The manganese and calcium ions of photosynthetic oxygen evolution

Synthetic Mn3Ce2O5-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The photosynthetic oxygen-evolving center (OEC) is a unique Mn4CaO5-cluster that catalyses water splitting into electrons, protons, and dioxygen. Precisely structural and functional mimicking of the OEC is a long-standing challenge and pressingly needed for understanding the structure-function relationship and catalytic mechanism of O-O bond formation. Herein we report two simple and robust artificial Mn3Ce2O5-complexes that display a remarkable structural similarity to the OEC in regarding of the ten-atom core (five metal ions and five oxygen bridges) and the alkyl carboxylate peripheral ligands. This Mn3Ce2O5-cluster can catalyse the water-splitting reaction on the surface of ITO electrode. These results clearly show that cerium can structurally and functionally replace both calcium and manganese in the cluster. Mass spectroscopic measurements demonstrate that the oxide bridges in the cluster are exchangeable and can be rapidly replaced by the isotopic oxygen of H2 18O in acetonitrile solution, which supports that the oxide bridge(s) may serve as the active site for the formation of O-O bond during the water-splitting reaction. These results would contribute to our understanding of the structure-reactivity relationship of both natural and artificial clusters and shed new light on the development of efficient water-splitting catalysts in artificial photosynthesis.

Theoretical elucidation of the structure, bonding, and reactivity of the CaMn4Ox clusters in the whole Kok cycle for water oxidation embedded in the oxygen evolving center of photosystem II. New molecular and quantum insights into the mechanism of the O-O bond formation

This paper reviews our historical developments of broken-symmetry (BS) and beyond BS methods that are applicable for theoretical investigations of metalloenzymes such as OEC in PSII. The BS hybrid DFT (HDFT) calculations starting from high-resolution (HR) XRD structure in the most stable S1 state have been performed to elucidate structure and bonding of whole possible intermediates of the CaMn4Ox cluster (1) in the Si (i = 0 ~ 4) states of the Kok cycle. The large-scale HDFT/MM computations starting from HR XRD have been performed to elucidate biomolecular system structures which are crucial for examination of possible water inlet and proton release pathways for water oxidation in OEC of PSII. DLPNO CCSD(T0) computations have been performed for elucidation of scope and reliability of relative energies among the intermediates by HDFT. These computations combined with EXAFS, XRD, XFEL, and EPR experimental results have elucidated the structure, bonding, and reactivity of the key intermediates, which are indispensable for understanding and explanation of the mechanism of water oxidation in OEC of PSII. Interplay between theory and experiments have elucidated important roles of four degrees of freedom, spin, charge, orbital, and nuclear motion for understanding and explanation of the chemical reactivity of 1 embedded in protein matrix, indicating the participations of the Ca(H2O)n ion and tyrosine(Yz)-O radical as a one-electron acceptor for the O-O bond formation. The Ca-assisted Yz-coupled O-O bond formation mechanisms for water oxidation are consistent with recent XES and very recent time-resolved SFX XFEL and FTIR results.

Solar energy conversion by photosystem II: principles and structures

Photosynthetic water oxidation by Photosystem II (PSII) is a fascinating process because it sustains life on Earth and serves as a blue print for scalable synthetic catalysts required for renewable energy applications. The biophysical, computational, and structural description of this process, which started more than 50 years ago, has made tremendous progress over the past two decades, with its high-resolution crystal structures being available not only of the dark-stable state of PSII, but of all the semi-stable reaction intermediates and even some transient states. Here, we summarize the current knowledge on PSII with emphasis on the basic principles that govern the conversion of light energy to chemical energy in PSII, as well as on the illustration of the molecular structures that enable these reactions. The important remaining questions regarding the mechanism of biological water oxidation are highlighted, and one possible pathway for this fundamental reaction is described at a molecular level.

Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl^-) are essential for oxygen evolution by PSII, and two Cl^- ions have been found to specifically bind near the Mn₄CaO₅ cluster in the OEC. The retention of these Cl^- ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl^- ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl^- ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl^- retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl^- ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl^- retention by these extrinsic subunits.

D139N mutation of PsbP enhances the oxygen-evolving activity of photosystem II through stabilized binding of a chloride ion

Photosystem II (PSII) is a multisubunit membrane protein complex that catalyzes light-driven oxidation of water to molecular oxygen. The chloride ion (Cl-) has long been known as an essential cofactor for oxygen evolution by PSII, and two Cl- ions (Cl-1 and Cl-2) have been found to specifically bind near the Mn4CaO5 cluster within the oxygen-evolving center (OEC). However, despite intensive studies on these Cl- ions, little is known about the function of Cl-2, the Cl- ion that is associated with the backbone nitrogens of D1-Asn338, D1-Phe339, and CP43-Glu354. In green plant PSII, the membrane extrinsic subunits-PsbP and PsbQ-are responsible for Cl- retention within the OEC. The Loop 4 region of PsbP, consisting of highly conserved residues Thr135-Gly142, is inserted close to Cl-2, but its importance has not been examined to date. Here, we investigated the importance of PsbP-Loop 4 using spinach PSII membranes reconstituted with spinach PsbP proteins harboring mutations in this region. Mutations in PsbP-Loop 4 had remarkable effects on the rate of oxygen evolution by PSII. Moreover, we found that a specific mutation, PsbP-D139N, significantly enhances the oxygen-evolving activity in the absence of PsbQ, but not significantly in its presence. The D139N mutation increased the Cl- retention ability of PsbP and induced a unique structural change in the OEC, as indicated by light-induced Fourier transform infrared (FTIR) difference spectroscopy and theoretical calculations. Our findings provide insight into the functional significance of Cl-2 in the water-oxidizing reaction of PSII.

From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem II

The manganese cluster of photosystem II has been the focus of intense research aiming to understand the mechanism of H₂O-oxidation. Great effort has also been applied to investigating its oxidative photoassembly process, termed photoactivation that involves the light-driven incorporation of metal ions into the active Mn₄CaO₅ cluster. The knowledge gained on these topics has fundamental scientific significance, but may also provide the blueprints for the development of biomimetic devices capable of splitting water for solar energy applications. Accordingly, synthetic chemical approaches inspired by the native Mn cluster are actively being explored, for which the native catalyst is a useful benchmark. For both the natural and artificial catalysts, the assembly process of incorporating Mn ions into catalytically active Mn oxide complexes is an oxidative process. In both cases this process appears to share certain chemical features, such as producing an optimal fraction of open coordination sites on the metals to facilitate the binding of substrate water, as well as the involvement of alkali metals (e.g., Ca²⁺) to facilitate assembly and activate water-splitting catalysis. This review discusses the structure and formation of the metal cluster of the PSII H₂O-oxidizing complex in the context of what is known about the formation and chemical properties of different Mn oxides. Additionally, the evolutionary origin of the Mn₄CaO₅ is considered in light of hypotheses that soluble Mn²⁺ was an ancient source of reductant for some early photosynthetic reaction centers ('photomanganotrophy'), and recent evidence that PSII can form Mn oxides with structural resemblance to the geologically abundant birnessite class of minerals. A new functional role for Ca²⁺ to facilitate sustained Mn²⁺ oxidation during photomanganotrophy is proposed, which may explain proposed physiological intermediates during the likely evolutionary transition from anoxygenic to oxygenic photosynthesis.

Excitation-energy transfer in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120 as studied by time-resolved fluorescence spectroscopy

Heterocysts are formed in filamentous heterocystous cyanobacteria under nitrogen-starvation conditions, and possess a very low amount of photosystem II (PSII) complexes than vegetative cells. Molecular, morphological, and biochemical characterizations of heterocysts have been investigated; however, excitation-energy dynamics in heterocysts are still unknown. In this study, we examined excitation-energy-relaxation processes of pigment-protein complexes in heterocysts isolated from the cyanobacterium Anabaena sp. PCC 7120. Thylakoid membranes from the heterocysts showed no oxygen-evolving activity under our experimental conditions and no thermoluminescence-glow curve originating from charge recombination of S2QA-. Two dimensional blue-native/SDS-PAGE analysis exhibits tetrameric, dimeric, and monomeric photosystem I (PSI) complexes but almost no dimeric and monomeric PSII complexes in the heterocyst thylakoids. The steady-state fluorescence spectrum of the heterocyst thylakoids at 77 K displays both characteristic PSI fluorescence and unusual PSII fluorescence different from the fluorescence of PSII dimer and monomer complexes. Time-resolved fluorescence spectra at 77 K, followed by fluorescence decay-associated spectra, showed different PSII and PSI fluorescence bands between heterocysts and vegetative thylakoids. Based on these findings, we discuss excitation-energy-transfer mechanisms in the heterocysts.

Rare-Earth Elements Can Structurally and Energetically Replace the Calcium in a Synthetic Mn4CaO4-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosynthesis is a unique biological Mn₄CaO₅ cluster catalyzing the water-splitting reaction. A great current challenge is to achieve a robust and precise mimic of the OEC in the laboratory. Herein, we report synthetic Mn₄XO₄ clusters (X = calcium, yttrium, gadolinium) that closely resemble the OEC with regard to the main metal-oxide core and peripheral ligands, as well as the oxidation states of the four Mn ions and the redox potential of the cluster. We demonstrate that rare-earth elements can structurally replace the calcium in neutral Mn₄XO₄ clusters. All three Mn₄XO₄ clusters with different redox-inactive metal ions display essentially the same redox properties, challenging the conventional view that the Lewis acidity of the redox-inactive metal ions could modulate the redox potential of the heteronuclear-oxide clusters. The new synthetic rare-earth element-containing Mn₄XO₄ clusters reported here provide robust and structurally well-defined chemical models and shed new light on the design of new water-splitting catalysts in artificial photosynthesis.

Structural Reorganization of a Synthetic Mimic of the Oxygen-Evolving Center in Multiple Redox Transitions Revealed by Electrochemical FTIR Spectra

In photosynthesis, the protein-bound natural oxygen-evolving center (OEC) undergoes multiple oxidation-state transitions in the light-driven water splitting reactions with a stepwise change in the oxidation potential. Because the protein is vulnerable to electrochemical oxidation, the multiple oxidation/reduction-state transitions can hardly be achieved by electrochemical oxidation with a continuous change in the oxidation potential. An OEC mimic that can undergo four redox transitions has been synthesized (Zhang, C., Science, 2015, 348, 690-693). Here we report an electrochemical FTIR spectroscopic study of this synthetic complex at its multiple oxidation states in the low-frequency region for Mn-O bonds. Compared with those of the native OEC induced by pulsed laser flashes, our results also show the existence of two structural isomers in the S₂ state, with the closed cubane conformer being more stable than the open cubane conformer, in contrast to that of the native OEC in which the open form is more stable.

Transcriptomic Analysis of the Photosynthetic, Respiration, and Aerenchyma Adaptation Strategies in Bermudagrass (Cynodon dactylon) under Different Submergence Stress

Submergence impedes photosynthesis and respiration but facilitates aerenchyma formation in bermudagrass. Still, the regulatory genes underlying these physiological responses are unclear in the literature. To identify differentially expressed genes (DEGs) related to these physiological mechanisms, we studied the expression of DEGs in aboveground and underground tissues of bermudagrass after a 7 d treatment under control (CK), shallow submergence (SS), and deep submergence (DS). Results show that compared with CK, 12276 and 12559 DEGs were identified under SS and DS, respectively. Among them, the DEGs closely related to the metabolism of chlorophyll biosynthesis, light-harvesting, protein complex, and carbon fixation were down-regulated in SS and DS. Meanwhile, a large number of DEGs involved in starch and sucrose hydrolase activities, glycolysis/gluconeogenesis, tricarboxylic acid (TCA) cycle, and oxidative phosphorylation were down-regulated in aboveground tissues of bermudagrass in SS and DS. Whereas in underground tissues of bermudagrass these DEGs were all up-regulated under SS, only beta-fructofuranosidase and α-amylase related genes were up-regulated under DS. In addition, we found that DEGs associated with ethylene signaling, Ca²⁺-ROS signaling, and cell wall modification were also up-regulated during aerenchyma formation in underground tissues of bermudagrass under SS and DS. These results provide the basis for further exploration of the regulatory and functional genes related to the adaptability of bermudagrass to submergence.

Proton and Water Transfer Pathways in the S2 → S3 Transition of the Water-Oxidizing Complex in Photosystem II: Time-Resolved Infrared Analysis of the Effects of D1-N298A Mutation and NO3- Substitution

Photosynthetic water oxidation is performed through a light-driven cycle of five intermediates (S₀-S₄ states) in photosystem II (PSII). The S₂ → S₃ transition, which involves concerted water and proton transfer, is a key process for understanding the water oxidation mechanism. Here, to identify the water and proton transfer pathways during the S₂ → S₃ transition, we examined the effects of D1-N298A mutation and NO₃^- substitution for Cl^-, which perturbed the O1 and Cl channels, respectively, on the S₂ → S₃ kinetics using time-resolved infrared spectroscopy. The S₂ → S₃ transition was retarded both upon NO₃^- substitution and upon D1-N298A mutation, whereas it was unaffected by further NO₃^- substitution in N298A PSII. The H/D kinetic isotope effect in N298A PSII was relatively small, revealing that water transfer is a rate-limiting step in this mutant. From these results, it was suggested that during the S₂ → S₃ transition, water delivery and proton release occur through the O1 and Cl channels, respectively.

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn₄CaO₅ cluster, with participation of the redox-active tyrosine residue (Y_Z) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between Y_Z and D1-His190 likely renders Y_Z kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at Y_Z remains elusive owing to the transient nature of its intermediate states involving Y_Z⋅. Herein, we employ a combination of high-resolution two-dimensional ¹⁴N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the Y_Z residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.

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Structural factors are critical in the design of artificial photosynthetic systems

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Correlation between hyperfine couplings of the N atoms and electron spin density

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Spin density distribution affected by charge delocalization and explicit waters

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Spin density modulation by electronic coupling as observed with P₆₈₀ and Y_Z in PSII

Mimicking the Catalytic Center for the Water-Splitting Reaction in Photosystem II

The oxygen-evolving center (OEC) in photosystem II (PSII) of plants, algae and cyanobacteria is a unique natural catalyst that splits water into electrons, protons and dioxygen. The crystallographic studies of PSII have revealed that the OEC is an asymmetric Mn4CaO5-cluster. The understanding of the structure-function relationship of this natural Mn4CaO5-cluster is impeded mainly due to the complexity of the protein environment and lack of a rational chemical model as a reference. Although it has been a great challenge for chemists to synthesize the OEC in the laboratory, significant advances have been achieved recently. Different artificial complexes have been reported, especially a series of artificial Mn4CaO4-clusters that closely mimic both the geometric and electronic structures of the OEC in PSII, which provides a structurally well-defined chemical model to investigate the structure-function relationship of the natural Mn4CaO5-cluster. The deep investigations on this artificial Mn4CaO4-cluster could provide new insights into the mechanism of the water-splitting reaction in natural photosynthesis and may help the development of efficient catalysts for the water-splitting reaction in artificial photosynthesis.

FTIR Microspectroscopic Analysis of the Water Oxidation Reaction in a Single Photosystem II Microcrystal

Microcrystals of photosystem II (PSII) have recently been used to investigate the intermediate structures of the water oxidizing complex during water oxidation by serial femtosecond crystallography using X-ray free electron lasers. To clarify the water oxidation mechanism, it is crucial to know whether the reaction proceeds properly in the microcrystals. In this work, we monitored the water oxidation reaction in a single PSII microcrystal using Fourier transform infrared (FTIR) microspectroscopy with the transmission method. Flash-induced micro-FTIR difference spectra of S-state transitions in a PSII microcrystal showed features virtually identical to the corresponding spectra previously obtained using the attenuated total reflection method for multiple microcrystals, representing the reactions near the crystal surface, as well as the spectra in solution. This observation indicates that the reaction processes of water oxidation proceed with relatively high efficiencies retaining native intermediate structures in the entire inside of a PSII microcrystal.

Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”.

Pivotal role of the redox-active tyrosine in driving the water splitting catalyzed by photosystem II

TyrZ oxidation state triggers hydrogen bond modification in the water oxidation catalysis.

Sequestration of Radionuclides Radium-226 and Strontium-90 by Cyanobacteria Forming Intracellular Calcium Carbonates

²²⁶Ra is a naturally occurring radionuclide with a half-life of 1600 years. In contrast, ⁹⁰Sr is a radionuclide of sole anthropogenic origin, produced by nuclear fission reactions and has a half-life of 29 years; each of these radionuclides poses potential threats to human and ecosystem health. Here, the cyanobacterium Gloeomargarita lithophora, capable of forming intracellular amorphous calcium carbonate inclusions, was investigated for its ability to uptake ²²⁶Ra and ⁹⁰Sr. In BG-11 medium, G. lithophora accumulated 3.9 μg g^-1 of ²²⁶Ra within 144 h and 47.9 ng g^-1 of ⁹⁰Sr within 1 h, corresponding to ∼99% removal of trace radionuclides. The presence of high-concentration Ca²⁺ in the background media solution did not inhibit ⁹⁰Sr and ²²⁶Ra uptake by G. lithophora. In contrast, dead biomass of G. lithophora accumulated 0.8 μg g^-1 of ²²⁶Ra and 8.87 ng g^-1 of ⁹⁰Sr. Moreover, Synechocystis, a nonbiomineralizing cyanobacteria, removed only 14 and 25% of ²²⁶Ra and ⁹⁰Sr, respectively. This suggested that sequestration of ⁹⁰Sr and ²²⁶Ra was not intrinsic to all cyanobacteria but was likely a specific biological trait of G. lithophora related to the formation of intracellular amorphous Ca-carbonates. The unique ability of G. lithophora to uptake ⁹⁰Sr and ²²⁶Ra at high rates makes it an attractive candidate for further studies involving bioremediation of these radionuclides.

Evidence of high Ca uptake by cyanobacteria forming intracellular CaCO3 and impact on their growth

Several species of cyanobacteria biomineralizing intracellular amorphous calcium carbonates (ACC) were recently discovered. However, the mechanisms involved in this biomineralization process and the determinants discriminating species forming intracellular ACC from those not forming intracellular ACC remain unknown. Recently, it was hypothesized that the intensity of Ca uptake (i.e., how much Ca was scavenged from the extracellular solution) might be a major parameter controlling the capability of a cyanobacterium to form intracellular ACC. Here, we tested this hypothesis by systematically measuring the Ca uptake by a set of 52 cyanobacterial strains cultured in the same growth medium. The results evidenced a dichotomy among cyanobacteria regarding Ca sequestration capabilities, with all strains forming intracellular ACC incorporating significantly more calcium than strains not forming ACC. Moreover, Ca provided at a concentration of 50 μM in BG-11 was shown to be limiting for the growth of some of the strains forming intracellular ACC, suggesting an overlooked quantitative role of Ca for these strains. All cyanobacteria forming intracellular ACC contained at least one gene coding for a mechanosensitive channel, which might be involved in Ca influx, as well as at least one gene coding for a Ca²⁺ /H⁺ exchanger and membrane proteins of the UPF0016 family, which might be involved in active Ca transport either from the cytosol to the extracellular solution or the cytosol toward an intracellular compartment. Overall, massive Ca sequestration may have an indirect role by allowing the formation of intracellular ACC. The latter may be beneficial to the growth of the cells as a storage of inorganic C and/or a buffer of intracellular pH. Moreover, high Ca scavenging by cyanobacteria biomineralizing intracellular ACC, a trait shared with endolithic cyanobacteria, suggests that these cyanobacteria should be considered as potentially significant geochemical reservoirs of Ca.

Effect of oxygen on the non-photochemical quenching of vascular plants and potential oxygen deficiency in the stroma of PsbS-knock-out rice

The excessive and harmful light energy absorbed by the photosystem (PS) II of higher plants is dissipated as heat through a protective mechanism termed non-photochemical quenching (NPQ) of chlorophyll fluorescence. PsbS-knock-out (KO) mutants lack the trans-thylakoid proton gradient (ΔpH)-dependent part of NPQ. To elucidate the molecular mechanism of NPQ, we investigated its dependency on oxygen. The development of NPQ in wild-type (WT) rice under low-oxygen (LO) conditions was reduced to more than 50% of its original value. However, under high-oxygen (HO) conditions, the NPQ of both WT and PsbS-KO mutants recovered. Moreover, WT and PsbS-KO mutant leaves infiltrated with the ΔpH dissipating uncoupler nigericin showed increased NPQ values under HO conditions. The experiments using intact chloroplasts and protoplasts of Arabidopsis thaliana supported that the LO effects observed in rice leaves were not due to carbon dioxide deficiency. There was a noticeable 90% reduction in the half-time of P700 oxidation rate in LO-treated leaves compared with that of WT control leaves, but the HO treatment did not significantly change the half-time of P700 oxidation rate. Overall, the results obtained here indicate that the stroma of the PsbS-KO plants could be potentially under O₂ deficiency. Because the functions of PsbS in rice leaves are likely to be similar to those in other higher plants, our findings offer novel insights into the role of oxygen in the development of NPQ.

A geminivirus betasatellite encoded βC1 protein interacts with PsbP and subverts PsbP-mediated antiviral defence in plants

Geminivirus disease complexes potentially interfere with plants physiology and cause disastrous effects on a wide range of economically important crops throughout the world. Diverse geminivirus betasatellite associations exacerbate the epidemic threat for global food security. Our previous study showed that βC1, the pathogenicity determinant of geminivirus betasatellites induce symptom development by disrupting the ultrastructure and function of chloroplasts. Here we explored the betasatellite-virus-chloroplast interaction in the scope of viral pathogenesis as well as plant defence responses, using Nicotiana benthamiana-Radish leaf curl betasatellite (RaLCB) as the model system. We have shown an interaction between RaLCB-encoded βC1 and one of the extrinsic subunit proteins of oxygen-evolving complex of photosystem II both in vitro and in vivo. Further, we demonstrate a novel function of the Nicotiana benthamiana oxygen-evolving enhancer protein 2 (PsbP), in that it binds DNA, including geminivirus DNA. Transient silencing of PsbP in N. benthamiana plants enhances pathogenicity and viral DNA accumulation. Overexpression of PsbP impedes disease development during the early phase of infection, suggesting that PsbP is involved in generation of defence response during geminivirus infection. In addition, βC1-PsbP interaction hampers non-specific binding of PsbP to the geminivirus DNA. Our findings suggest that betasatellite-encoded βC1 protein accomplishes counter-defence by physical interaction with PsbP reducing the ability of PsbP to bind geminivirus DNA to establish infection.

Structural isomers of the S2 state in photosystem II: do they exist at room temperature and are they important for function?

In nature, an oxo-bridged Mn₄ CaO₅ cluster embedded in photosystem II (PSII), a membrane-bound multi-subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light-induced charge separations in the reaction center of PSII. The Mn₄ CaO₅ cluster accumulates four oxidizing equivalents to enable the four-electron four-proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S-states, S₀  - S₄ in the Kok cycle. One important question related to the catalytic mechanism of the oxygen-evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S-states and if such equilibria are essential for the mechanism of the O-O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S₂ state of PSII with structural data collected of the S₁ , S₂ and S₃ states by serial crystallography at neutral pH (∼6.5) using an X-ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S₂ state, the room temperature crystallography data can be well-described by just one S₂ structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.

Tracing the Pathways of Waters and Protons in Photosystem II and Cytochrome c Oxidase

Photosystem II (PSII) uses water as the terminal electron donor, producing oxygen in the Mn4CaO5 oxygen evolving complex (OEC), while cytochrome c oxidase (CcO) reduces O2 to water in its heme–Cu binuclear center (BNC). Each protein is oriented in the membrane to add to the proton gradient. The OEC, which releases protons, is located near the P-side (positive, at low-pH) of the membrane. In contrast, the BNC is in the middle of CcO, so the protons needed for O2 reduction must be transferred from the N-side (negative, at high pH). In addition, CcO pumps protons from N- to P-side, coupled to the O2 reduction chemistry, to store additional energy. Thus, proton transfers are directly coupled to the OEC and BNC redox chemistry, as well as needed for CcO proton pumping. The simulations that study the changes in proton affinity of the redox active sites and the surrounding protein at different states of the reaction cycle, as well as the changes in hydration that modulate proton transfer paths, are described.

Oxygenic photosynthesis: history, status and perspective

Cyanobacteria and plants carry out oxygenic photosynthesis. They use water to generate the atmospheric oxygen we breathe and carbon dioxide to produce the biomass serving as food, feed, fibre and fuel. This paper scans the emergence of structural and mechanistic understanding of oxygen evolution over the past 50 years. It reviews speculative concepts and the stepped insight provided by novel experimental and theoretical techniques. Driven by sunlight photosystem II oxidizes the catalyst of water oxidation, a hetero-metallic Mn4CaO5(H2O)4 cluster. Mn3Ca are arranged in cubanoid and one Mn dangles out. By accumulation of four oxidizing equivalents before initiating dioxygen formation it matches the four-electron chemistry from water to dioxygen to the one-electron chemistry of the photo-sensitizer. Potentially harmful intermediates are thereby occluded in space and time. Kinetic signatures of the catalytic cluster and its partners in the photo-reaction centre have been resolved, in the frequency domain ranging from acoustic waves via infra-red to X-ray radiation, and in the time domain from nano- to milli-seconds. X-ray structures to a resolution of 1.9 Å are available. Even time resolved X-ray structures have been obtained by clocking the reaction cycle by flashes of light and diffraction with femtosecond X-ray pulses. The terminal reaction cascade from two molecules of water to dioxygen involves the transfer of four electrons, two protons, one dioxygen and one water. A rigorous mechanistic analysis is challenging because of the kinetic enslaving at millisecond duration of six partial reactions (4e−, 1H+, 1O2). For the time being a peroxide-intermediate in the reaction cascade to dioxygen has been in focus, both experimentally and by quantum chemistry. Homo sapiens has relied on burning the products of oxygenic photosynthesis, recent and fossil. Mankind's total energy consumption amounts to almost one-fourth of the global photosynthetic productivity. If the average power consumption equalled one of those nations with the highest consumption per capita it was four times greater and matched the total productivity. It is obvious that biomass should be harvested for food, feed, fibre and platform chemicals rather than for fuel.

Synthesis and encapsulation of V(IV,V) compounds in silica nanoparticles targeting development of antioxidant and antiradical nanomaterials

The quest for effective treatments of oxidative stress has concentrated over the years on new nanomaterials with improved antioxidant and antiradical activity, thereby attracting broad research interest. In that regard, research efforts in our lab were launched to pursue such hybrid materials involving a) synthesis of silica gel matrices, b) evaluation of the suitability of atoxic matrices as potential carriers for the controlled release of V(IV)(VOSO₄), V(V)(NaVO₃) compounds and a newly synthesized heterometallic lithium-vanadium(IV,V) tetranuclear compound containing vanadium-bound hydroxycarboxylic 1,3-diamine-2-propanol-N,N,N',N'-tetraacetic acid (DPOT), and c) investigation of structural and textural properties of silica nanoparticles (NPs) by different and complementary characterization techniques, inquiring into the nature of the encapsulated vanadium species and their interaction with the siloxane matrix, collectively targeting novel antioxidant and antiradical nanomaterials biotechnology. The physicochemical characterization of the vanadium-loaded SiO₂ NPs led to the formulation of optimized material configuration linked to the delivery of the encapsulated antioxidant-antiradical load. Entrapment and drug release studies showed a) the competence of hybrid nanoparticles with respect to encapsulation efficiency of the vanadium compound (concentration dependence), b) congruence with the physicochemical features determined, and c) a well-defined release profile of NP load. Antioxidant properties and the free radical scavenging capacity of the new hybrid materials (containing VOSO₄, NaVO₃, and V-DPOT) were demonstrated through a) 2-diphenyl-1-picrylhydrazyl (DPPH) free radical, and b) intracellular-extracellular reactive oxygen species (ROS) assays, through UV-Visible spectroscopy techniques, collectively showing that the hybrid silica NPs (empty-loaded) could serve as an efficient platform for nanodrug formulations counteracting oxidative stress.

A tris-(manganese(iii))corrole–porphyrin–corrole triad: synthesis, characterization and catalytic epoxidation

A homotrimetallic manganese(iii) corrole–porphyrin–corrole triad has been synthesized and structurally characterized.

Chlorophyll a fluorescence and transcriptome reveal the toxicological effects of bisphenol A on an invasive cyanobacterium, Cylindrospermopsis raciborskii

Bisphenol A has attracted worldwide attention due to its harmful effects on humans, animals and plants. In this study, the toxicological effects of BPA on Cylindrospermopsis raciborskii were assessed based on chlorophyll a fluorescence and transcriptome analyses. The results showed that the growth of C. raciborskii was significantly inhibited when BPA exceeded 0.1 mg L^-1. A marked rise of phase J was observed at a concentration greater than 0.1 mg L^-1, while a K phase appeared at 20 mg L^-1. The chlorophyll a fluorescence parameters of RC/CS₀, F₀, φ_P0, φ_E0, and ψ₀, underwent a significant decline under all treatments of BPA, whereas a significant increase in both V_J and M₀ occurred under all concentrations of BPA. Additionally, ABS/RC and DIo/RC markedly increased at 10 mg L^-1 and 20 mg L^-1. The transcriptome analysis revealed that the genes of photosynthesis, including psbA, psbB, psbC, psbD, apcA, apcB, cpcA, and cpcB, as well as those of chlorophyll and carotenoid biosynthesis, namely hemN, acsF, chlL, chlN, chlP, crtB, pds, were all down-regulated. Moreover, BPA also inhibited the oxidative phosphorylation, glycolysis/gluconeogenesis, citrate cycle (TCA cycle), and fatty acid metabolism in C. raciborskii. Taken together, these results suggest BPA can negatively affect the expression of multiple genes and the vital energy metabolism process to arrest the growth and photosynthesis of C. raciborskii.

Fourier Transform Infrared Analysis of the S-State Cycle of Water Oxidation in the Microcrystals of Photosystem II

Photosynthetic water oxidation is performed in photosystem II (PSII) through a light-driven cycle of intermediates called S states (S₀-S₄) at the water oxidizing center. Time-resolved serial femtosecond crystallography (SFX) has recently been applied to the microcrystals of PSII to obtain the structural information on these intermediates. However, it remains unanswered whether the reactions efficiently proceed throughout the S-state cycle retaining the native structures of the intermediates in PSII crystals. We investigated the water oxidation reactions in the PSII microcrystals using flash-induced Fourier transform infrared (FTIR) difference spectroscopy. In comparison with the FTIR spectra in solution, it was shown that all of the metastable intermediates in the microcrystals retained their native structures, and the efficiencies of the S-state transitions remained relatively high, although those of the S₂ → S₃ and S₃ → S₀ transitions were slightly lowered possibly due to some restriction of water movement in the crystals.

Photosynthetic water splitting by the Mn4Ca2+OX catalyst of photosystem II: its structure, robustness and mechanism

The biological energy cycle of our planet is driven by photosynthesis whereby sunlight is absorbed by chlorophyll and other accessory pigments. The excitation energy is then efficiently transferred to a reaction centre where charge separation occurs in a few picoseconds. In the case of photosystem II (PSII), the energy of the charge transfer state is used to split water into oxygen and reducing equivalents. This is accomplished by the relatively low energy content of four photons of visible light. PSII is a large multi-subunit membrane protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Four high energy electrons, together with four protons (4H+), are used to reduce plastoquinone (PQ), the terminal electron acceptor of PSII, to plastoquinol (PQH2). PQH2 passes its reducing equivalents to an electron transfer chain which feeds into photosystem I (PSI) where they gain additional reducing potential from a second light reaction which is necessary to drive CO2 reduction. The catalytic centre of PSII consists of a cluster of four Mn ions and a Ca2+ linked by oxo bonds. In addition, there are seven amino acid ligands. In this Article, I discuss the structure of this metal cluster, its stability and the probability that an acid-base (nucleophilic-electrophilic) mechanism catalyses the water splitting reaction on the surface of the metal-cluster. Evidence for this mechanism is presented from studies on water splitting catalysts consisting of organo-complexes of ruthenium and manganese and also by comparison with the enzymology of carbon monoxide dehydrogenase (CODH). Finally the relevance of our understanding of PSII is discussed in terms of artificial photosynthesis with emphasis on inorganic water splitting catalysts as oxygen generating photoelectrodes.

Effect of different methods of Ca2+ extraction from PSII oxygen-evolving complex on the QA- oxidation kinetics

Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster Mn₄CaO₅ of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca²⁺ cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca²⁺ extraction from OEC on the kinetics of the reduced primary electron acceptor (Q_A^-) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from Q_A^- to the secondary quinone acceptor Q_B. Electron transfer from Q_A^- to Q_B in photosystem II membranes with an occupied Q_B site was slowed down by a factor of 8. However, addition of EGTA or CaCl₂ to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of Q_A^- oxidation by Q_B in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca²⁺ from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the Q_A^- to Q_B electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca²⁺ depletion.

Natural and Artificial Mn4 Ca Cluster for the Water Splitting Reaction

The oxygen-evolving center (OEC) in photosystem II (PSII) is a unique biological catalyst that splits water into electrons, protons, and O₂ by using solar energy. Recent crystallographic studies have revealed that the structure of the OEC is an asymmetric Mn₄ Ca cluster, which provides a blueprint to develop man-made water-splitting catalysts for artificial photosynthesis. Although it is a great challenge to mimic the whole structure and function of the OEC in the laboratory, significant advances have recently been achieved. In this Minireview, recent progress on mimicking the natural OEC is discussed. New strategies are suggested to construct more stable and efficient new generation of catalytic materials for the water splitting reaction based on the artificial Mn₄ Ca cluster in the future.

Inhibitory and Non-Inhibitory NH3 Binding at the Water-Oxidizing Manganese Complex of Photosystem II Suggests Possible Sites and a Rearrangement Mode of Substrate Water Molecules

The identity and rearrangements of substrate water molecules in photosystem II (PSII) water oxidation are of great mechanistic interest and addressed herein by comprehensive analysis of NH₄⁺/NH₃ binding. Time-resolved detection of O₂ formation and recombination fluorescence as well as Fourier transform infrared (FTIR) difference spectroscopy on plant PSII membrane particles reveals the following. (1) Partial inhibition in NH₄Cl buffer occurs with a pH-independent binding constant of ∼25 mM, which does not result from decelerated O₂ formation, but from complete blockage of a major PSII fraction (∼60%) after reaching the Mn(IV)₄ (S₃) state. (2) The non-inhibited PSII fraction advances through the reaction cycle, but modified nuclear rearrangements are suggested by FTIR difference spectroscopy. (3) Partial inhibition can be explained by anticooperative (mutually exclusive) NH₃ binding to one inhibitory and one non-inhibitory site; these two sites may correspond to two water molecules terminally bound to the "dangling" Mn ion. (4) Unexpectedly strong modifications of the FTIR difference spectra suggest that in the non-inhibited PSII, ammonia binding obliterates the need for some of the nuclear rearrangements occurring in the S₂-S₃ transition as well as their reversal in the O₂ formation transition, in line with the carousel mechanism [Askerka, M., et al. (2015) Biochemistry 54, 5783]. (5) We observe the same partial inhibition of PSII by NH₄Cl also for thylakoid membranes prepared from mesophilic and thermophilic cyanobacteria, suggesting that the results described above are valid for plant and cyanobacterial PSII.

D1-Asn-298 in photosystem II is involved in a hydrogen-bond network near the redox-active tyrosine YZ for proton exit during water oxidation

In photosynthetic water oxidation, two water molecules are converted into one oxygen molecule and four protons at the Mn₄CaO₅ cluster in photosystem II (PSII) via the S-state cycle. Efficient proton exit from the catalytic site to the lumen is essential for this process. However, the exit pathways of individual protons through the PSII proteins remain to be identified. In this study, we examined the involvement of a hydrogen-bond network near the redox-active tyrosine Y_Z in proton transfer during the S-state cycle. We focused on spectroscopic analyses of a site-directed variant of D1-Asn-298, a residue involved in a hydrogen-bond network near Y_Z We found that the D1-N298A mutant of Synechocystis sp. PCC 6803 exhibits an O₂ evolution activity of ∼10% of the wild-type. D1-N298A and the wild-type D1 had very similar features of thermoluminescence glow curves and of an FTIR difference spectrum upon Y_Z oxidation, suggesting that the hydrogen-bonded structure of Y_Z and electron transfer from the Mn₄CaO₅ cluster to Y_Z were little affected by substitution. In the D1-N298A mutant, however, the flash-number dependence of delayed luminescence showed a monotonic increase without oscillation, and FTIR difference spectra of the S-state cycle indicated partial and significant inhibition of the S₂ → S₃ and S₃ → S₀ transitions, respectively. These results suggest that the D1-N298A substitution inhibits the proton transfer processes in the S₂ → S₃ and S₃ → S₀ transitions. This in turn indicates that the hydrogen-bond network near Y_Z can be functional as a proton transfer pathway during photosynthetic water oxidation.

Frequently asked questions about chlorophyll fluorescence, the sequel

Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.

Genetically introduced hydrogen bond interactions reveal an asymmetric charge distribution on the radical cation of the special-pair chlorophyll P680

The special-pair chlorophyll (Chl) P680 in photosystem II has an extremely high redox potential (E_m ) to enable water oxidation in photosynthesis. Significant positive-charge localization on one of the Chl constituents, P_D1 or P_D2, in P680⁺ has been proposed to contribute to this high E_m To identify the Chl molecule on which the charge is mainly localized, we genetically introduced a hydrogen bond to the 13¹-keto C=O group of P_D1 and P_D2 by changing the nearby D1-Val-157 and D2-Val-156 residues to His, respectively. Successful hydrogen bond formation at P_D1 and P_D2 in the obtained D1-V157H and D2-V156H mutants, respectively, was monitored by detecting 13¹-keto C=O vibrations in Fourier transfer infrared (FTIR) difference spectra upon oxidation of P680 and the symmetrically located redox-active tyrosines Y_Z and Y_D, and they were simulated by quantum-chemical calculations. Analysis of the P680⁺/P680 FTIR difference spectra of D1-V157H and D2-V156H showed that upon P680⁺ formation, the 13¹-keto C=O frequency upshifts by a much larger extent in P_D1 (23 cm^-1) than in P_D2 (<9 cm^-1). In addition, thermoluminescence measurements revealed that the D1-V157H mutation increased the E_m of P680 to a larger extent than did the D2-V156H mutation. These results, together with the previous results for the mutants of the His ligands of P_D1 and P_D2, lead to a definite conclusion that a charge is mainly localized to P_D1 in P680<sup/>.

The O2-Evolving Complex of Photosystem II: Recent Insights from Quantum Mechanics/Molecular Mechanics (QM/MM), Extended X-ray Absorption Fine Structure (EXAFS), and Femtosecond X-ray Crystallography Data

Efficient photoelectrochemical water oxidation may open a way to produce energy from renewable solar power. In biology, generation of fuel due to water oxidation happens efficiently on an immense scale during the light reactions of photosynthesis. To oxidize water, photosynthetic organisms have evolved a highly conserved protein complex, Photosystem II. Within that complex, water oxidation happens at the CaMn₄O₅ inorganic catalytic cluster, the so-called oxygen-evolving complex (OEC), which cycles through storage "S" states as it accumulates oxidizing equivalents and produces molecular oxygen. In recent years, there has been significant progress in understanding the OEC as it evolves through the catalytic cycle. Studies have combined conventional and femtosecond X-ray crystallography with extended X-ray absorption fine structure (EXAFS) and quantum mechanics/molecular mechanics (QM/MM) methods and have addressed changes in protonation states of μ-oxo bridges and the coordination of substrate water through the analysis of ammonia binding as a chemical analog of water. These advances are thought to be critical to understanding the catalytic cycle since protonation states regulate the relative stability of different redox states and the geometry of the OEC. Therefore, establishing the mechanism for substrate water binding and the nature of protonation/redox state transitions in the OEC is essential for understanding the catalytic cycle of O₂ evolution. The structure of the dark-stable S₁ state has been a target for X-ray crystallography for the past 15 years. However, traditional X-ray crystallography has been hampered by radiation-induced reduction of the OEC. Very recently, a revolutionary X-ray free electron laser (XFEL) technique was applied to PSII to reveal atomic positions at 1.95 Å without radiation damage, which brought us closer than ever to establishing the ultimate structure of the OEC in the S₁ state. However, the atom positions in this crystal structure are still not consistent with high-resolution EXAFS spectroscopy, partially due to the poorly resolved oxygen positions next to Mn centers and partial reduction due to extended dark adaptation of the sample. These inconsistencies led to the new models of the OEC with an alternative low oxidation state and raised questions on the protonation state of the cluster, especially the O5 μ-oxo bridge. This Account summarizes the most recent models of the OEC that emerged from QM/MM, EXAFS and femtosecond X-ray crystallography methods. When PSII in the S₁ state is exposed to light, the S₁ state is advanced to the higher oxidation states and eventually binds substrate water molecules. Identifying the substrate waters is of paramount importance for establishing the water-oxidation mechanism but is complicated by a large number of spectroscopically similar waters. Water analogues can, therefore, be helpful because they serve as spectroscopic markers that help to track the motion of the substrate waters. Due to a close structural and electronic similarity to water, ammonia has been of particular interest. We review three competing hypotheses on substrate water/ammonia binding and compile theoretical and experimental evidence to support them. Binding of ammonia as a sixth ligand to Mn4 during the S₁ → S₂ transition seems to satisfy most of the criteria, especially the most compelling recent EPR data on D1-D61A mutated PSII. Such a binding mode suggests delivery of water from the "narrow" channel through a "carousel" rearrangement of waters around Mn4 upon the S₂ → S₃ transition. An alternative hypothesis suggests water delivery through the "large" channel on the Ca side. However, both water delivery paths lead to a similar S₃ structure, seemingly reaching consensus on the nature of the last detectable S-state intermediate in the Kok cycle before O₂ evolution.

Conversion of photosystem II dimer to monomers during photoinhibition is tightly coupled with decrease in oxygen-evolving activity in the diatom Chaetoceros gracilis

The rapid turnover of photosystem II (PSII) in diatoms is thought to be at an exceptionally high rate compared with other oxyphototrophs; however, its molecular mechanisms are largely unknown. In this study, we examined the photodamage and repair processes of PSII in the marine centric diatom Chaetoceros gracilis incubated at 30 or 300 μmol photons m^-2 s^-1 in the presence of a de novo protein-synthesis inhibitor. When de novo protein synthesis was blocked by chloramphenicol (Cm), oxygen-evolving activity gradually decreased even at 30 μmol photons m^-2 s^-1 and could not be detected at 12 h. PSII inactivation was enhanced by higher illumination. Using Cm-treated cells, the conversion of PSII dimer to monomers was observed by blue native PAGE. The rate of PSII monomerization was very similar to that of the decrease in oxygen-evolving activity under both light conditions. Immunological detection of D1 protein in the Cm-treated cells showed that the rate of D1 degradation was slower than that of the former two events, although it was more rapid than that observed in other oxyphototrophs. Thus, the three accelerated events, especially PSII monomerization, appear to cause the unusually high rate of PSII turnover in diatoms.