Variable Fluorescence Measurements Research Articles

Photorespiration in eelgrass (Zostera marina L.): A photoprotection mechanism for survival in a CO2-limited world.

Photorespiration, commonly viewed as a loss in photosynthetic productivity of C3 plants, is expected to decline with increasing atmospheric CO2, even though photorespiration plays an important role in the oxidative stress responses. This study aimed to quantify the role of photorespiration and alternative photoprotection mechanisms in Zostera marina L. (eelgrass), a carbon-limited marine C3 plant, in response to ocean acidification. Plants were grown in controlled outdoor aquaria at different [CO2]aq ranging from ~55 (ambient) to ~2121 μM for 13 months and compared for differences in leaf photochemistry by simultaneous measurements of O2 flux and variable fluorescence. At ambient [CO2], photosynthesis was carbon limited and the excess photon absorption was diverted both to photorespiration and non-photochemical quenching (NPQ). The dynamic range of NPQ regulation in ambient grown plants, in response to instantaneous changes in [CO2]aq, suggested considerable tolerance for fluctuating environmental conditions. However, 60 to 80% of maximum photosynthetic capacity of ambient plants was diverted to photorespiration resulting in limited carbon fixation. The photosynthesis to respiration ratio (P E : R D) of ambient grown plants increased 6-fold when measured under high CO2 because photorespiration was virtually suppressed. Plants acclimated to high CO2 maintained 4-fold higher P E : R D than ambient grown plants as a result of a 60% reduction in photorespiration. The O2 production efficiency per unit chlorophyll was not affected by the CO2 environment in which the plants were grown. Yet, CO2 enrichment decreased the light level to initiate NPQ activity and downregulated the biomass specific pigment content by 50% and area specific pigment content by 30%. Thus, phenotypic acclimation to ocean carbonation in eelgrass, indicating the coupling between the regulation of photosynthetic structure and metabolic carbon demands, involved the downregulation of light harvesting by the photosynthetic apparatus, a reduction in the role of photorespiration and an increase in the role of NPQ in photoprotection. The quasi-mechanistic model developed in this study permits integration of photosynthetic and morphological acclimation to ocean carbonation into seagrass productivity models, by adjusting the limits of the photosynthetic parameters based on substrate availability and physiological capacity.

Shifts in growth light optima among diatom species support their succession during the spring bloom in the Arctic

Abstract Diatoms of the Arctic Ocean annually experience extreme changes of light environment linked to photoperiodic cycles and seasonal variations of the snow and sea‐ice cover extent and thickness which attenuate light penetration in the water column. Arctic diatom communities exploit this complex seasonal dynamic through a well‐documented species succession during spring, beginning in sea‐ice and culminating in massive phytoplankton blooms underneath sea‐ice and in the marginal ice zone. The pattern of diatom taxa sequentially dominating this succession is relatively well conserved interannually, and taxonomic shifts seem to align with habitat transitions. To understand whether differential photoadaptation strategies among diatom taxa explain these recurring succession sequences, we coupled laboratory experiments with field work in Baffin Bay at 67.5°N. Based on field data, we selected five diatom species typical of different ecological niches and measured their growth rates under light intensity ranges representative of their natural habitats. To characterize their photoacclimative responses, we sampled pigments and total particulate carbon, and conducted 14C‐uptake photosynthesis response curves and variable fluorescence measurements. We documented a gradient in species respective light intensity for maximal growth suggesting divergent light response plasticity, which for the most part align with species sequential dominance. Other photophysiological parameters supported this ecophysiological framing, although contrasts were always clear only between succession endmembers, Nitzschia frigida and Chaetoceros neogracilis. To validate that these photoacclimative responses are representative of in situ dynamics, we compared them to the chlorophyll a‐specific light‐limited slope (α*) and saturated rate of photosynthesis (), monitored in Baffin Bay on sea‐ice and planktonic communities. This complementary approach confirmed that unusual responses in α* and as a function of light history intensity are similar between sentinel sympagic species N. frigida and natural ice‐core communities. While no light‐history‐dependent trends were observed in planktonic communities, their α* and values were in the range of measurements from our monospecific cultures. Synthesis. Our results suggest that Arctic diatoms species photoadaptation strategy is tuned to the light environment of the habitats in which they dominate and indeed drives the seasonal taxonomic succession.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. II. Structure and Functions.

In Part I, by using 31P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (HII) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery. Electrochromic absorbance transients showed that the TM fragments were re-sealed, and the vesicles largely retained their impermeabilities after lipase treatments—in line with the low susceptibility of the bilayer against the same treatment, as reflected by our 31P-NMR spectroscopy. Signatures of HII-phase could not be discerned with small-angle X-ray scattering—but traces of HII structures, without long-range order, were found by freeze-fracture electron microscopy (FF-EM) and cryo-electron tomography (CET). EM and CET images also revealed the presence of small vesicles and fusion of membrane particles, which might account for one of the isotropic phases. Interaction of VDE (violaxanthin de-epoxidase, detected by Western blot technique in both membrane fragments) with TM lipids might account for the other isotropic phase. In general, non-bilayer lipids are proposed to play role in the self-assembly of the highly organized yet dynamic TM network in chloroplasts.

Photosynthesis ecophysiology of polar Vaucheria sp. – inter-annual comparison

The Vaucheria sp. microbial mats represent the most important primary producer in the tidal flat in Adventdalen, Svalbard. Its photosynthetic activity was monitored ex situ in a microcosm in late Arctic summer in 2016 and 2017 using variable chlorophyll fluorescence measurements with blue and red excitation lights. The effective quantum yield (FPSII) was measured, and the photosynthetic relative electron transport rate (rETR) was calculated. During the measurement period, the microclimate data, air temperature and photosynthetically active radiation (PAR), were recorded as well. Year 2016 was slightly warmer than year 2017. Despite of higher maximum PAR values found in 2016, the mean irradiance reached higher values in 2017 than 2016. When using red light excitation, the rETR and effective quantum yield values were lower than those measured using blue excitation light in 2016. However, opposite results were recorded in 2017, indicating thus rather sample-specific differences. According to redundancy analysis, the PAR was confirmed as the main driver of photosynthesis in late Arctic summer in both years. No serious photoinhibition, expressed as serious systematic decline of the rETR, was observed in both years indicating rapid photoacclimation of Vaucheria sp. photosynthesis to changing light environment. The air temperature was found to be less important driver of the photosynthetic activity. The inter-annual comparisons showed increased photosynthetic activity in 2017, probably caused by higher PAR in 2017, by differences in microcosmos design and/or heterogeneity of samples.

Photoacclimation strategies in northeastern Atlantic seagrasses: Integrating responses across plant organizational levels

Seagrasses live in highly variable light environments and adjust to these variations by expressing acclimatory responses at different plant organizational levels (meadow, shoot, leaf and chloroplast level). Yet, comparative studies, to identify species’ strategies, and integration of the relative importance of photoacclimatory adjustments at different levels are still missing. The variation in photoacclimatory responses at the chloroplast and leaf level were studied along individual leaves of Cymodocea nodosa, Zostera marina and Z. noltei, including measurements of variable chlorophyll fluorescence, photosynthesis, photoprotective capacities, non-photochemical quenching and D1-protein repair, and assessments of variation in leaf anatomy and chloroplast distribution. Our results show that the slower-growing C. nodosa expressed rather limited physiological and biochemical adjustments in response to light availability, while both species of faster-growing Zostera showed high variability along the leaves. In contrast, the inverse pattern was found for leaf anatomical adjustments in response to light availability, which were more pronounced in C. nodosa. This integrative plant organizational level approach shows that seagrasses differ in their photoacclimatory strategies and that these are linked to the species’ life history strategies, information that will be critical for predicting the responses of seagrasses to disturbances and to accordingly develop adequate management strategies.

Electron requirements for carbon incorporation along a diel light cycle in three marine diatom species.

Diatoms account for about 40% of primary production in highly productive ecosystems. The development of a new generation of fluorometers has made it possible to improve estimation of the electron transport rate from photosystem II, which, when coupled with the carbon incorporation rate enables estimation of the electrons required for carbon fixation. The aim of this study was to investigate the daily dynamics of these electron requirements as a function of the diel light cycle in three relevant diatom species and to apprehend if the method of estimating the electron transport rate can lead to different pictures of the dynamics. The results confirmed the species-dependent capacity for photoacclimation under increasing light levels. Despite daily variations in the photosynthetic parameters, the results of this study underline the low daily variability of the electron requirements estimated using functional absorption of the photosystem II compared to an estimation based on a specific absorption cross section of chlorophyll a. The stability of the electron requirements throughout the day would suggest it is potentially possible to estimate high-frequency primary production by using autonomous variable fluorescence measurements from ships-of-opportunity or moorings, without taking potential daily variation in this parameter into consideration, but this result has to be confirmed on natural phytoplankton assemblages. The results obtained in this study confirm the low electron requirements of diatoms to perform photosynthesis, and suggest a potential additional source of energy for carbon fixation, as recently described in the literature for this class.

Light Sheet Microscopy Imaging of Light Absorption and Photosynthesis Distribution in Plant Tissue.

In vivo variable chlorophyll fluorescence measurements of photosystem II (PSII) quantum yields in optically dense systems are complicated by steep tissue light gradients due to scattering and absorption. Consequently, externally measured effective PSII quantum yields may be composed of signals derived from cells differentially exposed to actinic light, where cells located deeper inside tissues receive lower irradiance than cells closer to the surface and can display distinct photophysiological status. We demonstrate how measured distributions of PSII quantum yields in plant tissue change under natural tissue light gradients as compared with conventionally measured quantum yields with even exposure to actinic light. This was achieved by applying actinic irradiance perpendicular to one side of thallus cross sections of the aquatic macrophyte Fucus vesiculosus with laser light sheets of defined spectral composition, while imaging variable chlorophyll fluorescence from cross sections with a microscope-mounted pulse amplitude-modulated imaging system. We show that quantum yields are highly affected by light gradients and that traditional surface-based variable chlorophyll fluorescence measurements result in substantial underestimations and/or overestimations, depending on incident actinic irradiance. We present a method for using chlorophyll fluorescence profiles in combination with integrating sphere measurements of reflectance and transmittance to calculate depth-resolved photon absorption profiles, which can be used to correct apparent PSII electron transport rates to photons absorbed by PSII. Absorption profiles of the investigated aquatic macrophyte were different in shape from what is typically observed in terrestrial leaves, and based on this finding, we discuss strategies for optimizing photon absorption via modulation of the structural organization of phytoelements according to in situ light environments.

Light availability rather than Fe controls the magnitude of massive phytoplankton bloom in the Amundsen Sea polynyas, Antarctica

AbstractAmundsen Sea polynyas are among the most productive, yet climate‐sensitive ecosystems in the Southern Ocean and host massive annual phytoplankton blooms. These blooms are believed to be controlled by iron fluxes from melting ice and icebergs and by intrusion of nutrient‐rich Circumpolar Deep Water, however the interplay between iron effects and other controls, such as light availability, has not yet been quantified. Here, we examine phytoplankton photophysiology in relation to Fe stress and physical forcing in two largest polynyas, Amundsen Sea Polynya (ASP) and Pine Island Polynya (PIP), using the combination of high‐resolution variable fluorescence measurements, fluorescence lifetime analysis, photosynthetic rates, and Fe‐enrichment incubations. These analyses revealed strong Fe stress in the ASP, whereas the PIP showed virtually no signatures of Fe limitation. In spite of enhanced iron availability in the PIP, chlorophyll biomass remained ∼ 30–50% lower than in the Fe‐stressed ASP. This apparent paradox would not have been observed if iron were the main control of phytoplankton bloom in the Amundsen Sea. Long‐term satellite‐based climatology records revealed that the ASP is exposed to significantly higher solar irradiance levels throughout the summer season, as compared to the PIP region, suggesting that light availability controls the magnitude of phytoplankton blooms in the Amundsen Sea. Our data suggests that higher Fe availability (e.g., due to higher melting rates of ice sheets) would not necessarily increase primary productivity in this region. Furthermore, stronger wind‐driven vertical mixing in expanding ice‐free areas may lead to reduction in light availability and productivity in the future.

Photoregulation in a Kleptochloroplastidic Dinoflagellate, Dinophysis acuta.

Some phagotrophic organisms can retain chloroplasts of their photosynthetic prey as so-called kleptochloroplasts and maintain their function for shorter or longer periods of time. Here we show for the first time that the dinoflagellate Dinophysis acuta takes control over “third-hand” chloroplasts obtained from its ciliate prey Mesodinium spp. that originally ingested the cryptophyte chloroplasts. With its kleptochloroplasts, D. acuta can synthesize photosynthetic as well as photoprotective pigments under long-term starvation in the light. Variable chlorophyll fluorescence measurements showed that the kleptochloroplasts were fully functional during 1 month of prey starvation, while the chlorophyll a-specific inorganic carbon uptake decreased within days of prey starvation under an irradiance of 100 μmol photons m-2 s-1. While D. acuta cells can regulate their pigmentation and function of kleptochloroplasts they apparently lose the ability to maintain high inorganic carbon fixation rates.

In situ Dynamics of O2, pH, Light, and Photosynthesis in Ikaite Tufa Columns (Ikka Fjord, Greenland)-A Unique Microbial Habitat.

The Ikka Fjord (SW Greenland) harbors a unique microbial habitat in the form of several hundred submarine tufa columns composed of ikaite, a special hexahydrate form of calcium carbonate that precipitates when alkaline phosphate- and carbonate-enriched spring water seeping out of the sea floor meets cold seawater. While several unique heterotrophic microbes have been isolated from the tufa columns, the microbial activity, and the boundary conditions for microbial growth in ikaite have remained unexplored. We present the first detailed in situ characterization of the physico-chemical microenvironment and activity of oxygenic phototrophs thriving within the ikaite columns. In situ underwater microsensor measurements of pH, temperature, and irradiance in the porous ikaite crystal matrix, revealed an extreme microenvironment characterized by low temperatures, strong light attenuation, and gradients of pH changing from pH 9 at the outer column surface to above pH 10 over the first 1–2 cm of the ikaite. This outer layer of the freshly deposited ikaite matrix contained densely pigmented yellow and green zones harboring a diverse phototrophic community dominated by diatoms and cyanobacteria, respectively, as shown by amplicon sequencing. In situ O2 measurements, as well as underwater variable chlorophyll fluorescence measurements of photosynthetic activity, demonstrated high levels of oxygenic photosynthesis in this extreme gradient environment with strong irradiance-driven O2 dynamics ranging from anoxia to hyperoxic conditions in the ikaite matrix, albeit the local formation of gas bubbles buffered the day-night dynamics of O2 in the tufa columns. The microbial phototrophs in the ikaite matrix are embedded in exopolymers forming endolithic biofilms that may interact with mineral formation and cementing of ikaite crystals.

Chloramphenicol Mediates Superoxide Production in Photosystem II and Enhances Its Photodamage in Isolated Membrane Particles.

Chloramphenicol (CAP) is an inhibitor of protein synthesis, which is frequently used to decouple photodamage and protein synthesis dependent repair of Photosystem II during the process of photoinhibition. It has been reported earlier that CAP is able to mediate superoxide production by transferring electrons from the acceptor side of Photosystem I to oxygen. Here we investigated the interaction of CAP with Photosystem II electron transport processes by oxygen uptake and variable chlorophyll fluorescence measurements. Our data show that CAP can accept electrons at the acceptor side of Photosystem II, most likely from Pheophytin, and deliver them to molecular oxygen leading to superoxide production. In addition, the presence of CAP enhances photodamage of Photosystem II electron transport in isolated membrane particles, which effect is reversible by superoxide dismutase. It is concluded that CAP acts as electron acceptor in Photosystem II and mediates its superoxide dependent photodamage. This effect has potential implications for the application of CAP in photoinhibitory studies in intact systems.

Using solar energy suboptimally

Phytoplankton How efficient are phytoplankton at converting sunlight into the products of photosynthesis? The two other pathways that that absorbed energy can take are emission back to the environment by fluorescence or conversion to heat. Lin et al. measured phytoplankton fluorescence lifetimes in the laboratory and combined them with satellite measurements of variable chlorophyll fluorescence. Combined, they determined the quantum yields of photochemistry and fluorescence in four ocean basins. Approximately 60% of absorbed solar energy is converted to heat, a figure 50% higher than has been determined for conditions of optimal growth. Science , this issue p. [264][1] [1]: /lookup/doi/10.1126/science.aab2213

Effect of biofilm on fluorescence measurements derived from fast repetition rate fluorometers

This study evaluates, for the first time, the influence of biofilms on single and double optical window (SOW and DOW, respectively) fast repetition rate fluorometer (FRRF) measurements of microalgal photosystem-II initial fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv = Fm – F0), quantum yield (Fv/Fm) and functional absorption cross section (σPSII)]. Biofilms with chlorophyll > 0.1 μg cm−2 and > 0.3 μgcm−2 on SOW and DOW, respectively, produced a substantial increase in fluorescence. However, the relative magnitude of biofouling effects depended on sample chlorophyll concentrations, being more critical at concentrations < 1 mg m−3. In DOW-FRRF, biofilms affected F0 (increased) and Fv/Fm (decreased) but not Fv and σPSII, whereas in SOW-FRRF, biofilms increased fluorescence and showed a variable effect on Fv/Fm and σPSII, because only biofilms on SOW attained actual Fm. As a result, the biofilm effect was substantial on SOW-FRRF measurements. On the other hand, the neutral-density filters (representing non-chlorophyll containing biofilms) with different transmission levels reduced the fluorescence signal. Correction procedures for the above photosystem-II parameters are proposed here.

Removing the light history signal from normalized variable fluorescence (Fv/Fm) measurements on marine phytoplankton

Variable fluorescence normalized to maximal fluorescence, Fv/Fm, determined by Fast Repetition Rate Fluorometry (FRRF) is being increasingly used to compare photosynthetic electron transport capacity in natural phytoplankton communities. Interpreting results of such studies is, however, complicated by the fact that both nutrient status and light history (photoinhibition under in situ conditions) are known to influence Fv/Fm. Thus, the value of Fv/Fm measurements in the field would be greatly enhanced if the light history signal could be separated from other influences. Here, both field and laboratory studies demonstrate that dark treatment (30 min‐4 h) before FRRF measurement is not sufficient to remove a light history signal in Fv/Fm. The signal could, however, be essentially eliminated by incubation of samples in low light prior to Fv/Fm determination. For the study conditions tested, the most effective treatment for removal of the signal was 4 h at 50 µmol m‐2 s‐1. However, the effectiveness of the light treatment in removing the light history was influenced by temperature. Therefore, no universal protocol for eliminating the light history signal can be developed, but recommendations are given for developing site‐specific approaches for separating the light history signal from other factors influencing Fv/Fm. Carrying out light incubations before determining Fv/Fm not only provides the possibility for eliminating a light history signal in the measurements but the difference between Fv/Fm measured after light and dark incubations appears also to be a potentially useful indicator of the degree of photoinhibition experienced by phytoplankton under natural conditions.

An inter-basin comparison of nutrient limitation and the irradiance response of pulse-amplitude modulated (PAM) fluorescence in Lake Erie phytoplankton

This research examined the application of the maximum quantum efficiency (Fv/Fm) and relative electron transport rate versus irradiance curves (rETR) as a rapid, sensitive assessment of Lake Erie phytoplankton nutrient status. I evaluated the potential benefits of the variable fluorescence parameters by comparing these parameters with chemical and physiological nutrient status assays. I tested the hypothesis that Fv/Fm and rETR curves could diagnose nutrient status in natural lake phytoplankton and be capable of discriminating which inorganic nutrient is limited temporally and spatially. Fv/Fm was on average highest in the more eutrophic west basin (WB) and lowest in the more oligotrophic central basin (CB). According to the chemical and physiological indicators, P deficiency was most severe in the CB during summer stratification and N deficiency was strongest in the WB during isothermal conditions. Like Fv/Fm, rETR at light saturation (rETRmax) and the initial slope of the rETR versus irradiance curve (α) decreased as the severity of N and P deficiency increased. Amendment with N or P stimulated increased Fv/Fm, rETRmax, and α in N- and P-limited samples, respectively, and abolished the photoinhibition apparent in rETR curves of nutrient-limited samples. These results supported the view that the N and P deficiency assays, and corresponding variations of variable fluorescence parameters, were valid indicators of widely variable N and P deficiency in the phytoplankton, and could be used to provide a promising tool in determining phytoplankton nutrient status. Contrary to my hopes, it did not appear that rETR–irradiance curves could discriminate between N and P deficiency. Identification of the most limiting nutrient still demanded additional information beyond the variable fluorescence measurements.

Increased photosystem II stability promotes H2production in sulfur-deprivedChlamydomonas reinhardtii

Photobiological H2 production is an attractive option for renewable solar fuels. Sulfur-deprived cells of Chlamydomonas reinhardtii have been shown to produce hydrogen with the highest efficiency among photobiological systems. We have investigated the photosynthetic reactions during sulfur deprivation and H2 production in the wild-type and state transition mutant 6 (Stm6) mutant of Chlamydomonas reinhardtii. The incubation period (130 h) was dissected into different phases, and changes in the amount and functional status of photosystem II (PSII) were investigated in vivo by electron paramagnetic resonance spectroscopy and variable fluorescence measurements. In the wild type it was found that the amount of PSII is decreased to 25% of the original level; the electron transport from PSII was completely blocked during the anaerobic phase preceding H2 formation. This block was released during the H2 production phase, indicating that the hydrogenase withdraws electrons from the plastoquinone pool. This partly removes the block in PSII electron transport, thereby permitting electron flow from water oxidation to hydrogenase. In the Stm6 mutant, which has higher respiration and H2 evolution than the wild type, PSII was analogously but much less affected. The addition of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea revealed that ∼80% of the H2 production was inhibited in both strains. We conclude that (i) at least in the earlier stages, most of the electrons delivered to the hydrogenase originate from water oxidation by PSII, (ii) a faster onset of anaerobiosis preserves PSII from irreversible photoinhibition, and (iii) mutants with enhanced respiratory activity should be considered for better photobiological H2 production.

Optimization of variable fluorescence measurements of phytoplankton communities with cyanobacteria

Excitation–emission fluorescence matrices of phytoplankton communities were simulated from laboratory-grown algae and cyanobacteria cultures, to define the optical configurations of theoretical fluorometers that either minimize or maximize the representation of these phytoplankton groups in community variable fluorescence measurements. Excitation sources that match the photosystem II (PSII) action spectrum of cyanobacteria do not necessarily lead to equal representation of cyanobacteria in community fluorescence. In communities with an equal share of algae and cyanobacteria, inducible PSII fluorescence in algae can be retrieved from community fluorescence under blue excitation (450–470 nm) with high accuracy (R 2 = 1.00). The highest correlation between community and cyanobacterial variable fluorescence is obtained under orange-red excitation in the 590–650 nm range (R 2 = 0.54). Gaussian band decomposition reveals that in the presence of cyanobacteria, the emission detection slit must be narrow (up to 10 nm) and centred on PSII chlorophyll-a emission (~683 nm) to avoid severe dampening of the signal by weakly variable phycobilisomal fluorescence and non-variable photosystem I fluorescence. When these optimizations of the optical configuration of the fluorometer are followed, both cyanobacterial and algal cultures in nutrient replete exponential growth exhibit values of the maximum quantum yield of charge separation in PSII in the range of 0.65–0.7.

Photosynthetic characteristics of marine aerobic anoxygenic phototrophic bacteria Roseobacter and Erythrobacter strains

A coastal Roseobacter strain of marine aerobic anoxygenic phototrophic bacteria (AAnPB) was isolated and phylogenetically determined. The strain OBYS 0001 was characterized by its physiological and biochemical properties with reference to the Erythrobacter longus type strain NBRC 14126. When grown in batch cultures, the growth curves of the both strains were similar. Cellular bacteriochlorophyll a concentrations of the strains reached the maxima in the stationary growth conditions. In vivo fluorescence excitation/optical density spectra between 470 and 600nm for OBYS 0001 represented higher values than NBRC 14126. Variable fluorescence measurements revealed that the functional absorption cross section (σ) of the bacterial photosynthetic complexes for OBYS 0001 was significantly higher than that for NBRC 14126 under green excitation. These results suggest that Roseobacter can capture green light more efficiently than Erythrobacter for photosynthesis. The photochemical quantum efficiencies (F (v)/F (m)) of the bacterial photosynthetic complexes for OBYS 0001 were consistently lower than those for NBRC 14126. A relationship between the growth rate and F (v)/F (m) was significant for OBYS 0001, but that was not found for NBRC 14126. These results suggested that F (v)/F (m) for AAnPB could not be used as a proxy of the growth rate which is consistent with their mostly heterotrophic characters.

An Integrated Analysis of Molecular Acclimation to High Light in the Marine Diatom Phaeodactylum tricornutum

Photosynthetic diatoms are exposed to rapid and unpredictable changes in irradiance and spectral quality, and must be able to acclimate their light harvesting systems to varying light conditions. Molecular mechanisms behind light acclimation in diatoms are largely unknown. We set out to investigate the mechanisms of high light acclimation in Phaeodactylum tricornutum using an integrated approach involving global transcriptional profiling, metabolite profiling and variable fluorescence technique. Algae cultures were acclimated to low light (LL), after which the cultures were transferred to high light (HL). Molecular, metabolic and physiological responses were studied at time points 0.5 h, 3 h, 6 h, 12 h, 24 h and 48 h after transfer to HL conditions. The integrated results indicate that the acclimation mechanisms in diatoms can be divided into an initial response phase (0–0.5 h), an intermediate acclimation phase (3–12 h) and a late acclimation phase (12–48 h). The initial phase is recognized by strong and rapid regulation of genes encoding proteins involved in photosynthesis, pigment metabolism and reactive oxygen species (ROS) scavenging systems. A significant increase in light protecting metabolites occur together with the induction of transcriptional processes involved in protection of cellular structures at this early phase. During the following phases, the metabolite profiling display a pronounced decrease in light harvesting pigments, whereas the variable fluorescence measurements show that the photosynthetic capacity increases strongly during the late acclimation phase. We show that P. tricornutum is capable of swift and efficient execution of photoprotective mechanisms, followed by changes in the composition of the photosynthetic machinery that enable the diatoms to utilize the excess energy available in HL. Central molecular players in light protection and acclimation to high irradiance have been identified.

Transcriptional regulation of the bidirectional hydrogenase in the cyanobacterium Synechocystis 6803

To identify optimal conditions for renewable hydrogen production from sunlight and water we have studied transcriptional changes of the hoxEFUYH genes encoding the bidirectional hydrogenase in the cyanobacterium Synechocystis PCC 6803. Transcript abundance detection by real time polymerase chain reaction was supplemented with variable chlorophyll fluorescence measurements to monitor redox changes of the photosynthetic electron transport chain. Our main observations are: (i) abundance of hox transcripts decreases in the dark and recovers in the light. (ii) Inhibition of the Calvin cycle by glycolaldehyde suppresses hox gene transcription, which can be restored by the addition of electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethylurea and dibromothymoquinone. (iii) The transcript levels of all hox genes are increased in anoxia, with additional induction of hoxEF in darkness or in the presence of dibromothymoquinone. (iv) Plastoquinone pool redox changes are not correlated with hox transcript level changes. (v) Changes in the transcript levels of lexA and sll0359 genes, encoding putative regulators of hox genes, are only partly correlated with transcript changes of hox genes under different conditions. Our data demonstrate a previously unrecognized light- and oxygen-dependent regulation of hox gene transcription in Synechocystis PCC 6803, which is related to photosynthetic electron transport and to unidentified oxygen and redox sensors. We also conclude that neither LexA nor Sll0359 are likely to be exclusive regulators of hox gene transcription.