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Pyruvate, the end product of glycolysis, is an important metabolite for the tricarboxylic acid (TCA) cycle and lipid metabolism. Pyruvate is converted to acetyl-CoA, which enters the TCA cycle for citrate formation. Citrate is the precursor for isocitrate that undergoes oxidative decarboxylation reaction forming α-ketoglutarate (α-KG) coupled with NADP+ reduction catalyzed by the enzyme isocitrate dehydrogenase (IDH). In cyanobacteria, α-KG donates its carbon skeleton to the glutamine synthetase-glutamine oxoglutarate aminotransferase (GS-GOGAT) pathway, thereby linking carbon and nitrogen metabolisms. In this study, the effect of pyruvate on IDH activity, its protein content, and its mRNA expression level were analyzed. Amongst the concentrations studied (100 µM, 250 µM, and 500 µM), in 250 µM externally supplied pyruvate, IDH registered the highest activity as well as maximum protein and mRNA contents when compared to the control. At this concentration, all biochemical parameters of nitrogen and carbon metabolisms- heterocyst frequency, nitrogenase, and glutamine synthetase activities, photosynthetic pigments, OEC (oxygen evolving complex) activity, carbohydrate content, proteins, and total biomass were also found to be highest. However, at an exposure to 500 µM pyruvate, there was a reduction in all the above-mentioned parameters indicating that although pyruvate is a key metabolite in the cellular metabolism, it tends to become toxic beyond this concentration. The compromised state of the cells was also evident from the micrographs developed from SEM (Scanning electron microscope) and TEM (Transmission electron microscope) studies under 500 µM pyruvate exposure. [ABSTRACT FROM AUTHOR]

This manuscript describes the syntheses, structures and magnetism of MnIII–CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes. [ABSTRACT FROM AUTHOR]

This manuscript describes the syntheses, structures and magnetism of MnIII–CaII/SrII complexes which are compositionally relevant in the context of the oxygen evolving complex (OEC) of photosystem II (PS II). A series of trimetallic tetraoxo complexes containing redox-inactive metal ions CaII or SrII were synthesized using a tetranucleating ligand framework. The structural characteristics of these complexes, with the oxido ligands bridging the redox-inactive metals and the manganese centres, make them particularly relevant to biological and heterogeneous metal-oxido clusters. Electrochemical studies of these compounds show that the reduction potentials are highly dependent upon the Lewis acidity of the redox-inactive metal, identifying the chemical basis for the observed differences in electrochemistry. This correlation provides insights into the role of the CaII/SrII ion in modulating the redox potential of the OEC and of other redox-inactive ions in tuning the redox potentials of other metal-oxide electrocatalysts. Temperature dependent magnetic measurements have also been performed for the complexes. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) is an important molecule in regulating plant growth, development and photosynthetic performance. This study investigates the impact of varying concentrations (0–300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the photosynthetic apparatus in sorghum (Sorghum bicolor L. Albanus) and maize (Zea mays L. Kerala) under physiological conditions. Analysis of the chlorophyll fluorescence signals (using PAM and the JIP-test) revealed an increased amount of open PSII reaction centers (qP increased), but it did not affect the number of active reaction centers per PSII antenna chlorophyll (RC/ABS). In addition, the smaller SNP concentrations (up to 150 μM) alleviated the interaction of QA with plastoquine in maize, while at 300 μM it predominates the electron recombination on QAQB−, with the oxidized S2 (or S3) states of oxygen evolving in complex ways in both studied plant species. At the same time, SNP application stimulated the electron flux-reducing end electron acceptors at the PSI acceptor side per reaction center (REo/RC increased up to 26%) and the probability of their reduction (φRo increased up to 20%). An increase in MDA (by about 30%) and H2O2 contents was registered only at the highest SNP concentration (300 µM). At this concentration, SNP differentially affected the amount of P700+ in studied plant species, i.e., it increased (by 10%) in maize but decreased (by 16%) in sorghum. The effects of SNP on the functions of the photosynthetic apparatus were accompanied by an increase in carotenoid content in both studied plants. Additionally, data revealed that SNP-induced changes in the photosynthetic apparatus differed between maize and sorghum, suggesting species specificity for SNP's impact on plants. [ABSTRACT FROM AUTHOR]

Aquatic organisms are exposed to several compounds that occur in mixtures in the environment. Thus, it is important to investigate their impacts on organisms because these combined effects can be potentiated. Cobalt (Co) and nickel (Ni) are metals that occur in the environment and are used in human activities. To the best of our knowledge, there are no studies that investigated the combined effects of these metals on a freshwater Chlorophyceae. Therefore, this study analyzed the isolated and combined effects of Co and Ni in cell density, physiological and morphological parameters, reactive oxygen species (ROS), carbohydrates and photosynthetic parameters of the microalga Raphidocelis subcapitata. Data showed that Co affected the cell density from 0.25 mg Co L−1; the fluorescence of chlorophyll a (Chl a) (0.10 mg Co L−1); ROS production (0.50 mg Co L−1), total carbohydrates and efficiency of the oxygen evolving complex (OEC) at all tested concentrations; and the maximum quantum yield (ΦM) from 0.50 mg Co L−1. Ni exposure decreased ROS and cell density (0.35 mg Ni L−1); altered Chl a fluorescence and carbohydrates at all tested concentrations; and did not alter photosynthetic parameters. Regarding the Co-Ni mixtures, our data best fitted the concentration addition (CA) model and dose-ratio dependent (DR) deviation in which synergism was observed at low doses of Co and high doses of Ni and antagonism occurred at high doses of Co and low doses of Ni. The combined metals affected ROS production, carbohydrates, ΦM, OEC and morphological and physiological parameters. [ABSTRACT FROM AUTHOR]

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help in understanding the mechanism of O–O bond formation presumably occurring in the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscore the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species [(dpaq)MnIV(O)]+ (2) through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one electron to a formal manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital, whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from the metal and the ligand, leaving a single electron in the quinoline-dominant orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal–ligand (quinoline)-based oxidation is proposed to generate a formal Mn(VI) species (4), a non-heme analogue of the species ‘compound I’, formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O–O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide. [ABSTRACT FROM AUTHOR]

Green alga Chlorella ohadii is known for its ability to carry out photosynthesis under harsh conditions. Using cryogenic electron microscopy (cryoEM), we obtained a high-resolution structure of PSII at 2.72 Å. This structure revealed 64 subunits, which encompassed 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, a unique subunit arrangement was observed to protect the oxygen-evolving complex. This arrangement involved PsbO (OEE1), PsbP (OEE2), PsbB, and PsbU (a homolog of plant OEE3). PsbU interacted with PsbO, PsbC, and PsbP, thereby stabilizing the shield of the oxygen-evolving complex. Significant changes were also observed at the stromal electron acceptor side. PsbY, identified as a transmembrane helix, was situated alongside PsbF and PsbE, which enclosed cytochrome b559. Supported by the adjacent C-terminal helix of Psb10, these four transmembrane helices formed a bundle that shielded cytochrome b559 from the surrounding solvent. Moreover, the bulk of Psb10 formed a protective cap, which safeguarded the quinone site and likely contributed to the stacking of PSII complexes. Based on our findings, we propose a protective mechanism that prevents QB (plastoquinone B) from becoming fully reduced. This mechanism offers insights into the regulation of electron transfer within PSII. [ABSTRACT FROM AUTHOR]

Marine phytoplankton contribute substantially to the global flux of carbon from the atmosphere to the deep ocean. Sea surface temperatures will inevitably increase in line with global climate change, altering the performance of marine phytoplankton. Differing sensitivities of photosynthesis and respiration to temperature, will likely shift the strength of the future oceanic carbon sink. To further clarify the molecular mechanisms driving these alterations in phytoplankton function, shotgun proteomic analysis was carried out on the globally-occurring coccolithophore Emiliania huxleyi exposed to moderate- (23°C) and elevated-(28°C) warming. Compared to the control (17°C), growth of E. huxleyi increased under elevated temperatures, with higher rates recorded under moderate-relative to elevated-warming. Proteomic analysis revealed a significant modification of the E. huxleyi cellular proteome as temperatures increased: at lower temperature, ribosomal proteins and photosynthetic machinery appeared abundant, as rates of protein translation and photosynthetic performance are restricted by low temperatures. As temperatures increased, evidence of heat stress was observed in the photosystem, characterized by a relative down-regulation of the Photosystem II oxygen evolving complex and ATP synthase. Acclimation to elevated warming (28°C) revealed a substantial alteration to carbon metabolism. Here, E. huxleyi made use of the glyoxylate cycle and succinate metabolism to optimize carbon use, maintain growth and maximize ATP production in heat-damaged mitochondria, enabling cultures to maintain growth at levels significantly higher than those recorded in the control (17°C). Based on the metabolic changes observed, we can predict that warming may benefit photosynthetic carbon fixation by E. huxleyi in the sub-optimal to optimal thermal range. Past the thermal optima, increasing rates of respiration and costs of repair will likely constrain growth, causing a possible decline in the contribution of this species to the oceanic carbon sink depending on the evolvability of these temperature thresholds. [ABSTRACT FROM AUTHOR]

Dramatic climate change has led to an increase in the intensity and frequency of drought episodes and, together with the high light conditions of the Mediterranean area, detrimentally influences crop production. Salicylic acid (SA) has been shown to supress phototoxicity, offering photosystem II (PSII) photoprotection. In the current study, we attempted to reveal the mechanism by which SA is improving PSII efficiency in oregano seedlings under moderate drought stress (MoDS). Foliar application of SA decreased chlorophyll content under normal growth conditions, but under MoDS increased chlorophyll content, compared to H2O-sprayed oregano seedlings. SA improved the PSII efficiency of oregano seedlings under normal growth conditions at high light (HL), and under MoDS, at both low light (LL) and HL. The mechanism by which, under normal growth conditions and HL, SA sprayed oregano seedlings compared to H2O-sprayed exhibited a more efficient PSII photochemistry, was the increased (17%) fraction of open PSII reaction centers (qp), and the increased (7%) efficiency of these open reaction centers (Fv′/Fm′), which resulted in an enhanced (24%) electron transport rate (ETR). SA application under MoDS, by modulating chlorophyll content, resulted in optimized antenna size and enhanced effective quantum yield of PSII photochemistry (ΦPSII) under both LL (7%) and HL (25%), compared to non-SA-sprayed oregano seedlings. This increased effective quantum yield of PSII photochemistry (ΦPSII) was due to the enhanced efficiency of the oxygen evolving complex (OEC), and the increased fraction of open PSII reaction centers (qp), which resulted in an increased electron transport rate (ETR) and a lower amount of singlet oxygen (1O2) production with less excess excitation energy (EXC). [ABSTRACT FROM AUTHOR]

Summary: Singlet oxygen (1O2) is a harmful species that functions also as a signaling molecule. In chloroplasts, 1O2 is produced via charge recombination reactions in photosystem II, but which recombination pathway(s) produce triplet Chl and 1O2 remains open. Furthermore, the role of 1O2 in photoinhibition is not clear.We compared temperature dependences of 1O2 production, photoinhibition, and recombination pathways.1O2 production by pumpkin thylakoids increased from −2 to +35°C, ruling out recombination of the primary charge pair as a main contributor. S2QA− or S2QB− recombination pathways, in turn, had too steep temperature dependences. Instead, the temperature dependence of 1O2 production matched that of misses (failures of the oxygen (O2) evolving complex to advance an S‐state). Photoinhibition in vitro and in vivo (also in Synechocystis), and in the presence or absence of O2, had the same temperature dependence, but ultraviolet (UV)‐radiation‐caused photoinhibition showed a weaker temperature response.We suggest that the miss‐associated recombination of P680+QA− is the main producer of 1O2. Our results indicate three parallel photoinhibition mechanisms. The manganese mechanism dominates in UV radiation but also functions in white light. Mechanisms that depend on light absorption by Chls, having 1O2 or long‐lived P680+ as damaging agents, dominate in red light. [ABSTRACT FROM AUTHOR]

Ever since the discovery that Mn was required for oxygen evolution in plants by Pirson in 1937 and the period‐four oscillation in flash‐induced oxygen evolution by Joliot and Kok in the 1970s, understanding of this process has advanced enormously using state‐of‐the‐art methods. The most recent in this series of innovative techniques was the introduction of X‐ray free‐electron lasers (XFELs) a decade ago, which led to another quantum leap in the understanding in this field, by enabling operando X‐ray structural and X‐ray spectroscopy studies at room temperature. This review summarizes the current understanding of the structure of Photosystem II (PS II) and its catalytic centre, the Mn4CaO5 complex, in the intermediate Si (i = 0–4)‐states of the Kok cycle, obtained using XFELs. [ABSTRACT FROM AUTHOR]

The understanding of light‐induced biological water oxidation in oxygenic photosynthesis is of great importance both for biology and (bio)technological applications. The chemically difficult multistep reaction takes place at a unique protein‐bound tetra‐manganese/calcium cluster in photosystem II whose structure has been elucidated by X‐ray crystallography (Umena et al. Nature 2011, 473, 55). The cluster moves through several intermediate states in the catalytic cycle. A detailed understanding of these intermediates requires information about the spatial and electronic structure of the Mn4Ca complex; the latter is only available from spectroscopic techniques. Here, the important role of Electron Paramagnetic Resonance (EPR) and related double resonance techniques (ENDOR, EDNMR), complemented by quantum chemical calculations, is described. This has led to the elucidation of the cluster's redox and protonation states, the valence and spin states of the manganese ions and the interactions between them, and contributed substantially to the understanding of the role of the protein surrounding, as well as the binding and processing of the substrate water molecules, the O‐O bond formation and dioxygen release. Based on these data, models for the water oxidation cycle are developed. [ABSTRACT FROM AUTHOR]

Photosystem II (PSII) of grapevine leaves is easily damaged under heat stress, but no such injury is observed when the leaves are heated in low light. To elucidate the mechanisms, we compared the photosynthetic characteristics of grapevine seedlings under heat treatments (42 °C) for 4 h in the dark or low light (200 μmol m−2 s−1). At 42 °C in the dark, the PSII maximum quantum yield (Fv/Fm) decreased significantly with the increase in time but did not change much in low light. The JIP (chlorophyll a fluorescence rise kinetics) test results showed that low light significantly alleviated the damage to the oxygen evolving complexes (OECs; the K-step was less visible) by heat stress. Further, in the presence of de novo D1 protein synthesis inhibitor chloramphenicol, Fv/Fm did not differ significantly between dark and light treatments under heat stress. The 50% re-reduction (RR50) of P700+ on cessation of far-red illumination was faster after light treatment than that in the dark. After exposure to 25 °C in a low light for 15 min, Y(NO) (the constitutive non-regulatory non-photochemical quenching) treated by heat stress and darkness was higher than that by heat stress and light. Overall, our results suggested that enhanced CEFs around PSI in low light could assist PSII against heat damage by maintaining the rate of PSII repair and inhibiting the non-radiative charge recombination in PSII reaction centers. [ABSTRACT FROM AUTHOR]

We report the failure of coupled-cluster valence-bond (CCVB) theory with two-pair configurations [D. W. Small and M. Head-Gordon, J. Chem. Phys. 130, 084103 (2009)] for open-shell (OS) spin-frustrated systems where including three-pair configurations is necessary to properly describe strong spin-correlations. We extend OS-CCVB by augmenting the model with three-pair configurations within the independent amplitude approximation. The resulting new electronic structure model, OS-CCVB+i3, involves only a quadratic number of independent wavefunction parameters. It includes the recently reported closed-shell CCVB+i3 as a special case. Its cost is dominated by integral transformations, and it is capable of breaking multiple bonds exactly for all systems examined so far. The strength of OS-CCVB+i3 is highlighted in realistic systems including the [CaMn3O4] cubane subunit of the oxygen-evolving complex and a molecular magnet with the [Cr9] core unit as well as model systems such as N3, V3O3, and P5. We show that OS-CCVB+i3 is only slightly dependent on the underlying perfect-pairing reference, while OS-CCVB shows a stronger dependence. We also emphasize the compactness of the OS-CCVB+i3 wavefunction compared to the heat-bath configuration interaction wavefunction, a recently introduced soft exponential-scaling approach. [ABSTRACT FROM AUTHOR]

In this study, the differences in chlorophyll fluorescence transient (OJIP) and modulated 820 nm reflection (MR820) of cucumber leaves were probed to demonstrate an insight into the precise influence of melatonin (MT) on cucumber photosystems under low temperature stress. We pre-treated cucumber seedlings with different levels of MT (0, 25, 50, 100, 200, and 400 µmol ⋅ L-1) before imposing low temperature stress (10 °C/6 °C). The results indicated that moderate concentrations of MT had a positive effect on the growth of low temperature-stressed cucumber seedlings. Under low temperature stress conditions, 100 µmol ⋅ L-1 (MT 100) improved the performance of the active photosystem II (PSII) reaction centers (PIabs), the oxygen evolving complex activity (OEC centers) and electron transport between PSII and PSI, mainly by decreasing the L-band, K-band, and G-band, but showed differences with different duration of low temperature stress. In addition, these indicators related to quantum yield and energy flux of PSII regulated by MT indicated that MT (MT 100) effectively protected the electron transport and energy distribution in the photosystem. According to the results of WO-I ≥ 1 and MR820 signals, MT also affected PSI activity. MT 100 decreased the minimal value of MR/MRO and the oxidation rate of plastocyanin (PC) and PSI reaction center (P700) (Vox), while increased ΔMRslow/MRO and deoxidation rates of PC+ and P700+ (Vred). The loss of the slow phase of MT 200 and MT 400-treated plants in the MR820 kinetics was due to the complete prevention of electron movement from PSII to re-reduce the PC+ and P700+. These results suggest that appropriate MT concentration (100 mmol ⋅ L-1) can improve the photosynthetic performance of PS II and electron transport from primary quinone electron acceptor (QA) to secondary quinone electron acceptor (QB), promote the balance of energy distribution, strengthen the connectivity of PSI and PSII, improve the electron flow of PSII via QA to PC+ and P700+ from reaching PSI by regulating multiple sites of electron transport chain in photosynthesis, and increase the pool size and reduction rates of PSI in low temperature-stressed cucumber plants, All these modifications by MT 100 treatment promoted the photosynthetic electron transfer smoothly, and further restored the cucumber plant growth under low temperature stress. Therefore, we conclude that spraying MT at an appropriate concentration is beneficial for protecting the photosynthetic electron transport chain, while spraying high concentrations of MT has a negative effect on regulating the low temperature tolerance in cucumber. [ABSTRACT FROM AUTHOR]

Zostera marina , a critical keystone marine angiosperm species in coastal seagrass meadows, possesses a photosensitive oxygen evolving complex (OEC). In harsh environments, the photoinactivation of the Z. marina OEC may lead to population declines. However, the factors underlying this photosensitivity remain unclear. Therefore, this study was undertaken to elucidate the elements contributing to Z. marina OEC photosensitivity. Our results demonstrated a gradual decrease in photosystem II performance towards shorter wavelengths, especially blue light and ultraviolet radiation. This phenomenon was characterized by a reduction in F v / F m and the rate of O 2 evolution, as well as increased fluorescence at 0.3 ms on the OJIP curve. Furthermore, exposure to shorter light wavelengths and longer exposure durations significantly reduced the relative abundance of the OEC peripheral proteins, indicating OEC inactivation. Analyses of light-screening substances revealed that carotenoids, which increased most notably under 420 nm light, might primarily serve as thermal dissipators instead of efficient light filters. In contrast, anthocyanins reacted least to short-wavelength light, in terms of changes to both their content and the expression of genes related to their biosynthesis. Additionally, the levels of aromatically acylated anthocyanins remained consistent across blue-, white-, and red-light treatments. These findings suggest that OEC photoinactivation in Z. marina may be linked to inadequate protection against short-wavelength light, a consequence of insufficient synthesis and aromatic acylation modification of anthocyanins. • The photoinactivation of Z. marina oxygen-evolving complex is wavelength dependent. • Light exposure causes differential expression of anthocyanin biosynthetic genes. • Anthocyanins of Z. marina have a low efficiency in synthesis under blue light. • The low efficiency of anthocyanins may be related to the CRY2 photoreceptor absence. [ABSTRACT FROM AUTHOR]

Compared with only the salt-treated plants, all of these growth parameters of Si-treated plants were improved in both cotton genotypes, indicating a positive effect of Si on enhancing salt tolerance of cotton. Keywords: salinity; silicon; chloroplast ultrastructure; antioxidant enzymes; photosynthetic performance EN salinity silicon chloroplast ultrastructure antioxidant enzymes photosynthetic performance N.PAG N.PAG 20 08/29/22 20220801 NES 220801 1. Since plant roots only absorb mono silicic acid (Si (OH) SB 4 sb , pH < 9), and Si in the soil is mainly in the form of silicate or aluminum silicate, exogenous Si as fertilizer is applied despite the abundant Si in soil [[11], [21]]. The application of the Si fertilizer markedly lowered the K-point fluorescence value of the salt-stressed cotton seedling (Figure 9D and Figure 10E), indicating that Si can alleviate the damage of oxygen-evolving complexes caused by salt stress to a certain extent, and relieve the suppression of the photosynthetic electron production and transport due to salt stress [[44]]. [Extracted from the article]

Salicylic acid (SA), an essential plant hormone, has received much attention due to its role in modulating the adverse effects of biotic and abiotic stresses, acting as an antioxidant and plant growth regulator. However, its role in photosynthesis under non stress conditions is controversial. By chlorophyll fluorescence imaging analysis, we evaluated the consequences of foliar applied 1 mM SA on photosystem II (PSII) efficiency of tomato (Solanum lycopersicum L.) plants and estimated the reactive oxygen species (ROS) generation. Tomato leaves sprayed with 1 mM SA displayed lower chlorophyll content, but the absorbed light energy was preferentially converted into photochemical energy rather than dissipated as thermal energy by non-photochemical quenching (NPQ), indicating photoprotective effects provided by the foliar applied SA. This decreased NPQ, after 72 h treatment by 1 mM SA, resulted in an increased electron transport rate (ETR). The molecular mechanism by which the absorbed light energy was more efficiently directed to photochemistry in the SA treated leaves was the increased fraction of the open PSII reaction centers (qp), and the increased efficiency of open reaction centers (Fv'/Fm'). SA induced a decrease in chlorophyll content, resulting in a decrease in non-regulated energy dissipated in PSII (ΦNO) under high light (HL) treatment, suggesting a lower amount of triplet excited state chlorophyll (3Chl*) molecules available to produce singlet oxygen (1O2). Yet, the increased efficiency, compared to the control, of the oxygen evolving complex (OEC) on the donor side of PSII, associated with lower formation of hydrogen peroxide (H2O2), also contributed to less creation of ROS. We conclude that under non stress conditions, foliar applied SA decreased chlorophyll content and suppressed phototoxicity, offering PSII photoprotection; thus, it can be regarded as a mechanism that reduces photoinhibition and photodamage, improving PSII efficiency in crop plants. [ABSTRACT FROM AUTHOR]

Nickel acts as an essential trace nutrient or toxicant for organisms, depending on its concentration. The increased concentrations of nickel, due to anthropogenic activity, in the aquatic environment are potential threats to aquatic organisms. However, the knowledge on toxic mechanisms of nickel to microalgae remains incompletely understood. In the present study, we investigated the toxic effects of nickel in the cosmopolitan diatom Phaeodactylum tricornutum via evaluation of physiological and transcriptome responses. The results showed that the median effective concentration-72 h (EC50-72 h) and EC50-96 h of nickel was 2.48 ± 0.33 and 1.85 ± 0.17 mg/L, respectively. The P. tricornutum cell abundance and photosynthesis significantly decreased by 1 mg/L of nickel. Results from photosynthetic parameters including efficiency of the oxygen evolving complex (OEC) of photosystem II (PSII) (Fv/F0), maximum photosynthetic efficiency of PS II (Fv/Fm), electron transport rate (ETR), actual photosynthetic efficiency of PS II (Y(II)), non-photochemical quenching (NPQ), and photochemical quenching (qP) indicated that OEC of PS II might be impaired by nickel. The transcriptome data also reveal that OEC apparatus coding gene PS II oxygen-evolving enhancer protein 2 (PsbP) was regulated by nickel. Moreover, induced reactive oxygen species (ROS) production and chlorophyll a content were also detected under nickel stress. Transcriptome analysis revealed that nickel affected a variety of differentially expressed genes (DEGs) that involved in redox homeostasis, nitrogen metabolisms, fatty acids, and DNA metabolism. However, thiol-disulfide redox system might play important roles in nickel-induced oxidative stress resistance. This study improved the understanding of the toxic effect of nickel on the diatom P. tricornutum. [ABSTRACT FROM AUTHOR]

A method that separately quantifies the PSII with inactive oxygen-evolving complex (OEC) and active D1 retaining the primary quinone acceptor (QA)-reducing activity from the PSII with damaged D1 in the leaf was developed using PAM fluorometry. It is necessary to fully reduce QA to obtain Fm, the maximum fluorescence. However, QA in PSII with inactive OEC and active D1 would not be fully reduced by a saturating flash. We used the acceptor-side inhibitor DCMU to fully reduce QA. Leaves of cucumber (Cucumis sativus L.) were chilled at 4°C in dark or illuminated with UV-A to selectively inactivate OEC. After these treatments, Fv/Fm, the maximum quantum yield, in the leaves vacuum-infiltrated with DCMU were greater than those in water-infiltrated leaves. In contrast, when the leaves were illuminated by red light to photodamage D1, Fv/Fm did not differ between DCMU- and water-infiltrated leaves. These results indicate relevance of the present evaluation of the fraction of PSII with inactive OEC and active D1. Several examinations in the laboratory and glasshouse showed that PSII with inactive OEC and active D1 was only rarely observed. The present simple method would serve as a useful tool to clarify the details of the PSII photoinhibition. [ABSTRACT FROM AUTHOR]

The aim of the paper was to synthesize Mn/Ca entities that are structurally similar to the reported oxygen‐evolving complex (OEC) of photosystem II (PSII). Here we have ventured Schiff base combinations of racemic (±)‐3‐Aminopropane‐1,2‐diol and Salicyladehyde (H3L1)/o‐Vanillin (H3L2) with varied ratios of ligand : CaIIsalt : MnIIIsalt. Interestingly, H3L2 : CaCl2 : MnCl2 (2 : 1 : 1) afforded [{MnIII2CaII2(HL2)2Cl2(μ3‐OH)2(H2O)6}Cl2 ⋅ 2H2O] (3) whereas H3L1 : CaCl2 : Mn(acetate)2 (2 : 1 : 1) and H3L1 : CaCl2 : MnCl2 (2 : 1 : 2), generated [{MnIV(HL1)2}2 ⋅ 6H2O](1) and [{MnIII3MnII(HL1)3(μ2‐Cl)3(μ3‐O)} ⋅ 2CH3OH ⋅ 2H2O] (2) respectively. More importantly, we have been able to get a structure which offers partial similarity to reported structure of the OEC. Variable temperature magnetic susceptibility measurements have been performed in the 1.8–30 K region for all the compounds. Overall, these studies provide a critical understanding for the rational design and construction of synthetic OEC. [ABSTRACT FROM AUTHOR]

Molecular hydrogen (H2) is considered an ideal energy vector and a clean fuel, due to its zero-carbon combustion. Nevertheless, despite hydrogen is the most and one of the most abundant elements in the universe and in earth crust, respectively, it is always combined with other elements in our planet and never appears in its elemental state. This means that H2 must be produced through, almost always, endergonic processes, whose sustainability depend not only on the starting material but also on the source of energy necessary for these processes to occur. Colors have been assigned to identify the level of sustainability of H2 production with the green one indicating H2 produced from water using a renewable source of energy, preferably sunlight. Redox water splitting (WS) into H2 (hydrogen evolution reaction, HER) and O2 (oxygen evolution reaction, OER) is, nevertheless, an extremely difficult process not only from the thermodynamic but also from the kinetic point of view. Relevant kinetic barriers are present in both sides of the redox process, especially in OER. For this reason, performing WS in an efficient manner requires the development of active and robust catalysts capable of offering alternative reaction pathways to WS, lowering down the unfavorable kinetic barriers and thus maximizing the energy conversion efficiency. Inspiration for developing efficient catalysts for HER and OER has traditionally derived from Nature, who, over the course of many billions of years, according to the evolutionary theory, has assembled two molecular catalytic pools, namely oxygen evolving complex and ferredoxin/ferredoxin NADP+ reductase, which offer viable kinetic pathways to both OER and reduction of NADP+ (the "biological form" of H2). In reality, after several attempts of mimicking natural catalysts, the efforts of the researchers have been addressed to different molecular systems, which exhibit best performances, unfortunately often based on noble-metal atoms, especially for OER. In this contribution we review the journey of the development of molecular catalysts for both HER and the OER, highlighting selected systems, which have brought us to the current level of knowledge. [ABSTRACT FROM AUTHOR]

The α-Ag2WO4 (hexagonal rod-shaped) is a multifunctional material with interesting physical and chemical properties, such as good electronic, photocatalytic, anticancer and microbicidal performance. Considering this, its use can contribute to the presence and accumulation of this compound in freshwater ecosystems. Therefore, the present study investigated the effects of α-Ag2WO4 on the freshwater Chlorophyceae Raphidocelis subcapitata, at the level of cell density, chlorophyll a (Chl a), total carbohydrate contents, and photosynthetic activity (maximum quantum yield and oxygen-evolving complex—OEC). The α-Ag2WO4 reduced cell density by ~ 48% already in the first 24 h of exposure at 31.76 μg L−1 (highest concentration). Moreover, at 31.76 μg L−1, we observed a drastic reduction in the maximum quantum yield, and impact in the oxygen evolving complex at 24 h and 48 h. However, our results indicated a possible recovery of the photosynthetic activity in the surviving algal cells at 72 and 96 h. The contents of chlorophyll a (Chl a) and total carbohydrates decreased significantly (Dunnett's test, p < 0.05) at 4.11, 5.84, 10.55, and 10.67 μg L−1 treatments and increased significantly (Dunnett's test, p < 0.05) at the highest concentration (31.76 μg L−1), which is possibly a mechanism for the algal cells to optimize the amount of energy to be used in the photosynthetic process and maintaining the integrity of the cell wall. This study contributes to clarifying how α-Ag2WO4 interacts with R. subcapitata¸ showing the toxicity mechanism of photosynthetic activity. This can help predict the fate and effect of these composites by providing a basis for their ecological risk assessment. [ABSTRACT FROM AUTHOR]

Nanoparticles have applications in many sectors in the society. ZnWO 4 nanoparticles (ZnWO 4 -NPs) have potential in the fabrication of sensors, lasers, and batteries, and in environmental remediation. Thus, these NPs may reach aquatic ecosystems. However, we still do not know their effects on aquatic biota and, to our knowledge, this is the first study that evaluates the toxicity of ZnWO 4 -NPs in a eukaryotic organism. We evaluated the toxicity of ZnWO 4 -NPs on the green microalga Raphidocelis subcapitata for 96 h, in terms of growth, cell parameters, photosynthesis, and biochemical analysis. Results show that most of Zn was presented in its particulate form, with low amounts of Zn2+, resulting in toxicity at higher levels. The growth was affected from 8.4 mg L−1, with 96h–IC50 of 23.34 mg L−1. The chlorophyll a (Chl a) content increased at 30.2 mg L−1, while the fluorescence of Chl a (FL3-H) decreased at 15.2 mg L−1. We observed increased ROS levels at 44.4 mg L−1. Regarding photosynthesis, the NPs affected the oxygen evolving complex (OEC) and the efficiency of the photosystem II at 22.9 mg L−1. At 44.4 mg L−1 the qP decreased, indicating closure of reaction centers, probably affecting carbon assimilation, which explains the decay of carbohydrates. There was a decrease of qN (non-regulated energy dissipation, not used in photosynthesis), NPQ (regulated energy dissipation) and Y(NPQ) (regulated energy dissipation via heat), indicating damage to the photoprotection system; and an increase in Y(NO), which is the non-regulated energy dissipation via heat and fluorescence. The results showed that ZnWO 4 -NPs can affect the growth and physiological and biochemical parameters of the chlorophycean R. subcapitata. Microalgae are the base of aquatic food chains, the toxicity of emerging contaminants on microalgae can affect entire ecosystems. Therefore, our study can provide some help for better protection of aquatic ecosystems. [Display omitted] • ZnWO 4 -NPs presented low free ion (Zn2+) release. • Cell parameters: 96h-IC50 was 23.34 mg L−1; increased cell size and granularity. • Decrease of Chl a fluorescence and carbohydrates; and increase of Chl a content. • Efficiency loss of the photosystem II and of the oxygen evolving complex. • Increased non-regulated energy dissipation; closure of reaction centers. [ABSTRACT FROM AUTHOR]

Cerium is the most abundant rare earth metal in the earth's crust, and it has deleterious effects on aquatic ecosystems from fertilizer runoff. Scenedesmus obliquus is an oil‐rich microalga that grows rapidly and is sensitive to many kinds of toxins. Given that microalgae are useful indicators of eutrophication and toxic stress, it was found that lower concentrations of cerium (0.50–5.00 mg·L−1) stimulated algal growth and increased chlorophyll a content, whereas higher concentrations (above 50.00 mg·L−1) had an inhibitory effect on algal growth and chlorophyll a content. The algal growth rate and chlorophyll a content peaked at a cerium concentration of 5.00 mg·L−1. Both the donor and acceptor sides of photosystem II (PSII) reaction centers were sensitive to cerium‐induced stress. Specifically, high concentrations of cerium damaged the oxygen evolving complex and PSII reaction center and suppressed electron transport at the donor and receptor side of the reaction center, influencing the absorption, transfer, and application of light energy in S. obliquus XJ002. In addition, we established a simple method to quantify the intracellular lipid content of S. obliquus XJ002, and the optimum staining conditions for Nile red were as follows: volume percentage of dimethyl sulfoxide was 2%, the concentration of Nile red was 2.0 µg·mL−1, and the staining time of Nile red was 5 min. The addition of cerium resulted in a significant increase in the total lipid content of XJ002. When the concentration of cerium was 50 mg·L−1, the total lipid content was 16.26% higher than the control group. This information will enhance our ability to utilize microelement fertilizer in biomass accumulation programs and will help to further reveal the key regulatory factors in the lipid metabolism, and would lay the foundation for promoting the research of microalgae bioenergy. [ABSTRACT FROM AUTHOR]

Water electrolysis powered by renewable electricity will likely be critical to a future hydrogen economy. However, the typical use of strongly acidic or alkaline electrolytes necessitates the use of expensive materials, while bubbles add to capital and operational costs, due to blocking of the electrode surface and the necessary use of pumps and gas-liquid separators. Here ' bubble-free ' oxygen evolution at mild pH is carried out using an electrocatalyst that mimics photosystem II (PSII). The bubble-free electrode includes a gas-extracting Gore-Tex® membrane. Edge-functionalised graphene (EFG) is included to mimic the metal-binding local protein environment, and the tyrosine residue, in the oxygen evolving complex (OEC) of PSII, while MnO x and Ca2+ are incorporated to mimic the Mn 4 CaO 5 cluster. Interaction between EFG, MnO x , and Ca2+ results in a significant, 130 mV fall in the overpotential required to drive electrocatalytic oxygen evolution at 10 mA cm−2, compared to the electrode without these biomimetic components. [Display omitted] • An electrocatalyst that mimics photosystem II is developed. • The biomimetic electrocatalyst is incorporated into a ' bubble-free ' electrode. • Enhanced oxygen evolution occurs due to the interaction of graphene, MnO x , and Ca2+. [ABSTRACT FROM AUTHOR]

The ever‐growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO43− templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] ⋅ 38H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] ⋅ 45H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] ⋅ 61H2O (Co20) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well‐known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s−1 Co9 currently represents the fastest Co‐GT‐based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4. [ABSTRACT FROM AUTHOR]

We propose a phenomenological model to explain photoluminescence degradation of silicon nanocrystals under singlet oxygen generation in gaseous and liquid systems. The model considers coupled rate equations, which take into account the exciton radiative recombination in silicon nanocrystals, photosensitization of singlet oxygen generation, defect formation on the surface of silicon nanocrystals as well as quenching processes for both excitons and singlet oxygen molecules. The model describes well the experimentally observed power law dependences of the photoluminescence intensity, singlet oxygen concentration, and lifetime versus photoexcitation time. The defect concentration in silicon nanocrystals increases by power law with a fractional exponent, which depends on the singlet oxygen concentration and ambient conditions. The obtained results are discussed in a view of optimization of the photosensitized singlet oxygen generation for biomedical applications. [ABSTRACT FROM AUTHOR]

Photosystem II (PSII) plays a key role in water-splitting and oxygen evolution. X-ray crystallography has revealed its atomic structure and some intermediate structures. However, these structures are in the crystalline state and its final state structure has not been solved. Here we analyzed the structure of PSII in solution at 1.95 Å resolution by single-particle cryo-electron microscopy (cryo-EM). The structure obtained is similar to the crystal structure, but a PsbY subunit was visible in the cryo-EM structure, indicating that it represents its physiological state more closely. Electron beam damage was observed at a high-dose in the regions that were easily affected by redox states, and reducing the beam dosage by reducing frames from 50 to 2 yielded a similar resolution but reduced the damage remarkably. This study will serve as a good indicator for determining damage-free cryo-EM structures of not only PSII but also all biological samples, especially redox-active metalloproteins. Kato, Miyazaki, Hamaguchi et al. report the structure of Photosystem II in solution at 1.95 Å resolution by single-particle cryo-electron microscopy. They find that reducing the electron beam dosage decreases the electron beam damage while keeping the resolution of the cryo-EM structure, providing insights into the best practice for the determination of cryo-EM structures. [ABSTRACT FROM AUTHOR]

In order to strengthen the inhibitory effect of UVC/H 2 O 2 on Microcystis aeruginosa growth, this study designed a novel strategy for inducing an advanced oxidation process in algal cells by splitting UVC irradiation into two rounds. The first irradiation of UVC upon adding H 2 O 2 facilitated the delivery of H 2 O 2 into the cell cytoplasm, which induced an intracellular advanced oxidation process after the second irradiation of UVC. The intermittent treatment of UVC/H 2 O 2 could further attack the Ca-Mn and Fe-S clusters in the photosynthetic electron transport chain. In contrast, conventional simultaneous treatment of UVC/H 2 O 2 only attacked the interaction subunits between PSII cores and the phycobilisome. The block of the photosynthetic electron transport chain, shedding of the Ca-Mn cluster, and damage of the Fe-S cluster gave rise to massive intracellular H 2 O 2 , O 2 •−, and HO•. Consequently, ROS acted as a mediator and led to caspase-3(-like) activation and the subsequent initiation of apoptosis-like cell death. The remarkable functional mechanisms make the intermittent treatment of UVC/H 2 O 2 an ideal method for the practical application of suppressing HABs (target-selective, long-lasting, cost-minimized, and eco-friendly). [Display omitted] • A method for suppressing harmful algal bloom (target-selective, long-lasting, cost-minimised, and eco-friendly). • Simultaneous treatment of UVC/H 2 O 2 attacked interaction subunits between PSII cores and the phycobilisome. • The intermittent treatment of UVC/H 2 O 2 attacked Fe-S cluster in photosystem and Ca-Mn cluster in oxygen evolving complex. • Variation of intracellular H 2 O 2 , O 2 •−, and HO• was investigated. • Microcystin-LR synthetic ability was strongly suppressed for at least 40 days. [ABSTRACT FROM AUTHOR]

Keywords: Algae; biotechnology; cyanobacteria; Govindjee; photosynthesis EN Algae biotechnology cyanobacteria Govindjee photosynthesis 275 333 59 01/02/21 20201201 NES 201201 This issue of the I New Zealand Journal of Botany i is the second of a two-part Special Issue arising from the 3rd New Zealand Symposium on the Biology and Biotechnological Applications of Microalgae and Photosynthetic Prokaryotes. Govindjee has continued to make scientific contributions through collaboration with colleagues all across the world and has generously sought to highlight the contributions of others by taking the lead in inviting others to join in writing memorials for his colleagues, and his own former graduate students (see e.g. Govindjee, Munday, et al. [50]; Govindjee et al. [54]; Govindjee, Nonomura, et al. [53]; Govindjee, Zilinskas, et al. [69]). From that point, we started a collaboration that led to the co-authorship of numerous papers (see e.g. Papageorgiou and Govindjee [133]; Kana et al. [88]), and reviews on the biophysical aspects of photosynthesis (see e.g. Papageorgiou and Govindjee [134]), co-hosting of many international conferences, and an edited book on the basics and applications of Chl I a i fluorescence (Papageorgiou and Govindjee [135], reprinted in 2010). I especially remember several times at Gordon Conferences and Photosynthesis Congresses in beautiful outdoor surroundings listening to him reminisce about his experiences with some of the giants of the early days of photosynthesis, including Robert Emerson (Govindjee's first PhD advisor before his death in a plane crash on the way to Harvard), Eugene Rabinowitch (who was Govindjee's advisor after the tragedy; see Govindjee et al. [53]), Otto Warburg (Nickelsen and Govindjee [127]), Louis (Lou) N. M. Duysens (Govindjee and Pulles [56]), and Jack Meyers. [Extracted from the article]

The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol-1 for HO-H bond dissociation and 210 kJ mol-1 for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol-1 for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with activation barriers of 218 and 233 kJ mol-1, which also fall in the visible light region. A comparison of the full reaction path with that on diiron dioxide hexacarbonyls Fe2O2(CO)6 is traced [ABSTRACT FROM AUTHOR]

Molecular quantum mechanics (MQM) investigations have been performed for elucidation of fundamental principles of the photo-induced water oxidation in oxygen evolving complex (OEC) of photosystem II (PSII). To this end, as a first theoretical step, broken symmetry (BS) quantum mechanics (QM) and QM(BS)/molecular mechanics (MM) calculations have been conducted for elucidation of geometrical, electronic and spin structures of the CaMn4Ox (X = 5, 6) cluster in the five steps Si (i = 0∼4) of the Kok cycle for water oxidation. The QM and QM/MM calculations have provided full optimised geometries of short-lived key intermediates and transition state structure for the O-O bond formation in the native solar-energy conversion. The interplay between theory and experiment have clearly indicated that the CaMn4O5 cluster in OEC of PSII exhibits typical physicochemical properties of strong correlation electron system (SCES) confined with effective protein field. Our QM and QM/MM computational results for key intermediates and transition structure for the O-O bond formation in the Kok cycle are now plentiful for derivation of fundamental principles (FP) for understanding of photo-induced water oxidation in OEC of PSII. We summarize twenty nine fundamental principles (FPs) in systematic manner by QM and QM/MM calculations for understanding of water oxidation in Oxygen Evolving Complex(OEC) of Photosystem II(PSII). One of our final aims is theoretical design of the next-generation artificial photosystem materials composed of abundant metal ions. [ABSTRACT FROM AUTHOR]

High‐valent metal−oxo species are of great interest as they serve as a robust catalyst for various organic transformations, and at the same time, they offer significant insight into the reactivity of various metalloenzymes. Formation of Mn−Oxo species is of great interest as they are involved in the Oxygen Evolving Complex of Photosystem II, and various bio‐mimic models were synthesized to understand its reactivity. In this context, using urea decorated amine ligands, Borovik et al. have reported the facile formation of MnIII=O and MnIV=O species from [MnIIH2buea]2− (here H2buea=tris[(N′‐tert‐butylureayl)‐N‐ethyl]amine) precursor complex using oxygen as the oxidant. While reactivity of these species is thoroughly studied, mechanism of formation of such species is scarcely explored. In this work, we have attempted to establish the formation of these species from the MnII precursor using the experimental conditions. Our calculations reveal the following fundamental steps in the formation of such species: i) O2 activation by MnII lead to formation of MnII−superoxide species wherein the oxidation state of the MnII found to be intact upon O2 binding facilitated by the deprotonated nitrogen atom present in the cavity (ii) in the second step, superoxo species is converted to MnII−hydroperoxo species, [MnIIH2buea(OOH)]2− using dimethylacetamide solvent as source for HAT reaction (iii) presence of water molecule found to aid the O−O bond cleavage in [MnIIH2buea(OOH)]2− species leading to the formation of the putative MnIII=O species, [MnIIIH3buea(O)]2− (iv) one‐electron oxidation of MnIII=O, leads to the formation of [MnIVH3buea(O)]− species and this step is endothermic and need some external oxidants for its formation. While various spin‐states and their roles are explored, our calculations reveal that the Mn atom prefers to be in the high‐spin state across the potential energy surface studied. However, the nature of the formation is strongly correlated to the spin state arising from the radical nature present in the O2 moiety and also in the deprotonated nitrogen atom. This offers a unique multistate reactivity channel for the formation these species easing various kinetic barriers across the potential energy surface. Further, we have also computed the spectral parameters for the experimentally observed species, which are in agreement with the reported data offering confidence on the mechanism established. To this end, our study unveils a facile formation of high‐valent Mn−Oxo species using O2 as oxidant and role of water molecules in the formation of such species, and these mechanistic insights are likely to have implications beyond the example studied here. [ABSTRACT FROM AUTHOR]

Summary: Cl− has long been known as a micronutrient for oxygenic photosynthetic resulting from its role an essential cofactor for photosystem II (PSII). Evidence on the in vivo Cl− distribution in Spinacia oleracea leaves and chloroplasts shows that sufficient Cl− is present for the involvement in PSII function, as indicated by in vitro studies on, among other organisms, S. oleracea PsII. There is also sufficient Cl− to function, with K+, in parsing the H+ electrochemical potential difference (proton motive force) across the illuminated thylakoid membrane into electrical potential difference and pH difference components. However, recent in vitro work on PSII from S. oleracea shows that oxygen evolving complex (OEC) synthesis, and resynthesis after photodamage, requires significantly higher Cl− concentrations than would satisfy the function of assembled PSII O2 evolution of the synthesised PSII with the OEC. The low Cl− affinity of OEC (re‐)assembly could be a component limiting the rate of OEC (re‐)assembly. [ABSTRACT FROM AUTHOR]

Domain-based local pair natural orbital (DLPNO) coupled cluster single and double (CCSD) methods with perturbative triples (T) correction with NormalPNO were used to compute energies for twelve different S1 structures of the CaMn4O5 cluster in the oxygen evolving complex (OEC) of photosystem II (PSII). The DLPNO-CCSD(T0) calculations with TightPNO for the important six structures among them revealed that the right (R)-opened S1XYZW structures were more stable than the corresponding left (L)-opened structures (X = O(5), Y = W2, Z = W1, and W = O(4)) of CaMn4O5. The three different S1 structures belonging to the R-opened type (S1acca, S1bbca, and S1abcb, where O2- = a, OH- = b and H2O = c) were found nearly degenerated in energy, indicating the possibility of the coexistence of different structures in the S1 state. The DLPNO-CCSD(T0) calculations with TightPNO supported the proposal of a dynamic equilibrium model based on the multi-intermediate structures for the S1 state, which is also in agreement with EPR and other experimental and hybrid DFT computational results. Implications of the computational results are discussed in relation to scope and applicability of NormalPNO and TightPNO for the CCSD(T0) calculations of strongly correlated electron systems such as 3d transition-metal complexes. [ABSTRACT FROM AUTHOR]

Continuous agricultural and other activities of human being have caused changes in the chemistry of global climate, as a consequence many biotic and abiotic stresses that reduce photosynthetic capacity of plants have emerged. Therefore, the aim of this review is to review photosynthesis limiting stresses under climate change scenarios and role of chlorophyll fluorescence on this stress. Different research findings indicate that UV-B radiation cause changes in biological processes of plants such as damage to the internal structure of photosynthesis or control its cellular process. Other type of stress is drought stress, which inhibits photosynthetic electron transport through Photosystem II and damages the oxygen evolving complex of PSII. On the other way, high night temperature stresses are increasing, and it can cause constant suppression of net CO2 assimilation rate in both C3 and C4 plants. Photoinhibition is considered as reactive oxygen species induced reduction of the primary acceptor of PSII plastoquinone (QA) or change in recombination between acceptor and donor side of P S I I. For assessing the effect of stress on photosynthesis, chlorophyll fluorescence is mostly used parameter and also it is an effective indicator of photosynthesis limiting stress and their mechanisms. [ABSTRACT FROM AUTHOR]

Catalytic oxidation of water by manganese complexes is of great interest because it can serve as a model for the natural oxygen evolving complex, Mn4CaO5. Complex compounds are highly active and tunable, but their limited stability and solubility restrict them for practical water oxidation applications. In the present article we have prepared surface anchored Mn(II)‐complexes, [MnII(H2O)2(X)2(dcbpy)] (where dcbpy= 4,4′‐dicarboxy‐2,2′‐bipyridine; X=acetate, sulphate, chloride ions) on gold electrode surface using self‐assembly process and applied for the electrocatalytic oxidation of water in neutral pH. The modified electrode was characterized by using different spectral, microscopic and electrochemical techniques. The Mn(II)‐acetate containing gold electrode (Mn(II)‐dcbpy‐Au) electrocatalytically oxidized water at a current density of 1.6 mA cm−2 with low overpotential ∼ 310 mV and a Tafel slope of 56 mV/decade. The modified electrode showed high electrochemical stability during controlled potential electrolysis. It was also observed that the Mn(II)‐acetate immobilized on dcbpy‐Au electrode showed higher electrocatalytic activity in oxidation of water than Mn(II)‐chloride or Mn(II)‐sulfate containing dcbpy‐Au electrode. The newly in‐situ generated Mn(II)‐complex modified electrode is inexpensive and highly efficient for the catalytic oxidation of water than the surface bound Mn‐based electrocatalysts reported previously. [ABSTRACT FROM AUTHOR]

Although UV-C radiation has been in use for killing unwanted cyanobacteria, experiments with lower doses of UV-C radiation instead showed induction of growth related parameters and enhanced biomass production in the cyanobacterium Nostoc muscorum Meg1. When the cyanobacterial cultures were exposed to UV-C radiation of varying doses (6, 12 and 18 mJ/cm2), concentrations of various photo-absorbing pigments, RuBisCO and D1 protein of PSII; activities of oxygen evolving complex, nitrogenase and glutamine synthetase were significantly increased upon 6 and 12 mJ/cm2 UV-C radiation exposures. Resulting higher photosynthetic performance was evident from the augmentation in carbohydrate content by ∼49% under single exposure to 6 mJ/cm2 UV-C by fifteenth day. The increased performances of both RuBisCO and D1 proteins were in part also due to induction at the genetic level as seen from the increase in their mRNA and protein levels under treatment. Similar increase was also observed in protein (16%) and in lipid contents (43%) that reflected an upsurge in the total biomass. Highest biomass (463 mg/L/d) was noted in culture exposed to 6 mJ/cm2 UV-C radiation, representing a ∼25% increase. Furthermore the possibility of this organism using part of the incident UV-C radiation as an additional source of energy was deduced from an experiment where the thylakoid membranes excited within UV (226–400 nm) range showed emission at longer wavelengths with an emission maximum at ∼640 nm. Thus this work provides evidence that lower UV-C doses can potentially augment cyanobacterial growth and use of unstandardized UV-C doses for restricting cyanobacterial growth may in fact produce contrary result. • Lower doses of UV-C radiation induced CO 2 and N 2 -fixation in cyanobacteria. • This in turn led to increase in biomass production. • The organism could use UV radiation as auxiliary energy for photosynthesis. • This indicated not all UV-C radiation doses are detrimental to cyanobacteria. • Hence caution is required while using UV-C in controlling cyanobacterial bloom. [ABSTRACT FROM AUTHOR]

• Reliable Mn Mn distances of the oxygen-evolving complex of photosystem II (PSII-OEC) were discussed. • SFX results of the S 3 state reported by the Okayama and Berkeley groups are substantially comparable. • QM/MM results are consistent with the SFX results in the entire S states. • Assignments of the protonation states of PSII-OEC are not conclusive at the 2 Å resolution structures. The fully optimized geometrical structures of the CaMn 4 O x (x = 5, 6) clusters in the S i (i = 0–3) states of the Kok cycle of photosynthetic water oxidation by the large-scale quantum mechanics/molecular mechanics (QM/MM) calculations were compared to recent experimental results based on serial femtosecond crystallography (SFX). The Mn Mn and Ca Mn distances obtained by the QM/MM calculations were found to be totally comparable to the SFX experiments, elucidating the entire Kok cycle involving the S 4 transition state during the O O bond formation for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII). [ABSTRACT FROM AUTHOR]

The ability to harvest light to drive chemical reactions and gain energy provided microbes access to high energy electron donors which fueled primary productivity, biogeochemical cycles, and microbial evolution. Oxygenic photosynthesis is often cited as the most important microbial innovation—the emergence of oxygen-evolving photosynthesis, aided by geologic events, is credited with tipping the scale from a reducing early Earth to an oxygenated world that eventually lead to complex life. Anoxygenic photosynthesis predates oxygen-evolving photosynthesis and played a key role in developing and fine-tuning the photosystem architecture of modern oxygenic phototrophs. The release of oxygen as a by-product of metabolic activity would have caused oxidative damage to anaerobic microbiota that evolved under the anoxic, reducing conditions of early Earth. Photosynthetic machinery is particularly susceptible to the adverse effects of oxygen and reactive oxygen species and these effects are compounded by light. As a result, phototrophs employ additional detoxification mechanisms to mitigate oxidative stress and have evolved alternative oxygen-dependent enzymes for chlorophyll biosynthesis. Phylogenetic reconstruction studies and biochemical characterization suggest photosynthetic reactions centers, particularly in Cyanobacteria, evolved to both increase efficiency of electron transfer and avoid photodamage caused by chlorophyll radicals that is acute in the presence of oxygen. Here we review the oxygen and reactive oxygen species detoxification mechanisms observed in extant anoxygenic and oxygenic photosynthetic bacteria as well as the emergence of these mechanisms over evolutionary time. We examine the distribution of phototrophs in modern systems and phylogenetic reconstructions to evaluate the emergence of mechanisms to mediate oxidative damage and highlight changes in photosystems and reaction centers, chlorophyll biosynthesis, and niche space in response to oxygen production. This synthesis supports an emergence of H 2 S-driven anoxygenic photosynthesis in Cyanobacteria prior to the evolution of oxygenic photosynthesis and underscores a role for the former metabolism in fueling fine-tuning of the oxygen evolving complex and mechanisms to repair oxidative damage. In contrast, we note the lack of elaborate mechanisms to deal with oxygen in non-cyanobacterial anoxygenic phototrophs suggesting these microbes have occupied similar niche space throughout Earth's history. • The ability to harvest light provided microbes access to high energy electron donors which fueled microbial evolution. • Photosynthetic machinery is particularly susceptible to the adverse effects of oxygen these effects are compounded by light. • Phototrophs employ additional detoxification mechanisms to mitigate oxidative stress. • Cyanobacterial anoxygenic photosynthesis could have fueled fine-tuning of the oxygen evolving complex. • Anoxygenic phototrophs lack of elaborate mechanisms to deal with oxygen. [ABSTRACT FROM AUTHOR]

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn3O4⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand‐free tetrameric CanMn4−nO4+ (n=0–4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas‐phase reactivity experiments with D2O and H218O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono‐calcium cluster CaMn3O4+ is observed to oxidize water via elimination of hydrogen peroxide. First‐principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn3O4+ as well as the role of calcium. [ABSTRACT FROM AUTHOR]

The optimized geometries of the CaMn4OX (X = 5, 6) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) by large‐scale quantum mechanics (QM) and molecular mechanics (MM) calculations are compared with recent serial femtosecond crystallography (SFX) results for the Si (i = 0–3) states. The valence states of four Mn ions by the QM/MM calculations are also examined in relation to the experimental results by the X‐ray emission spectroscopy (XES) for the Si intermediates. Geometrical and valence structures of right‐opened Mn‐hydroxide, Mn‐oxo and Mn‐peroxide intermediates in the S3 state are investigated in detail in relation to recent SFX and XES experiments for the S3 state. Interplay between theory and experiment indicates that the Mn‐oxo intermediate is a new possible candidate for the S3 state. Implications of the computational results are discussed in relation to possible mechanisms of the oxygenoxygen bond formation for water oxidation in OEC of PSII. [ABSTRACT FROM AUTHOR]

Understanding the structural modification experienced by the Mn4CaO5 oxygen‐evolving complex of photosystem II along the Kok‐Joliot's cycle has been a challenge for both theory and experiments since many decades. In particular, differential infrared spectroscopy was extensively used to probe the surroundings of the reaction center, to catch spectral changes between different S‐states along the catalytic cycle. Because of the complexity of the signals, only a limited quantity of identified peaks have been assigned so far, also because of the difficulty of a direct comparison with theoretical calculations. In the present work, we critically reconsider the comparison between differential vibrational spectroscopy and theoretical calculations performed on the structural models of the photosystem II active site and an inorganic structural mimic. Several factors are currently limiting the reliability of a quantitative comparison, such as intrinsic errors associated to theoretical methods, and most of all, the uncertainty attributed to the lack of knowledge about the localization of the underlying structural changes. Critical points in this comparison are extensively discussed. Comparing several computational data of differential S2/S1 infrared spectroscopy, we have identified weak and strong points in their interpretation when compared with experimental spectra. [ABSTRACT FROM AUTHOR]

The PsbO protein is an essential extrinsic subunit of photosystem II, the pigment–protein complex responsible for light‐driven water splitting. Water oxidation in photosystem II supplies electrons to the photosynthetic electron transfer chain and is accompanied by proton release and oxygen evolution. While the electron transfer steps in this process are well defined and characterized, the driving forces acting on the liberated protons, their dynamics and their destiny are all largely unknown. It was suggested that PsbO undergoes proton‐induced conformational changes and forms hydrogen bond networks that ensure prompt proton removal from the catalytic site of water oxidation, i.e. the Mn4CaO5 cluster. This work reports the purification and characterization of heterologously expressed PsbO from green algae Chlamydomonas reinhardtii and two isoforms from the higher plant Solanum tuberosum (PsbO1 and PsbO2). A comparison to the spinach PsbO reveals striking similarities in intrinsic protein fluorescence and CD spectra, reflecting the near‐identical secondary structure of the proteins from algae and higher plants. Titration experiments using the hydrophobic fluorescence probe ANS revealed that eukaryotic PsbO proteins exhibit acid–base hysteresis. This hysteresis is a dynamic effect accompanied by changes in the accessibility of the protein's hydrophobic core and is not due to reversible oligomerization or unfolding of the PsbO protein. These results confirm the hypothesis that pH‐dependent dynamic behavior at physiological pH ranges is a common feature of PsbO proteins and causes reversible opening and closing of their β‐barrel domain in response to the fluctuating acidity of the thylakoid lumen. [ABSTRACT FROM AUTHOR]

This essay for EurJIC's special issue on "Redox Catalysis for Artificial Photosynthesis" introduces the reader to the field of water oxidation using molecular catalysts. The most essential challenge our society must address during the 21st century is perhaps the realization of a system for producing sustainable energy on the global scale. Currently, there exists an urgent need to develop effective and economical carbon‐neutral or carbon‐free energy technologies. The production of solar fuels through water splitting constitutes a key enabling element. The construction of robust and efficient catalysts for oxidation of water is therefore essential. In this essay the progress and mechanistic considerations pertaining to molecular water oxidation catalysts are described and discussed from a personal perspective. [ABSTRACT FROM AUTHOR]