral summer. Combined exposure to pCO2 and elevated temperature impaired calcification and photosynthesis in the two Halimeda species due to changes in the microenvironment around the algal segments and a reduction in physiological performance. There were no significant changes in controls over the 5-week experiment, but there was a 50-70 % decrease in photochemical efficiency (maximum quantum yield), a 70-80 % decrease in O2 production and a threefold reduction in calcification rate in the elevated CO2 and high temperature treatment. Calcification in these species is closely coupled with photosynthesis, such that a decrease in photosynthetic efficiency leads to a decrease in calcification. Although pH seems to be the main factor affecting Halimeda species, heat stress also has an impact on their photosystem II photochemical efficiency. There was a strong combined effect of elevated CO2 and temperature in both species, where exposure to elevated CO2 or temperature alone decreased photosynthesis and calcification, but exposure to both elevated CO2 and temperature caused a greater decline in photosynthesis and calcification than in each stress individually. Our study shows that ocean acidification and ocean warming are drivers of calcification and photosynthesis inhibition in Halimeda. Predicted climate change scenarios for 2100 would therefore severely affect the fitness of Halimeda, which can result in a strongly reduced production of carbonate sediments on coral reefs under such changed climate conditions. 2012 Springer-Verlag.
Seymour, J.R., Doblin, M.A., Jeffries, T.C., Brown, M.V., Newton, K., Ralph, P.J., Baird, M. & Mitchell, J.G. 2012, 'Contrasting microbial assemblages in adjacent water masses associated with the East Australian Current', Environmental Microbiology Reports, vol. 4, no. 5, pp. 548-555.
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Different oceanographic provinces host discrete microbial assemblages that are adapted to local physicochemical conditions. We sequenced and compared the metagenomes of two microbial communities inhabiting adjacent water masses in the Tasman Sea, where the recent strengthening of the East Australian Current (EAC) has altered the ecology of coastal environments. Despite the comparable latitude of the samples, significant phylogenetic differences were apparent, including shifts in the relative frequency of matches to Cyanobacteria, Crenarchaeota and Euryarchaeota. Fine-scale variability in the structure of SAR11, Prochlorococcus and Synechococcus populations, with more matches to 'warm-water' ecotypes observed in the EAC, indicates the EAC may drive an intrusion of tropical microbes into temperate regions of the Tasman Sea. Furthermore, significant shifts in the relative importance of 17 metabolic categories indicate that the EAC prokaryotic community has different physiological properties than surrounding waters. 2012 Society for Applied Microbiology and Blackwell Publishing Ltd.
Verhoeven, M.P.C., Kelaher, B.P., Bishop, M.J. & Ralph, P.J. 2012, 'Epiphyte grazing enhances productivity of remnant seagrass patches', Austral Ecology, vol. 37, no. 8, pp. 885-892.
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Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P.australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C.fragum density, while P.australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above-ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting. 2012 The Authors. Austral Ecology 2012 Ecological Society of Australia.
McMinn, A., Ashworth, C., Bhagooli, R., Martin, A., Salleh, S., Ralph, P.J. & Ryan, K.G. 2012, 'Antarctic Coastal Microalgal Primary Production And Photosynthesis', Marine Biology, vol. 159, no. 12, pp. 2827-2837.
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Primary production in coastal Antarctica is primarily contributed from three sources: sea ice algae, phytoplankton, and microphytobenthos. Compared to other eastern Antarctic sites, the sea ice microalgal biomass at Casey Station, in spring 2005 was rela
Wangpraseurt, D., Larkum, A.W.D., Ralph, P.J. & Khl, M. 2012, 'Light gradients and optical microniches in coral tissues', Frontiers in Microbiology, vol. 3, no. AUG.
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Light quantity and quality are among the most important factors determining the physiology and stress response of zooxanthellate corals. Yet, almost nothing is known about the light field that Symbiodinium experiences withintheircoralhost, andthebasic opticalproperties of coral tissue are unknown.We used scalar irradiance microprobes to characterize vertical and lateral light gradients within and across tissues of several coral species. Our results revealed the presence of steep light gradients with photosynthetically available radiation decreasing by about one order of magnitude from the tissue surface to the coral skeleton. Surface scalar irradiance was consistently higher over polyp tissue than over coenosarc tissue in faviid corals. Coral bleaching increased surface scalar irradiance by ~150% (between 500 and 700 nm) relative to a healthy coral. Photosynthesis peaked around 300 ?m within the tissue, which corresponded to a zone exhibiting strongest depletion of scalar irradiance. Deeper coral tissue layers, e.g., ~1000 ?m into aboral polyp tissues, harbor optical microniches, where only ~10% of the incident irradiance remains. We conclude that the optical microenvironment of corals exhibits strong lateral and vertical gradients of scalar irradiance, which are affected by both tissue and skeleton optical properties. Our results imply that zooxanthellae populations inhabit a strongly heterogeneous light environment and highlight the presence of different optical microniches in corals; an important finding for understanding the photobiology, stress response, as well as the phenotypic and genotypic plasticity of coral symbionts. 2012 Wangpraseurt, Larkum, Ralph and Khl.
Petrou, K., Doblin, M.A. & Ralph, P.J. 2011, 'Heterogeneity in the photoprotective capacity of three Antarctic diatoms during short-term changes in salinity and temperature', Marine Biology, vol. 158, no. 5, pp. 1029-1041.
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The Antarctic marine ecosystem changes seasonally, forming a temporal continuum of specialised niche habitats including open ocean, sea ice and meltwater environments. The ability for phytoplankton to acclimate rapidly to the changed conditions of these environments depends on the species' physiology and photosynthetic plasticity and may ultimately determine their long-term ecological niche adaptation. This study investigated the photophysiological plasticity and rapid acclimation response of three Antarctic diatoms-Fragilariopsis cylindrus, Pseudo-nitzschia subcurvata and Chaetoceros sp.-to a selected range of temperatures and salinities representative of the sea ice, meltwater and pelagic habitats in the Antarctic. Fragilariopsis cylindrus displayed physiological traits typical of adaptation to the sea ice environment. Equally, this species showed photosynthetic plasticity, acclimating to the range of environmental conditions, explaining the prevalence of this species in all Antarctic habitats. Pseudo-nitzschia subcurvata displayed a preference for the meltwater environment, but unlike F. cylindrus, photoprotective capacity was low and regulated via changes in PSII antenna size. Chaetoceros sp. had high plasticity in non-photochemical quenching, suggesting adaptation to variable light conditions experienced in the wind-mixed pelagic environment. While only capturing short-term responses, this study highlights the diversity in photoprotective capacity that exists amongst three dominant Antarctic diatom species and provides insight into links between ecological niche adaptation and species' distribution. 2011 Springer-Verlag.
Behrendt, L., Larkum, A.W.D., Norman, A., Qvortrup, K., Chen, M., Ralph, P., Srensen, S.J., Trampe, E. & Khl, M. 2011, 'Endolithic chlorophyll d-containing phototrophs', ISME Journal, vol. 5, no. 6, pp. 1072-1076.
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Cyanobacteria in the genus Acaryochloris are the only known oxyphototrophs that have exchanged chlorophyll a (Chl a) with Chl d as their primary photopigment, facilitating oxygenic photosynthesis with near infrared (NIR) light. Yet their ecology and natural habitats are largely unknown. We used hyperspectral and variable chlorophyll fluorescence imaging, scanning electron microscopy, photopigment analysis and DNA sequencing to show that Acaryochloris-like cyanobacteria thrive underneath crustose coralline algae in a widespread endolithic habitat on coral reefs. This finding suggests an important role of Chl d-containing cyanobacteria in a range of hitherto unexplored endolithic habitats, where NIR light-driven oxygenic photosynthesis may be significant. 2011 International Society for Microbial Ecology. All rights reserved.
Ulstrup, K.E., Khl, M., van Oppen, M.J.H., Cooper, T.F. & Ralph, P.J. 2011, 'Variation in photosynthesis and respiration in geographically distinct populations of two reef-building coral species', Aquatic Biology, vol. 12, no. 3, pp. 241-248.
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Studies of the regulation and importance of physiological processes such as coral photosynthesis and respiration on coral reefs require knowledge of spatio-temporal patterns of variability at different scales. Oxygen microelectrodes were used to measure photosynthesis and dark respiration of 2 corals, Pocillopora damicornis and Turbinaria reniformis, in the northern (Lizard Island) and central (Davies and Broadhurst Reefs) regions of the Great Barrier Reef (GBR) in winter and summer. Genetic characterisation of Symbiodinium revealed that P. damicornis hosted a single symbiont type (Symbiodinium C1) in both regions, whereas T. reniformis harboured 2 types, dependent on location. Colonies at Lizard Island harboured Symbiodinium D, whereas colonies at Davies Reef harboured Symbiodinium C2. Rates of gross photosynthesis were greater in the central than in the northern GBR in summer. A similar pattern was detected for dark respiration rates in T. reniformis. No seasonal change in either photosynthesis or dark respiration was evident in the northern GBR, possibly due to less annual variability in light conditions, and for T. reniformis, additionally the presence of Symbiodinium D. These results highlight that environmental conditions coupled with regional-scale distribution of Symbiodinium are likely to exert important influences on respiration and photosynthetic performance of reef-building corals. & copy Inter-Research 2011.
Earp, A., Hanson, C.E., Ralph, P.J., Brando, V.E., Allen, S., Baird, M., Clementson, L., Daniel, P., Dekker, A.G., Fearns, P.R.C.S., Parslow, J., Strutton, P.G., Thompson, P.A., Underwood, M., Weeks, S. & Doblin, M.A. 2011, 'Review of fluorescent standards for calibration of in situ fluorometers: Recommendations applied in coastal and ocean observing programs', Optics Express, vol. 19, no. 27, pp. 26768-26782.
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Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic compounds, as well as to infer geophysical parameters such as chlorophyll or CDOM concentration, but measurements are susceptible to variation caused by biofouling, instrument design, sensor drift, operating environment, and calibration rigor. To collect high quality data, such sensors need frequent checking and regular calibration. In this study, a wide variety of both liquid and solid fluorescent materials were trialed to assess their suitability as reference standards for performance assessment of in situ fluorometers. Criteria used to evaluate the standards included the spectral excitation/emission responses of the materials relative to fluorescence sensors and to targeted ocean properties, the linearity of the fluorometer's optical response with increasing concentration, stability and consistency, availability and ease of use, as well as cost. Findings are summarized as a series of recommended reference standards for sensors deployed on stationary and mobile platforms, to suit a variety of in situ coastal to ocean sensor configurations. Repeated determinations of chlorophyll scale factor using the recommended liquid standard, Fluorescein, achieved an accuracy of 2.5%. Repeated measurements with the recommended solid standard, Plexiglas Satinice plum 4H01 DC (polymethylmethacrylate), over an 18 day period varied from the mean value by 1.0% for chlorophyll sensors and 3.3% for CDOM sensors. 2011 Optical Society of America.
Petrou, K., Hill, R., Doblin, M.A., McMinn, A., Johnson, R., Wright, S.W. & Ralph, P.J. 2011, 'Photoprotection of sea-ice microalgal communities from the east antarctic pack ice', Journal of Phycology, vol. 47, no. 1, pp. 77-86.
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All photosynthetic organisms endeavor to balance energy supply with demand. For sea-ice diatoms, as with all marine photoautotrophs, light is the most important factor for determining growth and carbon-fixation rates. Light varies from extremely low to often relatively high irradiances within the sea-ice environment, meaning that sea-ice algae require moderate physiological plasticity that is necessary for rapid light acclimation and photoprotection. This study investigated photoprotective mechanisms employed by bottom Antarctic sea-ice algae in response to relatively high irradiances to understand how they acclimate to the environmental conditions presented during early spring, as the light climate begins to intensify and snow and sea-ice thinning commences. The sea-ice microalgae displayed high photosynthetic plasticity to increased irradiance, with a rapid decline in photochemical efficiency that was completely reversible when placed under low light. Similarly, the photoprotective xanthophyll pigment diatoxanthin (Dt) was immediately activated but reversed during recovery under low light. The xanthophyll inhibitor dithiothreitol (DTT) and state transition inhibitor sodium fluoride (NaF) were used in under-ice in situ incubations and revealed that nonphotochemical quenching (NPQ) via xanthophyll-cycle activation was the preferred method for light acclimation and photoprotection by bottom sea-ice algae. This study showed that bottom sea-ice algae from the east Antarctic possess a high level of plasticity in their light-acclimation capabilities and identified the xanthophyll cycle as a critical mechanism in photoprotection and the preferred means by which sea-ice diatoms regulate energy flow to PSII. 2011 Phycological Society of America.
Petrou, K. & Ralph, P.J. 2011, 'Photosynthesis and net primary productivity in three Antarctic diatoms: Possible significance for their distribution in the Antarctic marine ecosystem', Marine Ecology Progress Series, vol. 437, pp. 27-40.
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Photosynthesis and net primary productivity were measured in 3 Antarctic diatoms, Fragilariopsis cylindrus, Pseudo-nitzschia subcurvata and Chaetoceros sp., exposed to rapid changes in temperature and salinity representing a range of conditions found during a seasonal cycle. Measured differences in fluorescence-derived photosynthetic activity and oxygen evolution suggested that some alternative electron cycling activity was present under high irradiances. F. cylindrus displayed the highest rates of relative electron transport and net primary productivity under all salinity and temperature combinations and showed adaptive traits towards the sea-icelike environment. P. subcurvata displayed a preference for low saline conditions where production rates were greatest. However, there was evidence of photosynthetic sensitivity to the lowest temperatures and highest salinities, suggesting a lack of adaptation for dealing with sea-ice-like conditions. Chaetoceros sp. showed high plasticity, acclimating well to all conditions but performing best under pelagic conditions. The study shows species-specific sensitivities to environmental change, highlighting photosynthetic capacity as a potentially important mechanism in ecological niche adaptation. When these data were modelled over different seasons, integrated daily net primary production was greatest under summer pelagic conditions. The findings from this study support the general observations of light control and seasonal development of net primary productivity and species succession in the Antarctic marine ecosystem. Inter-Research 2011.
Doblin, M.A., Petrou, K.L., Shelly, K., Westwood, K., van den Enden, R., Wright, S., Griffiths, B. & Ralph, P.J. 2011, 'Diel variation of chlorophyll-a fluorescence, phytoplankton pigments and productivity in the Sub-Antarctic and Polar Front Zones south of Tasmania, Australia', Deep-Sea Research Part II: Topical Studies in Oceanography, vol. 58, no. 21-22, pp. 2189-2199.
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Marine primary production is a fundamental measure of the ocean's capacity to convert carbon dioxide to particulate organic carbon for the marine foodweb, and as such is an essential variable used in ecosystem and biogeochemical models to assess trophic dynamics and carbon cycling. The Sub-Antarctic Zone (SAZ) is a major sink for atmospheric carbon and exhibits large gradients in ocean conditions on both temporal and spatial scales. In this dynamic system, an understanding of small-scale temporal changes is critical for modelling primary production at larger scales. Thus, we investigated diel effects on maximum quantum yield of PSII (FV/FM), photosynthetic pigment pools and primary productivity in the western (Diel 1) and eastern SAZ region (Diel 3) south of Tasmania, Australia, and compared this to a station at the polar front (Diel 2). Phytoplankton in the eastern SAZ had the greatest diel response, with cells showing decreased FV/FM and increased biosynthesis and transformation of xanthophyll and other photoprotective pigments during the day, but only in the surface waters (0 and 10m). Diel responses diminished by 30m. Cells in the western SAZ had similar responses across the depths sampled, increasing their FV/FM during the night and increasing their xanthophyll pigment content during the day. Phytoplankton at the polar front (Diel 2) showed intermediate diel-related variations in photophysiology, with xanthophyll conversion and increases in photoprotective pigments during the day but constant FV/FM. These diel changes at all sampling stations had little impact on carbon fixation rates, although cells sampled from the deep chlorophyll maximum at the polar front had significantly lower maximum carbon fixation and minimum saturating irradiance (Ek) compared to the other depths and stations. Considering the oceanographic context, cells at Diel 1 and 2 received less light and were more deeply mixed than cells at Diel 3, causing a dampening of the diel response. These results highlight that phytoplankton in the SAZ is regulated by the physical processes of mixing and light provision, but short-term diel effects on maximum quantum yield of PSII and photoprotective pigments may not propagate to changes in carbon fixation, particularly when cells are nutrient replete. If however, the more stratified eastern SAZ (which had the greatest diel responses) is indicative of how the SAZ region might respond to climate change, then diel effects may become more prominent in the future. 2011.
Petrou, K., Hassler, C.S., Doblin, M.A., Shelly, K., Schoemann, V., van den Enden, R., Wright, S. & Ralph, P.J. 2011, 'Iron-limitation and high light stress on phytoplankton populations from the Australian Sub-Antarctic Zone (SAZ)', Deep-Sea Research Part II: Topical Studies in Oceanography, vol. 58, no. 21-22, pp. 2200-2211.
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The high nutrient low chlorophyll (HNLC) surface waters of the Southern Ocean are characterised by high concentrations of nitrate and phosphate, low concentrations of dissolved iron and deep vertical mixing. Future climate scenarios predict increased surface temperatures and ocean stratification in the region. These changes to vertical mixing will result in a slowdown of nutrient supply to surface waters and an increase in the integrated irradiance in the upper mixed layer. To investigate the influence of iron-limitation and high irradiance on phytoplankton growth and physiology, a 6-day shipboard incubation experiment was conducted during the Sub-Antarctic Zone Sensitivity to Environmental Change (SAZ Sense) voyage using phytoplankton populations from the upper mixed layer in the north-eastern SAZ region. Iron-limitation was induced with an organic siderophore and was compared with a 1nM iron-enriched incubation and an unamended treatment (under silicate replete conditions). As expected, iron enrichment led to dominance by large diatoms and enhanced photosynthetic performance, while the iron-limited community showed a decline in total chl a and photochemical efficiency. Under the added stress of high light, the iron-limited community was able to cope with the shift from in situ (<150?molphotonsm -2s -1) to incubation (mean=765?molphotonsm -2s -1) irradiance by increasing the proportion of photoprotective pigments and diverting excess light energy via energy-dependent quenching (q E). The responses to iron-limitation under high light showed that the phytoplankton community was able to acclimate to these conditions, but exhibited an overall decline in photosynthetic activity. Data presented here suggest the community shifts, in particular the decrease in diatoms, and the decline in photosynthetic performance of phytoplankton under low iron-high irradiance conditions has the potential to impact future ocean productivity and biogeochemical cycling. 2011 Elsevier Ltd.
Jimenez, I.M., Ku?l, M., Larkum, A.W.D. & Ralph, P.J. 2011, 'Effects of flow and colony morphology on the thermal boundary layer of corals', Journal of the Royal Society Interface, vol. 8, no. 65, pp. 1785-1795.
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The thermal microenvironment of corals and the thermal effects of changing flow and radiation are critical to understanding heat-induced coral bleaching, a stress response resulting from the destruction of the symbiosis between corals and their photosynthetic microalgae. Temperature microsensor measurements at the surface of illuminated stony corals with uneven surface topography (Leptastrea purpurea and Platygyra sinensis) revealed millimetre- scale variations in surface temperature and thermal boundary layer (TBL) that may help understand the patchy nature of coral bleaching within single colonies. The effect of water flow on the thermal microenvironment was investigated in hemispherical and branching corals (Porites lobata and Stylophora pistillata, respectively) in a flow chamber experiment. For both coral types, the thickness of the TBL decreased exponentially from 2.5 mm at quasi-stagnant flow (0.3 cm s -1), to 1 mm at 5 cm s -1, with an exponent approximately 0.5 consistent with predictions from the heat transfer theory for simple geometrical objects and typical of laminar boundary layer processes. Measurements of mass transfer across the diffusive boundary layer using O 2 microelectrodes revealed a greater exponent for mass transfer when compared with heat transfer, indicating that heat and mass transfer at the surface of corals are not exactly analogous processes. 2011 The Royal Society.
Hill, R., Brown, C.M., DeZeeuw, K., Campbell, D.A. & Ralph, P.J. 2011, 'Increased rate of D1 repair in coral symbionts during bleaching is insufficient to counter accelerated photo-inactivation', LIMNOLOGY AND OCEANOGRAPHY, vol. 56, no. 1, pp. 139-146.
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Sinutok, S., Hill, R., Doblin, M.A., Wuhrer, R. & Ralph, P.J. 2011, 'Warmer more acidic conditions cause decreased productivity and calcification in subtropical coral reef sediment-dwelling calcifiers', Limnology and Oceanography, vol. 56, no. 4, pp. 1200-1212.
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The effects of elevated CO 2 and temperature on photosynthesis and calcification in the calcifying algae Halimeda macroloba and Halimeda cylindracea and the symbiont-bearing benthic foraminifera Marginopora vertebralis were investigated through exposure to a combination of four temperatures (28C, 30C, 32C, and 34C) and four CO 2 levels (39, 61, 101, and 203 Pa; pH 8.1, 7.9, 7.7, and 7.4, respectively). Elevated CO 2 caused a profound decline in photosynthetic efficiency (F V: F M), calcification, and growth in all species. After five weeks at 34C under all CO 2 levels, all species died. Chlorophyll (Chl) a and b concentration in Halimeda spp. Significantly decreased in 203 Pa, 32C and 34C treatments, but Chl a and Chl c 2 concentration in M. vertebralis was not affected by temperature alone, with significant declines in the 61, 101, and 203 Pa treatments at 28C. Significant decreases in F V: F M in all species were found after 5 weeks of exposure to elevated CO 2 (203 Pa in all temperature treatments) and temperature (32C and 34C in all pH treatments). The rate of oxygen prodCtion declined at 61, 101, and 203 Pa in all temperature treatments for all species. The elevated CO 2 and temperature treatments greatly reduced calcification (growth and crystal size) in M. vertebralis and, to a lesser extent, in Halimeda spp. These findings indicate that 32C and 101 Pa CO 2, are the upper limits for survival of these species on Heron Island reef, and we conclude that these species will be highly vulnerable to the predicted future climate change scenarios of elevated temperature and ocean acidification. 2011, by the American Society of Limnology and Oceanography, Inc.
Herrera-Silveira, J.A., Cebrian, J., Hauxwell, J., Ramirez-Ramirez, J. & Ralph, P.J. 2010, 'Evidence of negative impacts of ecological tourism on turtlegrass (Thalassia testudinum) beds in a marine protected area of the Mexican Caribbean', Aquatic Ecology, vol. 44, no. 1, pp. 23-31.
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Many marine protected areas (MPAs) have been established in recent years. Some MPAs are open to tourists to foster environmental education and generate revenue for the MPA. This has been coined ecological tourism?. Here, we examine the impact of ecological tourism on turtlegrass (Thalassia testudinum) health in one area of the Costa Occidental de Isla Mujeres, Punta Cancn y Punta Nizuc? MPA in the Mexican Caribbean. A heavily visited location was compared with an unvisited location. Turtlegrass leaves at the visited location were sparser, shorter, grew more slowly, and had more epiphytes than at the unvisited location. Vertical and horizontal rhizomes of turtlegrass also grew more slowly at the visited than at the unvisited location. There is reasonable evidence to suggest that the observed differences are likely due to the deleterious impacts of novice and careless snorkelers. If continuing, these impacts could cause severe degradation of the visited areas in this MPA and, thus, changes in management policies seem in order.
Csszr, N.B.M., Ralph, P.J., Frankham, R., Berkelmans, R. & van Oppen, M.J.H. 2010, 'Estimating the Potential for Adaptation of Corals to Climate Warming', PLoS ONE, vol. 5, no. 3.
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The persistence of tropical coral reefs is threatened by rapidly increasing climate warming, causing a functional breakdown of the obligate symbiosis between corals and their algal photosymbionts (Symbiodinium) through a process known as coral bleaching. Yet the potential of the coral-algal symbiosis to genetically adapt in an evolutionary sense to warming oceans is unknown. Using a quantitative genetics approach, we estimated the proportion of the variance in thermal tolerance traits that has a genetic basis (i.e. heritability) as a proxy for their adaptive potential in the widespread Indo-Pacific reef-building coral Acropora millepora. We chose two physiologically different populations that associate respectively with one thermotolerant (Symbiodinium clade D) and one less tolerant symbiont type (Symbiodinium C2). In both symbiont types, pulse amplitude modulated (PAM) fluorometry and high performance liquid chromatography (HPLC) analysis revealed significant heritabilities for traits related to both photosynthesis and photoprotective pigment profile. However, quantitative real-time polymerase chain reaction (qRT-PCR) assays showed a lack of heritability in both coral host populations for their own expression of fundamental stress genes. Coral colony growth, contributed to by both symbiotic partners, displayed heritability. High heritabilities for functional key traits of algal symbionts, along with their short clonal generation time and high population sizes allow for their rapid thermal adaptation. However, the low overall heritability of coral host traits, along with the corals' long generation time, raise concern about the timely adaptation of the coral-algal symbiosis in the face of continued rapid climate warming. 2010 Csszr et al.
McMinn, A., Pankowskii, A., Ashworth, C., Bhagooli, R., Ralph, P.J. & Ryan, K. 2010, 'In situ net primary productivity and photosynthesis of Antarctic sea ice algal, phytoplankton and benthic algal communities', Marine Biology, vol. 157, no. 6, pp. 1345-1356.
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Primary production at Antarctic coastal sites is contributed from sea ice algae, phytoplankton and benthic algae. Oxygen microelectrodes were used to estimate sea ice and benthic primary production at several sites around Casey, a coastal area in eastern Antarctica. Maximum oxygen export from sea ice was 0.95 mmol O2 m-2 h-1 (*11.7 mg C m-2 h-1) while from the sediment it was 6.08 mmol O2 m-2 h-1 (*70.8 mg C m-2 h-1). When the ice was present O2 export from the benthos was either low or negative. Sea ice algae assimilation rates were up to 3.77 mg C (mg Chl-a)-1 h-1 while those from the benthos were up to 1.53 mg C (mg Chl-a)-1 h-1. The contribution of the major components of primary productivity was assessed using fluorometric techniques. When the ice was present approximately 5565% of total daily primary production occurred in the sea ice with the remainder unequally partitioned between the sediment and the water column. When the ice was absent, the benthos contributed nearly 90% of the primary production.
Petrou, K., Hill, R., Brown, C.M., Campbell, D.A., Doblin, M.A. & Ralph, P.J. 2010, 'Rapid photoprotection in sea-ice diatoms from the East Antarctic pack ice', Limnology and Oceanography, vol. 55, no. 3, pp. 1400-1407.
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Photoinhibition and D1 protein re-synthesis were investigated in bottom-dwelling sea-ice microalgal communities from the East Antarctic pack ice during early spring. Bottom-dwelling sea-ice microalgal communities were dominated by diatoms that exhibited rapid photoprotection when exposed to a range of different light levels (10 ?mol photons m-2 s -1, 50 ?mol photons m-2 s-1, 100 ?mol photons m-2 s-1, and 200 ?mol photons m-2 s-1). Photosynthetic capacity of photosystem II (PSII) dropped significantly over 3 h under 200 ?mol photons m-2 s-1, but largely recovered when placed in a low-light environment (10 ?mol photons m-2 s-1) for an additional 3 h. PSII repair rates increased with increasing irradiance, and the D1-protein pool remained steady even under high light (200 ?mol photons m-2 s-1). Sea-ice diatoms showed a low intrinsic susceptibility to photoinactivation of PSII across all the light treatments, and a strong and irradiance-dependent induction of nonphotochemical quenching, which did not depend upon chloroplast protein synthesis, was also seen. These highly plastic organisms, once thought to be adapted to shade, are in fact well equipped to withstand rapid and relatively large changes in light at low temperatures with minimal long-term effect on their photosynthetic machinery. 2010, by the American Society of Limnology and Oceanography, Inc.
Lilley, R.M., Ralph, P.J. & Larkum, A.W.D. 2010, 'The determination of activity of the enzyme Rubisco in cell extracts of the dinoflagellate alga Symbiodinium sp. by manganese chemiluminescence and its response to short-term thermal stress of the alga', Plant, Cell and Environment, vol. 33, no. 6, pp. 995-1004.
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The dinoflagellate alga Symbiodinium sp., living in symbiosis with corals, clams and other invertebrates, is a primary producer in coral reefs and other marine ecosystems. The function of the carbon-fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in dinoflagellates is difficult to study because its activity is rapidly lost after extraction from the cell. We report procedures for the extraction of Rubisco from Symbiodinium cells and for stable storage. We describe a continuous assay for Rubisco activity in these crude cell extracts using the Mn2+ chemiluminescence of Rubisco oxygenase. Chemiluminescence time courses exhibited initial transients resembling bacterial Form II Rubisco, followed by several minutes of linearly decreasing activity. The initial activity was determined from extrapolation of this linear section of the time course. The activity of fast-frozen cell extracts was stable at -80 C and, after thawing and storage on ice, remained stable for up to 1 h before declining non-linearly. Crude cell extracts bound [14C] 2-carboxy-D-arabitinol 1,5-bisphosphate to a high molecular mass fraction separable by gel filtration chromatography. After pre-treatment of Symbiodinium cell cultures in darkness at temperatures above 30 C, the extracted Rubisco activities decreased, with almost complete loss of activity above 36 C. The implications for the sensitivity to elevated temperature of Symbiodinium photosynthesis are assessed. 2010 Blackwell Publishing Ltd.
Wright, J.T., Byers, J.E., Koukoumaftsis, L.P., Ralph, P.J. & Gribben, P.E. 2010, 'Native species behaviour mitigates the impact of habitat-forming invasive seaweed', Oecologia, vol. 163, no. 2, pp. 527-534.
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Habitat-forming invasive species cause large, novel changes to the abiotic environment. These changes may elicit important behavioural responses in native fauna, yet little is known about mechanisms driving this behaviour and how such trait-mediated responses influence the fitness of native species. Low dissolved oxygen is a key abiotic change created by the habitat-forming invasive seaweed, Caulerpa taxifolia, which influences an important behavioural response (burrowing depth) in the native infaunal bivalve Anadara trapezia. In Caulerpa-colonised areas, Anadara often emerged completely from the sediment, and we experimentally demonstrate that water column hypoxia beneath the Caulerpa canopy is the mechanism instigating this "pop-up" behaviour. Importantly, pop-up in Caulerpa allowed similar survivorship to that in unvegetated sediment; however, when we prevented Anadara from popping-up, they suffered >50% mortality in just 1 month. Our findings not only highlight the substantial environmental alteration by Caulerpa, but also an important role for the behaviour of native species in mitigating the effects of habitat-forming invasive species. 2010 Springer-Verlag.
Seery, C.R. & Ralph, P.J. 2010, 'Ecological relevance of a chlorophyll a fluorescence ecotoxicological endpoint', Toxicological and Environmental Chemistry, vol. 92, no. 8, pp. 1529-1540.
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With the increasing use of new techniques to assess toxic stress and the effects of pollution, it is important to ensure that any "new" endpoint employed is able to demonstrate a level of eco-relevance. Though recent application of chlorophyll a fluorescence parameters to macroalgal bioassays has been successful, a level of eco-relevance for such use has not been experimentally shown. As such, this article presents a series of experiments designed to establish a link between fluorescence parameters and higher-level effects (supra-individual level). The results show that fluorescence parameters are capable of predicting germination success of macroalgal gametes and can do so much sooner than the actual event of germination. Furthermore, a decline in ?F/F?m is shown to be strongly associated with a reduced germination success. Thus, a level of eco-relevance has been established for the fluorescence endpoint that is at least equal to that of the widely accepted germination-based macroalgal bioassays. Taylor & Francis.
Hill, R., Ulstrup, K.E. & Ralph, P.J. 2009, 'Temperature induced changes in thylakoid membrane thermostability of cultured, freshly isolated, and expelled zooxanthellae from scleractinian corals', Bulletin of Marine Science, vol. 85, no. 3, pp. 223-244.
Coral bleaching events are characterized by a dysfunction between the cnidar- ian coral host and the symbiotic dinoflagellate algae, known as zooxanthellae (genus Symbiodinium). Elevated temperature and intense light induce coral bleaching, where zooxanthellae are expelled from the host tissue. The primary cellular process in zooxanthellae which leads to coral bleaching is unresolved, and here, we investigated the sensitivity of the thylakoid membrane in a Symbiodinium culture and in genetically identified freshly isolated and expelled Symbiodinium cells. The fluorescence-temperature curve technique was used to measure the critical temperature (Tc) at which irreversible damage to the thylakoid membrane occurred. The accuracy of this technique was confirmed through the collection of scanning transmission electron micrographs which demonstrated the clear relationship between Tc and thylakoid membrane degradation. Analysis of 10 coral species with a diverse range of genetically distinct Symbiodinium communities showed a decline in Tc from summer to winter. A Symbiodinium culture and fragments of Pocillopora damicornis (Linnaeus, 1758) were exposed to a series of light and temperature treat ments, where Tc increased from approximately 37 C to 42 C upon exposure to elevated temperature. Under bleaching conditions, the thermostability of the thylakoid membrane increased within 4 hrs by 5.1 C, to a temperature far above bleaching thresholds, in both freshly isolated and photosynthetically competent zooxanthellae expelled from P. damicornis under these conditions. It is demonstrated that the thermostability of the thylakoid membrane increases in cultured, freshly isolated, and expelled zooxanthellae exposed to bleaching stress, suggesting it is not the pri mary site of impact during coral bleaching events. 2009 Rosenstiel School of Marine and Atmospheric Science of the University of Miami.
Buxton, L.J., Badger, M. & Ralph, P.J. 2009, 'Effects Of Moderate Heat Stress And Dissolved Inorganic Carbon Concentration On Photosynthesis And Respiration Of Symbiodinium Sp (Dinophyceae) In Culture And In Symbiosis', Journal of Phycology, vol. 45, no. 2, pp. 357-365.
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The influence of temperature and inorganic carbon (C-i) concentration on photosynthesis was examined in whole corals and samples of cultured symbiotic dinoflagellates (Symbiodinium sp.) using combined measurements from a membrane inlet mass spectrometer and chl a fluorometer. In whole corals, O-2 production at 26 degrees C was significantly limited at C-i concentrations below ambient seawater (similar to 2.2 mM). Further additions of C-i up to similar to 10 mM caused no further stimulation of oxygenic photosynthesis. Following exposure to 30 degrees C (2 d), net oxygen production decreased significantly in whole corals, as a result of reduced production of photosynthetically derived oxygen rather than increased host consumption. Whole corals maintained a rate of oxygen evolution around eight times lower than cultured Symbiodinium sp. at inorganic carbon concentrations < 2 mM, but cultures displayed greater levels of photoinhibition following heat treatment (30 degrees C, 2 d). Whole corals and cultured zooxanthellae differed considerably in their responses to C-i concentration and moderate heat stress, demonstrating that cultured Symbiodinium make an incongruous model for those in hospite. Reduced net oxygen evolution, in whole corals, under conditions of low C-i (< 2 mM) has been interpreted in terms of possible sink limitation leading to increased nonphotochemical energy dissipation. The advantages of combined measurement of net gas exchange and fluorometry offered by this method are discussed.
Strom, D., Ralph, P.J. & Stauber, J.L. 2009, 'Development of a toxicity identification evaluation protocol using chlorophyll-a fluorescence in a marine microalga', Archives of Environmental Contamination and Toxicology, vol. 56, no. 1, pp. 30-38.
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Growth inhibition bioassays with the microalga Nitzschia closterium have recently been applied in marine Toxicity Identification Evaluation (TIE) testing. However, the 48-h test duration can result in substantial loss of toxicants over time, which might lead to an underestimation of the sample toxicity. Although shorter-term microalgal bioassays can minimize such losses, there are few bioassays available and none are adapted for marine TIE testing. The acute (5-min) chlorophyll-a fluorescence bioassay is one alternative; however, this bioassay was developed for detecting herbicides in freshwater aquatic systems and its suitability for marine TIE testing was not known. In this study, a chlorophyll-a fluorescence bioassay using the marine microalga Isochrysis galbana was able to detect contaminants other than herbicides at environmentally relevant concentrations and tolerated the physical and chemical manipulations needed for a Phase I TIE. Phase I TIE procedures were successfully developed using this chlorophyll-a fluorescence bioassay and used to identify all classes of contaminants present in a synthetic mixture of known chemical composition. In addition, TIEs with both the acute fluorescence bioassay and the standard growth inhibition bioassay identified the same classes of toxicants in a sample of an unknown complex effluent. Even though the acute chlorophyll-a fluorescence end point was less sensitive than the chronic cell division end point, TIEs with the chlorophyll-a fluorescence bioassay provided a rapid and attractive alternative to longer-duration bioassays. 2008 Springer Science+Business Media, LLC.
Baird, A.H., Bhagooli, R., Ralph, P.J. & Takahashi, S. 2009, 'Coral bleaching: the role of the host', Trends in Ecology and Evolution, vol. 24, no. 1, pp. 16-20.
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Coral bleaching caused by global warming is one of the major threats to coral reefs. Very recently, research has focused on the possibility of corals switching symbionts as a means of adjusting to accelerating increases in sea surface temperature. Although symbionts are clearly of fundamental importance, many aspects of coral bleaching cannot be readily explained by differences in symbionts among coral species. Here we outline several potential mechanisms by which the host might influence the bleaching response, and conclude that predicting the fate of corals in response to climate change requires both members of the symbiosis to be considered equally. 2008 Elsevier Ltd. All rights reserved.
Collier, C.J., Lavery, P.S., Ralph, P.J. & Masini, R.J. 2009, 'Shade-induced response and recovery of the seagrass Posidonia sinuosa', Journal of Experimental Marine Biology and Ecology, vol. 370, no. 1-2, pp. 89-103.
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The effect of shading on the seagrass Posidonia sinuosa Cambridge et Kuo was investigated to identify mechanisms that prolong its survival during periods of low light and permit its subsequent recovery. We also tested whether the responses were consistent in plants growing at different depths. Shade treatments were low (LS; 70 - 100% of ambient Photosynthetic Photon Flux Density), medium (MS; 12 - 39%) and heavy (HS; 5 - 4%) at the shallow (3 - 4m) site, whilst the deep (7 - 8m) site had no HS treatment. HS at the shallow and MS at the deep site were below minimum light requirements (MLR) for the long-term survival of P. sinuosa. Physiological, morphological and growth attributes were repeatedly measured during 198 d of shade treatments and a subsequent 384 d recovery period at ambient PPFD. Shoot density declined by 82% within 105 d under HS treatment, though 6% of shoots remained after 198 d. We estimate that complete shoot loss in HS would have taken 2years. Rhizome sugar concentrations declined to 32 - 52% of the controls at the end of the most severe shading treatments but after shoot loss, sugar concentrations declined more slowly or increased, suggesting a return to positive carbon balance. In the treatments below MLR, shading induced changes in physiological, morphological and growth characteristics, including reduced leaf length and width, reduced ?13C and photosynthetic adaptation to low light (increased ?, reduced Ek and ETRmax), though not consistently. After removal of shading, photosynthetic characteristics became more typical of high light adaptation, possibly induced by greater light penetration through the thinned canopy, including reversal of the changes in ?, Ek and ETRmax and induction of non-photochemical quenching. Carbohydrate concentrations increased to ambient concentrations within 115 d at ambient PPFD. Recovery of shoot density was slow, remaining significantly reduced in the MS and HS treatments after 384 d recovery. Shoot density at the end of shading is an important determinant of the rate seagrass meadows will recover and we estimated that the moderately and heavily shaded meadows would require 3.5 to 5years to recover. 2008 Elsevier B.V. All rights reserved.
Khl, M., Holst, G., Larkum, A.W.D. & Ralph, P.J. 2008, 'Imaging of oxygen dynamics within the endolithic algal community of the massive coral Porites lobata', Journal of Phycology, vol. 44, no. 3, pp. 541-550.
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We used transparent planar oxygen optodes and a luminescence lifetime imaging system to map (at a pixel resolution of <200 ?m) the two-dimensional distribution of O2 within the skeleton of a Porites lobata colony. The O2 distribution was closely correlated to the distribution of the predominant endolithic microalga, Ostreobium quekettii Bornet et Flahault that formed a distinct green band inside the skeleton. Oxygen production followed the outline of the Ostreobium band, and photosynthetic O2 production was detected at only 0.2 ?mol photons m-2 s-1, while saturation occurred at ?37 ?mol photons m-2 s-1. Oxygen levels varied from ?60% to 0% air saturation in the illuminated section of the coral skeleton in comparison to the darkened section. The O2 production within the Ostreobium band was lower in the region below the upward facing surface of the coral and elevated on the sides. Oxygen consumption in darkness was also greatest within the Ostreobium zone, as well as in the white skeleton zone immediately below the corallites. The rate of O2 depletion was not constant within zones and between zones, showing pronounced heterogeneity in endolithic respiration. When the coral was placed in darkness after a period of illumination, O 2 levels declined by 50% within 20 min and approached steady-state after 40-50 min in darkness. Our study demonstrates the use of an important new tool in endolith photobiology and presents the first data of spatially resolved O2 concentration and its correlation to the physical structures and specific zones responsible for O2 production and consumption within the coral skeleton. 2008 Phycological Society of America.
Hill, R. & Ralph, P.J. 2008, 'Dark-induced reduction of the plastoquinone pool in zooxanthellae of scleractinian corals and implications for measurements of chlorophyll a fluorescence', Symbiosis, vol. 46, no. 1, pp. 45-56.
Fluorometric measurements of maximum quantum yield (Fv/F m) and fast induction curves (FICs) require coral samples to be dark-adapted (DA). Pathways causing dark-reduction of the plastoquinone (PQ) pool are shown here to be active in corals. Early morning sunlight and far-red light successfully increased Fv/Fm and lowered the O and J steps of FICs in corals that were darkened overnight. The thick-tissued massive coral, Cyphastrea serailia, was shown to be more prone to reduction of the PQ pool, with significant reductions in Fv/Fm occurring after 10 min of DA, and elevated J steps occurring within 200 s following a far-red flash. In thinner-tissued branching species, Pocillopora damicornis and Acropora nobilis, elevation of the J step also occurred within 200 s of DA, but a drop in Fv/Fm was only manifested after 30 min. Pre-exposure to far-red light is an effective and simple procedure to ensure determination of the true maximum quantum yield of Photosystem II (PSII) and accurate FICs which require a fully oxidised inter-system electron transport chain and open PSII reaction centres. 2008 Balaban.
Hill, R. & Ralph, P.J. 2008, 'Impact of bleaching stress on the function of the oxygen evolving complex of zooxanthellae from scleractinian corals', Journal of Phycology, vol. 44, no. 2, pp. 299-310.
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Global climate change is leading to the rise of ocean temperatures and is triggering mass coral bleaching events on reefs around the world. The expulsion of the symbiotic dinoflagellate algae is believed to occur as a result of damage to the photosynthetic apparatus of these symbionts, although the specific site of initial impact is yet to be conclusively resolved. Here, the sensitivity of the oxygen evolving complex (OEC) to bleaching stress was studied as well as its natural variation between seasons. The artificial electron donor, diphenyl carbazide (DPC), was added to cultured, freshly isolated and expelled (bleaching treatments only) zooxanthellae suspensions. Chl a fluorescence and oxygen production measurements showed that upon addition of DPC, no restoration of diminished photochemical efficiency occurred under control or bleaching conditions. This result was consistent between 12 h and 5 d bleaching treatments on Pocilloporadamicornis, indicating that the OEC is not the primary site of damage, and that zooxanthellae expulsion from the host is a nonselective process with respect to the functioning of the OEC. Further experiments measuring fast induction curves (FICs) revealed that in both summer and winter, the temperature when OEC function was lost occurred between 7C and 14C above the sea surface temperature. FIC and oxygen production measurements of P. damicornis during exposure to bleaching stress demonstrated that the thermotolerance of the OEC increased above the temperature of the bleaching treatment over a 4 h period. This finding indicates that the OEC has the capacity to acclimate between seasons and remains functional at temperatures well above bleaching thresholds. 2008 Phycological Society of America.
Roff, G., Ulstrup, K.E., Fine, M., Ralph, P.J. & Hoegh-Guldberg, O. 2008, 'Spatial heterogeneity of photosynthetic activity within diseased corals from the Great Barrier Reef', Journal of Phycology, vol. 44, no. 2, pp. 526-538.
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Morphological diagnosis and descriptions of seven disease-like syndromes affecting scleractinian corals were characterized from the southern Great Barrier Reef (GBR). Chl a fluorescence of PSII was measured using an Imaging-PAM (pulse amplitude modulated) fluorometer, enabling visualization of the two-dimensional variability in the photophysiology of endosymbiotic dinoflagellates (zooxanthellae) by measuring rapid light curves. Three of four syndromes associated with active tissue loss (type a) were spatially homogenous (white syndrome, brown band, and skeletal eroding band), with no impact on the photochemical function of zooxanthellae populations at or behind the lesion borders. However, a decline in maximum quantum yield (Fv/F m) and elevated levels of maximum nonphotochemical quenching (NPQmax) occurred in visually healthy tissue of black band disease adjacent to the lesion borders, possibly due to hypoxic conditions caused by the black band cyanobacterial mat. Two out of three syndromes associated with pathological change of intact tissue with no active tissue loss (type b) showed variable photophysiological responses (neoplasia and pigmentation response). Only the bleached foci associated with white patch syndrome appeared to impact primarily on the symbiotic dinoflagellates, as evidenced by declines in minimum fluorescence (F0) and maximum quantum yield (Fv/F m), with no indication of degeneration in the host tissues. Our results suggest that for the majority of coral syndromes from the GBR, pathogenesis occurs in the host tissue, while the impact on the zooxanthellae populations residing in affected corals is minimal. 2008 Phycological Society of America.
Petrou, K., Doblin, M.A., Smith, R.A., Ralph, P.J., Shelly, K. & Beardall, J. 2008, 'State transitions and nonphotochemical quenching during a nutrient-induced fluorescence transient in phosphorus-starved Dunaliella tertiolecta', Journal of Phycology, vol. 44, no. 5, pp. 1204-1211.
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Assessments of nutrient-limitation in microalgae using chl a fluorescence have revealed that nitrogen and phosphorus depletion can be detected as a change in chl a fluorescence signal when nutrient-starved algae are resupplied with the limiting nutrient. This photokinetic phenomenon is known as a nutrient-induced fluorescence transient, or NIFT. Cultures of the unicellular marine chlorophyte Dunaliella tertiolecta Butcher were grown under phosphate starvation to investigate the photophysiological mechanism behind the NIFT response. A combination of low temperature (77 K) fluorescence, photosynthetic inhibitors, and nonphotochemical quenching analyses were used to determine that the NIFT response is associated with changes in energy distribution between PSI and PSII and light-stress-induced nonphotochemical quenching (NPQ). Previous studies point to state transitions as the likely mechanism behind the NIFT response; however, our results show that state transitions are not solely responsible for this phenomenon. This study shows that an interaction of at least two physiological processes is involved in the rapid quenching of chl a fluorescence observed in P-starved D. tertiolecta: (1) state transitions to provide the nutrient-deficient cell with metabolic energy for inorganic phosphate (P i)-uptake and (2) energy-dependent quenching to allow the nutrient-stressed cell to avoid photodamage from excess light energy during nutrient uptake. 2008 Phycological Society of America.
Collier, C.J., Lavery, P.S., Ralph, P.J. & Masini, R.J. 2008, 'Physiological characteristics of the seagrass Posidonia sinuosa along a depth-related gradient of light availability', Marine Ecology Progress Series, vol. 353, pp. 65-79.
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Physiological plasticity has the capacity to prolong seagrass survival under reduced light conditions. However, when light is attenuated across a depth gradient, the relative importance of this over other mechanisms for tolerating long-term light reduction (such as morphological adjustments and shoot loss) has been guestioned. This study aimed to describe a number of the physiological characteristics of Posidonia sinuosa Cambridge et Kuo along a depth-related gradient of light availability (1.6 to 9.0 m depth) and infer how these characteristics are important for the long-term maintenance of the meadow. Rapid light curve-derived parameters, light harvesting pigments, photoprotective pigments and nutrient and carbohydrate concentrations exhibited few differences among depth strata, but showed some (albeit limited) adjustment between the seasons. It was inferred that some physiological plasticity is possible in P. sinuosa but that differences in the depth-related gradient of long-term light availability were not sufficient to induce physiological differences, even at the depth limit. Shoot density reductions, which reduce the effects of self-shading, possibly offset depth-related light reductions. Because the physiological characteristics we examined did not explain the adaptations by P. sinuosa to the long-term gradient of light availability, they may not be useful indicators of long-term light reduction. Inter-Research 2008.
Ulstrup, K.E., Hill, R., Van Oppen, M.J.H., Larkum, A.W.D. & Ralph, P.J. 2008, 'Seasonal variation in the photo-physiology of homogeneous and heterogeneous Symbiodinium consortia in two scleractinian corals', Marine Ecology Progress Series, vol. 361, pp. 139-150.
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Seasonal variation in the composition of the algal endosymbiont community and photophysiology was determined in the corals Pocillopora damicornis, which show high local fidelity to one symbiont type (Symbiodinium C1), and Acropora valida, with a mixed Symbiodinium symbiont community, comprising members of both clades A and C. The relative abundances of Symbiodinium types varied over time. A significant decline in symbiont densities in both coral species during the summer of 2005 coincided with a NOAA 'hotspot' warning for Heron Island. This also coincided with a relative increase in the presence and dominance of clade A in A. valida, particularly in sun-adapted surfaces. The effective quantum yield of Photosystem II (?PSII) suggested that sun-adapted surfaces of P. damicornis are more sensitive than shade-adapted surfaces to combined effects of higher temperature and irradiance in summer. Xanthophyll cycling was greater in P. damicornis than A. valida, irrespective of branch position and sampling time; this may be a mechanism by which P. damicornis compensates for its fidelity to Symbiodinium C1. Furthermore, xanthophyll de-epoxidation in P. damicornis symbionts was greater in sun-adapted than shade-adapted surfaces, correlating with non-photochemical quenching (NPQRLC). No variation was found in A. valida, indicating that resident symbiont communities may not have been physiologically compromised, perhaps as a result of changes in the composition of the Symbiodinium community consortia. Inter-Research 2008.
Jimenez, I.M., Khl, M., Larkum, A.W.D. & Ralph, P.J. 2008, 'Heat budget and thermal microenvironment of shallow-water corals: Do massive corals get warmer than branching corals?', Limnology and Oceanography, vol. 53, no. 4, pp. 1548-1561.
Coral surface temperature was investigated with multiple temperature sensors mounted on hemispherical and branching corals under (a) artificial lighting and controlled flow; (b) natural sunlight and controlled flow; and (c) in situ conditions in a shallow lagoon, under naturally fluctuating irradiance, water flow, and temperature. Under high irradiance and low flow conditions, hemispherical corals were 0.6C warmer than the surrounding water. Hemispherical corals reached higher temperatures than branching corals, by a measure of 0.2C to 0.4C. Microsensor temperature measurements showed the presence of a thermal boundary layer (TBL). The TBL thickness was flow dependent, and under low flow conditions, a TBL up to 3 mm thick limited heat transfer to the ambient water. Combined microsensor measurements of temperature and oxygen showed that the TBL was approximately four times thicker than the diffusive boundary layer, as predicted from heat and mass transfer theory. A simple conceptual model describes coral surface temperature as a function of heat fluxes between coral tissue, skeleton, and surroundings. The slope of the predicted linear relationship between coral temperature and solar irradiance is fixed by the efficiencies of light absorption and the heat losses to the skeleton and the water. Although spectral absorptivity may play a significant role in coral warming, shape-related differences in thermal properties can cause hemispherical corals to reach higher temperatures than branching corals. Shape-related differences in thermal histories may thus help explain differences in susceptibility to coral bleaching between branching and hemispherical coral species. 2008, by the American Society of Limnology and Oceanography, Inc.
Collier, C.J., Lavery, P.S., Masini, R.J. & Ralph, P.J. 2007, 'Morphological, growth and meadow characteristics of the seagrass Posidonia sinuosa along a depth-related gradient of light availability', Marine Ecology Progress Series, vol. 337, pp. 103-115.
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Morphological and growth characteristics of the meadow-forming seagrass Posidonia sinuosa (Cambridge et Kuo), were measured along a depth-related gradient of light to infer its response to long-term differences in light availability. Morphometric measurements were carried out at 6 depths between 1.6 and 9.0 m in summer and winter at Cockburn Sound and summer only at Warnbro Sound in south-western Australia. The minimum light requirement for P. sinuosa of 8.5 % sub-surface light was among the lower range reported for seagrasses. Its slow growth rate (0.5-1.5 mgdry shoot-1 d -1), relative to similarly sized species, may contribute to the low light requirements of this species. Shoot density, leaf area index and biomass showed pronounced and consistent differences among depths (up to 88-fold reduction of above-ground biomass from shallow to deep sites). At the deeper sites, the reduced shoot density probably reduces respiratory demand and alleviates self-shading. Morphological differences (leaf length, width and thickness and number of leaves per shoot) did not follow a clear and consistent trend with depth. Despite a 70 % reduction in light availability at the canopy level between the shallowest and deepest sites, leaf growth rate was unaffected by depth during summer, and in winter differed between only a few depths. We propose that the reduction in shoot density partially alleviates the effects of self-shading and permits comparable leaf growth rates across the depth range. These results suggest that for interpreting long-term responses to light availability, shoot density is the most sensitive of the morphological characteristics measured here. Inter-Research 2007.
Ralph, P.J., Durako, M.J., Enrquez, S., Collier, C.J. & Doblin, M.A. 2007, 'Impact of light limitation on seagrasses', Journal of Experimental Marine Biology and Ecology, vol. 350, no. 1-2, pp. 176-193.
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Seagrass distribution is controlled by light availability, especially at the deepest edge of the meadow. Light attenuation due to both natural and anthropogenically-driven processes leads to reduced photosynthesis. Adaptation allows seagrasses to exist under these sub-optimal conditions. Understanding the minimum quantum requirements for growth (MQR) is revealed when light conditions are insufficient to maintain a positive carbon balance, leading to a decline in seagrass growth and distribution. Respiratory demands of photosynthetic and non-photosynthetic tissues strongly influence the carbon balance, as do resource allocations between above- and below-ground biomass. Seagrass light acclimation occurs on varying temporal scales, as well as across spatial scales, from the position along a single leaf blade to within the canopy and finally across the meadow. Leaf absorptance is regulated by factors such as pigment content, morphology and physical properties. Chlorophyll content and morphological characteristics of leaves such as leaf thickness change at the deepest edge. We present a series of conceptual models describing the factors driving the light climate and seagrass responses under current and future conditions, with special attention on the deepest edge of the meadow. Crown Copyright 2007.
Ralph, P.J., Smith, R.A., MacInnis-Ng, C.M.O. & Seery, C.R. 2007, 'Use of fluorescence-based ecotoxicological bioassays in monitoring toxicants and pollution in aquatic systems: Review', Toxicological and Environmental Chemistry, vol. 89, no. 4, pp. 589-607.
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Chlorophyll a fluorescence has the potential to become a valuable ecotoxicological endpoint, which could be used with a range of aquatic phototrophs. Chlorophyll a fluorescence bioassays have been applied in the assessment of heavy metals, herbicides, petrochemicals and nutrients. The strengths of this endpoint are that it is rapid, non-invasive and non-destructive, while the major weakness is the lack of clear ecological relevance. We provide an overview of chlorophyll a fluorescence applications in ecotoxicology. We reviewed test conditions, parameters and protocols used to date and found standardised protocols to be lacking. The most favoured fluorescence parameters were maximum quantum yield (Fv/Fm) and effective quantum yield (?PSII); microalgae were the most widely used tested organism, herbicides the most commonly tested toxicant, while most studies lacked a summary statistic (such as EC50). We recommend that future research in aquatic chlorophyll a fluorescence ecotoxicology focus on standardisation of test protocols and statistical techniques. 2007 Taylor & Francis.
Ralph, P.J., Ryan, K.G., Martin, A. & Fenton, G. 2007, 'Melting out of sea ice causes greater photosynthetic stress in algae than freezing in', Journal of Phycology, vol. 43, no. 5, pp. 948-956.
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Sea ice is the dominant feature of polar oceans and contains significant quantities of microalgae. When sea ice forms and melts, the microalgal cells within the ice matrix are exposed to altered salinity and irradiance conditions, and subsequently, their photosynthetic apparatuses become stressed. To simulate the effect of ice formation and melting, samples of sea-ice algae from Cape Hallett (Antarctica) were exposed to altered salinity conditions and incubated under different levels of irradiance. The physiological condition of their photosynthetic apparatuses was monitored using fast and slow fluorescence-induction kinetics. Sea-ice algae exhibited the least photosynthetic stress when maintained in 35 and 51 salinity, whereas 16, 21, and 65 treatments resulted in significant photosynthetic stress. The greatest photosynthetic impact appeared on PSII, resulting in substantial closure of PSII reaction centers when exposed to extreme salinity treatments. Salinity stress to sea-ice algae was light dependent, such that incubated samples only suffered photosynthetic damage when irradiance was applied. Analysis of fast-induction curves showed reductions in J, I, and P transients (or steps) associated with combined salinity and irradiance stress. This stress manifests itself in the limited capacity for the reduction of the primary electron receptor, QA, and the plastoquinone pool, which ultimately inhibited effective quantum yield of PSII and electron transport rate. These results suggest that sea-ice algae undergo greater photosynthetic stress during the process of melting into the hyposaline meltwater lens at the ice edge during summer than do microalgae cells during their incorporation into the ice matrix during the process of freezing. 2007 Phycological Society of America.
Ralph, P.J., Larkum, A.W.D. & Khl, M. 2007, 'Photobiology of endolithic microorganisms in living coral skeletons: 1. Pigmentation, spectral reflectance and variable chlorophyll fluorescence analysis of endoliths in the massive corals Cyphastrea serailia, Porites lutea and Goniastrea australensis', Marine Biology, vol. 152, no. 2, pp. 395-404.
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We used microscopy, reflectance spectroscopy, pigment analysis, and photosynthesis-irradiance curves measured with variable fluorescence techniques to characterise the endolithic communities of phototrophic microorganisms in the skeleton of three massive corals from a shallow reef flat. Microscopic observations and reflectance spectra showed the presence of up to four distinct bands of photosynthetic microorganisms at different depths within the coral skeleton. Endolithic communities closer to the coral surface exhibited higher photosynthetic electron transport rates and a green zone dominated by Ostreobium quekettii nearest the surface had the greatest chlorophyll pigment concentration. However, Ostreobium was also present and photosynthetically active in the colourless band between the coral tissue and the green band. The spectral properties and pigment density of the endolithic bands were also found to closely correlate to photosynthetic rates as assessed by fluorometry. All endolithic communities were extremely shade-adapted, and photosynthesis was saturated at irradiances <7 ?mol photons m-2s-1. 2007 Springer-Verlag.
Ulstrup, K.E., Van Oppen, M.J.H., Khl, M. & Ralph, P.J. 2007, 'Inter-polyp genetic and physiological characterisation of Symbiodinium in an Acropora valida colony', Marine Biology, vol. 153, no. 2, pp. 225-234.
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Corals harbouring genetically mixed communities of endosymbiotic algae (Symbiodinium) often show distribution patterns in accordance with differences in light climate across an individual colony. However, the physiology of these genetically characterised communities is not well understood. Single stranded conformation polymorphism (SSCP) and real time quantitative polymerase chain reaction (qPCR) analyses were used to examine the genetic diversity of the Symbiodinium community in hospite across an individual colony of Acropora valida at the spatial scale of single polyps. The physiological characteristics of the polyps were examined prior to sampling with a combined O2 microelectrode with a fibre-optic microprobe (combined sensor diameter 50-100 ?m) enabling simultaneous measurements of O2 concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface as a function of increasing irradiances. Both sun- and shade-adapted polyps were found to harbour either Symbiodinium clade C types alone or clades A and C simultaneously. Polyps were grouped in two categories according to (1) their orientation towardps light, or (2) their symbiont community composition. Physiological differences were not detected between sun- and shade-adapted polyps, but O2 concentration at 1,100 ?mol photons m-2 s-1 was higher in polyps that harboured both clades A and C symbionts than in polyps that harboured clade C only. These results suggest that the acclimatisation of zooxanthellae of individual polyps of an A. valida colony to ambient light levels may not be the only determinant of the photosynthetic capacity of zooxanthellae. Here, we found that photosynthetic capacity is also likely to have a strong genetic basis and differs between genetically distinct Symbiodinium types. 2007 Springer-Verlag.
Bishop, M.J., Kelaher, B.P., Alquezar, R., York, P.H., Ralph, P.J. & Skilbeck, C.G. 2007, 'Trophic cul-de-sac, Pyrazus ebeninus, limits trophic transfer through an estuarine detritus-based food web', Oikos, vol. 116, no. 3, pp. 427-438.
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The importance to food-webs of trophic cul-de-sacs, species that channel energy flow away from higher trophic levels, is seldom considered outside of the pelagic systems in which they were first identified. On intertidal mudflats, inputs of detritus from saltmarshes, macroalgae or microphytobenthos are generally regarded as a major structuring force underpinning food-webs and there has been no consideration of trophic cul-de-sacs to date. A fully orthogonal three-factor experiment manipulating the density of the abundant gastropod, Pyrazus ebeninus, detritus and macrobenthic predators on a Sydney mudflat revealed large deleterious effects of the gastropod, irrespective of detrital loading or the presence of predators. Two months after experimental manipulation, the standing-stock of microphytobenthos in plots with high (44 per m2) densities of P. ebeninus was 20% less than in plots with low (4 per m2) densities. Increasing densities of P. ebeninus from low to high halved the abundance of macroinvertebrates and the average number of species. In contrast, the addition of detritus had differing effects on microphytobenthos (positively affected) and macroinvertebrates (negatively affected). Over the two-months of our experiment, no predatory mortality of P. ebeninus was observed and high densities of P. ebeninus decreased impacts of predators on macroinvertebrate abundances. Given that the dynamics of southeast Australian mudflats are driven more by disturbance than seasonality in predators and their interactions with prey, it is likely that Pyrazus would be similarly resistant to predation and have negative effects on benthic assemblages at other times of the year, outside of our study period. Thus, in reducing microphytobenthos and the abundance and species richness of macrofauna, high abundances of the detritivore P. ebeninus may severely limit the flow of energy up the food chain to commercially-important species. This study therefore suggests that trophic cul-de-sacs are not limited to the eutrophied pelagic systems in which they were first identified, but may exist in other systems as well. OIKOS.
McMinn, A., Ryan, K.G., Ralph, P.J. & Pankowski, A. 2007, 'Spring sea ice photosynthesis, primary productivity and biomass distribution in eastern Antarctica, 2002-2004', Marine Biology, vol. 151, no. 3, pp. 985-995.
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While it is known that Antarctic sea ice biomass and productivity are highly variable over small spatial and temporal scales, there have been very few measurements from eastern Antarctic. Here we attempt to quantify the biomass and productivity and relate patterns of variability to sea ice latitude ice thickness and vertical distribution. Sea ice algal biomass in spring in 2002, 2003 and 2004 was low, in the range 0.01-8.41 mg Chl a m-2, with a mean and standard deviation of 2.08 1.74 mg Chl a m-2 (n = 199). An increased concentration of algae at the bottom of the ice was most pronounced in thicker ice. There was little evidence to suggest that there was a gradient of biomass distribution with latitude. Maximum in situ production in 2002 was approximately 2.6 mg C m-2 h-1 with assimilation numbers of 0.73 mg C (mg Chl a)-1 h-1. Assimilation numbers determined by the 14C incubations in 2002 varied between 0.031 and 0.457 mg C (mg Chl a)-1 h-1. Maximum fluorescence quantum yields of the incubated ice samples in 2002 were 0.470 0.041 with Ek indices between 19 and 44 ?mol photons m-2 s-1. These findings are consistent with the shade-adapted character of ice algal communities. In 2004 maximum in situ production was 5.9 mg C m-2 h-1 with an assimilation number of 5.4 mg C (mg Chl a)-1 h-1. Sea ice biomass increased with ice thickness but showed no correlation with latitude or the time the ice was collected. Forty-four percent of the biomass was located in bottom communities and these were more commonly found in thicker ice. Surface communities were uncommon. 2006 Springer-Verlag.
Hill, R. & Ralph, P.J. 2007, 'Post-bleaching viability of expelled zooxanthellae from the scleractinian coral Pocillopora damicornis', Marine Ecology Progress Series, vol. 352, pp. 137-144.
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Coral bleaching events have been linked to elevated seawater temperatures in combination with intense light and can be characterised by the loss of symbionts (zooxanthellae, genus Symbiodinium) from the host tissue, as well as a reduction in photosynthetic pigments in these zooxanthellae. The long-term (days) viability of expelled zooxanthellae in the water column from the scleractinian coral Pocillopora damicornis was explored in this study through measurements of photosynthetic health and morphological condition. After initial expulsion, zooxanthellae were found to be photosynthetically competent and structurally intact. However, within 6 to 12 h following this time, photosystem II photochemical efficiency dramatically declined in these cells and photosynthetic damage was gradually manifested in the loss of structural integrity of the cell. The time of expulsion during bleaching exposure, as well as ambient water temperature, greatly influenced survivorship. Expelled zooxanthellae were collected at 4 different time intervals (0-6, 6-12, 12-24 and 24-36 h) following the onset of exposure to bleaching conditions (32C and 400 ?mol photons m-2 s-1) and then maintained at 28, 30 or 32C and 100 ?mol photons m-2 s-1 for up to 96 h. Those cells expelled within the first 6 h of bleaching and held at 28C (lagoon temperature) had the greatest longevity, although even in this treatment, long-term photosynthetic viability was restricted to 5 d in the water column. This suggests that unless expelled zooxanthellae inhabit other environments of coral reefs (such as sediments) which may be more favourable for survival, their capacity for persistence in the environment is extremely limited. Inter-Research 2007.
Seery, C.R., Gunthorpe, L. & Ralph, P.J. 2006, 'Herbicide impact on Hormosira banksii gametes measured by fluorescence and germination bioassays', Environmental Pollution, vol. 140, no. 1, pp. 43-51.
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The innovative bioassay described here involves chlorophyll a fluorescence measurements of gametes from the macroalgae, Hormosira banksii, where gametes (eggs) were exposed to Diuron, Irgarol and Bromacil. Response was assessed as percent inhibition from control of effective quantum yield (?F/Fm?) of photosystem II, herein referred to as % PSII Inhibition. This was measured with the dual-channelled pulse amplitude modulated (PAM) fluorometer, ToxY-PAM. The fluorescence bioassay was run simultaneously with an established H. banksii germination bioassay to compare sensitivity, precision, and time-to-result. The fluorescence bioassay gave highly sensitive results evidenced by EC 50s (% PSII Inhibition) for Diuron, Irgarol and Bromacil being three, four and three orders of magnitude (respectively) lower than EC50s generated from the germination bioassays. Precision of the fluorescence bioassay was demonstrated with low coefficient of variations (<30%) for all three toxicants. With regard to time, the fluorescence bioassay gave results within 6 h, as opposed to more than 50 h for the germination bioassay. 2005 Elsevier Ltd. All rights reserved.
Charles, A.L., Markich, S.J. & Ralph, P.J. 2006, 'Toxicity of uranium and copper individually, and in combination, to a tropical freshwater macrophyte (Lemna aequinoctialis)', Chemosphere, vol. 62, no. 8, pp. 1224-1233.
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Copper (Cu) and uranium (U) are of potential ecotoxicological concern to tropical freshwater biota in northern Australia, as a result of mining activities. Few data are available on the toxicity of U, and no data are available on the toxic interaction of
Ulstrup, K.E., Ralph, P.J., Larkum, A.W.D. & Khl, M. 2006, 'Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis', Marine Biology, vol. 149, no. 6, pp. 1325-1335.
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We investigated heterogeneity of light acclimation of photosynthesis in sun- and shade-adapted coenosarc and polyp tissues of Pocillopora damicornis. The zooxanthellar community within P. damicornis colonies at Heron Island is genetically uniform, yet they showed a large degree of plasticity in their photo-physiological acclimation linked to light microclimates characterised by fibre-optic microprobes. Microscale scalar irradiance measurements showed higher absorption in polyp than coenosarc tissues and higher absorption in the more densely pigmented shade-adapted polyps than in sun-adapted polyps. The combination of an O2 microelectrode with a fibre-optic microprobe (combined sensor diameter 50-100 ?m) enabled parallel measurements of O 2 concentration, gross photosynthesis rate and photosystem II (PSII) quantum yield at the coral surface under steady-state conditions as a function of increasing irradiances. Lower O2 levels at the tissue surface and higher compensation irradiance indicated a higher respiration activity in sun-adapted polyp tissue as compared to shade-adapted polyps. Shade-adapted coenosarc and polyp tissues exhibited lower maxima of relative electron transport rates (rETRmax) (8415 and 4110, respectively) than sun-adapted coenosarc and polyp tissues (13614 and 7713, respectively). Shade-adapted tissues showed stronger decrease of rETR at high scalar irradiances as compared to sun-adapted tissues. The relationship between the relative PSII electron transport and the rate of gross photosynthesis, as well as O2 concentration, was non-linear in sun-adapted tissues over the entire irradiance range, whereas for shade-adapted tissues the relationship became non-linear at medium to high scalar irradiances >200 ?mol photons m-2 s-1. This suggests that rETR measurements should be used with caution in corals as a proxy for photosynthesis rates. The apparently high rates of photosynthesis (oxygen evolution rates) suggest that there must be a considerable electron transport rate through the photosystems that is not observed by the rETR measurements. This may be accounted for by vertical heterogeneity of zooxanthellae in the tissue and the operation of an alternative electron pathway such as cyclic electron flow around PSII. Springer-Verlag 2006.
Ryan, K.G., Hegseth, E.N., Martin, A., Davy, S.K., O'Toole, R., Ralph, P.J., McMinn, A. & Thorn, C.J. 2006, 'Comparison of the microalgal community within fast ice at two sites along the Ross Sea coast, Antarctica', Antarctic Science, vol. 18, no. 4, pp. 583-594.
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Diverse microbial communities survive within the sea ice matrix and are integral to the energy base of the Southern Ocean. Here we describe initial findings of a four season survey (between 1999-2004) of community structure and biomass of microalgae within the sea ice and in the underlying water column at Cape Evans and Cape Hallett, in the Ross Sea, Antarctica as part of the Latitudinal Gradient Project. At Cape Evans, bottom-ice chlorophyll a levels ranged from 4.4 to 173 mg Chl a m-2. Dominant species were Nitzschia stellata, N. lecointei, and Entomoneis kjellmanii, while the proportion of Berkeleya adeliensis increased steadily during spring. Despite being obtained later in the season, the Cape Hallett data show considerably lower standing stocks of chlorophyll ranging from 0.11 to 36.8 mg Chl a m-2. This difference was attributed to a strong current, which may have ablated much of the bottom ice biomass and provided biomass to the water below. This loss of algae from the bottom of the ice may explain why the ice community contributed only 2% of the standing stock in the total water column. Dominant species at Cape Hallett were Nitzschia stellata, Fragilariopsis curta and Cylindrotheca closterium. The low biomass at Cape Hallett and the prevalence of smaller-celled diatoms in the bottom ice community indicate that the ice here is more typical of pack ice than fast ice. Further data will allow us to quantify and model the extent to which ice-driven dynamics control the structure and function of the sea ice ecosystem and to assess its resilience to changing sea ice conditions. Antarctic Science Ltd.
Ulstrup, K., Ralph, P.J., Larkum, A. & Kuhl, M. 2006, 'Intra-colonial variability in light acclimation of zooxanthellae in coral tissues of Pocillopora damicornis Received:', Marine Biology, vol. 149, pp. 1325-1335.
Ryan, K.G., Hegseth, E., Martin, A., Davy, S., O'Toole, R., Ralph, P.J., McMinn, A. & Thorn, C. 2006, 'Comparison of the microalgal community within fast ice two sites along the Ross Sea coast, Antarctia', Antarctic Science, vol. 18, no. 4, pp. 583-594.
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Hill, R. & Ralph, P.J. 2006, 'Photosystem II heterogeneity of in hospite zooxanthellae in scleractinian corals exposed to bleaching conditions', Photochemistry and Photobiology, vol. 82, no. 6, pp. 1577-1585.
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Increased ocean temperatures are thought to be triggering mass coral bleaching events around the world. The intracellular symbiotic zooxanthellae (genus Symbiodinium) are expelled from the coral host, which is believed to be a response to photosynthetic damage within these symbionts. Several sites of impact have been proposed, and here we probe the functional heterogeneity of Photosystem II (PSII) in three coral species exposed to bleaching conditions. As length of exposure to bleaching conditions (32C and 350 ?mol photons m-2 s-1) increased, the QA- reoxidation kinetics showed a rise in the proportion of inactive PSII centers (PSIIX), where QB was unable to accept electrons. PSIIX contributed up to 20% of the total PSII centers in Pocillopora damicornis, 35% in Acropora nobilis and 14% in Cyphastrea serailia. Changes in Fv/Fm and amplitude of the J step along fast induction curves were found to be highly dependent upon the proportion of . PSII X centers within the total pool of PSII reaction centers. Determination of PSII antenna size revealed that under control conditions in the three coral species up to 60% of PSII centers were lacking peripheral light-harvesting complexes (PSII?). In P. damicornis, the proportion of PSII? increased under bleaching conditions and this could be a photoprotective mechanism in response to excess light. The rapid increases in PSIIX and PSII? observed in these corals under bleaching conditions indicates these physiological processes are involved in the initial photochemical damage to zooxanthellae. 2006 American Society for Photobiology.
Ulstrup, K.E., Berkelmans, R., Ralph, P.J. & Van Oppen, M.J.H. 2006, 'Variation in bleaching sensitivity of two coral species across a latitudinal gradient on the Great Barrier Reef: The role of zooxanthellae', Marine Ecology Progress Series, vol. 314, pp. 135-148.
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The ability of corals to cope with environmental change, such as increased temperature, relies on the physiological mechanisms of acclimatisation and long-term genetic adaptation. We experimentally examined the bleaching sensitivity exhibited by 2 species of coral, Pocillopora damicornis and Turbinaria reniformis, at 3 locations across a latitudinal gradient of almost 6 degrees on the Great Barrier Reef (GBR). Target bleaching temperature was reached by using a ramping rate of 0.2C/h. We found that the bleaching sensitivity and recovery of both species differed between corals with clade D symbionts and those with clade C. However, in P. damicornis bleaching susceptibility corresponded more strongly with latitude than with zooxanthella type and hence, temperature history, suggesting that local adaptation has occurred. The observed bleaching sensitivity was shown by a decrease in photochemical efficiency (Fv/Fm) in both species of coral. The rate of recovery in T. reniformis was highest in explants containing clade D symbionts. The occurrence of clade D in the northern section of the GBR may reflect a long-term response to high sea water temperatures, while the presence of clade D in low abundance in T. reniformis at Heralds Prong Reef and Percy Island may be a result of recent bleaching events. Inter-Research 2006.
Ralph, P.J., McMinn, A., Ryan, K.G. & Ashworth, C. 2005, 'Short-term effect of temperature on the photokinetics of microalgae from the surface layers of Antarctic pack ice', Journal of Phycology, vol. 41, no. 4, pp. 763-769.
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Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above -5C and at irradiances up to 350 ?mol photons m -2s-1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to -10C, provided the irradiance remained under 50 ?mol photonsm-2s-1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETR max]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 ?mol electronsm-2s -1, which occurred at -1.8 C. Effective quantum yield of PSII and rETRmax of the halotolerant brine microalgae exhibited a temperature-dependent pattern, where both parameters were higher at -1.8 C and lower at -10 C. Relative ETRmax at temperatures above -5C were stable across a wide range of irradiance. 2005 Phycological Society of America.
Macinnis-Ng, C.M.O., Morrison, D.A. & Ralph, P.J. 2005, 'Temporal and spatial variation in the morphology of the brown macroalga Hormosira banksii (Fucales, Phaeophyta)', Botanica Marina, vol. 48, no. 3, pp. 198-207.
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Hormosira banksii is a morphologically variable macroalgal species from southeastern and southern Australia, which has been previously categorised into ecoforms according to habitat. This study is by far the largest quantitative evaluation of morphological variation in H. banksii, covering 74 sites from South Australia, Victoria, New South Wales and Tasmania. Morphological features from 505 samples were analysed using principal components analysis, with the patterns identified being statistically assessed with a Monte Carlo permutation test. There was considerable morphological variation between samples taken at several marine (but not estuarine) sites in both 1994 and 1999. However, this variation was not consistent across either morphological features or populations, and presumably represents random fluctuations. Analysis of the entire dataset demonstrated a significant difference between samples growing in marine and estuarine habitats. Further assessment of variation within these two groups revealed some significantly different populations based on geographical locations but not habitat variation. While this study presents strong evidence for two distinct taxa within H. banksii (marine versus estuarine populations), the taxonomic status of this species should not be altered until genetic studies have been conducted. 2005 by Walter de Gruyter.
Hill, R., Frankart, C. & Ralph, P.J. 2005, 'Impact of bleaching conditions on the components of non-photochemical quenching in the zooxanthellae of a coral', Journal of Experimental Marine Biology and Ecology, vol. 322, no. 1, pp. 83-92.
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Mass coral bleaching events are a worldwide phenomenon, which generally occur during periods of elevated sea surface temperature and intense sunlight. These conditions result in a decline in photochemical efficiency of symbiotic microalgae (zooxanthellae) which ultimately leads to the expulsion of these symbionts. The physiological mechanism which triggers the release of the zooxanthellae has yet to be adequately determined. Under bleaching conditions, non-photochemical quenching (NPQ) is used to dissipate excess energy from photosystem II (PSII). NPQ was partitioned into three components, (energy dependent quenching [qE], state transition quenching [qT] and photoinhibitory quenching [qI]), based on relaxation kinetics upon addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and darkening. This investigation revealed that for corals not exposed to bleaching stress, qE was the principle means of energy dissipation (?60% of the total NPQ). In corals exposed to either high-light (475 ?mol photons m-2 s-1 and 25C) or elevated temperature (225 ?mol photons m-2 s -1 and 32C) treatments, the dominant component of NPQ was qE and the relative proportions did not change during the exposure period (1-8 h). When exposed to bleaching conditions (475 ?mol photons m-2 s -1 and 32C) the contribution of the different components changed after 4 h and the total NPQ increased. At this time, the contribution of qT to the total NPQ significantly increased to equal that of qE (40%), suggesting state transitions become more important under such conditions. Throughout the exposure period in all treatments, no change in the proportion of qI was observed. 2005 Elsevier B.V. All rights reserved.
Hill, R. & Ralph, P.J. 2005, 'Diel and seasonal changes in fluorescence rise kinetics of three scleractinian corals', Functional Plant Biology, vol. 32, no. 6, pp. 549-559.
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The effect of diel oscillations in light on the photosynthetic response of three coral species during summer and winter was studied. Fast induction curves revealed detailed information on primary photochemistry as well as redox states of electron acceptors in photosystem II (PSII). The comparison between seasons revealed that similar physiological mechanisms were operating in response to high-light conditions throughout the year and that environmental variables, such as temperature, had no measurable effect between seasons. A diurnal hysteresis was seen in both seasons in F v/F m as well as in the fast induction curves, where photosynthetic capacity was lower in the afternoon than in the morning when light intensities were the same. This suggests the operation of dynamic down regulation, following exposure to midday high light. Fast induction curve analysis revealed a decline in the O, J, I and P steps towards midday and a rapid recovery by the late afternoon. The decrease in J and its rapid recovery indicated a drop in the rate of Q A reduction as a result of an increase in non-photochemical quenching (NPQ). The P step increased in amplitude in the first hours of sunlight, which suggests an increased oxidation of the plastoquinone (PQ) pool and a greater capacity for electron transport. Similarly, a rise in F v/F m was observed within the first hour of sunlight. This response was attributed to the dark reduction of the PQ pool, induced by night time anaerobic conditions and possibly oxygen-dependent chlororespiration, which would lead to a state 2 transition. The early morning removal of chlororespiration and hypoxic conditions would have returned the photosystems to state 1, resulting in the increased photochemical efficiency of the zooxanthellae. CSIRO 2005.
Ralph, P.J. & Gademann, R. 2005, 'Rapid light curves: A powerful tool to assess photosynthetic activity', Aquatic Botany, vol. 82, no. 3, pp. 222-237.
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Rapid light curves provide detailed information on the saturation characteristics of electron transport, as well as the overall photosynthetic performance of a plant. Rapid light curves were collected from samples of Zostera marina grown under low and high-light conditions (50 and 300 ?mol photons m-2 s-1) and the distinctive patterns of RLC parameters are discussed, in terms of differential sink capacity and PSII reaction centre closure. Derived cardinal points of a rapid light curve (?, Ek and rETRmax) describe the photosynthetic capacity of a seagrass leaf, its light adaptation state and its capacity to tolerate short-term changes in light. The shapes of the corresponding F and F?m curves also provide information on the development of the trans-thylakoid proton gradient and thermal energy dissipation. Low-light leaves showed limited photosynthetic capacity and reduced activity of non-photochemical quenching pathways, whereas photosynthesis of high light leaves were not limited and showed an elevated level of non-photochemical quenching, possibly associated with xanthophyll cycle activity. Light-dark kinetics are also discussed in relation to relaxation of non-photochemical quenching and its various components. A curve fitting model is recommended based on the double exponential decay function. In this paper, we explain the fundamental aspects of a RLC, describe how it reflects the response to light exposure of a leaf, how to interpret these curves, and how to quantitatively describe and compare RLCs. 2005 Elsevier B.V. All rights reserved.
Ralph, P.J., Larkum, A.W.D. & Khl, M. 2005, 'Temporal patterns in effective quantum yield of individual zooxanthellae expelled during bleaching', Journal of Experimental Marine Biology and Ecology, vol. 316, no. 1, pp. 17-28.
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Bleaching is a worldwide phenomenon affecting coral reefs. During elevated temperature and light conditions (bleaching), expelled zooxanthellae show distinct patterns in photosynthetic health. An innovative new device was used to collect individual expelled zooxanthellae, when a coral was exposed to bleaching conditions. This has provided new insight into the photosynthetic condition and abundance of expelled zooxanthellae. It has been assumed that expelled zooxanthellae were dead or moribund; however, we have found individual cells can have healthy effective quantum yields (?PSII) >0.65 after 8 h of bleaching conditions (500 ?mol photons m-2 s -1, 33C). The population of expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis showed distinct patterns in the frequency distribution of ?PSII over time and between locations (sun versus shade) within a colony. During the first 4 h of exposure to bleaching conditions, only 5% of expelled individual cells from P. damicornis were photosynthetically inactive (?PSII<0.05), whereas for C. serailia, this was 30%. The overall photosynthetic health of expelled zooxanthellae from C. serailia was better than P. damicornis (0.530.13 and 0.380.13 after 8 h, respectively). This was generally reflected by the in hospite measurement of the coral, yet, the in hospite cells always had a higher ?PSII than expelled cells, suggesting that host tissue provided added photoprotection for the zooxanthellae. 2005 Elsevier B.V. All rights reserved.
Ulstrup, K.E., Hill, R. & Ralph, P.J. 2005, 'Photosynthetic impact of hypoxia on in hospite zooxanthellae in the scleractinian coral Pocillopora damicornis', Marine Ecology Progress Series, vol. 286, pp. 125-132.
Shallow water coral reefs may experience hypoxia under conditions of calm weather doldrums. Anaerobic responses of endosymbionts (i.e. zooxanthellae) within Pocillopora damicornis coral colonies were tested using both slow and fast chlorophyll a fluorescence induction kinetics. Zooxanthellae were examined in hospite when exposed to control conditions (26C, 200 ?mol photons m-2 s-1, 100% air-saturation, 4 cm s-1 flow) and to 2 treatments of reduced air content (40 and 0%), achieved by controlling the N2:O2 ratio in water circulating at 2 cm s -1. Furthermore, the impact of water flow on photosynthesis was examined at 0% air saturation by turning off the flow entirely (0 cm s -1), thereby mimicking the environmental conditions of calm weather doldrums. Corals exposed to depleted air content (0 % with and without flow) showed a significant decrease (p < 0.001) in effective quantum yield (?PSII) in comparison with controls. Maximum quantum yield was significantly reduced when gas exchange was inhibited (0% without flow), whereas non-photochemical quenching (NPQ) was not affected. Fast polyphasic fluorescence transients of chlorophyll a fluorescence showed a significant increase in minimum dark-adapted fluorescence, F0, when corals were exposed to anaerobic conditions. Furthermore, an increase in the J peak (2 ms) corresponding to the reduction of the primary electron acceptor, QA, was observed in 0% air-saturation with flow. We found that the most sensitive parameters for detecting physiological change associated with hypoxia were ?PSII using slow (pulse-amplitude modulation) fluorescence kinetics, as well as an increase in the O peak, ?Po(electron transport efficiency before QA), and an elevation of the J peak on a double-normalised transient using fast (Plant Efficiency Analyser) induction kinetics. Inter-Research 2005.
Bengtson Nash, S.M., Schreiber, U., Ralph, P.J. & Mller, J.F. 2005, 'The combined SPE:ToxY-PAM phytotoxicity assay; application and appraisal of a novel biomonitoring tool for the aquatic environment', Biosensors and Bioelectronics, vol. 20, no. 7, pp. 1443-1451.
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Mounting concerns regarding the environmental impact of herbicides has meant a growing requirement for accurate, timely information regarding herbicide residue contamination of, in particular, aquatic systems. Conventional methods of detection remain limited in terms of practicality due to high costs of operation and the specialised information that analysis provides. A new phytotoxicity bioassay was trialled for the detection of herbicide residues in filter-purified (Milli-Q) as well as natural waters. The performance of the system, which combines solid-phase extraction (SPE) with the ToxY-PAM dual-channel yield analyser (Heinz Walz GmbH), was tested alongside the traditional method of liquid chromatography-mass spectrometry (LC-MS). The assay methodology was found to be highly sensitive (LOD 0.1 ng L -1 diuron) with good reproducibility. The study showed that the assay protocol is time effective and can be employed for the aquatic screening of herbicide residues in purified as well as natural waters. 2004 Elsevier B.V. All rights reserved.
Kuehl, M., Chen, M., Ralph, P.J., Schreiber, U. & Larkum, A. 2005, 'A niche for cyanobacteria containing chlorophyll d', Nature, vol. 433, no. 1, pp. 820-820.
The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.
Ralph, P.J., Schreiber, U., Gademann, R., Khl, M. & Larkum, A.W.D. 2005, 'Coral photobiology studied with a new imaging pulse amplitude modulated fluorometer', Journal of Phycology, vol. 41, no. 2, pp. 335-342.
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A new high-resolution imaging fluorometer (Imaging-PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards & Haime (massive), and Pavona decussata Dana (plate). Images of fluorescence parameters (F, Fm?, effective quantum yield, optimal quantum yield, electron transport rate, relative photosynthetic rate, and non-photochemical quenching) allowed heterogeneity to be detected in terms of position on colony and indicated that the photosynthetic activity of polyp and coenosarc tissues responded differently to changing light for all three species. The Imaging-PAM offers a special routine, with which images of PAR absorption (absorptivity) are obtained. In this way, for the first time it has become possible to derive images of the relative photosynthesis rate. Polyps had a lower PAR absorptivity than coenosarc tissue for A. nobilis and P. decussata, whereas G. australiensis showed the opposite pattern. Acropora nobilis showed heterogeneity along the longitudinal axis of the branch, which could be differentiated from the effect of variations in illumination across the rugose and curved surface. Diel changes were apparent and influenced the longitudinal heterogeneity along the A. nobilis branch. Images were also obtained showing the degree of photoinhibition caused by high-light stress across a coral surface at a hitherto unobtainable level of resolution. 2005 Phycological Society of America.
Ralph, P.J., Macinnis-Ng, C.M.O. & Frankart, C. 2005, 'Fluorescence imaging application: Effect of leaf age on seagrass photokinetics', Aquatic Botany, vol. 81, no. 1, pp. 69-84.
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We used the Imaging-PAM fluorometer to map spatial variability of photosynthesis in three seagrass species, Halophila ovalis, Zostera capricorni and Posidonia australis. Photosynthesis was described by relative photosynthetic rate (PS/50), effective quantum yield (?PSII), non-photochemical quenching (NPQ and qN), electron transport rate (ETR) and leaf absorptivity. Photosynthetic patterns were linked to leaf age and light climate but patterns were not consistent across species. Longitudinal heterogeneity in photosynthesis was apparent along the leaves of all three species while lateral spatial heterogeneity was found only across Z. capricorni and H. ovalis leaves. Age of leaf tissue, determined by longitudinal location on the leaf, strongly influenced photosynthetic activity of Z. capricorni and P. australis. A comparison of H. ovalis leaves of differing maturity demonstrated the influence of leaf age on photosynthetic activity, yet a comparison of Z. capricorni leaves of differing maturity showed no leaf-age effects. Variations in stress-induced changes across a seagrass leaf can be used to identify areas or particular regions of the leaf, which are more susceptible to photodamage. Clear evidence of substantial within-leaf heterogeneity in photosynthetic activity (i.e., a two-fold variation in half saturation constant along a leaf of P. australis) has serious implications for use of small sections of leaf for photosynthetic incubations (such as O2 or single-point chlorophyll a fluorescence measurements). 2004 Elsevier B.V. All rights reserved.
Ralph, P.J., Schreiber, U., Gademann, R., Kuehl, M. & Larkum, A. 2005, 'Coral photobiology studied with a new imaging pulse amplitude modulated fluorometer', Journal Of Phycology, vol. 41, no. 2, pp. 335-342.
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A new high-resolution imaging fluorometer (Imaging-PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards A Haime (
Khl, M., Chen, M., Ralph, P.J., Schreiber, U. & Larkum, A.W.D. 2005, 'Ecology: A niche for cyanobacteria containing chlorophyll d', Nature, vol. 433, no. 7028, pp. 820-820.
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Macinnis-Ng, C.M.O. & Ralph, P.J. 2004, 'In situ impact of multiple pulses of metal and herbicide on the seagrass, Zostera capricorni', Aquatic Toxicology, vol. 67, no. 3, pp. 227-237.
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Tides and freshwater inflow which influence water movement in estuarine areas govern the exposure-regime of pollutants. In this experiment, we examined the in situ impact of double pulses of copper and the herbicide Irgarol 1051 on the photosynthesis of the seagrass, Zostera capricorni. Despite a 4-day recovery period between the two 10h pulses of toxicant, the effective quantum yield of photosystem II (?F/Fm?) and total chlorophyll concentrations indicated that multiple-pulses had a greater impact than a single pulse. During the first exposure period, samples exposed to Irgarol 1051 had ?F/Fm? values as low as zero while controls remained around 0.6 relative units. After the second exposure period, treated samples recovered to only 0.4 relative units. Samples exposed to copper had ?F/Fm? values around 0.3 relative units during the first exposure period and while these samples recovered before the second dose, they remained below 0.2 relative units after the second exposure period. Alternate samples were also exposed to one toxicant, allowed to recover and then exposed to the other toxicant. ?F/Fm? values indicated that copper exposure followed by Irgarol 1051 exposure was more toxic than Irgarol 1051 exposure followed by copper exposure. 2004 Elsevier B.V. All rights reserved.
Hill, R., Larkum, A.W.D., Frankart, C., Khl, M. & Ralph, P.J. 2004, 'Loss of functional photosystem ii reaction centres in zooxanthellae of corals exposed to bleaching conditions: Using fluorescence rise kinetics', Photosynthesis Research, vol. 82, no. 1, pp. 59-72.
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Mass coral bleaching is linked to elevated sea surface temperatures, 1-2C above average, during periods of intense light. These conditions induce the expulsion of zooxanthellae from the coral host in response to photosynthetic damage in the algal symbionts. The mechanism that triggers this release has not been clearly established and to further our knowledge of this process, fluorescence rise kinetics have been studied for the first time. Corals that were exposed to elevated temperature (33C) and light (280 ?mol photons m -2 s -1), showed distinct changes in the fast polyphasic induction of chlorophyll-a fluorescence, indicating biophysical changes in the photochemical processes. The fluorescence rise over the first 2000ms was monitored in three species of corals for up to 8 h, with a PEA fluorometer and an imaging-PAM. Pocillopora damicornis showed the least impact on photosynthetic apparatus, while Acropora nobilis was the most sensitive, with Cyphastrea serailia intermediate between the other two species. A. nobilis showed a remarkable capacity for recovery from bleaching conditions. For all three species, a steady decline in the slope of the initial rise and the height of the J-transient was observed, indicating the loss of functional Photosystem II (PS II) centres under elevated-temperature conditions. A significant loss of PS II centres was confirmed by a decline in photochemical quenching when exposed to bleaching stress. Non-photochemical quenching was identified as a significant mechanism for dissipating excess energy as heat under the bleaching conditions. Photophosphorylation could explain this decline in PS II activity. State transitions, a component of non-photochemical quenching, was a probable cause of the high non-photochemical quenching during bleaching and this mechanism is associated with the phosphorylation-induced dissociation of the light harvesting complexes from the PS II reaction centres. This reversible process may account for the coral recovery, particularly in A. nobilis.
Hill, R., Schreiber, U., Gademann, R., Larkum, A.W.D., Khl, M. & Ralph, P.J. 2004, 'Spatial heterogeneity of photosynthesis and the effect of temperature-induced bleaching conditions in three species of corals', Marine Biology, vol. 144, no. 4, pp. 633-640.
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Heterogeneity in photosynthetic performance between polyp and coenosarc tissue in corals was shown using a new variable fluorescence imaging system (Imaging-PAM) with three species of coral, Acropora nobilis, Cyphastrea serailia and Pocillopora damicornis. In comparison to earlier studies with fibre-optic microprobes for fluorescence analysis, the Imaging-PAM enables greater accuracy by allowing different tissues to be better defined and by providing many more data points within a given time. Spatial variability of photosynthetic performance from the tip to the distal parts was revealed in one species of branching coral, A. nobilis. The effect of bleaching conditions (33C vs. 27C) was studied over a period of 8 h. Marked changes in fluorescence parameters were observed for all three species. Although a decline in ?PSII (effective quantum yield) and Yi (the first effective quantum yield obtained from a rapid light curve) were observed, P. damicornis showed no visual signs of bleaching on the Imaging-PAM after this time. In A. nobilis and C. serailia, visual signs of bleaching over the 8 h period were accompanied by marked changes in F (light-adapted fluorescence yield), NPQ (non-photochemical quenching) and Ek (minimum saturating irradiance), as well as ?PSII and Yi. These changes were most marked over the first 5 h. The most sensitive species was A. nobilis, which after 8 h at 33C had reached a ?PSII value of almost zero across its whole surface. Differential bleaching responses between polyps and coenosarc tissue were found in P. damicornis, but not in A. nobilis and C. serailia. NPQ increased with exposure time to 33C in both the latter species, accompanied by a decreasing Ek, suggesting that the xanthophyll cycle is entrained as a mechanism for reducing the effects of the bleaching conditions. Springer-Verlag 2004.
Ryan, K.G., Ralph, P.J. & McMinn, A. 2004, 'Acclimation of Antarctic bottom-ice algal communities to lowered salinities during melting', Polar Biology, vol. 27, pp. 679-686.
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Macinnis-Ng, C.M.O. & Ralph, P.J. 2004, 'Variations in sensitivity to copper and zinc among three isolated populations of the seagrass, Zostera capricorni', Journal of Experimental Marine Biology and Ecology, vol. 302, no. 1, pp. 63-83.
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Metal accumulation in seagrass is well documented, but toxic impacts and mechanisms of tolerance in seagrass are not well understood. We looked at the impacts of 10 h exposure to copper and zinc for three isolated populations of Zostera capricorni in the Sydney (Australia) region. Photosynthetic efficiency (measured as the effective quantum yield, ?F/Fm?) and chlorophyll pigment concentrations showed different sensitivities to metal impacts at the three geographically isolated sites. Seagrasses from the least developed estuary were the most sensitive to metals and the two more developed estuaries had more tolerant populations. Determination of metal concentrations in the leaves showed that there was no difference in metal exclusion as the sensitive seagrass accumulated no more metal than the tolerant seagrass. Equally, background levels of copper and zinc in the sediments and seagrass tissue could not explain the differences in tolerance. We discuss some other possible mechanisms of tolerance. The outcomes suggest that assessing metal content in seagrass tissue may not demonstrate degree of photosynthetic impact. 2003 Elsevier B.V. All rights reserved.
Macinnis-Ng, C.M.O. & Ralph, P.J. 2003, 'In situ impact of petrochemicals on the photosynthesis of the seagrass Zostera capricorni', Marine Pollution Bulletin, vol. 46, no. 11, pp. 1395-1407.
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We used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of pulsed exposures of aged crude oil (Champion Crude), dispersant (VDC) and an oil+dispersant mixture on the seagrass Zostera capricorni Aschers in laboratory and field experiments, using custom-made chambers. Samples were exposed for 10 h to 0.25% and 0.1% concentrations of aged crude oil and dispersant as well as mixtures of 0.25% oil+0.05% dispersant and 0.1% oil+0.02% dispersant. During this time and for the subsequent four day recovery period, the maximum and effective quantum yields of photosystem II (Fv/Fm and ?F/Fm ? respectively) were measured. In the laboratory experiments, both values declined in response to oil exposure and remained low during the recovery period. Dispersant exposure caused a decline in both values during the recovery period, while the mixture of aged crude oil+dispersant had little impact on both quantum yields. In situ samples were less sensitive than laboratory samples, showing no photosynthetic impact due to dispersant and oil+dispersant mixture. Despite an initial decline in ?F/Fm ?, in situ oil-exposed samples recovered by the end of the experiment. Chlorophyll pigment analysis showed only limited ongoing impact in both laboratory and field situations. This study suggests that laboratory experiments may overestimate the ongoing impact of petrochemicals on seagrass whilst the dispersant VDC can reduce the impact of oil on seagrass photosynthesis. 2003 Elsevier Ltd. All rights reserved.
Macinnis-Ng, C.M.O. & Ralph, P.J. 2003, 'Short-term response and recovery of Zostera capricorni photosynthesis after herbicide exposure', Aquatic Botany, vol. 76, no. 1, pp. 1-15.
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We used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of pulsed exposure to catastrophic levels of the herbicides Atrazine, Diuron and Irgarol 1051 on the seagrass Zostera capricorni Aschers. in laboratory and field experiments. Custom-made in situ chambers were developed so seagrasses could be dosed within the meadow. Zostera capricorni was exposed to 10 and 100 ?g l-1 herbicide solutions for 10 h. During this time and for the subsequent 4-day recovery period, chlorophyll a fluorescence parameters (maximum quantum yield: Fv/Fm and effective quantum yield: ?F?m) were measured. Laboratory samples exposed to these herbicides were severely impacted during the exposure period and most treatments did not recover fully. ?F/F?m was a more sensitive indicator of herbicide impact than Fv/Fm. In situ samples were also severely impacted by Irgarol and Diuron exposure whereas samples recovered completely after exposure to Atrazine at the same concentrations as the laboratory experiments. Total chlorophyll concentrations showed only limited impact in both laboratory and field situations. This study suggests that laboratory experiments may overestimate the on-going impact of herbicides on seagrass. 2003 Elsevier Science B.V. All rights reserved.
Hohnberg, D., Ralph, P.J. & Jones, H. 2003, 'Toxicity of the herbicide atrazine at environmental concentrations to Vallisneria Gigantea, assessed using chlorophyll fluorescence', Australasian Journal of Ecotoxicology, vol. 9, pp. 93-100.
Choinski Jr, J.S., Ralph, P. & Eamus, D. 2003, 'Changes in photosynthesis during leaf expansion in Corymbia gummifera', Australian Journal of Botany, vol. 51, no. 1, pp. 111-118.
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Growth, pigment levels and various photosynthesis parameters were measured in expanding leaves of Corymbia gummifera (Solander ex Gaertner) Hochreutiner. C. gummifera trees were studied growing in sandstone plateau woodland communities in Royal National Park, New South Wales, in a recently burned open habitat. Young leaves (horizontally oriented to maximise light exposure) were found to be conspicuously red until they reached approximately 75% of their full size. As the leaves expanded, anthocyanin content declined and chlorophyll levels proportionately increased. Young red leaves showed net negative carbon assimilation rates, although CO2 assimilation rate, transpiration rate, stomatal conductance, actual quantum yield of PSII (?PSII) and apparent electron transport rate (ETR) all increased in a similar pattern as the leaves expanded. Measurements of maximum quantum yield of dark-adapted leaves (Fv/Fm) were also correlated with leaf area. Younger leaves had lower Fv/Fm ratios than did mature leaves, whether measured at midday or 2 h after sunset, indicating that young leaves exhibited some degree of chronic photoinhibition. It is concluded that C. gummifera exhibits a transient red pattern of anthocyanin expression and that photosynthesis is limited in young leaves because of low stomatal conductance, low chlorophyll content, immature chloroplasts and an attenuation of light caused by anthocyanins.
Ralph, P.J., Gademann, R., Larkum, A.W.D. & Khl, M. 2002, 'Spatial heterogeneity in active chlorophyll fluorescence and PSII activity of coral tissues', Marine Biology, vol. 141, no. 4, pp. 639-646.
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Chlorophyll-a fluorescence was measured in six species of coral, using pulse-amplitude-modulated fluorometers employing fibre-optic probes with diameters of 8 mm, 1 mm and 140 ?m. The 8-mm probe integrated responses over a large area, giving more weight to coenosarc than polyp tissue for Acropora nobilis. With 1-mm and 140-?m fibre-optic probes, the photosynthetic responses of zooxanthellae in the coenosarc and the polyp tissue of Acropora nobilis were distinguished. The polyp tissue exhibited a lower maximum in relative electron transport rate than did the coenosarc tissue, and was subject to down-regulation at higher irradiances. Coenosarc and polyp tissue (both containing zooxanthellae) showed a wide range of responses in the other corals. Down-regulation of photosynthesis in a single polyp of Pocillopora damicornis was followed after exposure to moderate irradiance, with recovery occurring over a further 4 h of shade conditions. All the corals (Acropora millepora, A. nobilis, Cyphastrea serailia, Montipora tuberculosa, Pocillopora damicornis and Porites cylindrica) showed evidence of strong down-regulation of photosynthesis under high irradiance, and little evidence of photoinhibitory damage to photosystem II.
Ralph, P.J., Polk, S.M., Moore, K.A., Orth, R.J. & Smith Jr, W.O. 2002, 'Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance', Journal of Experimental Marine Biology and Ecology, vol. 271, no. 2, pp. 189-207.
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Changes in the photobiology and photosynthetic pigments of the seagrass Zostera marina from Chesapeake Bay (USA) were examined under a range of natural and manipulated irradiance regimes. Photosynthetic activity was assessed using chlorophyll-a fluorescence, and photosynthetic pigments were measured by HPLC. Large changes in the violaxanthin, zeaxanthin, and antheraxanthin content were concomitant with the modulation of non-photochemical quenching (NPQ). Photokinetics (F v/F m, rapid light curves (RLC), and non-photochemical quenching) varied as a result of oscillating irradiance and were highly correlated to xanthophyll pigment content. Zeaxanthin and antheraxanthin concentrations increased under elevated light conditions, while violaxanthin increased in darkened conditions. Unusually high concentrations of antheraxanthin were found in Z. marina under a wide range of light conditions, and this was associated with the partial conversion of violaxanthin to zeaxanthin. These results support the idea that xanthophyll intermediate pigments induce a photoprotective response during exposure to high irradiances in this seagrass. 2002 Elsevier Science B.V. All rights reserved.
Schreiber, U., Gademann, R., Bird, P., Ralph, P.J., Larkum, A. & Kuehl, M. 2002, 'Apparent light requirement for activation of photosynthesis upon rehydration of desicated beachrock microbial mats', Journal of Phycology, vol. 38, no. N/A, pp. 125-134.
Macinnis-Ng, C.M. & Ralph, P.J. 2002, 'Towards a more ecologically relevant assessment of the impact of heavy metals on the photosynthesis of seagrass, Zostera capricorni', Marine Pollution Bulletin, vol. 45, no. N/A, pp. 100-106.
This in situ study used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of copper, cadmium, lead and zinc on the seagrass Zostera capricorni. Custom-made portable in situ exposure (PIE) chambers were developed so seagrasses could be dosed within the meadow. Z. capricorni was exposed to 0.1 and 1 mg l-1 of metal solutions for 10 h. During this time and for the subsequent four-day recovery period, the effective quantum yield of photosystem II (PS II) (?F/Fm?) was measured. While the results were variable, copper and zinc exposed samples had a depressed ?F/Fm? during the exposure period. Samples exposed to zinc recovered to pre-exposure levels but those exposed to copper did not. Cadmium and lead did not impact on the chlorophyll a fluorescence and the chlorophyll pigment data supported these findings. This study presents an innovative new application of chlorophyll a fluorescence stress assessment.
Ralph, P.J. & Short, F.T. 2002, 'Impact of the wasting disease pathogen, Labyrinthula zosterae, on the photobiology of eelgrass Zostera marina', Marine Ecology Progress Series, vol. 226, pp. 265-271.
Labyrinthula zosterae is clearly shown to be a primary pathogen of eelgrass Zostera marina L., not merely a secondary infection of senescent leaves or an indication of decomposition. The results of this investigation using a Diving-PAM fluorometer indicate that the regions of tissue photosynthetically compromised by Labyrinthula are substantially larger than previously thought. Labyrinthula moves through Zostera marina tissue at a rate of up to 0.8 mm h-1 during daylight periods. The photosynthetic efficiency of apparently healthy green leaf tissue can be reduced by almost 50% in areas up to 3 mm from a necrotic region infected with Labyrinthula. Once a necrotic spot expands to bisect the eelgrass leaf, the condition of all acropetal tissue is diminished; leaf tissue up to 5 cm away has severely reduced photosynthetic activity.
Schreiber, U., Gademann, R., Bird, P., Ralph, P.J., Larkum, A.W.D. & Khl, M. 2002, 'Apparent light requirement for activation of photosynthesis upon rehydration of desiccated beachrock microbial mats', Journal of Phycology, vol. 38, no. 1, pp. 125-134.
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Photosynthetic electron transport of beachrock microbial mats growing in the intertidal zone of Heron Island (Great Barrier Reef, Australia) was investigated with a pulse amplitude modulation chl fluorometer providing four different excitation wave-lengths for preferential excitation of the major algal groups (cyanobacteria, green algae, diatoms/dinoflagellates). A new type of fiberoptic emitter-detector unit (PHYTO-EDF) was used to measure chl fluorescence at the sample surface. Fluorescence signals mainly originated from cyanobacteria, which could be almost selectively assessed by 640-nm excitation. Even after desiccation for long time periods under full sunlight, beachrock showed rapid recovery of photosynthesis after rehydration in the light (t1/2 ? 15 min). However, when rehydrated in the dark, the quantum yield of energy conversion of PSII remained zero over extended periods of time. Parallel measurements of O2 concentration with an oxygen microoptode revealed zero oxygen concentration in the surface layer of rehydrated beachrock in the dark. Upon illumination, O2 concentration increased in parallel with PSII quantum yield and decreased again to zero in the dark. It is proposed that oxygen is required for preventing complete dark reduction of the PSII acceptor pools via the NADPH-dehydrogenase/chlororespiration pathway. This hypothesis is supported by the observation that PSII quantum yield could be partially induced in the dark by flushing with molecular oxygen.
Ralph, P.J., Gademann, R. & Larkum, A.W.D. 2001, 'Zooxanthellae expelled from bleached corals at 33C are photosynthetically competent', Marine Ecology Progress Series, vol. 220, pp. 163-168.
While a number of factors have been linked to coral bleaching, such as high light, high temperature, low salinity, and UV exposure, the best explanation for recent coral bleaching events are small temperature excursions of 1 to 2C above summer sea-surface temperatures in the tropics which induce the dinoflagellate symbionts (zooxanthellae) to be expelled from the host. The mechanism that triggers this expulsion of the algal symbionts is not resolved, but has been attributed to damage to the photosynthetic mechanism of the zooxanthellae. In the present investigation we addressed the question of whether such expelled zooxanthellae are indeed impaired irreversibly in their photosynthesis. We employed a Microscopy Pulse Amplitude-Modulated (PAM) fluorometer, by which individual zooxanthellae can be examined to study photosynthesis in zooxanthellae expelled when corals are subjected to a temperature of 33C. We show that the expelled zooxanthellae from Cyphastrea serailia were largely unaffected in their photosynthesis and could be heated to 37C before showing temperature-induced photosynthetic impairment. These results suggest strongly that the early events that trigger temperature-induced expulsion of zooxanthellae involve a dysfunction in the interaction of the zooxanthellae and the coral host tissue, and not a dysfunction in the zooxanthellae per se.
Ralph, P.J. 2000, 'Herbicide toxicity of Halophila ovalis assessed by chlorophyll a fluorescence', Aquatic Botany, vol. 66, no. 2, pp. 141-152.
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Coastal habitats are increasingly being exposed to herbicide contamination from urban and agricultural catchments. The response of a seagrass, Halophila ovalis (R. Br.) Hook. f. to four herbicides was assessed using chlorophyll a fluorescence. The herbicides tested were atrazine, simazine, DCMU (10, 100 ?g 1-1 and 1 mg 1-1) and glyphosate (1, 10 and 100 mg 1-1). Atrazine, simazine and DCMU all had substantial impacts on the chlorophyll a fluorescence responses, whereas glyphosate at concentrations two orders of magnitude higher than the other herbicides, showed no significant effect. These herbicides affected photosynthesis by reducing electron transport of H. ovalis as follows: DCMU > atrazine > simazine > glyphosate (from greatest to least inhibition). Up to 100 mg 1-1 glyphosate did not significantly affect the photosynthetic capacity. Photosynthetic pigment analysis suggested that the photosystems may have been disrupted.
Haynes, D., Ralph, P.J., Pranges, J. & Dennison, B. 2000, 'The Impact of the Herbicide Diuron on Photosynthesis in Three Species of Tropical Seagrass', Marine Pollution Bulletin, vol. 41, no. 0, pp. 288-293.
The impact and recovery from exposure to the herbicide diuron [DCMU; 3-(30,40-dichlorophenyl)-1,1-dimethyl- urea] was assessed for three tropical seagrasses, maintained in outdoor aquaria over a 10-day period. Photosynthetic stress was detected using chlorophyll a Puorescence, measured with a Diving-PAM (pulse am- plitude modulated ?Puorometer). Exposure to 10 and 100 lg ly1 diuron resulted in a decline in e?ective quantum yield (F=Fm0 ) within 2 h of herbicide exposure in Cy- modocea serrulata, Halophila ovalis and Zostera capri- corni. E?ective quantum yield also declined over the Rrst 24 h of exposure in H. ovalis at even lower diuron con- centrations (0.1 and 1.0 lg ly1). E?ective quantum yield in H. ovalis and Z. capricorni was signiRcantly depressed at all diuron concentrations (0.1?}100 lg ly1) after 5 days exposure, whereas e?ective quantum yield in C. serrulata was only signiRcantly lower in plants exposed to highest diuron concentrations (10 and 100 lg ly1). E?ective quantum yield depression was present 5 days after plants exposed to 10 and 100 lg ly1 diuron were returned to fresh seawater. These results indicate that exposure to herbicide concentrations present in nearshore Queensland sediments present a potential risk to seagrass functioning.
Ralph, P.J. 1999, 'Photosynthetic response of Halophila ovalis (R. Br.) Hook. f. to combined environmental stress', Aquatic Botany, vol. 65, no. 1-4, pp. 83-96.
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Combinations of stresses showed an additive effect in comparison to the individual stress responses. It is apparent from these results that thermal, elevated-light or osmotic stress increases the sensitivity of Halophila ovalis to any of the other stress factors. Photosynthetic stress was detected using chlorophyll a fluorescence. Quantum yield was consistently the most effective measure of photosynthetic stress from combination stress exposures. Chlorophyll pigment analysis supported the general decline in photosynthetic capacity as indicated by the chlorophyll fluorescence; however, several anomalies did occur.
Ralph, P.J. 1999, 'Light-induced photoinhibitory stress responses of laboratory-cultured Halophila ovalis', Botanica Marina, vol. 42, no. 1, pp. 11-22.
This paper details experiments performed to investigate the short-term stress effects of both high and lowlight regimes on laboratory-cultured Halophila ovalis using chlorophyll fluorescence. Increasing irradiance up to 400 ?mol quanta m-2 s-1 on laboratory-cultured H. ovalis caused a range of photoprotective (PSII downregulation) responses, depending on intensity and the exposure period. The 400 ?mol quanta m-2 s-1 treatment appeared to inhibit the photosynthetic rate, although this was not permanent, as the tissue rapidly recovered. Low-light conditions over periods of up to 10 days did not adversely affect H. ovalis. The efficiency of photon capture increased, and the proportion of open PSII reaction centres was greater under lower PPFD conditions. The rate of onset and the degree of impact of the stress as identified by chlorophyll fluorescence was noticeably faster (within 1 h) at elevated PPFD, as opposed to reduced light conditions. Recovery from high-light stress was rapid, as demonstrated by the qP and quantum yield responses; however, the qN coefficient was much slower to adjust to altered ambient light regimes. The range of PPFD investigated indicated the plants' capacity to adjust to modified light regimes, and also showed the significance of chlorophyll fluorescence response to environmentally relevant PPFD levels. Pigment analysis supported the conclusions drawn from the chlorophyll fluorescence results. The 400 ?mol quanta m-2 s-1 treatment resulted in a decrease in chlorophyll concentrations, with chlorophyll b suffering a greater decline. The 200 ?mol quanta m-2 s-1 treatment was similar to the control with respect to pigment concentration. Carotenoids were not affected by elevated light. Reduced light caused an increase in both the chlorophyll and carotenoid pigments.
Ralph, P.J., Gademann, R., Larkum, A.W.D. & Schreiber, U. 1999, 'In situ underwater measurements of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts', Marine Ecology Progress Series, vol. 180, pp. 139-147.
Chlorophyll fluorescence was used to assess the in situ photosynthesis of a range of reef-dwelling endosymbionts. Such non-intrusive in situ measurements became possible after the recent development of a submersible pulse modulated fluorometer (DIVING-PAM, Walz, Germany). Several corals, a clam [Tridacna maxima) and an anemone (Heteractis sp.) all showed strong chlorophyll a fluorescence signals originating from the dinoflagellate endosymbionts. Quenching analysis by the saturation pulse method revealed high quantum yields and light response curves characteristic of physiologically healthy sun plants. Rapid light curves (RLC) were applied to assess the light saturation behaviour of the different organisms in their rapidly changing natural environment. The 3 corals (Acropora aspera, Goniastrea sp. and Porites sp.), the clam T. maxima, and the anemone Heteractis sp. all showed high photosynthetic activity. The corals had a maximum electron transport rate of 180 to 270 ?mol electrons m-2 s-1, the Heteractis sp. displayed a maximum rate of approximately 120 ?mol electrons m-2 s-1, whilst the T, maxima showed no saturation up to 1900 ?mol quanta m-2 s-1, where a rate of 325 ?mol electrons m-2 s-1 was observed. Three species of corals showed varying degrees of an apparent mid-day depression, occurring during the summer peak irradiance at low tide. Because quantum efficiency rapidly recovered during the afternoon period, when the irradiance levels decreased again, it is concluded from these preliminary investigations that the depression resulted mainly from down-regulation of photosystem II. Depth did not appear to influence the RLCs of A. aspera growing at 2, 5 and 10 m. Coral morphology influenced the apparent electron transport rate at different locations within a single colony.
Ralph, P.J. & Burchett, M.D. 1998, 'Photosynthetic response of Halophila ovalis to heavy metal stress', Environmental Pollution, vol. 103, no. 1, pp. 91-101.
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This paper deals with the photosynthetic effects of a range of heavy metals on the seagrass Halophila ovalis. In this study, the photosynthetic response of laboratory-cultured H. ovalis to four heavy metals (Cu, Cd, Pb, Zn) was investigated. The results indicated clearly that chlorophyll a fluorescence was effective in monitoring the onset and development of stress, and occasional recovery, of H. ovalis when exposed to a wide range of heavy metals. Heavy metals in concentrations from 1 to 10 mg litre-1 produced several acute toxic responses. They had a variety of effects on the photosynthetic processes of this seagrass, with Cu and Zn having substantially greater effects than Pb and Cd. Quantum yield was the most sensitive measure of the photosynthetic processes affected by all heavy metals tested. With some exceptions, photosynthetic pigment content generally confirmed the chlorophyll a fluorescence responses. Copyright (C) 1998 Elsevier Science Ltd.
Ralph, P.J., Gademann, R. & Dennison, W.C. 1998, 'In situ seagrass photosynthesis measured using a submersible, pulse-amplitude modulated fluorometer', Marine Biology, vol. 132, no. 3, pp. 367-373.
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Assessments of photosynthetic activity in marine plants can now be made in situ using a newly developed, submersible, pulse-amplitude modulated (PAM) fluorometer: Diving-PAM. PAM fluorometry provides a measure of chlorophyll a fluorescence using rapid-light curves in which the electron-transport rate can be determined for plants exposed to ambient light conditions. This technique was used to compare the photosynthetic responses of seagrasses near Rottnest Island, Western Australia. Several fluorescence parameters were measured as a function of time of day and water depth; electron-transport rate (ETR), quantum yield, photochemical quenching and non-photochemical quenching and Photosystem II (PSII) photochemical efficiency (F(v):F(m) ratio) were measured. Results indicate that recent light-history plays a crucial role in seagrass photosynthetic responses. Maximum ETR of Posidonia australis, Amphibolis antarctica and Halophila ovalis is influenced by the irradiance during the diurnal cycle, with low rates at dawn and dusk (< 10 ?mol electron m-2 s-1), highest rates in late morning (40 to 60 ?mol electron m-2 s-1) and a mid-day depression. Maximum ETR and PSII photochemical efficiency varied widely between seagrass species and were not correlated. A comparison of photochemical to non-photochemical quenching indicated that seagrasses in shallow water receiving high light have a high capacity for non-photochemical quenching (e.g. light protection) compared to seagrasses in deep water. These results indicate that in situ measurements of photosynthesis will provide new insights into the mechanisms and adaptive responses of marine plants.
Ralph, P.J. 1998, 'Photosynthetic response of laboratory-cultured Halophila ovalis to thermal stress', Marine Ecology Progress Series, vol. 171, pp. 123-130.
Chlorophyll a fluorescence was able to rapidly detect responses of laboratory-cultured Halophila ovalis to acute changes in temperature. Six heating (27.5, 30.0, 32.5, 35.0, 37.5 and 40.0C) and 6 chilling (10.0, 12.5, 15.0, 17.5, 20.0 and 22.5C) stress levels were used over a 96 h exposure period, followed by a 5 d recovery period, to provide a comprehensive measure of the overall thermal stress effects and responses. The acute (5 h) response of H. ovalis to thermal shock was characterised by a change in photochemical quenching, whilst after 96 h the effective and maximum quantum yields were able to detect temperature changes of 2.5C. Maximum fluorescence declined for both heating and chilling, whilst minimum fluorescence was stable for chilling and increased with moderate heating. H. ovalis was susceptible to thermal stress outside the optimum photosynthetic range of 25 to 30C, where extreme temperatures (10.0, 12.5, 37.5 and 40C) caused a complete collapse of the PSII electron transport system. When thermal stress was applied in darkness, chlorophyll a fluorescence was not able to detect the onset of thermal stress (except at 40.0C). H. ovalis tolerated thermal shock from 15 to 30C for up to 96 h, and was able to completely recover on return to standard growth conditions.
Ralph, P.J. 1998, 'Photosynthetic responses of Halophila ovalis (R. Br.) Hook. f. to osmotic stress', Journal of Experimental Marine Biology and Ecology, vol. 227, no. 2, pp. 203-220.
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Chlorophyll fluorescence was used to monitor the onset, development and recovery from hyperand hypo-osmotic stress effects of the seagrass, Halophila ovalis (R. Br.) Hook. f. H. ovalis was able to tolerate rapid transfer from normal (35 ppt) seawater to salinities of 25% to 150% seawater without significant photosynthetic stress symptoms. Exposure to fresh water caused acute stress: Within 24 h, all specimens exposed to fresh water showed substantial photosynthetic damage, and after 4 days were photosynthetically inactive. Hyper-osmotic conditions caused more severe stress responses, with H. ovalis tolerating 150% seawater, whereas sudden exposure to 200% and 250% seawater caused irreparable photosynthetic damage within 96 h. The severity of the extreme osmotic stress responses was light-dependent, as exposure to a similar level of osmotic stress in darkness apparently resulted in less damage. Recovery was limited to the 25% and 50% seawater treatments, whereas the 150% treatment survived but did not recover to the control level. Fresh water, 200% and 250% seawater treatments completely quenched photosynthesis during the 5-day experimental period, and, therefore, recovery was not possible. Although recovery from osmotic stress was demonstrated, the extreme treatments resulted in premature senescence of the entire leaf and petiole. Photosynthetic pigments were also affected by osmotic stress, and the damage caused by osmotic stress was partially light-dependent. Photosynthetic pigments thus confirmed the chlorophyll fluorescence responses.
Ralph, P.J., Morrison, D.A. & Addison, A. 1998, 'A quantitative study of the patterns of morphological variation within Hormosira banksii (Turner) Decaisne (Fucales: Phaeophyta) in South-Eastern Australia', Journal of Experimental Marine Biology and Ecology, vol. 225, no. 2, pp. 285-300.
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Hormosira banksii shows a considerable degree of morphological variability throughout its range in south-eastern Australia, apparently in relation to the local habitat, and there have been several previous qualitative attempts to categorize this variation by recognizing ecoforms. From our quantitative morphometric analyses of plants from 21 sites covering 300 km of coastline in south-eastern Australia, using multiple discriminant function analysis based on seven vesicle characteristics (measuring size and shape), there is very little evidence of intergrading forms. The morphological variation is not multivariately continuous, as has been previously suggested, although each individual attribute does show more-or- less continuous variation, and the morphological variation is not a simple reflection of habitat but reflects more complex microhabitat relationships. The morphological forms that we recognize are multivariate, and thus all of the attributes need to be considered. In particular, volume (or surface area:volume ratio) is usually a very good discriminator between groups, indicating that both size and shape are important for defining the groups. We recognize two main phenotypically distinct groups, comprising plants from sheltered estuarine situations and those from exposed marine rock platforms. The vesicles of plants from the estuarine habitats are more-or-less spherical (length ? diameter), with a volume approximately 3-10 times that of vesicles from the marine plants; it is thus probable that the variation in the vesicle dimensions of these morphs can be linked to desiccation resistance. The estuarine and marine habitats are likely to be ecologically isolated from each other, and there is therefore unlikely to be a great deal of gene flow between these plants; if they are treated as separate species, then they would be H. banksii (Turner) Decaisne and H. sieberi (Bory) Decaisne, respectively. Three relatively distinct morphometric forms were also recognizable within the marine group, including plants from the bottom of rock pools; from rock pool edges and sublittoral regions; and from the surface of rock pools and exposed parts of the platforms. Two less distinctive morphometric forms were recognizable within the estuarine group, including plants from tidal flats and those from mangrove forests. The taxonomic status of these microhabitat forms remains uncertain.
Ralph, P.J. & Burchett, M.D. 1998, 'Impact of petrochemicals on the photosynthesis of Halophila ovalis using chlorophyll fluorescence', Marine Pollution Bulletin, vol. 36, no. 6, pp. 429-436.
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Laboratory-cultured Halophila ovalis showed tolerance to petrochemical exposure up to 1% (w/v) solution of Bass Strait crude oil, an oil dispersant (Corexit 9527) and a mixture of crude oil and dispersant. Quantum yield, as measured by chlorophyll fluorescence, was the most sensitive measure of the photosynthetic processes affected by petrochemicals. The results indicated clearly that chlorophyll fluorescence was effective at monitoring the onset and development of stress and recovery of H. ovalis when exposed to crude oil, dispersant and a mixture of the two compounds. Photosynthetic pigment content generally confirmed the chlorophyll fluorescence response; however, several anomalies occurred.
Schreiber, U., Gademann, R., Ralph, P.J. & Larkum, A.W.D. 1997, 'Assessment of photosynthetic performance of Prochloron in Lissoclinum patella in hospite by chlorophyll fluorescence measurements', Plant and Cell Physiology, vol. 38, no. 8, pp. 945-951.
Two new PAM fluorometers (pulse amplitude modulated) were used in an investigation of photosynthetic performance of Prochloron resident as a symbiont in the ascidian Lissoclinum patella, growing in a coral reef of Heron Island on the Great Barrier Reef. With a new DIVING-PAM in situ measurements of effective PSII quantum yield (?F/Fm') as a function of quantum flux density (rapid light curves) were carried out in 2.5 m depth in the reef and in a seawater tank. Photosynthetic electron transport rates were measured on in hospite Prochloron both in situ and in collected material. Both light-limited and light-saturated yields were exceptionally high. Maximal yields (Fv/Fm) were ~0.83. A new TEACHING-PAM was employed for analysing dark-light induction and light-dark relaxation kinetics in collected samples with Prochloron in hospite. Considerable variability in kinetic responses was observed which was found to be at least in part due to differences in O2 concentration. It is suggested that endogenous reductants feed electrons into the intersystem transport chain, which normally is reoxidized by O2 (chlororespiration), and that in the dark, the reduction level of PSII acceptors is increased due to a decline in O2 concentration. The pattern of fluorescence responses differed markedly from those found in cyanobacteria and provides new insights into light-harvesting responses of a photosynthetic prokaryote with a membrane bound light-harvesting system, as contrasted with an extrinsic light-harvesting system.
Ralph, P.J. & Burchett, M.D. 1995, 'Photosynthetic responses of the seagrass Halophila ovalis (R. Br.) Hook. f. to high irradiance stress, using chlorophyll a fluorescence', Aquatic Botany, vol. 51, no. 1-2, pp. 55-66.
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With the increasing threat of destruction to diminishing seagrass resources, there is a need for an accurate stress monitoring procedure to be developed for species of this community. In this study chlorophyll a fluorescence (20C) was used to monitor the stress responses of the seagrass Halophila ovalis (R. Br.) Hook. f. when exposed to increased irradiance. Leaf tissue was exposed to the following light treatments: 100, 500 and 1000 ?mol m-2 s-1 for 10, 20, 40, 60 and 120 min. Exposure to the higher irradiances resulted in significant photoinhibitory responses for both fluorescence and oxygen evolution measurements. There was evidence of both photoinhibitory responses; photoprotection and photodamage. Photoprotective processes appear to be operating in all 500 ?mol m-2 s-1 treatments, and in the 1000 ?mol m-2 s-1 treatments for at least the first 60 min. Responses included an elevated initial fluorescence, accompanied by a reduction in the variable/maximum fluorescence ratio, maximum fluorescence and variable fluorescence. Photodamage appeared to be involved after 120 min exposure at 1000 ?mol m-2 s-1, where all fluorescence parameters including initial fluorescence were quenched and the maximum oxygen evolution rate saturated at a lower irradiance. A direct linear relationship between variable/maximum fluorescence ratio and the light saturated photosynthetic rate was demonstrated for H. ovalis. The oxygen evolution data corroborate the evidence of the stress responses indicated with the fluorescence results. Generally, the longer the exposure period or higher the irradiance, the greater the damage to the tissue and the associated stress symptoms. These results demonstrate the potential of chlorophyll a fluorescence to assess the relative photoinhibitory response of H. ovalis to short-term exposure to increased irradiance. 1995.
Ralph, P.J., Burchett, M.D. & Pulkownik, A. 1992, 'Distribution of extractable carbohydrate reserves within the rhizome of the seagrass Posidonia australis Hook. f.', Aquatic Botany, vol. 42, no. 4, pp. 385-392.
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Rhizomes of Posidonia australis Hook. f., collected from Botany Bay on the eastern coast of Australia, were analyzed for extractable carbohydrates. The concentration of extractable carbohydrate in the stelar tissue of the rhizome was significantly higher than that in the surrounding cortex. Reduced concentrations of extractable carbohydrate reserves were found in juvenile tissue and tissue adjacent to the apical meristem. Unexpanded internodes contained higher concentrations of extractable carbohydrate than expanded internodes. Tissue from equivalent positions along both the lateral and parental axis of a rhizome branch showed similar extractable carbohydrate levels and had similar patterns of internodal length. The pattern of carbohydrate storage within the rhizomatous tissue of P. australis is closely related to the age and health of the plant. 1992.


Ralph, P.J. & Sinutok, S. AccessUTS Pty Limited 2013, Vallisneria and submerged macrophyte management in Penrith Lakes, Sydney.
Ralph, P.J., Skilbeck, G. & Sinutok, S. AccessUTS Pty Limited 2013, Vallisneria and submerged macrophyte management in the Penrith Lakes Scheme, Sydney.
Petrou, K., Jimenez Denness, I.M., Chartrand, K.M., Ralph, P.J. & Rasheed, A. DEEDI Publication 2011, Seagrass Health Study - Phase II August Update, pp. 1-54, Cairns.
Chartrand, K.M., McKenna, S.A., Petrou, K., Jimenez Denness, I.M., Franklin, J., Sankey, T.L., Hedge, S.A., Rasheed, M. & Ralph, P.J. DEEDI Publication 2010, Port Curtis Benthic Primary Producer Habitat Assessment and Health Studies Update: Interim Report December 2010, pp. 1-128, Cairns.
Ralph, P.J., Wilson, K., Hill, R. & Petrou, K. Institute for Water and Environmental Resource Management, and Department of Environmental Sciences, 2007, Effects of increased temperature pulses on temperate seagrass: progress reports 1-4, pp. 1-14, Sydney.