Charpy-Roubaud, C & Larkum, AWD 2005, 'Dinitrogen fixation by exposed communities on the rim of Tikehau atoll (Tuamotu Archipelago, French Polynesia)', Coral Reefs, vol. 24, no. 4, pp. 622-628.
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Various types of sub-aerially exposed microbial mats, including emergent mats, beach sand, beach rock and Kopara mats, are widespread on the 78 km (25 km2) of rim surrounding the Tikehau atoll lagoon. These mats form laminated accretions or diffuse microbial communities growing under high insolation and temperatures, and are therefore subject to desiccation. Both heterocystous and non-heterocystous cyanobacteria occur in these mats. Using acetylene reduction techniques, nitrogenase activity was observed at all sites over a period of 5 years and was 3-17 times higher during daylight than at night in all communities except for beach rock. 15N2 measurements indicated a molar ratio of acetylene reduction to N2 fixed of 1.6 for all exposed communities. Estimated N2 fixation ranged from 1.44 to 8.0 mg N m-2 day-1 in these exposed communities (mean of 4.66 mg N m-2 day-1) with beachrock showing the highest rates. For the whole reef rim, daily N2 fixation amounted to 98.42 kg N day-1 which represents 28% of the rate of fixation in the entire lagoon (area 400 km2). © Springer-Verlag 2005.
Chen, M, Bibby, TS, Nield, J, Larkum, A & Barber, J 2005, 'Iron deficiency induces a chlorophyll d-binding Pcb antenna system around Photosystem I in Acaryochloris marina', BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS, vol. 1708, no. 3, pp. 367-374.
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The prochlorophyte-like cyanobacterium Acaryochloris marina contains two pcb genes, pcbA and pcbC, which encode chlorophyll (Chl) d-binding antenna proteins PcbA and PcbC, respectively. Using real-time reverse transcriptase polymerase chain reaction (RT-PCR), it is shown that when Acaryochloris cells are grown in an iron-deficient medium, the transcription of the pcbC gene is up-regulated compared to that of pcbA. Biochemical and immunological analyses indicated that under the same iron-deficient conditions, the level of Photosystem I (PSI) decreased compared with that of Photosystem II (PSII). Electron microscopy revealed that concomitant with these changes was the formation of Pcb-PSI supercomplexes which, in their largest form, were composed of 18 Pcb subunits forming a ring around the trimeric PSI reaction centre core. Mass spectrometry indicated that the PcbC protein is the main constituent of this outer PSI antenna system. It is therefore concluded that in Acaryochloris, the PcbC protein forms an antenna for PSI when iron levels become limiting and in this way compensates for the drop in the level of PSI relative to PSII which occurs under these conditions. © 2005 Elsevier B.V. All rights reserved.
Chen, M, Bibby, TS, Nield, J, Larkum, AWD & Barber, J 2005, 'Structure of a large photosystem II supercomplex from Acaryochloris marina', FEBS Letters, vol. 579, no. 5, pp. 1306-1310.
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Acaryochloris marina is a prochlorophyte‐like cyanobacterium containing both phycobilins and chlorophyll d as light harvesting pigments. We show that the chlorophyll d light harvesting system, composed of Pcb proteins, functionally associates with the photosystem II (PSII) reaction center (RC) core to form a giant supercomplex. This supercomplex has a molecular mass of about 2300 kDa and dimensions of 385 Å × 240 Å. It is composed of two PSII‐RC core dimers arranged end‐to‐end, flanked by eight symmetrically related Pcb proteins on each side. Thus each PSII‐RC monomer has four Pcb subunits acting as a light harvesting system which increases the absorption cross section of the PSII‐RC core by almost 200%.
Chen, M, Eggink, LL, Hoober, JK & Larkum, AWD 2005, 'Influence of Structure on Binding of Chlorophylls to Peptide Ligands', Journal of the American Chemical Society, vol. 127, no. 7, pp. 2052-2053.
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Four classes of chlorophyll (Chl), a, b, c, and d, are involved in photosynthesis within cyanobacteria, algae, and plants. These classes have different evolutionary origins, chemical properties, and biological functions. Our results demonstrate that peptide-bound ligands provided by the imidazole group of histidine and the charge-compensated glutamate-arginine ion pair readily form coordination bonds with Chls a and d but do not interact significantly with Chls b and c. These ligands are apparently not sufficiently strong Lewis bases to displace strongly coordinated water from Chls b and c. These differences determine specificity of binding of Chls in light-harvesting complexes and play an important role in assembly of stable Chl-protein complexes, which has had a profound impact on the evolution of photosynthetic organisms. Copyright © 2005 American Chemical Society.
Chen, M, Hiller, RG, Howe, CJ & Larkum, AWD 2005, 'Unique Origin and Lateral Transfer of Prokaryotic Chlorophyll-b and Chlorophyll-d Light-Harvesting Systems', Molecular Biology and Evolution, vol. 22, no. 1, pp. 21-28.
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pcb genes, encoding proteins binding light-harvesting chlorophylls, were cloned and sequenced from the Chl d-containing cyanobacterium, Acaryochloris marina, and the Chl b-containing cyanobacterium, Prochloron didemni. Both organisms contained two tandem pcb genes. Peptide fingerprinting confirmed the expression of one of the A. marina pcb genes. Phylogenetic tree reconstruction using distance-matrix and maximum-likelihood methods indicated a single origin of the pcb gene family, whether occurring in Chl b-containing or Chl d-containing organisms. This may indicate widespread lateral transfer of the Pcb protein-based light-harvesting system.
Chen, M, Telfer, A, Lin, S, Pascal, A, Larkum, AWD, Barber, J & Blankenship, RE 2005, 'The nature of the photosystem II reaction centre in the chlorophyll d-containing prokaryote, Acaryochloris marina', Photochemical & Photobiological Sciences, vol. 4, no. 12, pp. 1060-1064.
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Pigment–protein complexes enriched in photosystem II (PS II) have been isolated from the chlorophyll (Chl) d containing cyanobacterium, Acaryochloris marina. A small PS II-enriched particle, we call ‘crude reaction centre’, contained 20 Chl d, 0.5 Chl a and 1 redox active cytochrome b-559 per 2 pheophytin a, plus the D1 and D2 proteins. A larger PS II-enriched particle, we call ‘core’, additionally bound the antenna complexes, CP47 and CP43, and had a higher chlorophyll per pheophytin ratio. Pheophytin a could be photoreduced in the presence of a strong reductant, indicating that it is the primary electron acceptor in photosystem II of A. marina. A substoichiometric amount of Chl a (less than one chlorophyll a per 2 pheophytin a) strongly suggests that Chl a does not have an essential role in the photochemistry of PS II in this organism. We conclude that PS II, in A. marina, utilizes Chl d and not Chl a as primary electron donor and that the primary electron acceptor is one of two molecules of pheophytin a. © 2005 The Royal Society of Chemistry and Owner Societies.
Coyne, KJ, Handy, SM, Demir, E, Whereat, EB, Hutchins, DA, Portune, KJ, Doblin, MA & Cary, SC 2005, 'Improved quantitative real-time PCR assays for enumeration of harmful algal species in field samples using an exogenous DNA reference standard', LIMNOLOGY AND OCEANOGRAPHY-METHODS, vol. 3, no. 9, pp. 381-391.
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© 2005, by the American Society of Limnology and Oceanography, Inc. Quantitative real-time PCR (QPCR) is a powerful and sensitive method for quantitative detection of microorganisms. Application of this methodology for enumeration of harmful algal bloom (HAB) species has the potential to revolutionize our approach to HAB research, making it possible to identify correlations between cell abundances and factors that regulate bloom dynamics. Its application to ecological studies, however, has produced mixed results. QPCR assays typically rely on the generation of standard curves from plasmids or laboratory cultures that may be unrealistic when compared to amplification of DNA extracted from field samples. In addition, existing methods often fail to incorporate controls to assess variability in extraction and amplification efficiencies, or include controls that are sequence-specific and preclude the investigation of multiple species. Here, we describe the development and rigorous analysis of QPCR assays for two HAB species, Chattonella subsalsa and Heterosigma akashiwo, in which we introduce a known concentration of exogenous DNA plasmid into the extraction buffer as a reference standard. Since the target DNA is extracted in the presence of the reference standard, inherent variability in extraction and amplification efficiencies affect both target and standard equally. Furthermore, the reference standard is applicable to QPCR analysis of any microbial species. Using environmental bloom samples as calibrators, we evaluated the accuracy of the comparative Ct method for enumeration of target species in several field samples. Our investigation demonstrates that the comparative Ct method with an exogenous DNA reference standard provides both accurate and reproducible quantification of HAB species in environmental samples.
Drake, LA, Meyer, AE, Forsberg, RL, Baier, RE, Doblin, MA, Heinemann, S, Johnson, WP, Koch, M, Rublee, PA & Dobbs, FC 2005, 'Potential invasion of microorganisms and pathogens via 'interior hull fouling': biofilms inside ballast water tanks', BIOLOGICAL INVASIONS, vol. 7, no. 6, pp. 969-982.
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Surfaces submerged in an aquatic milieu are covered to some degree with biofilms - organic matrices that can contain bacteria, microalgae, and protozoans, sometimes including disease-causing forms. One unquantified risk of aquatic biological invatsions is the potential for biofilms within ships' ballast water tanks to harbour pathogens, and, in tuen, seed othe waters. To begin to evaluate this vector, we collected biofilm samples fromtanks' surfaces and deployed controlled-surface sampling units within tanks. We then measured a variety of microbial metrics within the biofilms to test the hypotheses that pathogens are present in biofilms and that biofilms have higher microbial densities compared to ballst water. Field experiments and sampling of coastwise and oceangoing ships arriving at ports in Chesapeake Bay and the North American Great Lakes showed the presence of abundant microorganisms, including pathogens, in biofilms. These results suggest that ballast-tank biofilms represent an additional risk of microbial invasion, provided they release cells into the water or they are sloughed off during normal ballasting operations.
Handy, SM, Coyne, KJ, Portune, KJ, Demir, E, Doblin, MA, Hare, CE, Cary, SC & Hutchins, DA 2005, 'Evaluating vertical migration behavior of harmful raphidophytes in the Delaware Inland Bays utilizing quantitative real-time PCR', AQUATIC MICROBIAL ECOLOGY, vol. 40, no. 2, pp. 121-132.
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Mixed blooms of 4 species of harmful raphidophytes (Chattonella cf. verruculosa, Chattonella subsalsa, Heterosigma akashiwo, and Fibrocapsa japonica) occur in the shallow (1 ti 2m) Delaware Inland Bays (DIB), USA. Raphidophytes vertically migrate in other deeper water ecosystems to utilise deep nutrient stocks at night, and thus obtain an advantage over non-migrating algae.Anoxic DIB sediments release high levels of bioavailable phosphate, which could potentially be used by vertically migrating flagellates. This study aimed to characterise and understamd the migration oatterns of DIB raphidiphytes, and determine whether benthic phosphate fluxes could provide the cells with P. We demonstrated vertical migration of isolated DIB raphidophyte cultures in the laboratory, where differences inthe response of C. subsalsa and H. akashiwo to light:dark period manipulations suggested possible diffeerneces in external versus endogenous regulation of migration behaviour in the 2 species. Natural blooms in the filed (enclosed in a mesocosm system) also exhibited patterns of diel vertical distributions of each species. Our data suggested that these 2 photoautotrophic species spend daylight hours near the surface and are found directly on the sediment surface at night. However, diel changes in particulate C:P ratios did not support the hypothesis that there is preferential uptake of sedimentary phosphate at night. Our results also suggested that the migration behaviour may have important implications for designing sampling strategies for monitoring programs. QPCR has a number of decisive advantages over traditional microscopic counting methods, making this a poweful tool for fine spatial temporal scale detection and enum,eration of vertically migrating harmful algal species.
Hill, R & Ralph, PJ 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 Fv/Fm 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 QA 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 Fv/Fm 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.
Hill, R, Frankart, C & Ralph, PJ 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 μmol photons m-2 s-1 and 25°C) or elevated temperature (225 μmol 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 μmol 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.
Kuhl, M, Chen, M, Ralph, PJ, Schreiber, U & Larkum, AWD 2005, 'A niche for cyanobacteria containing chlorophyll d', NATURE, vol. 433, no. 7028, pp. 820-820.
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The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.
Larkum, AWD & Kuhl, M 2005, 'Chlorophyll d: the puzzle resolved', TRENDS IN PLANT SCIENCE, vol. 10, no. 8, pp. 355-357.
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Chlorophyll a (Chl a) has always been regarded as the sole chlorophyll with a role in photochemical conversion in oxygen-evolving phototrophs, whereas chlorophyll d (Chl d), discovered in small quantities in red algae in 1943, was often regarded as an artefact of isolation. Now, as a result of discoveries over the past year, it has become clear that Chl d is the major chlorophyll of a freeliving and widely distributed cyanobacterium that lives in light environments depleted in visible light and enhanced in infrared radiation. Moreover, Chl d not only has a light-harvesting role but might also replace Chl a in the special pair of chlorophylls in both reactions centers of photosynthesis.
Macinnis-Ng, CMO, Morrison, DA & Ralph, PJ 2005, 'Temporal and spatial variation in the morphology of the brown macroalga Hormosira banksii (Fucales, Phaeophyta)', BOTANICA MARINA, vol. 48, no. 3, pp. 198-207.
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Hormosira banksii is a morphologically variable macroalgal species from southeastern and southern Australia, which has been previously categorised into ecoforms according to habitat. This study is by far the largest quantitative evaluation of morphological variation in H. banksii, covering 74 sites from South Australia, Victoria, New South Wales and Tasmania. Morphological features from 505 samples were analysed using principal components analysis, with the patterns identified being statistically assessed with a Monte Carlo permutation test. There was considerable morphological variation between samples taken at several marine (but not estuarine) sites in both 1994 and 1999. However, this variation was not consistent across either morphological features or populations, and presumably represents random fluctuations. Analysis of the entire dataset demonstrated a significant difference between samples growing in marine and estuarine habitats. Further assessment of variation within these two groups revealed some significantly different populations based on geographical locations but not habitat variation. While this study presents strong evidence for two distinct taxa within H. banksii (marine versus estuarine populations), the taxonomic status of this species should not be altered until genetic studies have been conducted. © 2005 by Walter de Gruyter.
Nasha, SMB, Schreiber, U, Ralph, PJ & Muller, JF 2005, 'The combined SPE : ToxY-PAM phytotoxicity assay; application and appraisal of a novel biomonitoring tool for the aquatic environment', BIOSENSORS & BIOELECTRONICS, vol. 20, no. 7, pp. 1443-1451.
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Mounting concerns regarding the environmental impact of herbicides has meant a growing requirement for accurate, timely information regarding herbicide residue contamination of, in particular, aquatic systems. Conventional methods of detection remain limited in terms of practicality due to high costs of operation and the specialised information that analysis provides. A new phytotoxicity bioassay was trialled for the detection of herbicide residues in filter-purified (Milli-Q) as well as natural waters. The performance of the system, which combines solid-phase extraction (SPE) with the ToxY-PAM dual-channel yield analyser (Heinz Walz GmbH), was tested alongside the traditional method of liquid chromatography-mass spectrometry (LC-MS). The assay methodology was found to be highly sensitive (LOD 0.1 ng L -1 diuron) with good reproducibility. The study showed that the assay protocol is time effective and can be employed for the aquatic screening of herbicide residues in purified as well as natural waters. © 2004 Elsevier B.V. All rights reserved.
Oren, A, Pri-El, N, Shapiro, O & Siboni, N 2005, 'Gas vesicles isolated from Halobacterium cells by lysis in hypotonic solution are structurally weakened', FEMS MICROBIOLOGY LETTERS, vol. 252, no. 2, pp. 337-341.
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Petrasek, Z, Schmitt, FJ, Theiss, C, Huyer, J, Chen, M, Larkum, A, Eichler, HJ, Kemnitz, K & Eckert, HJ 2005, 'Excitation energy transfer from phycobiliprotein to chlorophyll d in intact cells of Acaryochloris marina studied by time- and wavelength-resolved fluorescence spectroscopy', PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES, vol. 4, no. 12, pp. 1016-1022.
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The fluorescence decay spectra and the excitation energy transfer from the phycobiliproteins (PBP) to the chlorophyll-antennae of intact cells of the chlorophyll (Chl) d-dominated cyanobacterium Acaryochloris marina were investigated at 298 and 77 K by time- and wavelength-correlated single photon counting fluorescence spectroscopy. At 298 K it was found that (i) the fluorescence dynamics in A. marina is characterized by two emission peaks located at about 650 and 725 nm, (ii) the intensity of the 650 nm fluorescence depends strongly on the excitation wavelength, being high upon excitation of phycobiliprotein (PBP) at 632 nm but virtually absent upon excitation of chlorophyll at 430 nm, (iii) the 650 nm fluorescence band decayed predominantly with a lifetime of 70 ± 20 ps, (iv) the 725 nm fluorescence, which was observed independent of the excitation wavelength, can be described by a three-exponential decay kinetics with lifetimes depending on the open or the closed state (F0 or Fm) of the reaction centre of Photosystem II (PS II). Based on the results of this study, it is inferred that the excitation energy transfer from phycobiliproteins to Chl d of PS II in A. marina occurs with a time constant of about 70 ps, which is about three times faster than the energy transfer from the phycobilisomes to PS II in the Chl a-containing cyanobacterium Synechococcus 6301 (C. W. Mullineaux and A. R. Holzwarth, Biochim. Biophys. Acta, 1991, 68–78, 1098). A similar fast PBP to Chl d excitation energy transfer was also observed at 77 K. At 77 K a small long-lived fluorescence decay component with a lifetime of 14 ns was observed in the 640–700 nm spectral range. However, it has a rather featureless spectrum, not typical for Chl a, and was only observed upon excitation at 400 nm but not upon excitation at 632 and 654 nm. Thus, this long-lived fluorescence component cannot be used as an indicator that the primary PS II donor of Acaryochloris marina contains Chl a. © 2005 The Roya...
Ralph, PJ & Gademann, R 2005, 'Rapid light curves: A powerful tool to assess photosynthetic activity', AQUATIC BOTANY, vol. 82, no. 3, pp. 222-237.
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Rapid light curves provide detailed information on the saturation characteristics of electron transport, as well as the overall photosynthetic performance of a plant. Rapid light curves were collected from samples of Zostera marina grown under low and high-light conditions (50 and 300 μmol photons m-2 s-1) and the distinctive patterns of RLC parameters are discussed, in terms of differential sink capacity and PSII reaction centre closure. Derived cardinal points of a rapid light curve (α, Ek and rETRmax) describe the photosynthetic capacity of a seagrass leaf, its light adaptation state and its capacity to tolerate short-term changes in light. The shapes of the corresponding F and F′m curves also provide information on the development of the trans-thylakoid proton gradient and thermal energy dissipation. Low-light leaves showed limited photosynthetic capacity and reduced activity of non-photochemical quenching pathways, whereas photosynthesis of high light leaves were not limited and showed an elevated level of non-photochemical quenching, possibly associated with xanthophyll cycle activity. Light-dark kinetics are also discussed in relation to relaxation of non-photochemical quenching and its various components. A curve fitting model is recommended based on the double exponential decay function. In this paper, we explain the fundamental aspects of a RLC, describe how it reflects the response to light exposure of a leaf, how to interpret these curves, and how to quantitatively describe and compare RLCs. © 2005 Elsevier B.V. All rights reserved.
Ralph, PJ, Larkum, AWD & Kuhl, M 2005, 'Temporal patterns in effective quantum yield of individual zooxanthellae expelled during bleaching', JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY, vol. 316, no. 1, pp. 17-28.
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Bleaching is a worldwide phenomenon affecting coral reefs. During elevated temperature and light conditions (bleaching), expelled zooxanthellae show distinct patterns in photosynthetic health. An innovative new device was used to collect individual expelled zooxanthellae, when a coral was exposed to bleaching conditions. This has provided new insight into the photosynthetic condition and abundance of expelled zooxanthellae. It has been assumed that expelled zooxanthellae were dead or moribund; however, we have found individual cells can have healthy effective quantum yields (φPSII) >0.65 after 8 h of bleaching conditions (500 μmol photons m-2 s -1, 33°C). The population of expelled zooxanthellae from Cyphastrea serailia and Pocillopora damicornis showed distinct patterns in the frequency distribution of φPSII over time and between locations (sun versus shade) within a colony. During the first 4 h of exposure to bleaching conditions, only 5% of expelled individual cells from P. damicornis were photosynthetically inactive (φPSII<0.05), whereas for C. serailia, this was 30%. The overall photosynthetic health of expelled zooxanthellae from C. serailia was better than P. damicornis (0.53±0.13 and 0.38±0.13 after 8 h, respectively). This was generally reflected by the in hospite measurement of the coral, yet, the in hospite cells always had a higher φPSII than expelled cells, suggesting that host tissue provided added photoprotection for the zooxanthellae. © 2005 Elsevier B.V. All rights reserved.
Ralph, PJ, Macinnis-Ng, CMO & Frankart, C 2005, 'Fluorescence imaging application: effect of leaf age on seagrass photokinetics', AQUATIC BOTANY, vol. 81, no. 1, pp. 69-84.
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We used the Imaging-PAM fluorometer to map spatial variability of photosynthesis in three seagrass species, Halophila ovalis, Zostera capricorni and Posidonia australis. Photosynthesis was described by relative photosynthetic rate (PS/50), effective quantum yield (ΦPSII), non-photochemical quenching (NPQ and qN), electron transport rate (ETR) and leaf absorptivity. Photosynthetic patterns were linked to leaf age and light climate but patterns were not consistent across species. Longitudinal heterogeneity in photosynthesis was apparent along the leaves of all three species while lateral spatial heterogeneity was found only across Z. capricorni and H. ovalis leaves. Age of leaf tissue, determined by longitudinal location on the leaf, strongly influenced photosynthetic activity of Z. capricorni and P. australis. A comparison of H. ovalis leaves of differing maturity demonstrated the influence of leaf age on photosynthetic activity, yet a comparison of Z. capricorni leaves of differing maturity showed no leaf-age effects. Variations in stress-induced changes across a seagrass leaf can be used to identify areas or particular regions of the leaf, which are more susceptible to photodamage. Clear evidence of substantial within-leaf heterogeneity in photosynthetic activity (i.e., a two-fold variation in half saturation constant along a leaf of P. australis) has serious implications for use of small sections of leaf for photosynthetic incubations (such as O2 or single-point chlorophyll a fluorescence measurements). © 2004 Elsevier B.V. All rights reserved.
Ralph, PJ, McMinn, A, Ryan, KG & Ashworth, C 2005, 'Short-term effect of temperature on the photokinetics of microalgae from the surface layers of Antarctic pack ice', JOURNAL OF PHYCOLOGY, vol. 41, no. 4, pp. 763-769.
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Microalgae growing within brine channels (85 psu salinity) of the surface ice layers of Antarctic pack ice showed considerable photosynthetic tolerance to the extreme environmental condition. Brine microalgae exposed to temperatures above -5°C and at irradiances up to 350 μmol photons· m -2·s-1 showed no photosynthetic damage or limitations. Photosynthesis was limited (but not photoinhibited) when brine microalgae were exposed to -10°C, provided the irradiance remained under 50 μmol photons·m-2·s-1. The highest level of photosynthetic activity (maximum relative electron transport rate [rETR max]) in brine microalgae growing within the surface layer of sea ice was at approximately 18 μmol electrons·m-2·s -1, which occurred at -1.8° C. Effective quantum yield of PSII and rETRmax of the halotolerant brine microalgae exhibited a temperature-dependent pattern, where both parameters were higher at -1.8° C and lower at -10° C. Relative ETRmax at temperatures above -5°C were stable across a wide range of irradiance. © 2005 Phycological Society of America.
Ralph, PJ, Schreiber, U, Gademann, R, Kuhl, M & Larkum, AWD 2005, 'Coral photobiology studied with a new imaging pulse amplitude modulated fluorometer', JOURNAL OF PHYCOLOGY, vol. 41, no. 2, pp. 335-342.
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A new high-resolution imaging fluorometer (Imaging-PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards A Haime (
Rodriguez-Lanetty, M, Scaramuzzi, C, Quinnell, RG & Larkum, AWD 2005, 'Transport of symbiotic zooxanthellae in mesogleal canals of Zoanthus robustus?', Coral Reefs, vol. 24, no. 2, pp. 195-196.
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Seymour, JR, Seuront, L & Mitchell, JG 2005, 'Microscale and small-scale temporal dynamics of a coastal planktonic microbial community', MARINE ECOLOGY PROGRESS SERIES, vol. 300, pp. 21-37.
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The temporal dynamics of heterotrophic bacteria and Synechococcus-type cyanobacteria communities were studied in a coastal habitat characterised by strong hydrodynamic variability using 10 s (microscale) and 30 min (small-scale) sampling intervals. Flow cytometric analysis allowed for the discrimination of 3 populations of heterotrophic bacteria and the examination of the Synechococcus cell cycle. During the 11 h small-scale study, 2-fold changes in the total abundance of both the bacterial and Synechococcus communities were observed, and clear temporal patterns in the abundance, activity and cellular state of the 2 populations were evident. Cumulative sum analysis further revealed distinct periods and trends in the temporal dynamics of the bacterial and Synechococcus communities. Shifts in the abundance of all heterotrophic bacterial populations were significantly correlated to turbulent energy dissipation. No such correlation was evident for the Synechococcus population, which instead appeared to follow a diel cell cycle very similar in nature to patterns observed in other environments. In 2 microscale studies, conducted during dissimilar hydrodynamic conditions, approx. 2-fold shifts in the abundance of the bacterial and Synechococcus populations were also observed. Microscale temporal patterns were dominated by localised variability and the existence of hotspots in abundance and activity, although cumulative sum analysis also revealed more general trends, sometimes occurring over periods of several minutes.
Smith, DJ, Suggett, DJ & Baker, NR 2005, 'Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals?', Global Change Biology, vol. 11, no. 1, pp. 1-11.
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AbstractThe bleaching of corals in response to increases in temperature has resulted in significant coral reef degradation in many tropical marine ecosystems. This bleaching has frequently been attributed to photoinhibition of photosynthetic electron transport and the consequent photodamage to photosystem II (PSII) and the production of damaging reactive oxygen species (ROS) in the zooxanthellae (Symbiodinium spp.). However, these events may be because of perturbations of other processes occurring within the zooxanthellae or the host cells, and consequently constitute only secondary responses to temperature increase. The processes involved with the onset of photoinhibition of electron transport, photodamage to PSII and pigment bleaching in coral zooxanthellae are reviewed. Consideration is given to how increases in temperature might lead to perturbations of metabolic processes in the zooxanthellae and/or their host cells, which could trigger events leading to bleaching. It is concluded that production of ROS by the thylakoid photosynthetic apparatus in the zooxanthellae plays a major role in the onset of bleaching resulting from photoinhibition of photosynthesis, although it is not clear which particular ROS are involved. It is suggested that hydrogen peroxide generated in the zooxanthellae may have a signalling role in triggering the mechanisms that result in expulsion of zooxanthellae from corals.
Ulstrup, KE, Hill, R & Ralph, PJ 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°C, 200 μmol 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 (φPSII) 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 φPSII using slow (pulse-amplitude modulation) fluorescence kinetics, as well as an increase in the O peak, φPo(electron transport efficiency before QA), and an elevation of the J peak on a double-normalised transient using fast (Plant Efficiency Analyser) induction kinetics. © Inter-Research 2005.
