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Dr Ross Hill


Dr Ross Hill is an Associate of the Faculty of Science and an Honorary Member of the Plant Functional Biology & Climate Change Cluster.

Ross currently holds a highly competitive Vice-Chancellor's Postdoctoral Research Fellowship at The University of New South Wales.

Ross’s research primarily concentrates on the impacts of climate change to a range of photosynthetic marine organisms which produce calcium carbonate skeletons. Of primary concern is how these vulnerable species will respond to warmer and more acidic conditions and how climate change will affect their interaction with natural enemies, such as herbivory and disease. Experiments being conducted at the Sydney Institute of Marine Science (SIMS) are simulating a range of future climate change scenarios to determine the likely impacts for these important species.

Another focus of Ross’s research is exploring the photosynthetic mechanisms that lead to coral bleaching. Thermal stress is known to cause the expulsion of algal symbionts from corals, but the photosynthetic trigger of bleaching, the capacity for photoprotection and protein repair mechanisms remain unresolved. Ross's research also extends to Antarctica where he investigates the impacts of climate change on pack ice algae.

These algae live in an extreme environment yet form the base of the Antarctic food chain. This research is of high importance as it is critical that we understand what impacts climate change will have on such a unique and valuable ecosystem.

In 2003, Ross completed his undergraduate degree in Environmental Biology with First Class Honours and the University Medal. He subsequently completed his PhD at the University of Technology, Sydney on the photosynthetic impacts to symbiotic dinoflagellates in corals during bleaching events, and graduated in 2008. Ross then completed a five month, ARC-funded, Research Fellowship, involving a 6 week voyage to Antarctica.

From 2008-2012, Ross was a Lecturer and Chancellor’s Postdoctoral Research Fellow at the University of Technology, Sydney.


Professional Associations:

  • Councillor and webmanager for the Australian Coral Reef Society.
  • Member of the Sydney Institute of Marine Science
  • Invited speaker at the Coral Reef Targeted Research and Capacity Building for Management Workshop, Puerto Morelos, Mexico (2005)

Professional Memberships:
  • American Society of Limnology and Oceanography (ASLO)
  • Australasian Society for Phycology and Aquatic Botany (ASPAB)
  • Australian Coral Reef Society (ACRS)
  • Australian Marine Science Association (AMSA)
  • International Society for Reef Studies (ISRS)
  • International Symbiosis Society (ISS)

  • Professor David Kramer, Michigan State University. Photosynthetic antenna systems of Symbiodinium.
  • Associate Professor Douglas Campbell, Mt Allison University (Canada). Microalgal photosynthetic protein analyses, specifically D1 and Rubisco.
  • Emeritus Professor Anthony Larkum, The University of Sydney and University of Technology, Sydney. Photosynthesis, photoprotection and calcification in marine phototrophs.
  • Professor Michael Kühl, University of Copenhagen. Optical properties of corals and other tropical benthic marine calcifiers.
  • Dr Shunichi Takahashi, Australian National University. Mechanisms of photoinactivation and repair in Symbiodinium and the influence of thermal stress in promoting photoinhibition.
  • Professor Alfred Holzwarth and Dr Chavdar Slavov, Max-Planck-Institut für Bioanorganische Chemie. Characterising the potential of excess energy dissipation in the antennae of Symbiodinium.
  • Dr Björn Rost, Alfred Wegener Institute for Polar and Marine Research. Evaluating the vulnerability of temperate photosynthetic marine calcifiers to climate change.
  • Professor Ond?ej Prášil, Laboratory of Photosynthesis, Institute of Microbiology, Czech Academy of Sciences. Interpreting low-temperature fluorescence spectrum of dinoflagellate algae to reveal shifts in light harvesting antenna complexes.
  • Dr Christine Ferrier-Pagès, Centre Scientifique de Monaco. Research on coral calcification, growth and feeding.
  • Associate Professor Simon Davy, Dr Ken Ryan and Dr Laura Wicks, Victoria University of Wellington (New Zealand). Coral and anemone photobiology in high-latitude reefs.
  • Dr Imre Vass, Biological Research Centre, Hungarian Academy of Sciences. Sensitivity of photosynthetic processes in coral symbionts under thermal stress.
  • Dr Peter Scanes, Head of the Coastal Waters Unit at the NSW Department of Environment, Climate Change and Water. Building an understanding of seagrass resilience into estuarine management.
  • Associate Professor Dee Carter, The University of Sydney. Molecular ecology and photobiology of Symbiodinium and Chromera.
  • Dr Jane Williamson, Macquarie University. Impacts of ocean acidification to marine bivalves.
  • Dr Selina Ward, The University of Queensland. Climate change impacts on coral reproduction.
  • Dr Anna Scott, Southern Cross University. Climate change impacts on sea anemone reproduction, bleaching thresholds of adult anemones and recovery strategies.
  • Adrian Lutz, James Cook University and the Australian Institute of Marine Science. The biochemistry of electron carriers in the photosynthetic machinery of coral symbionts.
  • Professor Peter Steinberg and Dr Tilmann Harder, University of New South Wales. Calcification and herbivore avoidance in temperate photosynthetic marine calcifiers.

Ross Hill - Marine Climate Change video

Image of Ross Hill
Associate of the Faculty, Plant Functional Biology & Climate Change
B.Env.Biol. (UTS), B. Sc in Env. Sci (1st Class Hons & Univ. Medal), Doctor of Philosophy

Research Interests

  • Coral bleaching
  • Ocean acidification
  • Antarctic algae
  • Desalination
  • Climate change

Seagrass Inc
Ross Hill - Marine Climate Change video

Since 2004, Ross has taught the following subjects:

  • Biocomplexity
  • The Biosphere
  • Plant Physiology and Ecophysiology
  • Marine Primary Producers
  • Mapping and Remote Sensing
  • Environmental Management
  • Animal Function and Diversity
  • Cells, Genetics and Evolution


Hill, R. 2008, Coral Bleaching: photosynthetic impacts on symbiotic dinoflagellates. Coral Reefs and Climate Change, VDM Publishing House Ltd, Germany.


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.
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', ASPAB 2009 Conference, Townsville, Australia.
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 µmol 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.

Journal articles

Sinutok, S., Hill, R., Kühl, 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.
Jeans, J., Szabó, 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.
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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Schrameyer, V., Wangpraseurt, D., Hill, R., Kühl, 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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© 2014 Schrameyer et al. 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.
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.
Krämer, 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.
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 32°C. In contrast, the primary site of thermal inhibition occurred upstream of ferredoxin in clades A and B at 32°C, while at 34°C, 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.
Hill, R. & Scott, A. 2012, 'The influence of irradiance on the severity of thermal bleaching in sea anemones that host anemonefish', Coral Reefs, vol. 31, no. 1, pp. 273-284.
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Hill, R., Larkum, A.W.D., Prášil, O., Kramer, D.M., Szabó, 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., Kühl, 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.
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.
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 (28°C, 30°C, 32°C, and 34°C) 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 34°C under all CO 2 levels, all species died. Chlorophyll (Chl) a and b concentration in Halimeda spp. Significantly decreased in 203 Pa, 32°C and 34°C 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 28°C. 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 (32°C and 34°C in all pH treatments). The rate of oxygen prod°Ction 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 32°C 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.
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.
Wicks, L.C., Hill, R. & Davy, S.K. 2010, 'The influence of irradiance on tolerance to high and low temperature stress exhibited by Symbiodinium in the coral, Pocillopora damicornis, from the high-latitude reef of Lord Howe Island', Limnology and Oceanography, vol. 55, no. 6, pp. 2476-2486.
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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.
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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.
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.
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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 7°C and 14°C 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.
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.
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 (32°C and 400 ?mol photons m-2 s-1) and then maintained at 28, 30 or 32°C 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 28°C (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.
Scanes, P., Coade, G., Doherty, M. & Hill, R. 2007, 'Evaluation of the utility of water quality based indicators of estuarine lagoon condition in NSW, Australia', Estuarine, Coastal and Shelf Science, vol. 74, no. 1-2, pp. 306-319.
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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 (32°C 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.
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 25°C) or elevated temperature (225 ?mol photons m-2 s -1 and 32°C) 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 32°C) 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.
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.
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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 (26&deg;C, 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. &copy; Inter-Research 2005.
Hill, R., Larkum, A.W.D., Frankart, C., Kühl, 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, 1-2&deg;C 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 (33&deg;C) 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 cora...
Hill, R., Schreiber, U., Gademann, R., Larkum, A.W.D., Kühl, 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 (33&deg;C vs. 27&deg;C) 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 33&deg;C 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 33&deg;C 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. &copy; Springer-Verlag 2004.
Major, R.E., Pyke, G.H., Christy, M.T., Gowing, G. & Hill, R.S. 1994, 'Can nest predation explain the timing of the breeding season and the pattern of nest dispersion of New Holland honeyeaters?', Oikos, vol. 69, no. 3, pp. 364-372.


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.