I graduated as a biologist in 2005 from the University of Szeged, Hungary with a specialisation of plant cell biology and plant stress physiology.
I obtained my PhD degree in 2011 at the Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, Szeged, Hungary. My PhD research topic was to reveal the macro-organisation and structural flexibility of the light-harvesting pigment system in diatoms.
Since 2011, I am a post-doctoral fellow at UTS within the Climate Change Cluster (C3) and my research activity is to explore and characterise the light harvesting and photoprotective processes in various species of microalgae. My particular interest is to unravel the unique photosynthetic apparatus and photosynthetic productivity of coral-endosymbiont algae (Symbiodinium sp.) in response to climatic variation, especially focusing on their peridinin-chlorophyll proteins. I am also a member of the C3 Algal Biosystems Research team; I carry out various photobiology and photobioreactor projects on biofuel producing algae such as the prasinophycean alga Nannochloropsis sp. and diatoms (Chaetoceros sp.)
Key research achievements:
- a new marker of structural flexibility of photosynthesis under stress in diatoms (Bacillariophyta)
- Isoprene as a structural stabilisator of photosynthetic membranes
- Self-assembling artificial nanorods: new candidates for light collectors in solar fuel cells
- Unique properties of photosynthetic light harvesting in coral endosymbiont algae
- Enhancing biofuel production in microalgae
Member of the Australian Coral Reef Society
Current research interests
- light-harvesting, photosynthesis and photoprotection in diatoms, dinoflagellates, biofuel producing algae (Nannochloropsis sp.)
- optimising photosynthesis and growth of microalgae to enhance their secondary metabolite composition relevant to industrial applications
Past research interests
- dynamics of photosynthesis (supramolecular organisation of thylakoid membranes and light-harvesting antenna complexes of algae, characterised by various spectroscopic and biochemical methods
- photosynthetic thylakoid membrane dynamics under environmental stress
- microscopic analysis of photosynthetic membranes (transmission electron microscopy, fluorescence microscopy)
Iermak, I, Szabó, M & Zavafer, A 2020, 'Analysis of OJIP transients during photoinactivation of photosystem ii indicates the presence of multiple photosensitizers in vivo and in vitro', Photosynthetica, vol. 58, no. Special Issue, pp. 497-506.View/Download from: Publisher's site
© The authors. Generally, excessive excitation absorbed by the pigments is considered the cause of PSII photodamage. Previous studies of action spectra of PSII photodamage concluded that shorter wavelengths induce more damage, supporting the hypothesis of the existence of more than one photosensitizer. However, the relative influence of different photosensitizers is still inconclusive. In this work, we have revisited this question by inducing PSII photodamage in vivo and in vitro at two different wavelengths (460 and 660 nm) where the net absorption cross section was the same using equal irradiance. To correlate PSII photodamage with each wavelength band, we followed its time course using the OJIP transient of the chlorophyll fluorescence to determine the possible contributions of photoinhibition by different photosensitizers. We found evidence that at least two sites of photoinactivation of PSII exist.
Bates, H, Zavafer, A, Szabo, M & Ralph, PJ 2019, 'A guide to Open-JIP, a low-cost open-source chlorophyll fluorometer', PHOTOSYNTHESIS RESEARCH, vol. 142, no. 3, pp. 361-368.View/Download from: Publisher's site
Zavřel, T, Szabó, M, Tamburic, B, Evenhuis, C, Kuzhiumparambil, U, Literáková, P, Larkum, AWD, Raven, JA, Červený, J & Ralph, PJ 2018, 'Effect of carbon limitation on photosynthetic electron transport in Nannochloropsis oculata.', Journal of photochemistry and photobiology. B, Biology, vol. 181, pp. 31-43.View/Download from: Publisher's site
This study describes the impacts of inorganic carbon limitation on the photosynthetic efficiency and operation of photosynthetic electron transport pathways in the biofuel-candidate microalga Nannochloropsis oculata. Using a combination of highly-controlled cultivation setup (photobioreactor), variable chlorophyll a fluorescence and transient spectroscopy methods (electrochromic shift (ECS) and P700 redox kinetics), we showed that net photosynthesis and effective quantum yield of Photosystem II (PSII) decreased in N. oculata under carbon limitation. This was accompanied by a transient increase in total proton motive force and energy-dependent non-photochemical quenching as well as slightly elevated respiration. On the other hand, under carbon limitation the rapid increase in proton motive force (PMF, estimated from the total ECS signal) was also accompanied by reduced conductivity of ATP synthase to protons (estimated from the rate of ECS decay in dark after actinic illumination). This indicates that the slow operation of ATP synthase results in the transient build-up of PMF, which leads to the activation of fast energy dissipation mechanisms such as energy-dependent non-photochemical quenching. N. oculata also increased content of lipids under carbon limitation, which compensated for reduced NAPDH consumption during decreased CO2 fixation. The integrated knowledge of the underlying energetic regulation of photosynthetic processes attained with a combination of biophysical methods may be used to identify photo-physiological signatures of the onset of carbon limitation in microalgal cultivation systems, as well as to potentially identify microalgal strains that can better acclimate to carbon limitation.
Chartrand, KM, Szabó, M, Sinutok, S, Rasheed, MA & Ralph, PJ 2018, 'Living at the margins - The response of deep-water seagrasses to light and temperature renders them susceptible to acute impacts.', Marine environmental research, vol. 136, pp. 126-138.View/Download from: Publisher's site
Seagrasses inhabit environments where light varies at different timescales, nonetheless are acutely sensitive to reductions in light beyond some conditional bounds. Two tropical deep-water seagrasses, Halophila decipiens and Halophila spinulosa, from the Great Barrier Reef were tested for their response to defined light and temperature regimes to identify their growth requirements and potential thresholds of mortality. Species were exposed to two light intensities, saturating (75 μmol photons m-2 s-1) and limiting (25 μmol photons m-2 s-1) light and two temperature treatments (26 °C and 30 °C) over a four-week period. Wavelength-specific parameters of PSII photochemistry were evaluated for seagrass leaves, as well as shoot density, gas exchange, and pigment content. Both species were sustained under saturating light levels (3.2 mol photons m-2 d-1) while limiting light led to decreased shoot density for H. decipiens and H. spinulosa after two and four weeks, respectively. Wavelength-specific photochemistry was also affected under light-limiting treatments for both species while the functional absorption cross section was highly conserved. Photoacclimation and physiological adjustments by either species was not adequate to compensate for reduced irradiance suggesting these plants reside at the margins of their functional limits. As such, relatively short periods of light attenuating events, like dredging or flood plumes, may be detrimental to deep-water seagrass populations.
Davey, PA, Pernice, M, Ashworth, J, Kuzhiumparambil, U, Szabó, M, Dolferus, R & Ralph, PJ 2018, 'A new mechanistic understanding of light-limitation in the seagrass Zostera muelleri.', Marine Environmental Research, vol. 134, pp. 55-67.View/Download from: Publisher's site
In this study we investigated the effect of light-limitation (∼20 μmol photons m-2 s-1) on the southern hemisphere seagrass, Zostera muelleri. RNA sequencing, chlorophyll fluorometry and HPLC techniques were used to investigate how the leaf-specific transcriptome drives changes in photosynthesis and photo-pigments in Z. muelleri over 6 days. 1593 (7.51%) genes were differentially expressed on day 2 and 1481 (6.98%) genes were differentially expressed on day 6 of the experiment. Differential gene expression correlated with significant decreases in rETRMax, Ik, an increase in Yi (initial photosynthetic quantum yield of photosystem II), and significant changes in pigment composition. Regulation of carbohydrate metabolism was observed along with evidence that abscisic acid may serve a role in the low-light response of this seagrass. This study provides a novel understanding of how Z. muelleri responds to light-limitation in the marine water column and provides potential molecular markers for future conservation monitoring efforts.
Kim, M, Brodersen, KE, Szabó, M, Larkum, AWD, Raven, JA, Ralph, PJ & Pernice, M 2018, 'Low oxygen affects photophysiology and the level of expression of two-carbon metabolism genes in the seagrass Zostera muelleri.', Photosynthesis Research, vol. 136, no. 2, pp. 147-160.View/Download from: Publisher's site
Seagrasses are a diverse group of angiosperms that evolved to live in shallow coastal waters, an environment regularly subjected to changes in oxygen, carbon dioxide and irradiance. Zostera muelleri is the dominant species in south-eastern Australia, and is critical for healthy coastal ecosystems. Despite its ecological importance, little is known about the pathways of carbon fixation in Z. muelleri and their regulation in response to environmental changes. In this study, the response of Z. muelleri exposed to control and very low oxygen conditions was investigated by using (i) oxygen microsensors combined with a custom-made flow chamber to measure changes in photosynthesis and respiration, and (ii) reverse transcription quantitative real-time PCR to measure changes in expression levels of key genes involved in C4 metabolism. We found that very low levels of oxygen (i) altered the photophysiology of Z. muelleri, a characteristic of C3 mechanism of carbon assimilation, and (ii) decreased the expression levels of phosphoenolpyruvate carboxylase and carbonic anhydrase. These molecular-physiological results suggest that regulation of the photophysiology of Z. muelleri might involve a close integration between the C3 and C4, or other CO2 concentrating mechanisms metabolic pathways. Overall, this study highlights that the photophysiological response of Z. muelleri to changing oxygen in water is capable of rapid acclimation and the dynamic modulation of pathways should be considered when assessing seagrass primary production.
Goyen, S, Pernice, M, Szabó, M, Warner, ME, Ralph, PJ & Suggett, DJ 2017, 'A molecular physiology basis for functional diversity of hydrogen peroxide production amongst Symbiodinium spp. (Dinophyceae)', Marine Biology: international journal on life in oceans and coastal waters, vol. 164, no. 3.View/Download from: Publisher's site
Szabó, M, Larkum, AWD, Suggett, DJ, Vass, I, Sass, L, Osmond, B, Zavafer, A, Ralph, PJ & Chow, WS 2017, 'Non-intrusive assessment of photosystem II and photosystem I in whole coral tissues', Frontiers in Marine Science, vol. 4, pp. 1-12.View/Download from: Publisher's site
© 2017 Szabó, Larkum, Suggett, Vass, Sass, Osmond, Zavafer, Ralph and Chow. Reef building corals (phylum Cnidaria) harbor endosymbiotic dinoflagellate algae (genus Symbiodinium) that generate photosynthetic products to fuel their host's metabolism. Non-invasive techniques such as chlorophyll (Chl) fluorescence analyses of Photosystem II (PSII) have been widely used to estimate the photosynthetic performance of Symbiodinium in hospite. However, since the spatial origin of PSII chlorophyll fluorescence in coral tissues is uncertain, such signals give limited information on depth-integrated photosynthetic performance of the whole tissue. In contrast, detection of absorbance changes in the near infrared (NIR) region integrates signals from deeper tissue layers due to weak absorption and multiple scattering of NIR light. While extensively utilized in higher plants, NIR bio-optical techniques are seldom applied to corals. We have developed a non-intrusive measurement method to examine photochemistry of intact corals, based on redox kinetics of the primary electron donor in Photosystem I (P700) and chlorophyll fluorescence kinetics (Fast-Repetition Rate fluorometry, FRRf). Since the redox state of P700 depends on the operation of both PSI and PSII, important information can be obtained on the PSII-PSI intersystem electron transfer kinetics. Under moderate, sub-lethal heat stress treatments (33 ◦ C for~20 min), the coral Pavona decussata exhibited down-regulation of PSII electron transfer kinetics, indicated by slower rates of electron transport from Q A to plastoquinone (PQ) pool, and smaller relative size of oxidized PQ with concomitant decrease of a specifically-defined P700 kinetics area, which represents the active pool of PSII. The maximum quantum efficiency of PSII (F v /F m ) and functional absorption cross-section of PSII (σ PSII ) remained unchanged. Based on the coordinated response of P700 parameters and PSII-PSI electron transport properties, we propose that...
Rehman, AU, Szabó, M, Deák, Z, Sass, L, Larkum, A, Ralph, P & Vass, I 2016, 'Symbiodinium sp. cells produce light-induced intra- and extracellular singlet oxygen, which mediates photodamage of the photosynthetic apparatus and has the potential to interact with the animal host in coral symbiosis.', The New phytologist, vol. 212, pp. 472-484.View/Download from: Publisher's site
Coral bleaching is an important environmental phenomenon, whose mechanism has not yet been clarified. The involvement of reactive oxygen species (ROS) has been implicated, but direct evidence of what species are involved, their location and their mechanisms of production remains unknown. Histidine-mediated chemical trapping and singlet oxygen sensor green (SOSG) were used to detect intra- and extracellular singlet oxygen ((1) O2 ) in Symbiodinium cultures. Inhibition of the Calvin-Benson cycle by thermal stress or high light promotes intracellular (1) O2 formation. Histidine addition, which decreases the amount of intracellular (1) O2 , provides partial protection against photosystem II photoinactivation and chlorophyll (Chl) bleaching. (1) O2 production also occurs in cell-free medium of Symbiodinium cultures, an effect that is enhanced under heat and light stress and can be attributed to the excretion of (1) O2 -sensitizing metabolites from the cells. Confocal microscopy imaging using SOSG showed most extracellular (1) O2 around the cell surface, but it is also produced across the medium distant from the cells. We demonstrate, for the first time, both intra- and extracellular (1) O2 production in Symbiodinium cultures. Intracellular (1) O2 is associated with photosystem II photodamage and pigment bleaching, whereas extracellular (1) O2 has the potential to mediate the breakdown of symbiotic interaction between zooxanthellae and their animal host during coral bleaching.
Radford, D, Szabo, M, Raven, J & Ralph, PJ 2015, 'SATISFYING THE NUTRIENT TANK OF NANNOCHLOROPSIS OCULATA; CO-LIMITATION REDUCES FILLING EFFICIENCY', EUROPEAN JOURNAL OF PHYCOLOGY, vol. 50, pp. 138-139.
Suggett, DJ, Goyen, S, Evenhuis, C, Szabo, M, Pettay, DT, Warner, ME & Ralph, PJ 2015, 'Functional diversity of photobiological traits within the genus Symbiodinium appears to be governed by the interaction of cell size with cladal designation', New Phytologist, vol. 208, no. 2, pp. 370-381.View/Download from: Publisher's site
© 2015 New Phytologist Trust. Dinoflagellates of the genus Symbiodinium express broad diversity in both genetic identity (phylogeny) and photosynthetic function to presumably optimize ecological success across extreme light environments; however, whether differences in the primary photobiological characteristics that govern photosynthetic optimization are ultimately a function of phylogeny is entirely unresolved. We applied a novel fast repetition rate fluorometry approach to screen genetically distinct Symbiodinium types (n = 18) spanning five clades (A-D, F) for potential phylogenetic trends in factors modulating light absorption (effective cross-section, reaction center content) and utilization (photochemical vs dynamic nonphotochemical quenching; [1 - C] vs [1 - Q]) by photosystem II (PSII). The variability of PSII light absorption was independent of phylogenetic designation, but closely correlated with cell size across types, whereas PSII light utilization intriguingly followed one of three characteristic patterns: (1) similar reliance on [1 - C] and [1 - Q] or (2) preferential reliance on [1 - C] (mostly A, B types) vs (3) preferential reliance on [1 - Q] (mostly C, D, F types), and thus generally consistent with cladal designation. Our functional trait-based approach shows, for the first time, how Symbiodinium photosynthetic function is governed by the interplay between phylogenetically dependent and independent traits, and is potentially a means to reconcile complex biogeographic patterns of Symbiodinium phylogenetic diversity in nature.
Hill, R, Szabo, M, Rehman, A, Vass, I, Ralph, PJ & Larkum, A 2014, 'Inhibition of photosynthetic CO2 fixation in the coral Pocillopora damicornis and its relationship to thermal bleaching', Journal of Experimental Biology, vol. 217, pp. 2150-2162.View/Download from: Publisher's site
Two inhibitors of the CalvinBenson cycle [glycolaldehyde (GA) and potassium cyanide (KCN)] were used in cultured Symbiodinium cells and in nubbins of the coral Pocillopora damicornis to test the hypothesis that inhibition of the CalvinBenson cycle triggers coral bleaching. Inhibitor concentration range-finding trials aimed to determine the appropriate concentration to generate inhibition of the CalvinBenson cycle, but avoid other metabolic impacts to the symbiont and the animal host. Both 3 mmol l-1 GA and 20 µmol l-1 KCN caused minimal inhibition of host respiration, but did induce photosynthetic impairment, measured by a loss of photosystem II function and oxygen production. GA did not affect the severity of bleaching, nor induce bleaching in the absence of thermal stress, suggesting inhibition of the CalvinBenson cycle by GA does not initiate bleaching in P. damicornis. In contrast, KCN did activate a bleaching response through symbiont expulsion, which occurred in the presence and absence of thermal stress. While KCN is an inhibitor of the CalvinBenson cycle, it also promotes reactive oxygen species formation, and it is likely that this was the principal agent in the coral bleaching process. These findings do not support the hypothesis that temperature-induced inhibition of the CalvinBenson cycle alone induces coral bleaching.
Jeans, J, Szabo, M, Campbell, DA, Larkum, A, Ralph, PJ & 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.View/Download from: Publisher's site
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 610 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
Szabo, M, Parker, KB, Guruprasad, S, Kuzhiumparambil, U, Lilley, RM, Tamburic, B, Schliep, MT, Larkum, A, Schreiber, U, Raven, J & Ralph, PJ 2014, 'Photosynthetic acclimation of Nannochloropsis oculata investigated by multi-wavelength chlorophyll fluorescence analysis', Bioresource Technology, vol. 167, pp. 521-529.View/Download from: Publisher's site
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 (sII (?)), absolute electron transfer rates (ETR(II)), and non-photochemical quenching (NPQ) of chlorophyll fluorescence in both HL and LL cells. While sII(?) was not significantly different between the two growth conditions, HL cells upregulated ETR(II) 1.6 to 1.8-fold compared to LL cells, most significantly in the wavelength range of 440-540 nm. 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.
Szabo, M, Wangpraseurt, D, Tamburic, B, Larkum, A, Schreiber, U, Suggett, DJ, Kühl, M & Ralph, PJ 2014, 'Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis', Plant Physiology and Biochemistry, vol. 83, pp. 159-167.View/Download from: Publisher's site
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, sII(?), 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, sII of Symbiodinium was highest in the blue spectral region and showed a progressive decrease towards red wavelengths. Absolute values for sII at 440 nm 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 440 nm 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.
Tamburic, B, Guruprasad, S, Radford, DT, Szabo, M, Lilley, R, Larkum, A, Franklin, J, Kramer, D, Blackburn, S, Raven, J, Schliep, MT & Ralph, PJ 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, p. e86047.View/Download from: Publisher's site
Tamburic, B, Szabo, M, Tran, A, Larkum, A, Suggett, DJ & Ralph, PJ 2014, 'Action spectra of oxygen production and chlorophyll a fluorescence in the green microalga Nannochloropsis oculata', Bioresource Technology, vol. 169, pp. 320-327.View/Download from: Publisher's site
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 µmol photons m-2 s-1. Blue light (~414 nm) was absorbed more efficiently and directed to photosystem II more effectively than red light (~679 nm) at light intensities below the photosaturation limit. Conversion of absorbed photons into photosynthetic oxygen evolution was maximised at 625 nm; however, this maximum is unstable since neighbouring wavelengths (646 nm) 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.
Kanazawa, A, Blanchard, GJ, Szabo, M, Ralph, PJ & Kramer, DM 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.View/Download from: Publisher's site
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 peridininchlorophyll aprotein (PCP). It has been proposed that the appearance of a PCP-specific 77 K fluorescence emission band around 672675 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 achlorophyll c2peridininprotein-complex (acpPC) and associated photosystems (Reynolds et al., 2008 Proc Natl Acad Sci USA 105:1367413678).We have tested this model using time-resolved fluorescence decay kinetics in conjunction with global fitting to compare the timeevolution 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 nmwas, instead, caused by strong preferential exciton quenching of the membrane antenna complexes associatedwith 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.
Wangpraseurt, D, Tamburic, B, Szabo, M, Suggett, DJ, Ralph, PJ & Kuhl, M 2014, 'Spectral Effects on Symbiodinium Photobiology Studied with a Programmable Light Engine', PLoS One.View/Download from: Publisher's site
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 O2 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.
Hill, R, Larkum, A, Prasil, O, Kramer, DM, Szabo, M, Kumar, V & Ralph, PJ 2012, 'Light-induced dissociation of antenna complexes in the symbionts of scleractinian corals correlates with sensitivity to coral bleaching', Coral Reefs, vol. 31, pp. 963-975.View/Download from: Publisher's site
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, ß-carotene, as well as the dissociation of the light-harvesting chlorophyll complexes. The evidence is compatible with either the membrane-bound chlorophyll a-chlorophyll c 2-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 ß-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.
Chappaz-Gillot, C, Marek, PL, Blaive, BJ, Canard, G, Burck, J, Garab, G, Hahn, H, Javorfi, T, Kelemen, L, Krupke, R, Mossinger, D, Ormos, P, Reddy, CM, Roussel, C, Steinbach, G, Szabo, M, Ulrich, AS, Vanthuyne, N, Vijayaraghavan, A, Zupcanova, A & Balaban, TS 2012, 'Anisotropic organization and microscopic manipulation of self-assembling synthetic porphyrin microrods that mimic chlorosomes: Bacterial light-harvesting systems', Journal of the American Chemical Society, vol. 134, no. 2, pp. 944-954.View/Download from: Publisher's site
Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to micro-scale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems.(1,2) They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces.
Jajoo, A, Szabo, M, Zsiros, O & Garab, G 2012, 'Low pH induced structural reorganization in thylakoid membranes', Biochimica et Biophysica Acta, vol. 1817, pp. 1388-1391.View/Download from: Publisher's site
By using low temperature fluorescence spectroscopy, it has been shown that exposing chloroplast thylakoid membranes to acidic pH reversibly decreases the fluorescence of photosystem II while the fluorescence of photosystem I increases [P. Singh-Rawal et al. (2010) Evidence that pH can drive state transitions in isolated thylakoid membranes from spinach, Photochem Photobiol Sci, 9830-837]. In order to shed light on the origin of these changes, we performed circular dichroism (CD) spectroscopy on freshly isolated pea thylakoid membranes. We show that the magnitude of the psi-type CD, which is associated with the presence of chirally ordered macroarrays of the chromophores in intact thylakoid membranes, decreases gradually and reversibly upon gradually lowering the pH of the medium from 7.5 to 4.5 (psi, polymer or salt induced). The same treatment, as shown on thylakoid membranes washed in hypotonic low salt medium possessing no psi-type bands, induces no discernible change in the excitonic CD. These data show that while no change in the pigment-pigment interactions and thus in the molecular organization of the bulk protein complexes can be held responsible for the observed changes in the fluorescence, acidification of the medium significantly alters the macro-organization of the complexes; hence providing an explanation for the pH-induced redistribution of the excitation energy between the two photosystems. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial
Nagy, G, Szabo, M, Unnep, R, Kali, G, Miloslavina, Y, Lambrev, PH, Zsiros, O, Porcar, L, Timmins, P, Rosta, L & Garab, G 2012, 'Modulation of the multilamellar membrane organization and of the chiral macrodomains in the diatom Phaeodactylum tricornutum revealed by small-angle neutron scattering and circular dichroism spectroscopy', Photosynthesis Research, vol. 111, no. 1-2, pp. 71-79.View/Download from: Publisher's site
Diatoms possess effective photoprotection mechanisms, which may involve reorganizations in the photosynthetic machinery. We have shown earlier, by using circular dichroism(CD) spectroscopy, that in Phaeodactylum tricornutumthe pigment-protein complexes are arranged into chiral macrodomains, which have been proposed to be associated with the multilamellar organization of the thylakoid membranes and shown to be capable of undergoing light-induced reversible reorganizations (Szabo et al. Photosynth Res 95: 237, 2008). Recently, by using small-angle neutron scattering (SANS) on the same algal cells we have determined the repeat distances and revealed reversible light-induced reorganizations in the lamellar order of thylakoids (Nagy et al. Biochem J 436: 225, 2011). In this study, we show that in moderately heat-treated samples, the weakening of the lamellar order is accompanied by the diminishment of the psi-type CD signal associated with the long-range chiral order of the chromophores (psi, polymer or salt-induced). Further, we show that the light-induced reversible increase in the psi-type CD is associated with swelling in the membrane system, with magnitudes larger in high light than in low light. In contrast, shrinkage of the membrane system, induced by sorbitol, brings about a decrease in the psi-type CD signal; this shrinkage also diminishes the non-photochemical quenching capability of the cells. These data shed light on the origin of the psi-type CD signal, and confirm that both CD spectroscopy and SANS provide valuable information on the macro-organization of the thylakoid membranes and their dynamic properties; these parameters are evidently of interest with regard to the photoprotection in whole algal cells.
Nagy, G, Posselt, D, Kovacs, L, Holm, JK, Szabo, M, Ughy, B, Rosta, L, Peters, J, Timmins, P & Garab, G 2011, 'Reversible membrane reorganizations during photosynthesis in vivo: revealed by small-angle neutron scattering', Biochemical Journal, vol. 436, pp. 225-230.View/Download from: Publisher's site
In the present study, we determined characteristic repeat distances of the photosynthetic membranes in living cyanobacterial and eukaryotic algal cells, and in intact thylakoid membranes isolated from higher plants with time-resolved small-angle neutron scattering. This non-invasive technique reveals light-induced reversible reorganizations in the seconds-to-minutes time scale, which appear to be associated with functional changes in vivo.
Velikova, V, Varkonyi, Z, Szabo, M, Maslenkova, L, Nogues, I, Kovacs, L, Peeva, V, Busheva, M, Garab, G, Sharkey, TD & Loreto, F 2011, 'Increased thermostability of thylakoid membranes in isoprene-emitting leaves probed with three biophysical techniques', Plant Physiology, vol. 157, pp. 905-916.View/Download from: Publisher's site
Three biophysical approaches were used to get insight into increased thermostability of thylakoid membranes in isoprene- emittingplants.Arabidopsis (Arabidopsis thaliana) plants genetically modified to make isoprene and Platanus orientalis leaves, in which isoprene emission was chemically inhibited, were used. First, in the circular dichroism spectrum the transition temperature of the main band at 694 nm was higher in the presence of isoprene, indicating that the heat stability of chiral macrodomains of chloroplast membranes, and specifically the stability of ordered arrays of light-harvesting complex IIphotosystem II in the stacked region of the thylakoid grana, was improved in the presence of isoprene. Second, the decay of electrochromic absorbance changes resulting from the electric field component of the proton motive force (DA515) was evaluated following single-turnover saturating flashes. The decay of DA515 was faster in the absence of isoprene when leaves of Arabidopsis and Platanus were exposed to high temperature, indicating that isoprene protects the thylakoid membranes against leakiness at elevated temperature. Finally, thermoluminescence measurements revealed that S2QB 2 charge recombination was shifted to higher temperature in Arabidopsis and Platanus plants in the presence of isoprene, indicating higher activation energy for S2QB 2 redox pair, which enables isoprene-emitting plants to perform efficient primary photochemistry of photosystem II even at higher temperatures. The data provide biophysical evidence that isoprene improves the integrity and functionality of the thylakoid membranes at high temperature. These results contribute to our understanding of isoprene mechanism of action in plant protection against environmental stresses.
Szabo, M, Premvardhan, L, Lepetit, B, Goss, R, Wilhelm, C & Garab, G 2010, 'Functional heterogeneity of the fucoxanthins and fucoxanthin-chlorophyll proteins in diatom cells revealed by their electrochromic response and fluorescence and linear dichroism spectra', Chemical Physics, vol. 373, no. 1-2, pp. 110-114.View/Download from: Publisher's site
Electrochromic transient, liner dichroism and 77 K fluorescence excitation and emission spectra show that fucoxanthins and fucoxanthin-chlorophyll proteins exhibit functional heterogeneity in diatom cells.
Szabo, M, Lepetit, B, Goss, R, Wilhelm, C, Mustardy, L & Garab, G 2008, 'Structurally flexible macro-organization of the pigment-protein complexes of the diatom Phaeodactylum tricornutum', Photosynthesis Research, vol. 95, no. 2-3, pp. 237-245.View/Download from: Publisher's site
By means of circular dichroism (CD) spectroscopy, we have characterized the organization of the photosynthetic complexes of the diatom Phaeodactylum tricornutum at different levels of structural complexity: in intact cells, isolated thylakoid membranes and purified fucoxanthin chlorophyll protein (FCP) complexes. We found that the CD spectrum of whole cells was dominated by a large band at (+)698 nm, accompanied by a long tail from differential scattering, features typical for psi-type (polymerization or salt-induced) CD. The CD spectrum additionally contained intense (-)679 nm, (+)445 nm and (-)470 nm bands, which were also present in isolated thylakoid membranes and FCPs. While the latter two bands were evidently produced by excitonic interactions, the nature of the (-)679 nm band remained unclear. Electrochromic absorbance changes also revealed the existence of a CD-silent long-wavelength (~545 nm) absorbing fucoxanthin molecule with very high sensitivity to the transmembrane electrical field. In intact cells the main CD band at (+)698 nm appeared to be associated with the multilamellar organization of the thylakoid membranes. It was sensitive to the osmotic pressure and was selectively diminished at elevated temperatures and was capable of undergoing light-induced reversible changes.
Lepetit, B, Volke, D, Szabo, M, Hoffmann, R, Garab, G, Wilhelm, C & Goss, R 2007, 'Spectroscopic and molecular characterization of the oligomeric antenna of the diatom Phaeodactylum tricornutum', Biochemistry, vol. 46, no. 34, pp. 9813-9822.View/Download from: Publisher's site
The photosynthetic antenna system of diatoms contains fucoxanthin chlorophyll a/c binding proteins (FCPs), which are membrane intrinsic proteins showing high homology to the light harvesting complexes (LHC) of higher plants. In the present study, we used a mild solubilization of P. tricornutum thylakoid membranes in combination with sucrose density gradient centrifugation or gelfiltration and obtained an oligomeric FCP complex (FCPo). The spectroscopic characteristics and pigment stoichiometries of the FCPo complex were comparable to FCP complexes that were isolated after solubilization with higher detergent per chlorophyll ratios. The excitation energy transfer between the FCP-bound pigments was more efficient in the oligomeric FCPo complexes, indicating that these complexes may represent the native form of the diatom antenna system in the thylakoid membrane. Determination of the molecular masses of the two different FCP fractions by gelfiltration revealed that the FCP complexes consisted of trimers, whereas the FCPo complexes were either composed of six monomers or two tightly associated trimers. In contrast to vascular plants, stable functional monomers could not be isolated in P. tricornutum. Both types of FCP complexes showed two protein bands in SDS-gels with apparent molecular masses of 18 and 19 kDa, respectively. Sequence analysis by MS/MS revealed that the 19 kDa protein corresponded to the fcpC and fcpD genes, whereas the 18 kDa band contained the protein of the fcpE gene. The presence of an oligomeric antenna in diatoms is in line with the oligomeric organization of antenna complexes in different photoautotrophic groups.
Larkum, AWD, Pernice, M, Schliep, M, Davey, P, Szabo, M, Raven, JA, Lichtenberg, M, Brodersen, KE & Ralph, PJ 2018, 'Photosynthesis and metabolism of seagrasses' in Seagrasses of Australia: Structure, Ecology and Conservation, Springer, Switzerland, pp. 315-342.View/Download from: Publisher's site
© Springer International Publishing AG, part of Springer Nature 2018. Seagrasses have a unique leaf morphology where the major site for chloroplasts is in the epidermal cells, stomata are absent and aerenchyma is present inside the epidermis. This means that the major site for photosynthesis is in the epidermis. Furthermore the lack of stomata means that the route for carbon uptake is via inorganic carbon (C i ) uptake across the vestigial cuticle and through the outer plasma membranes. Since the leaf may at times be in an unstirred situation diffusion through an unstirred layer outside the leaf may be a large obstacle to carbon uptake. The existence of a carbon concentrating mechanism is discussed, but its existence to date is not proven. Active bicarbonate uptake across the plasmalemma does not seem to operate; an external carbonic anhydrase and an extrusion of protons seem to play a role in enhancing CO 2 uptake. There is some evidence that a C4 mechanism plays a role in carbon fixation but more evidence from "omics" is required. Photorespiration certainly occurs in seagrasses and an active xanthophyll cycle is present to cope with damaging high light, but both these biochemical mechanisms need further work. Finally, epiphytes pose a problem which impedes the uptake of C i and modifies the light environment inside the leaves.
Larkum, A, Szabo, M, Fitzpatrick, D & Raven, J 2017, 'Cyclic Electron Flow in Cyanobacteria and Eukaryotic Algae' in Photosynthesis and Bioenergetics, World Scientific, pp. 305-343.View/Download from: Publisher's site
In oxygenic photosynthesis light energy is largely captured in linear electron flow (LEF) between the photosystems and drives ATP formation via a thylakoid proton-driven ATP synthase. In addition, for over 50 years there has been good evidence that an additional cyclic electron flow (CEF) around photosystem I (PSI) is harnessed to provide extra ATP in addition to that produced by LEF. The evidence comes from all oxygenic organisms, cyanobacteria, eukaryotic algae and embryophytic plants. However, the CEF mechanism has been difficult to investigate because of the cyclic nature of the EF and confusion with other pathways not using oxygen as a terminal electron acceptor, and the MAPS, flavodiiron and chlororespiration pathways to oxygen. This article discusses the current evidence for CEF in all oxygenic organisms and suggests future experiments by which the situation can be clarified.
Lepetit, Volke, Szabo, Hoffmann, Garab, Wilhelm & Goss 2008, 'The Oligomeric Antenna of the Diatom P. tricornutum — Localisation of Diadinoxanthin Cycle Pigments' in Photosynthesis. Energy from the Sun 14th International Congress on Photosynthesis, Springer Science & Business Media.
The. Oligomeric. Antenna. of. the. Diatom. P. tricornutum. –. Localisation. of.
Diadinoxanthin. Cycle. Pigments. Bernard Lepetit 1 ... Abstract In the present
work a thorough examination of the oligomeric antenna of P. tricornutum was
Szabo, M, 'Macro-organization and structural flexibility of the photosynthetic pigment system in diatoms'.
- Prof. David Kramer, Michigan State University, USA
- Emeritus Prof. Anthony Larkum, The University of Sydney and University of Technology, Sydney.
- Prof. Wah Soon Chow, Australian National University, Canberra
- Prof. Imre Vass, Biological Research Centre, Hungary
- Research Leader Wayne O`Connor, Department of Primary Industries, PSFI
- Professor Roger Hiller, Macquarie University, Sydney