Photosystem II

Light intensities that limit electron flow induce rapid degradation of the photosystem II (PSII) reaction center D1 protein. The mechanism of this phenomenon is not known. We propose that at low excitation rates back electron flow and charge recombination between the Q B •− or Q A •− semiquinone acceptors and the oxidized S 2,3 states of the PSII donor side may cause oxidative damage via generation of active oxygen species. Therefore, damage per photochemical event should increase with decreasing rates of PSII excitation. To test this hypothesis, the effect of the dark interval between single turnover flashes on the inactivation of water oxidation, charge separation and recombination, and the degradation of D1 protein were determined in spinach thylakoids. PSII inactivation per flash increases as the dark interval between the flashes increases, and a plateau is reached at dark intervals, allowing complete charge recombination of the Q B •− /S 2,3 or Q A •− /S 2 states (about 200 and...

Redox-controlled phosphorylation of thylakoid membrane proteins represents a unique system for the regulation of light energy utilization in photosynthesis. The molecular mechanisms for this process remain unknown, but current views suggest that the plastoquinone pool directly controls the activation of the kinase. On the basis of enzyme activation by a pH shift in the darkness combined with flash photolysis, EPR, and optical spectroscopy we propose that activation occurs when plastoquinol occupies the quinol-oxidation (Qo) site of the cytochrome bf complex, having its high-potential path components in a reduced state. A linear correlation between kinase activation and accessibility of the Qo site to plastoquinol was established by quantification of the shift in the g y EPR signal of the Rieske Fe–S center resulting from displacement of the Qo-site plastoquinol by a quinone analog. Activity persists as long as one plastoquinol per cytochrome bf is still available. Withdrawal of one ...

Flash‐induced absorbance changes in the picosecond and nanosecond time range have been measured in the ‘D1‐D2‐cyt b‐559 complex’ isolated from photosystem II membranes. The results indicate the efficient formation of the primary radical pair P680+ pheophytin−, which had a lifetime of about 36 ns, and the presence of unconnected chlorophyll in this preparation. It is concluded that the complex contains the active photosystem II reaction center, and that this reaction center contains at most 4 chlorophyll a molecules.

A key step in the photosynthetic reactions in photosystem 11 of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem 11 reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 ± 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem 11 reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 ± 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at -165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem 11 reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+1is temperature in- dependent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.

EPR spectroscopy was applied to investigate the inhibition of electron transport in photosystem I1 by Cu2+ ions. Our results show that Cu2+ has inhibitory effects on both the donor and the acceptor side of photosystem 11. In the presence of Cu2+, neither EPR signal II,,, fast nor signal IIfast, which both reflect oxidation of tyrosinez, could be induced by illumination. This shows that Cu2+ inhibits electron

The reduction kinetics of EPR signal I&r,, in the presence of iodide were measured with EPR spectroscopy and the amount of 125I incorporated into the DZ-protein in the reaction center of photosystem II was quantitated in a PS II core complex preparation. The halftime of signal II,,, reduction in the dark was estimated at 31, 25 and 20 min in the presence of 0, 20 and 50 mM KI, respectively, reflecting the reduction of signal II sr0W by iodide. The amount of 1251 that was incorporated into the DZ-protein in the dark was correlated to the reduction of signal IIsroW by iodide. This correlation indicates that the site of iodide oxidation in the dark and the component giving rise to EPR signal II sroW are located on the DZ-protein in the PS II reaction center.

The oxygen-evolving activity of photosystem II (PSII) was inhibited by extraction of the Mn-cluster using hydroxylamine. The activity was recovered partially by the photo-activation treatment applied after addition of the exogenous Mn 2+ to the inhibited samples. The effect of Ca 2+ , Cl -and H + ions in the reaction medium on reconstitution of the oxygen-evolving activity of PSII was studied. The results can be useful for the understanding of the principles of artificial energy converters based on photosynthetic reaction centers and for the study of artificial photosynthesis.

The photosystem II (PSII) light harvesting complex (LHC) consists of a variety of pigment protein complexes which are involved in structural organization and regulation of photosynthetic unit. These LHC proteins encoded by a group of Lhcb genes are essential for the structural integrity of PSII supercomplex, the channeling the excitation energy to the reaction center of PSII and its redistribution to photosystem I by state transitions. Numerous studies with the help of recent technological advancements have enabled a significant progress in our understanding on the structure of PSII-LHCII supercomplexes and their mobilization under various light conditions. Here, we present a mini-review on the latest concepts and models depicting the structure of PSII-LHCII supercomplexes and the role of Lhcb proteins in their supra-molecular organization. Also we will review on the current understandings and remaining problems involved in the mobilization of the supercomplexes during state transitions and during high light illumination for controlling light energy distribution between the two photosystems.

Reversible phosphorylation of thylakoid light harvesting proteins is a mechanism to compensate for unbalanced excitation of PSI vs PSII under limiting light. In monocots, an additional phosphorylation event on the PSII antenna CP29 occurs upon exposure to excess light, enhancing resistance to light stress. Different from the case of the major LHCII antenna complex, the STN7 kinase and its related PPH1 phosphatase were proven not to be involved in CP29 phosphorylation, indicating that a different set of enzymes act in the high-light response. Here, we analyze a rice stn8 mutant in which both PSII core proteins and CP29 phosphorylation are suppressed in high light, implying that STN8 is the kinase catalyzing this reaction. In order to identify the phosphatase involved, we produced a recombinant enzyme encoded by the rice ortholog of AtPBCP, antagonist of AtSTN8, which catalyzes the dephosphorylation of PSII core proteins. The recombinant protein was active in dephosphorylating P-CP29....

The photosystem II (PSII) light harvesting complex (LHC) consists of a variety of pigment protein complexes which are involved in structural organization and regulation of photosynthetic unit. These LHC proteins encoded by a group of Lhcb genes are essential for the structural integrity of PSII supercomplex, the channeling the excitation energy to the reaction center of PSII and its redistribution to photosystem I by state transitions. Numerous studies with the help of recent technological advancements have enabled a significant progress in our understanding on the structure of PSII-LHCII supercomplexes and their mobilization under various light conditions. Here, we present a mini-review on the latest concepts and models depicting the structure of PSII-LHCII supercomplexes and the role of Lhcb proteins in their supra-molecular organization. Also we will review on the current understandings and remaining problems involved in the mobilization of the supercomplexes during state transitions and during high light illumination for controlling light energy distribution between the two photosystems.

PsbS is a 22-kDa Photosystem (PS) II protein involved in non-photochemical quenching (NPQ) of chlorophyll fluorescence. Rice (Oryza sativa L.) has two PsbS genes, PsbS1 and PsbS2. However, only inactivation of PsbS1, through a knockout (PsbS1-KO) or in RNAi transgenic plants, results in plants deficient in qE, the energy-dependent component of NPQ. In studies presented here, under fluctuating high light, growth of young seedlings lacking PsbS is retarded, and PSII in detached leaves of the mutants is more sensitive to photoinhibitory illumination compared with the wild type. Using both histochemical and fluorescent probes, we determined the levels of reactive oxygen species, including singlet oxygen, superoxide, and hydrogen peroxide, in leaves and thylakoids. The PsbS-deficient plants generated more superoxide and hydrogen peroxide in their chloroplasts. PSII complexes isolated from them produced more superoxide compared with the wild type, and PSII-driven superoxide production was...

and PSII repair cycle STN8 kinase and PSII core proteins phosphorylation Proteases and D1 protein degradation Reactive oxygen species (ROS) Ultra-violet radiation a b s t r a c t Photosystem II (PSII) is vulnerable to high light (HL) illumination resulting in photoinhibition. In addition to photoprotection mechanisms, plants have developed an efficient PSII repair mechanism to save themselves from irreversible damage to PSII under abiotic stresses including HL illumination. The phosphorylation/dephosphorylation cycle along with subsequent degradation of photodamaged D1 protein to be replaced by the insertion of a newly synthesized copy of D1 into the PSII complex, is the core function of the PSII repair cycle. The exact mechanism of this process is still under discussion. We describe the recent progress in identifying the kinases, phosphatases and proteases, and in understanding their involvement in the maintenance of thylakoid structure and the quality control of proteins by PSII repair cycle during photoinhibition.

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction center. Man-made systems are still far from replicating the complexity of PSII as the majority of PSII-mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Rupolypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru 4 POM). The resulting [PBI] 5 Ru 4 POM complex shows: a robust amphiphilic structure and dynamic aggregation into large 2D-paracrystalline domains, a red-shifted light-harvesting efficiency > 40%, and favorable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ> 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

for their valuable experimental and practical inputs given during the period of Airyscan detector adaptation in our laboratory. Finally, we also express our gratitude for the inspiration that have emerged throughout the project solution. The authors declare that they have no conflict of interest.

Cyanobacteria possess several photoprotective mechanisms involving relatively rapid (<few minutes) changes in the photosynthetic apparatus to dynamically adjust the amount of irradiance arriving at photosystem II (PS II). Photoprotection in cyanobacteria differs from that in plants and algae, with both of the latter possessing transmembrane chlorophyll- and carotenoid-binding light-harvesting complexes. Most cyanobacteria harvest light via a large extra-membrane complex, the phycobilisome (PBS) that contains blue and red phycobiliproteins. Thus far, three prominent and rapid processes have been identified that control effective PS II antenna size in cyanobacteria: state transitions, blue-green-light-induced thermal dissipation of excess energy absorbed by PBS, and PBS decoupling. While the latter two have a mostly photoprotective function, the process of state transitions also optimizes energy distribution between the two photosystems.

Primary photosynthetic reactions take place inside thylakoid membrane where light-to-chemical energy conversion 20 is catalyzed by two pigment-protein complexes, photosystem I (PSI) and photosystem II (PSII). Light absorption in 21 cyanobacteria is increased by pigment-protein supercomplexesphycobilisomes (PBSs) situated on thylakoid 22 membrane surfaces that transfer excitation energy into both photosystems. We have explored the localization of 23 PSI, PSII and PBSs in thylakoid membrane of native cyanobacteria cell Anabaena sp. 7120 by means of cryogenic con-24 focal microscopy. We have adapted a conventional temperature controlling stage to an Olympus FV1000 confocal 25 microscope. The presence of red shifted emission of chlorophylls from PSI has been confirmed by spectral measure-26 ments. Confocal fluorescence images of PSI (in a spectral range 710-750 nm), PSII (in a spectral range 690-705 nm) 27 and PBSs (in a spectral range 650-680 nm) were recorded at low temperature. Co-localization of images showed 28 spatial heterogeneity of PSI, PSII and PBSs over the thylakoid membrane, and three dominant areas were identified: 29 PSI-PSII-PBS supercomplex area, PSII-PBS supercomplex area and PSI area. The observed results were discussed with 30 regard to light-harvesting regulation in cyanobacteria.

Primary photosynthetic reactions take place inside thylakoid membrane where light-to-chemical energy conversion 20 is catalyzed by two pigment-protein complexes, photosystem I (PSI) and photosystem II (PSII). Light absorption in 21 cyanobacteria is increased by pigment-protein supercomplexesphycobilisomes (PBSs) situated on thylakoid 22 membrane surfaces that transfer excitation energy into both photosystems. We have explored the localization of 23 PSI, PSII and PBSs in thylakoid membrane of native cyanobacteria cell Anabaena sp. 7120 by means of cryogenic con-24 focal microscopy. We have adapted a conventional temperature controlling stage to an Olympus FV1000 confocal 25 microscope. The presence of red shifted emission of chlorophylls from PSI has been confirmed by spectral measure-26 ments. Confocal fluorescence images of PSI (in a spectral range 710-750 nm), PSII (in a spectral range 690-705 nm) 27 and PBSs (in a spectral range 650-680 nm) were recorded at low temperature. Co-localization of images showed 28 spatial heterogeneity of PSI, PSII and PBSs over the thylakoid membrane, and three dominant areas were identified: 29 PSI-PSII-PBS supercomplex area, PSII-PBS supercomplex area and PSI area. The observed results were discussed with 30 regard to light-harvesting regulation in cyanobacteria.

O 2 evolution from single turnover flashes of up to 96 Wmol absorbed quanta m 32 and from multiple turnover pulses of 8.6 and 38.6 ms duration and 12 800 and 850 Wmol absorbed quanta m 32 s 31 intensity, respectively, was measured in sunflower leaves with the help of zirconium O 2 analyser. O 2 evolution from one flash could be measured with 1% accuracy on the background of 10^50 Wmol O 2 mol 31 . Before the measurements leaves were pre-adapted either at 30^60 or 1700 Wmol quanta m 32 s 31 to induce different non-photochemical excitation quenching (q N ). Short (1 min) exposures at the high light that created only energy-dependent, q E type quenching, caused no changes in the O 2 yield from saturating flashes or pulses that could be related to the q E quenching, but the yield from low intensity flashes and pulses decreased considerably. Long 30^60min exposures at the high light induced a reversible inhibitory, q I type quenching that decreased the O 2 yield from both, saturating and limiting flashes and pulses (but more from the limiting ones), which reversed within 15 min under the low light. The results are in agreement with the notion that q E is caused by a quenching process in the PSII antenna and no changes occur in the PSII centres, but the reversible (15^30 min) q I quenching is accompanied by inactivation of a part of PSII centres.

Chl fluorescence induction (FI) was recorded in sunflower leaves pre-adapted to darkness or low preferentially PSI light, or inhibited by DCMU. For analysis the FI curves were plotted against the cumulative number of excitations quenched by PSII, n q , calculated as the cumulative complementary area above the FI curve. In the ?DCMU leaves n q was \1 per PSII, suggesting pre-reduction of Q A during the dark pre-exposure. A strongly sigmoidal FI curve was constructed by complementing (shifting) the recorded FI curves to n q = 1 excitation per PSII. The full FI curve in ?DCMU leaves was well fitted by a model assuming PSII antennae are excitonically connected in domains of four PSII. This result, obtained by gradually reducing Q A in PSII with pre-blocked Q B (by DCMU or PQH 2 ), differs from that obtained by gradually blocking the Q B site (by increasing DCMU or PQH 2 level) in leaves during (quasi)steady-state e -transport (Oja and Laisk, Photosynth Res 114, 15-28, 2012). Explanations are discussed. Donor side quenching was characterized by comparison of the total n q in one and the same dark-adapted leaf, which apparently increased with increasing PFD during FI. An explanation for the donor side quenching is proposed, based on electron transfer from excited P680* to oxidized tyrosine Z (TyrZ ox ). At high PFDs the donor side quenching at the J inflection of FI is due mainly to photochemical quenching by TyrZ ox . This quenching remains active for subsequent photons while TyrZ remains oxidized, following charge transfer to Q A . During further induction this quenching disappears as soon as PQ and Q A become reduced, charge separation becomes impossible and TyrZ is reduced by the water oxidizing complex.

Using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, we determined the excitation partitioning to photosystem I1 (PSII), the CO,/O, specificity of ribulose-l,5bisphosphate carboxylase/oxygenase, the dark respiration in the light, and the alternative electron transport rate to acceptors other than bisphosphoglycerate, and the transport resistance for CO, in the mesophyll cells for individual leaves of herbaceous and tree species. The specificity of ribulose-1,5-bisphosphate carboxylase/ oxygenase for CO, was determined from the slope of the O , dependente of the CO, compensation point between 1.5 and 21 ?& O,. Its value, on the basis of dissolved CO, and O , concentrations at 25.5"C, varied between 86 and 89. Dark respiration i n the light, estimated from the difference between the CO, compensation point and the CO, photocompensation point, was about 20 to 50% of the respiration rate in the dark. The excitation distribution to PSll was estimated from the extrapolation of the dependence of the PSll quantum yield on F/F, to F = O, where F is steady-state and F,,, is pulse-saturated fluorescence, and varied between 0.45 and 0.6. The alternative electron transport rate was found as the difference between the electron transport rates calculated from fluorescence and from gas exchange, and at low CO, concentrations and 10 to 21 % O*, it was 25 to 30% of the maximum electron transport. l h e calculated mesophyll diffusion resistance accounted for about 20 to 30% of the total mesophyll resistance, which also includes carboxylation resistance. Whole-leaf photosynthesis is limited by gas phase, mesophyll diffusion, and carboxylation resistances in nearly the same proportion in both herbaceous species and trees.

VIPP proteins aid thylakoid biogenesis and membrane maintenance in cyanobacteria, algae, and plants. Some members of the Chlorophyceae contain two VIPP paralogs termed VIPP1 and VIPP2, which originate from an early gene duplication event during the evolution of green algae. VIPP2 is barely expressed under nonstress conditions but accumulates in cells exposed to high light intensities or H 2 O 2 , during recovery from heat stress, and in mutants with defective integration (alb3.1) or translocation (secA) of thylakoid membrane proteins. Recombinant VIPP2 forms rod-like structures in vitro and shows a strong affinity for phosphatidylinositol phosphate. Under stress conditions, >70% of VIPP2 is present in membrane fractions and localizes to chloroplast membranes. A vipp2 knock-out mutant displays no growth phenotypes and no defects in the biogenesis or repair of photosystem II. However, after exposure to high light intensities, the vipp2 mutant accumulates less HSP22E/F and more LHCSR3 protein and transcript. This suggests that VIPP2 modulates a retrograde signal for the expression of nuclear genes HSP22E/F and LHCSR3. Immunoprecipitation of VIPP2 from solubilized cells and membrane-enriched fractions revealed major interactions with VIPP1 and minor interactions with HSP22E/F. Our data support a distinct role of VIPP2 in sensing and coping with chloroplast membrane stress. high light response, membrane stress, molecular chaperones, protein homeostasis, reactive oxygen species, retrograde signalling, thylakoid membrane biogenesis VIPP1 is a highly conserved protein found in cyanobacteria and chloroplasts. In cyanobacteria, VIPP1 localizes to the cytoplasm, the Jasmine Theis and Justus Niemeyer should be considered joint first author.

The photosynthetic efficiency in plants is affected by salinity. Focus of this study was to observe the consequences of salinity on the rate of photosynthesis in Moringa (Moringa oleifera L.) plants. Experiment was conducted under field conditions with 3 replicates and data of treated and non-treated plants was collected accordingly. Photosynthetic rate was affected by different levels of salt stress. The change in photosynthetic was attributes were determined by OJIP and light response curve calculations by using Fluor Pen [FP 100-PS (Photon system, Czech Republic)] and DUAL-PAM-100 (Walz, Germany). Salinity stress decreased chlorophyll a fluorescence characteristic. The significant quantity of electron transport (φEo), quantum yield of primary photochemistry (φPo), proficiency per trapped excitation (Ψo) and performance index of photosystem II (PSII). Performance index (PIABS) was also declined with salinity in M. oleifera. Our results showed that electron transport rate and photo...

Recently, we proposed a simple conceptual fluctuating antenna model (FAM), describing excitation diffusion and trapping in a continuous medium, where variations of the excitation transfer pathways are taken into account by the introduced fractional space dimension. Since then, this model has been successfully applied to simulate multi-exponential excitation quenching kinetics in a series of plant photosynthetic systems, purified from the thylakoid membranes, without invoking a radical pair state in the reaction centre. Here, we overview this model and its parameters obtained for various systems, and extend the area of its applications to several pigment–protein supercomplexes containing the photosystem I (PSI). We show that while the diffusion in the PSI core is virtually three-dimensional, the PSI core aggregates interconnected with other light-harvesting complexes (LHCI and/or LHCII) are characterized by a substantially reduced dimension, which indicates a smaller number of energy...

We have performed time-resolved fluorescence measurements on photosystem II (PSII) containing membranes (BBY particles) from spinach with open reaction centers. The decay kinetics can be fitted with two main decay components with an average decay time of 150 ps. Comparison with recent kinetic exciton annihilation data on the major light-harvesting complex of PSII (LHCII) suggests that excitation diffusion within the antenna contributes significantly to the overall charge separation time in PSII, which disagrees with previously proposed trap-limited models. To establish to which extent excitation diffusion contributes to the overall charge separation time, we propose a simple coarse-grained method, based on the supramolecular organization of PSII and LHCII in grana membranes, to model the energy migration and charge separation processes in PSII simultaneously in a transparent way. All simulations have in common that the charge separation is fast and nearly irreversible, corresponding to a significant drop in free energy upon primary charge separation, and that in PSII membranes energy migration imposes a larger kinetic barrier for the overall process than primary charge separation.

We assessed the photosynthetic responses of eight wheat varieties in conditions of a simulated heat wave in a transparent plastic tunnel for one week. We found that high temperatures (up to 38 °C at midday and above 20 °C at night) had a negative effect on the photosynthetic functions of the plants and provided differentiation of genotypes through sensitivity to heat. Measurements of gas exchange showed that the simulated heat wave led to a 40% decrease in photosynthetic activity on average in comparison to the control, with an unequal recovery of individual genotypes after a release from stress. Our results indicate that the ability to recover after heat stress was associated with an efficient regulation of linear electron transport and the prevention of over-reduction in the acceptor side of photosystem I.

1. Preparation of 200 nm polystyrene (PS) nanosphere suspension in ethanol which contain 0.1% vol of Tween 80 or Tween 20. 2. Spin coat the PS nanosphere suspension on a clean substrate at 1,000 rpm to form monolayer of PS nanosphere. 3. Coat the PS nanosphere monolayer with gold by using a metal evaporator. Optimize gold coverage. 4. Redisperstion of the Au-coated nanoparticles by sonication and centrifuge to concentrate the suspension. 5. Characterization of Janus colloids by using a scanning electron microscopy technique (SEM ).

We present a chemical route to covalently couple the photosystem I (PS I) to carbon nanotubes (CNTs). Small linker molecules are used to connect the PS I to the CNTs. Hybrid systems, consisting of CNTs and the PS I, promise new photo-induced transport phenomena due to the outstanding electro-optical properties of the robust cyanobacteria membrane protein PS I.

SummaryMassive degradation of photosynthetic proteins is the hallmark of leaf senescence; however the mechanism involved in chloroplast protein breakdown is not completely understood. As small ‘senescence‐associated vacuoles’ (SAVs) with intense proteolytic activity accumulate in senescing leaves of soybean and Arabidopsis, the main goal of this work was to determine whether SAVs are involved in the degradation of chloroplastic components. SAVs with protease activity were readily detected through confocal microscopy of naturally senescing leaves of tobacco (Nicotiana tabacum L.). In detached leaves incubated in darkness, acceleration of the chloroplast degradation rate by ethylene treatment correlated with a twofold increase in the number of SAVs per cell, compared to untreated leaves. In a tobacco line expressing GFP targeted to plastids, GFP was re‐located to SAVs in senescing leaves. SAVs were isolated by sucrose density gradient centrifugation. Isolated SAVs contained chloroplas...

Picocyanobacteria are the numerically dominant photoautotrophs of the oligotrophic regions of Earth's oceans. These organisms are characterized by their small size and highly reduced genomes. Strains partition to different light intensity and nutrient level niches, with differing photosynthetic apparatus stoichiometry, light harvesting machinery and susceptibility to photoinactivation. In this study, we grew three strains of picocyanobacteria: the low light, high nutrient strain Prochlorococcus marinus MIT 9313; the high light, low nutrient Prochlorococcus marinus MED 4; and the high light, high nutrient marine Synechococcus strain WH 8102; under low and high growth light levels. We then performed matched photophysiology, protein and transcript analyses. The strains differ significantly in their rates of Photosystem II repair under high light and in their capacity to remove the PsbA protein as the first step in the Photosystem II repair process. Notably, all strains remove the PsbD subunit at the same rate that they remove PsbA. When grown under low light, MIT 9313 loses active Photosystem II quickly when shifted to high light, but has no measurable capacity to remove PsbA. MED 4 and WH 8102 show less rapid loss of Photosystem II and considerable capacity to remove PsbA. MIT 9313 has less of the FtsH protease thought to be responsible for the removal of PsbA in other cyanobacteria. Furthermore, by transcript analysis the predominant FtsH isoform expressed in MIT 9313 is homologous to the FtsH 4 isoform characterized in the model strain Synechocystis PCC 6803, rather than the FtsH 2 and 3 isoforms thought to be responsible for PsbA degradation. MED 4 on the other hand shows high light inducible expression of the isoforms homologous to FtsH 2 and 3, consistent with its faster rate of PsbA removal. MIT 9313 has adapted to its low light environment by diverting resources away from Photosystem II content and repair.

For photosynthesis, oxygenic phototrophic organisms necessarily contain not only chlorophylls but also carotenoids. Various carotenoids have been identified in algae and taxonomic studies of algae have been conducted. In this review, the relationship between the distribution of chlorophylls and carotenoids and the phylogeny of sea and freshwater oxygenic phototrophs, including cyanobacteria, red algae, brown algae, and green algae, is summarized. These phototrophs contain division-or class-specific chlorophylls and carotenoids, such as fucoxanthin, peridinin, diadinoxanthin, and siphonaxanthin. The distribution of β-carotene and its derivatives, including β-carotene, zeaxanthin, violaxanthin, neoxanthin, diadinoxanthin, fucoxanthin, and peridinin (β-branch carotenoids), are limited to divisions of a part of Rhodophyta, Cryptophyta, Heterokontophyta, Haptophyta, and Dinophyta. Meanwhile, the distribution of α-carotene and its derivatives, such as lutein, loroxanthin, and siphonaxanthin (α-branch carotenoids), are limited to divisions of a part of Rhodophyta (macrophytic type), Cryptophyta, Euglenophyta, Chlorarachniophyta, and Chlorophyta. In addition, carotenogenesis pathways are also discussed based on the chemical structures of carotenoids and the known characteristics of carotenogenesis enzymes in other organisms. The specific genes and enzymes for carotenogenesis in algae are not yet known. Most carotenoids bind to membrane-bound pigment-protein complexes, such as reaction centers and light-harvesting complexes. Some carotenoids function in photosynthesis and are briefly summarized. Water-soluble peridinin-chlorophyll a-protein (PCP) and orange carotenoid protein (OCP) have also been characterized. This review is a summary and update from the previous review on the distribution of major carotenoids, primary carotenogenesis pathways, and the characteristics of carotenogenesis enzymes and genes.

The paper presents the results of the study of the effect of cadmium (CdSO4) ions on photosystem II (PSII) photochemical activity. The presence of Cd 2+ ions in the medium was found to significantly inhibit the oxygen evolution in the thylakoid membranes, indicating a clear correlation between the concentration of Cd 2+ ions and the inhibition of PSII photochemical activity. Additionally, the study identified that this inhibition is strongly dependent on the pH level of the medium. The inhibition of PSII photochemical activity occurred quickly at low cadmium concentrations (<1.0 mM), and weakly at higher concentrations (1.0-30 mM). The study also observed that the inhibition of O2 evolution by Cd 2+ ions was time-dependent, with a rapid 20-40% inhibition of oxygen evolution occurring when adding Cd 2+ . The photochemical activity of PSII was inhibited slowly during long-term incubation. The findings imply that PSII contains at least one high-affinity cadmium binding site.

To understand the mechanism of photosynthetic inhibition and generation of reactive oxygen species (ROS) in Symbiodinium types under stress, chemicals such as dichlorophenyl dimethylurea (DCMU) are widely used. Moreover, DCMU and recently menthol were used to generate aposymbiotic cnidarian hosts. While the effects of DCMU on Symbiodinium cells have been extensively studied, no studies have shown the mechanism behind menthol-induced coral bleaching. Moreover, no study has compared the effects of DCMU and menthol treatments on photosystem II (PSII) activity and generation of ROS in different Symbiodinium types. In this study, we utilized five freshly isolated Symbiodinium types (S. minutum (B1), S. goreaui (C1), C3, C15, and S. trenchii (D1a)) to compare the effects of DCMU and menthol treatments. Symbiodinium cells were exposed to DCMU and menthol at different concentrations for 4 h. Results showed that values of the 50% inhibitory concentration (IC 50 ) for PSII inhibition were 0.72∼1.96 mM for menthol-treated cells compared to 29∼74 pM for DCMU-treated cells. Diverse responses of Symbiodinium types were displayed in terms of PSII tolerance to menthol (S. minutum > S. trenchii = C15 > C3 = S. goreaui), and also in the response curves. In contrast, responses were not so diverse when the different types were treated with DCMU. Three of five mentholtreated Symbiodinium types showed instant and significant ROS generation when PSII activity was inhibited, compared to no ROS being generated in DCMU-treated Symbiodinium types. Both results indicated that menthol inhibited Symbiodinium PSII activity through Symbiodinium type-dependent mechanisms, which were also distinct from those with DCMU treatment. This study further confirmed that photosynthetic functions Symbiodinium have diverse responses to stress even within the same clade.

Determination of the thermal tolerance of Symbiodinium using the activation energy for inhibiting photosystem II activity. Zoological Studies 51(2): 137-142. Holobionts with different Symbiodinium clades or subclades display varying levels of thermal tolerance; however, an index to quantify and standardize this difference has not yet been formulated. In this study, the potential for the activation energy (Ea) to inhibit photosystem (PS)II being used to represent the heat tolerance of Symbiodinium was investigated. As the Ea required for PSII heat denaturation increased, the PSII apparatus in the algae remained stable at higher temperatures; thus, PSII activity was maintained at higher temperatures. The Ea was determined by fitting the kinetics data of the decrease in the maximum quantum yield (Fv/Fm) of freshly isolated Symbiodinium (FIS) at an elevated temperature to the Arrhenius equation. The results indicated that the PSII activity of FIS linearly decreased with an increase in the incubation time under thermal stress (r 2 > 0.95), and the rate of PSII denaturation significantly fit the Arrhenius equation (r 2 > 0.95) after a logarithmic transformation. Comparisons between 5 Symbiodinium subclades indicated that D1a, known as the most heat-tolerant subclade, showed the highest Ea value (348 ± 16 kJ/mole), which was significantly (p < 0.05) higher than those of B1, C1, C3, and C15 (126-262 kJ/mole). The reliability of the Ea calculation was confirmed by the low coefficient of variation (< 10%), suggesting that it can reliably be used to quantify the thermal tolerance of Symbiodinium.

V.V. conceived the original research plan, designed the experiment with inputs from J.-P.S. and A.G., performed the experiments with support from A.G., C.M., T.M.R., and M.A., analyzed the data and independently wrote the article with extensive inputs from A.G. and J.-P.S.; C.M. carried out the lipid analyses and analyzed the data; A.G. performed the statistical analyses and wrote the article; T.M.R. provided technical assistance to C.M.; M.A. performed the transmission electron microscopy study; A.W. and P.S.-K. supervised and complemented the writing; J.-P.S. conceived the original research plan, supervised the experiments, and wrote the article.

The effects of short-time fumigation (0-60 min) of intact spinach leaves with S0 2 (2 ppm) on the photosynthetic apparatus were investigated. A rather high S0 2 concentration was applied to monitor immediate effects on the fluorescence behaviour with the influence of repair processes or secondary types of damage being minimized. Three different types of in vivo chlorophyll fluorescence measurements were used: Rapid induction kinetics (Kautsky effect), slow induction kinetics with repetitive application of saturation pulses (saturation pulse method), and decay kinetics following a single turnover saturating flash. The slow induction kinetics with repetitive application of saturation pulses reacts in the most sensitive way indicating a primary damage at the level of the enzymatic reactions of the Calvin cycle. It is suggested that stromal acidification upon S0 2 uptake interferes with light activation of Calvin cycle enzymes. With longer fumigation times also damage at the level of photosystem II becomes apparent: A decrease in variable fluorescence yield reflects a lowering of photosystem II quantum yield, and the slowing down of fluorescence relaxation kinetics reveals an effect on the secondary electron transport from Q A to Q B . The detrimental effects of S0 2 depend to a great extent on the application of light during fumigation. Besides a light requirement for S0 2 uptake by stomata opening also the possibility of photoinhibitory damage is discussed. The susceptibility of leaves to photoinhibition may increase with a lowering of Calvin cycle activity by S0 2 .

Significance Metal clusters play important roles in a wide variety of proteins. In cyanobacteria, algae, and plants, photosystem II uses light energy to oxidize water and release O 2 at an active site that contains 1 calcium and 4 manganese atoms. This cluster must be built within the protein environment through a process known as photoassembly. Through experiments and simulations, we found that the efficiency of photoassembly was highly dependent on protons and chloride. Surprisingly, when the solvent was switched from H 2 O to deuterated water, D 2 O, the yield of photoassembly was higher. These results provide insights into the stepwise mechanism of photoassembly that can inform synthesis and repair strategies being developed for artificial photosynthesis technologies.

Photosystem I (PSI) is a multi-subunit, pigment-protein complex that catalyzes lightdriven electron transfer (ET) in its bi-branched reaction center (RC). Recently it was suggested that the initial charge separation (CS) event can take place independently within each ec2/ec3 chlorophyll pair. In order to improve our understanding of this phenomenon, we have generated new mutations in the PsaA and PsaB subunits near the electron transfer cofactor 2 (ec2 chlorophyll). PsaA-Asn604 accepts a hydrogen bond from the water molecule that is the axial ligand of ec2 B and the case is similar for PsaB-Asn591 and ec2 A . The second set of targeted sites was PsaA-Ala684 and PsaB-Ala664, whose methyl groups are present near ec2 A and ec2 B , respectively. We generated a number of mutants by targeting the selected protein residues. These mutations were expected to alter the energetics of the primary charge separation event. The PsaA-A684N mutants exhibited increased ET on the B-branch as compared to the A-branch in both in vivo and in vitro conditions. The transient electron paramagnetic resonance (EPR) spectroscopy revealed the formation of increased B-side radical pair (RP) at ambient and cryogenic temperatures. The ultrafast transient absorption spectroscopy and fluorescence decay measurement of the PsaA-A684N and PsaB-A664N showed a slight deceleration of energy trapping. Thus making mutations near ec2 on each branch resulted into modulation of the charge separation process. In the second set of mutants, where ec2 cofactor was target by substitution of PsaA-Asn604 or PsaB-Asn591 to other amino acids, a drop in energy trapping was observed. The quantum

Acaryochloris marina is an oxygenic cyanobacterium that utilizes far-red light for photosynthesis. It has an expanded genome, which helps in its adaptability to the environment, where it can survive on low energy photons. Its major light absorbing pigment is chlorophyll d and it has α-carotene as a major carotenoid. Light harvesting antenna includes the external phycobilin binding proteins, which are hexameric rods made of phycocyanin and allophycocyanins, while the small integral membrane bound chlorophyll binding proteins are also present. There is specific chlorophyll a molecule in both the reaction center of Photosystem I (PSI) and PSII, but majority of the reaction center consists of chlorophyll d. The composition of the PSII reaction center is debatable especially the role and position of chlorophyll a in it. Here we discuss the photosystems of this bacterium and its related biology.

Optical sensors for fluorescence of chlorophyll a (f-Chl a ) and phycocyanin (f-PC) are increasingly used as a proxy for biomass of algae and cyanobacteria, respectively. They provide measurements at highfrequency and modest cost. These sensors require site-specific calibration due to a range of interferences. Light intensity affects the fluorescence yield of cyanobacteria and algae through light harvesting regulation mechanisms, but is often neglected as a potential source of error for in-situ f-Chl a and f-PC measurements. We hypothesised that diel light variations would induce significant f-Chl a and f-PC suppression when compared to dark periods. We tested this hypothesis in a controlled experiment using three commercial fluorescence probes which continuously measured f-Chl a and f-PC from a culture of the cyanobacterium Dolichospermum variabilis as well as f-Chl a from a culture of the green alga Ankistrodesmus gracilis in a simulated natural light regime. Under light, all devices showed a significant ( p < 0.01) suppression of f-Chl a and f-PC compared to measurements in the dark. f-Chl a decreased by up to 79% and f-PC by up to 59% at maximum irradiance compared to dark-adapted periods. Suppression levels were higher during the second phase of the diel cycle (declining light), indicating that quenching is dependent on previous light exposure. Diel variations in light intensity must be considered as a significant source of bias for fluorescence probes used for algal monitoring. This is of high relevance as most monitoring activities take place during daytime and hence f-Chl a and f-PC are likely to be systematically underestimated under bright conditions. Compensation models, design modifications to fluorometers and sampling design are discussed as suitable alternatives to overcome light-induced fluorescence quenching.

Flash-induced redox reactions in spinach PS II core particles were investigated with absorbance difference spectroscopy in the UV-region and EPR spectroscopy. In the absence of artificial electron acceptors, electron transport was limited to a single turnover. Addition of the electron acceptors DCBQ and ferricyanide restored the characteristic period-four oscillation in the UV absorbance associated with the S-state cycle, but not the period-two oscillation indicative of the alternating appearance and disappearance of a semiquinone at the QB-site. In contrast to PS II membranes, all active centers were in state S 1 after dark adaptation. The absorbance increase associated with the S-state transitions on the first two flashes, attributed to the Z+S1--> ZS 2 and Z+S2----> ZS 3 transitions, respectively, had half-times of 95 and 380/xs, similar to those reported for PS II membrane fragments. The decrease due to the Z÷S3---> ZS 0 transition on the third flash had a half-time of 4.5 ms, as in salt-washed PS II membrane fragments. On the fourth flash a small, unresolved, increase of less than 3 /xs was observed, which might be due to the Z+S0--->ZS~ transition. The deactivation of the higher S-states was unusually fast and occurred within a few seconds and so was the oxidation of S O to S~ in the dark, which had a half-time of 2-3 min. The same lifetime was found for tyrosine D ÷, which appeared to be formed within milliseconds after the first flash in about 10% inactive centers and after the third and later flashes by active centers in Z+S3.

Tyrosine (Y D ) in the D2 reaction centre polypeptide of photosystem II (PSII) is redox-active and, under illumination, forms a dark-stable radical Y Å D . The origin of its stability and the functional role of Y Å D are not well understood. For understanding the electronic structure and reactivity of Y Å D , it is crucial to unambiguosly establish its hyperfine structure. There is considerable variation in the hyperfine data of Y Å D and their interpretation in literature. In the present study, the hyperfine structure of tyrosine radical Y Å D in PSII was probed by EPR in conjunction with carefully designed site specific isotope labelling. A comprehensive series of different selectively 2 H-, 13 C-or 17 O-labeled tyrosine were synthesized and incorporated in Spirodela oligorrhiza with more than 95% enrichment. The 13 C-and 17 O-hyperfine interactions were obtained from spectral simulations. From the anisotropy of the hyperfine interactions the spin densities at all phenoxyl ring positions were precisely obtained. Comparison of the absolute differences in individual spin densities between Y Å D and neutral tyrosine radical in vitro with those of computationally calculated spin densities yield excellent agreement for a well ordered hydrogen bond between Y Å D and the surrounding protein matrix with a bond length of 1.5 A A. Enantioselective labeling confirms that the b-methylene hydrogens of Y Å D in S. oligorrhiza are oriented in a highly constrained specific position making Y Å D strongly immobilized, thereby ensuring a firm hydrogen bond of the phenoxyl oxygen to the protein matrix.

Chlorophyll (Chl)-containing light-harvesting complexes (LHCs) in chloroplasts of plant and algal cells usually include an oxidized Chl (Chl b or c) in addition to Chl a. Oxidation of peripheral groups on the tetrapyrrole structure increases the Lewis acid strength of the central Mg atom. We propose that the resulting stronger coordination bonds between oxidized Chls and ligands in LHC apoproteins (LHCPs) stabilize the initial intermediates and thus promote assembly of LHCs within the chloroplast envelope.