A photosystem II-associated carbonic anhydrase regulates the efficiency of photosynthetic oxygen evolution

Highly Active Carbonic Anhydrase of the Thylakoid Lumen of Chlamydomonas reinhardtii

The green unicellular algae Chlamydomonas reinhardtii contains 12-13 carbonic anhydrases (CAs). For a long time, the two closely related α-CAs of the periplasmic membrane CAH1 and CAH2 were considered to be the CAs with the highest CO2 hydration activity. The recombinant protein α-CA CAH3 (rCAH3) from the thylakoid lumen obtained in the present study showed more than three times higher activity compared to CAH1 and more than 11 times higher compared to previous studies with rCAH3. Long-term sustainability of the enzyme was observed at alkaline pH (>8), with maintenance of half of its activity at 4 °C for up to 50 days. Thermostability of rCAH3 indicated the retention of the activity at 20 °C for one hour at pH 9-10 with its ~50% decrease at pH 6-7. However, the residual activity of rCAH3 after incubation at an extremely high temperature (75 °C) for 15 min led to the formation of the double-hump graph with maxima at pH 6 and 9. The enzyme demonstrated high sensitivity to ethoxyzolamide and acetazolamide at nM concentrations, to Zn2+ and Cu2+ cations at 1 mM concentrations, and L-cysteine was able to completely inhibit CA activity of rCAH3 through reduction of sulfhydryl groups. Esterase activity of rCAH3 was well detected with values comparable to those of bovine CAII, but with a maximum at pH 8 instead of pH 9, which is usual for bovine CAII. The results indicated that CAH3 may be the most active CA of C. reinhardtii and that its role in the photosynthetic apparatus function could have been underestimated in previous works.

CAH3 from Chlamydomonas reinhardtii: Unique Carbonic Anhydrase of the Thylakoid Lumen

CAH3 is the only carbonic anhydrase (CA) present in the thylakoid lumen of the green algae Chlamydomonas reinhardtii. The monomer of the enzyme has a molecular weight of ~29.5 kDa with high CA activity. Through its dehydration activity, CAH3 can be involved either in the carbon-concentrating mechanism supplying CO2 for RuBisCO in the pyrenoid or in supporting the maximal photosynthetic activity of photosystem II (PSII) by accelerating the removal of protons from the active center of the water-oxidizing complex. Both proposed roles are considered in this review, together with a description of the enzymatic parameters of native and recombinant CAH3, the crystal structure of the protein, and the possible use of lumenal CA as a tool for increasing biomass production in higher plants. The identified involvement of lumenal CAH3 in the function of PSII is still unique among green algae and higher plants and can be used to understand the mechanism(s) of the functional interconnection between PSII and the proposed CA(s) of the thylakoid lumen in other organisms.

Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

The intracellular accumulation of inorganic carbon (C_i) by microalgae and cyanobacteria under ambient atmospheric CO₂ levels was first documented in the 80s of the 20th Century. Hence, a third variety of the CO₂-concentrating mechanism (CCM), acting in aquatic photoautotrophs with the C₃ photosynthetic pathway, was revealed in addition to the then-known schemes of CCM, functioning in CAM and C₄ higher plants. Despite the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of microalgae and cyanobacteria for the CO₂ substrate and low CO₂/O₂ specificity, CCM allows them to perform efficient CO₂ fixation in the reductive pentose phosphate (RPP) cycle. CCM is based on the coordinated operation of strategically located carbonic anhydrases and CO₂/HCO₃^- uptake systems. This cooperation enables the intracellular accumulation of HCO₃^-, which is then employed to generate a high concentration of CO₂ molecules in the vicinity of Rubisco's active centers compensating up for the shortcomings of enzyme features. CCM functions as an add-on to the RPP cycle while also acting as an important regulatory link in the interaction of dark and light reactions of photosynthesis. This review summarizes recent advances in the study of CCM molecular and cellular organization in microalgae and cyanobacteria, as well as the fundamental principles of its functioning and regulation.

Effect of CO2 Content in Air on the Activity of Carbonic Anhydrases in Cytoplasm, Chloroplasts, and Mitochondria and the Expression Level of Carbonic Anhydrase Genes of the α- and β-Families in Arabidopsis thaliana Leaves

The carbonic anhydrase (CA) activities of the preparations of cytoplasm, mitochondria, chloroplast stroma, and chloroplast thylakoids, as well as the expression levels of genes encoding αCA1, αCA2, αCA4, βCA1, βCA2, βCA3, βCA4, βCA5, and βCA6, were measured in the leaves of Arabidopsis thaliana plants, acclimated to different CO₂ content in the air: low (150 ppm, lCO₂), normal (450 ppm, nCO₂), and high (1200 ppm, hCO₂). To evaluate the photosynthetic apparatus operation, the carbon assimilation and chlorophyll a fluorescence were measured under the same conditions. It was found that the CA activities of the preparations of cytoplasm, chloroplast stroma, and chloroplast thylakoids measured after two weeks of acclimation were higher, the lower CO₂ concentration in the air. That was preceded by an increase in the expression levels of genes encoding the cytoplasmic form of βCA1, and other cytoplasmic CAs, βCA2, βCA3, and βCA4, as well as of the chloroplast CAs, βCA5, and the stromal forms of βCA1 in a short-term range 1–2 days after the beginning of the acclimation. The dependence on the CO₂ content in the air was most noticeable for the CA activity of the preparations of the stroma; it was two orders higher in lCO₂ plants than in hCO₂ plants. The CA activity of thylakoid membranes from lCO₂ plants was higher than that in nCO₂ and hCO₂ plants; however, in these plants, a significant increase in the expression levels of the genes encoding αCA2 and αCA4 located in thylakoid membranes was not observed. The CA activity of mitochondria and the expression level of the mitochondrial βCA6 gene did not depend on the content of carbon dioxide. Taken together, the data implied that in the higher plants, the supply of inorganic carbon to carboxylation sites is carried out with the cooperative functioning of CAs located in the cytoplasm and CAs located in the chloroplasts.

Loss of carbonic anhydrase in the thylakoid lumen causes unusual moderate-light-induced rearrangement of the chloroplast in Chlamydomonas reinhardtii as a way of photosystem II photoprotection

Chlamydomonas reinhardtii cells have a single large cup-shaped chloroplast that can lose lobes under high light to prevent photodamage of the photosynthetic apparatus, including photosystem II (PSII). Here, under moderate light treatment, the development of the unusual morphology of the chloroplast is shown for mutant cia3, which is deficient in carbonic anhydrase (EC 4.2.1.1) CAH3 in the thylakoid lumen, while such light intensity is harmless for wild type (WT) cells for hours. Cia3 cells had more activated PSII photoprotective mechanisms and showed a tendency to shift in the balance of the PSII damage-repair cycle, whereas PSII retained the same photosynthetic efficiency as in the WT. These findings allow speculation about the unique PSII photoprotection strategy by rearranging the chloroplast in the absence of CAH3. CAH3, in turn, is suggested to be an important participant of the C. reinhardtii photosynthetic apparatus operation, functioning in close connection with PSII.

Unsolved Problems of Carbonic Anhydrases Functioning in Photosynthetic Cells of Higher C3 Plants

The review presents current data on carbonic anhydrases found in various compartments of photosynthetic cells of higher plants. The available data on expression of genes some of carbonic anhydrases and its dependence on environmental factors and plant age are considered. The existing hypotheses on the functions of carbonic anhydrases of plasma membrane, cytoplasm, as well as of stroma and thylakoids of chloroplast, first of all, the hypothesis on participation of these enzymes in supplying carbon dioxide molecules to ribulose-bisphosphate carboxylase (Rubisco) are analyzed. Difficulties of establishing physiological role of the plant cell carbonic anhydrase are discussed in detail.

Involvement of Carbonic Anhydrase CAH3 in the Structural and Functional Stabilization of the Water-Oxidizing Complex of Photosystem II from Chlamydomonas reinhardtii

The involvement of carbonic anhydrases (CA) and CA activity in the functioning of photosystem II (PSII) has been studied for a long time and has been shown in many works. However, so far only for CAH3 from Chlamydomonas reinhardtii there is evidence for its association with the donor side of PSII, where the CA activity of CAH3 can influence the functioning of the water-oxidizing complex (WOC). Our results suggest that CAH3 is also involved in the organization of the native structure of WOC independently of its CA activity. It was shown that in PSII preparations from wild type (WT) the high O2-evolving activity of WOC was observed up to 100 mM NaCl in the medium and practically did not decrease with increasing incubation time with NaCl. At the same time, the WOC function in PSII preparations from CAH3-deficient mutant cia3 is significantly inhibited already at NaCl concentrations above 35 mM, reaching 50% at 100 mM NaCl and increased incubation time. It is suggested that the absence of CAH3 in PSII from cia3 causes disruption of the native structure of WOC, allowing more pronounced conformational changes of its proteins and, consequently, suppression of the WOC active center function, when the ionic strength of the medium is increased. The results of Western blot analysis indicate a more difficult removal of PsbP protein from PSII of cia3 at higher NaCl concentrations, apparently due to the changes in the intermolecular interactions between proteins of WOC in the absence of CAH3. At the same time, the values of the maximum quantum yield of PSII did not practically differ between preparations from WT and cia3, indicating no effect of CAH3 on the photoinduced electron transfer in the reaction center of PSII. The obtained results indicate the involvement of the CAH3 protein in the native organization of the WOC and, as a consequence, in the stabilization of its functional state in PSII from C. reinhardtii.

Stress-Related Changes in the Expression and Activity of Plant Carbonic Anhydrases

The data on stress-related changes in the expression and activity of plant carbonic anhydrases (CAs) suggest that they are generally upregulated at moderate stress severity. This indicates probable involvement of CAs in adaptation to drought, high salinity, heat, high light, C_i deficit, and excess bicarbonate. The changes in CA levels under cold stress are less studied and generally represented by the downregulation of CAs excepting βCA2. Excess Cd²⁺ and deficit of Zn²⁺ specifically reduce CA activity and reduce its synthesis. Probable roles of βCAs in stress adaptation include stomatal closure, ROS scavenging and partial compensation for decreased mesophyll CO₂ conductance. βCAs play contrasting roles in pathogen responses, interacting with phytohormone signaling networks. Their role can be either negative or positive, probably depending on the host-pathogen system, pathogen initial titer, and levels of ·NO and ROS. It is still not clear why CAs are suppressed under severe stress levels. It should be noted, that the role of βCAs in the facilitation of CO₂ diffusion and their involvement in redox signaling or ROS detoxication are potentially antagonistic, as they are inactivated by oxidation or nitrosylation. Interestingly, some chloroplastic βCAs may be relocated to the cytoplasm under stress conditions, but the physiological meaning of this effect remains to be studied.

Bicarbonate-Mediated CO2 Formation on Both Sides of Photosystem II

The effect of bicarbonate (HCO3-) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO3- enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO2 and H2O. The other is for the electron-acceptor side, in which (i) reduction of the QA quinone leads to the release of HCO3- from its binding site on the non-heme iron and (ii) the Em potential of the QA/QA•- couple increases when HCO3- dissociates. This suggested a protective/regulatory role of HCO3- as it is known that increasing the Em of QA decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with 13C isotope labeling of HCO3- allows donor- and acceptor-side formation of CO2 by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under in vivo conditions.

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.

Is carbonic anhydrase activity of photosystem II required for its maximum electron transport rate?

It is known, that the multi-subunit complex of photosystem II (PSII) and some of its single proteins exhibit carbonic anhydrase activity. Previously, we have shown that PSII depletion of HCO3-/CO2 as well as the suppression of carbonic anhydrase activity of PSII by a known inhibitor of α‑carbonic anhydrases, acetazolamide (AZM), was accompanied by a decrease of electron transport rate on the PSII donor side. It was concluded that carbonic anhydrase activity was required for maximum photosynthetic activity of PSII but it was not excluded that AZM may have two independent mechanisms of action on PSII: specific and nonspecific. To investigate directly the specific influence of carbonic anhydrase inhibition on the photosynthetic activity in PSII we used another known inhibitor of α‑carbonic anhydrase, trifluoromethanesulfonamide (TFMSA), which molecular structure and physicochemical properties are quite different from those of AZM. In this work, we show for the first time that TFMSA inhibits PSII carbonic anhydrase activity and decreases rates of both the photo-induced changes of chlorophyll fluorescence yield and the photosynthetic oxygen evolution. The inhibitory effect of TFMSA on PSII photosynthetic activity was revealed only in the medium depleted of HCO3-/CO2. Addition of exogenous HCO3- or PSII electron donors led to disappearance of the TFMSA inhibitory effect on the electron transport in PSII, indicating that TFMSA inhibition site was located on the PSII donor side. These results show the specificity of TFMSA action on carbonic anhydrase and photosynthetic activities of PSII. In this work, we discuss the necessity of carbonic anhydrase activity for the maximum effectiveness of electron transport on the donor side of PSII.

Vyacheslav (Slava) Klimov (1945-2017): A scientist par excellence, a great human being, a friend, and a Renaissance man

Vyacheslav Vasilevich (V.V.) Klimov (or Slava, as most of us called him) was born on January 12, 1945 and passed away on May 9, 2017. He began his scientific career at the Bach Institute of Biochemistry of the USSR Academy of Sciences (Akademy Nauk (AN) SSSR), Moscow, Russia, and then, he was associated with the Institute of Photosynthesis, Pushchino, Moscow Region, for about 50 years. He worked in the field of biochemistry and biophysics of photosynthesis. He is known for his studies on the molecular organization of photosystem II (PSII). He was an eminent scientist in the field of photobiology, a well-respected professor, and, above all, an outstanding researcher. Further, he was one of the founding members of the Institute of Photosynthesis in Pushchino, Russia. To most, Slava Klimov was a great human being. He was one of the pioneers of research on the understanding of the mechanism of light energy conversion and of water oxidation in photosynthesis. Slava had many collaborations all over the world, and he is (and will be) very much missed by the scientific community and friends in Russia as well as around the World. We present here a brief biography and some comments on his research in photosynthesis. We remember him as a friendly and enthusiastic person who had an unflagging curiosity and energy to conduct outstanding research in many aspects of photosynthesis, especially that related to PSII.

An Update on the Metabolic Roles of Carbonic Anhydrases in the Model Alga Chlamydomonas reinhardtii

Carbonic anhydrases (CAs) are metalloenzymes that are omnipresent in nature. CAs catalyze the basic reaction of the reversible hydration of CO₂ to HCO₃⁻ and H⁺ in all living organisms. Photosynthetic organisms contain six evolutionarily different classes of CAs, which are namely: α-CAs, β-CAs, γ-CAs, δ-CAs, ζ-CAs, and θ-CAs. Many of the photosynthetic organisms contain multiple isoforms of each CA family. The model alga Chlamydomonas reinhardtii contains 15 CAs belonging to three different CA gene families. Of these 15 CAs, three belong to the α-CA gene family; nine belong to the β-CA gene family; and three belong to the γ-CA gene family. The multiple copies of the CAs in each gene family may be due to gene duplications within the particular CA gene family. The CAs of Chlamydomonas reinhardtii are localized in different subcellular compartments of this unicellular alga. The presence of a large number of CAs and their diverse subcellular localization within a single cell suggests the importance of these enzymes in the metabolic and biochemical roles they perform in this unicellular alga. In the present review, we update the information on the molecular biology of all 15 CAs and their metabolic and biochemical roles in Chlamydomonas reinhardtii. We also present a hypothetical model showing the known functions of CAs and predicting the functions of CAs for which precise metabolic roles are yet to be discovered.

The multifunctional face of plant carbonic anhydrase

Although most studies on the ubiquitous enzyme carbonic anhydrase (CA) have indicated its significant role in plants to facilitate the diffusion of CO₂ to the site of inorganic carbon fixation, it is becoming increasingly likely that carbonic anhydrase isoforms also have diverse unexplored functions in plant cells. This review lays emphasis on additional roles of CA associated with many physiological, biochemical and structural changes in plant metabolism. The presented findings have revealed essential functions of CA isoforms in plant adjustment to both abiotic and biotic agents and developmental stimuli. However, sometimes it is difficult to separate the non-photosynthetic from the photosynthetic-related role of CAs during post-stress impaired metabolism, and the preventive CA outcome might be due to the effect of these enzymes on improvement of photosynthetic capacity. Finally, taking into account the experimental evidence, the direct and indirect functional roles of CAs in mitigating negative effects of environmental conditions are presented.

Plant Carbonic Anhydrases: Structures, Locations, Evolution, and Physiological Roles

Carbonic anhydrases (CAs) are zinc metalloenzymes that catalyze the interconversion of CO2 and HCO3- and are ubiquitous in nature. Higher plants contain three evolutionarily distinct CA families, αCAs, βCAs, and γCAs, where each family is represented by multiple isoforms in all species. Alternative splicing of CA transcripts appears common; consequently, the number of functional CA isoforms in a species may exceed the number of genes. CAs are expressed in numerous plant tissues and in different cellular locations. The most prevalent CAs are those in the chloroplast, cytosol, and mitochondria. This diversity in location is paralleled in the many physiological and biochemical roles that CAs play in plants. In this review, the number and types of CAs in C3, C4, and crassulacean acid metabolism (CAM) plants are considered, and the roles of the α and γCAs are briefly discussed. The remainder of the review focuses on plant βCAs and includes the identification of homologs between species using phylogenetic approaches, a consideration of the inter- and intracellular localization of the proteins, along with the evidence for alternative splice forms. Current understanding of βCA tissue-specific expression patterns and what controls them are reviewed, and the physiological roles for which βCAs have been implicated are presented.

Carbon Supply and Photoacclimation Cross Talk in the Green Alga Chlamydomonas reinhardtii

Photosynthetic organisms are exposed to drastic changes in light conditions, which can affect their photosynthetic efficiency and induce photodamage. To face these changes, they have developed a series of acclimation mechanisms. In this work, we have studied the acclimation strategies of Chlamydomonas reinhardtii, a model green alga that can grow using various carbon sources and is thus an excellent system in which to study photosynthesis. Like other photosynthetic algae, it has evolved inducible mechanisms to adapt to conditions where carbon supply is limiting. We have analyzed how the carbon availability influences the composition and organization of the photosynthetic apparatus and the capacity of the cells to acclimate to different light conditions. Using electron microscopy, biochemical, and fluorescence measurements, we show that differences in CO2 availability not only have a strong effect on the induction of the carbon-concentrating mechanisms but also change the acclimation strategy of the cells to light. For example, while cells in limiting CO2 maintain a large antenna even in high light and switch on energy-dissipative mechanisms, cells in high CO2 reduce the amount of pigments per cell and the antenna size. Our results show the high plasticity of the photosynthetic apparatus of C. reinhardtii This alga is able to use various photoacclimation strategies, and the choice of which to activate strongly depends on the carbon availability.

Carbonic anhydrases in photosynthetic cells of higher plants

This review presents information about carbonic anhydrases, enzymes catalyzing the reversible hydration of carbon dioxide in aqueous solutions. The families of carbonic anhydrases are described, and data concerning the presence of their representatives in organisms of different classes, and especially in the higher plants, are considered. Proven and hypothetical functions of carbonic anhydrases in living organisms are listed. Particular attention is given to those functions of the enzyme that are relevant to photosynthetic reactions. These functions in algae are briefly described. Data about probable functions of carbonic anhydrases in plasma membrane, mitochondria, and chloroplast stroma of higher plants are discussed. Update concerning carbonic anhydrases in chloroplast thylakoids of higher plants, i.e. their quantity and possible participation in photosynthetic reactions, is given in detail.

Introducing an algal carbon-concentrating mechanism into higher plants: location and incorporation of key components

Many eukaryotic green algae possess biophysical carbon-concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO2 concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H(14) CO3 (-) uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild-type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.

CARBONIC ANHYDRASE ACTIVITY OF INTEGRAL-FUNCTIONAL COMPLEXES OF THYLAKOID MEMBRANES OF SPINACH CHLOROPLASTS

Isolated thylakoid membranes were disrupted by treatment with nonionic detergents digitonin or dodecyl maltoside. Solubilized polypeptide complexes were separated by native gel charge shift electrophoresis. The position of ATP-synthase complex and its isolated catalytic part (CF1) within gel was determined using the color reaction for ATPase activity. Due to the presence of cytochromes, the red band in unstained gels corresponded to the cytochrome b6f complex. Localization of the cytochrome b6f complex, ATP synthase and coupling CF1 in the native gel was confirmed by their subunit composition determined after SDS-electrophoretic analysis. Carbonic anhydrase (CA) activity in polypeptide zones of PS II, cytochrome b6f complex, and ATP-synthase CF1 was identified in native gels using indicator bromothymol blue. CA activity of isolated CF1 in solution was determined by infrared gas analysis as the rate of bicarbonate dehydration. The water-soluble acetazolamide, an inhibitor of CA, unlike lipophilic ethoxyzolamide inhibited CA activity of CF1 Thus, it was shown for the first time that ATP-synthase has a component which is capable of catalyzing the interconversion of forms of carbonic acid associated with proton exchange. The data obtained suggest the presence of multiple forms of carbonic anhydrase in the thylakoid membranes of spinach chloroplasts and confirm their involvement in the proton transfer to the ATP synthase.

The CO2 concentrating mechanism and photosynthetic carbon assimilation in limiting CO2 : how Chlamydomonas works against the gradient

The CO2 concentrating mechanism (CCM) represents an effective strategy for carbon acquisition that enables microalgae to survive and proliferate when the CO2 concentration limits photosynthesis. The CCM improves photosynthetic performance by raising the CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photorespiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and interconversion between different Ci species catalyzed by carbonic anhydrases (CAs) are key components in the CCM, and an array of molecular regulatory elements is present to facilitate the sensing of CO2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO2 conditions. This review intends to integrate our current understanding of the eukaryotic algal CCM and its interaction with carbon assimilation, based largely on Chlamydomonas as a model, and to illustrate how Chlamydomonas acclimates to limiting CO2 conditions and how its CCM is regulated.

Crystal structure and functional characterization of photosystem II-associated carbonic anhydrase CAH3 in Chlamydomonas reinhardtii

In oxygenic photosynthesis, light energy is stored in the form of chemical energy by converting CO2 and water into carbohydrates. The light-driven oxidation of water that provides the electrons and protons for the subsequent CO2 fixation takes place in photosystem II (PSII). Recent studies show that in higher plants, HCO3 (-) increases PSII activity by acting as a mobile acceptor of the protons produced by PSII. In the green alga Chlamydomonas reinhardtii, a luminal carbonic anhydrase, CrCAH3, was suggested to improve proton removal from PSII, possibly by rapid reformation of HCO3 (-) from CO2. In this study, we investigated the interplay between PSII and CrCAH3 by membrane inlet mass spectrometry and x-ray crystallography. Membrane inlet mass spectrometry measurements showed that CrCAH3 was most active at the slightly acidic pH values prevalent in the thylakoid lumen under illumination. Two crystal structures of CrCAH3 in complex with either acetazolamide or phosphate ions were determined at 2.6- and 2.7-Å resolution, respectively. CrCAH3 is a dimer at pH 4.1 that is stabilized by swapping of the N-terminal arms, a feature not previously observed in α-type carbonic anhydrases. The structure contains a disulfide bond, and redox titration of CrCAH3 function with dithiothreitol suggested a possible redox regulation of the enzyme. The stimulating effect of CrCAH3 and CO2/HCO3 (-) on PSII activity was demonstrated by comparing the flash-induced oxygen evolution pattern of wild-type and CrCAH3-less PSII preparations. We showed that CrCAH3 has unique structural features that allow this enzyme to maximize PSII activity at low pH and CO2 concentration.

Screening of novel chemical compounds as possible inhibitors of carbonic anhydrase and photosynthetic activity of photosystem II

Thirty novel chemical compounds were designed and synthesized expecting that they would be possible inhibitors. From this number eleven were organic bases, twenty-four were their organic derivatives and fourteen were metal complexes. Screening of these chemicals by their action on photosynthetic electron transfer (PET) and carbonic anhydrase (CA) activity (CAA) of photosystem II (PSII), α-CA, as well as β-CA was done. Several groups were revealed among them. Some of them are capable to suppress either one, two, three, or even all of the measured activities. As example, one of the Cu(II)-phenyl sulfonylhydrazone complexes (compound 25) suppresses CAA of α-CA by 88%, CAA of β-CA by 100% inhibition; CAA of PSII by 100% and the PSII photosynthetic activity by 66.2%. The Schiff base compounds (12, 15) and Cu(II)-phenyl sulfonylhydrazone complexes (25, 26) inhibited the CAA and PET of PSII significantly. The obtained data indicate that the PSII donor side is a target of the inhibitory action of these agents. Some physico- or electrochemical properties such as diffusion coefficient, number of transferred electrons, peak potential and heterogeneous standard rate constants of the compounds were determined in nonaqueous media. pKa values were also determined in nonaqueous and aqueous media. Availability in the studied group of novel chemical agents possessing different inhibitory activity allow in future to isolate the "active part" in the structure of the inhibitors responsible for different inhibitory mechanisms, as well as to determine the influence of side substituters on its inhibitory efficiency.

The presence of soluble carbonic anhydrase in the thylakoid lumen of chloroplasts from Arabidopsis leaves

Supernatant obtained after high-speed centrifugation of disrupted thylakoids that had been washed free from extrathylakoid carbonic anhydrases demonstrated carbonic anhydrase activity that was inhibited by the specific inhibitors acetazolamide and ethoxyzolamide. A distinctive feature of the effect of Triton X-100 on this activity also suggested that the source of the activity is a soluble protein. Native electrophoresis of a preparation obtained using chromatography with agarose/mafenide as an affinity sorbent revealed one protein band with carbonic anhydrase activity. The same protein was revealed in a mutant deficient in soluble stromal carbonic anhydrase β-CA1, and this indicated that the newly revealed carbonic anhydrase is not a product of the At3g01500 gene. These data imply the presence of soluble carbonic anhydrase in the thylakoid lumen of higher plants.

Mobile hydrogen carbonate acts as proton acceptor in photosynthetic water oxidation

Cyanobacteria, algae, and plants oxidize water to the O2 we breathe, and consume CO2 during the synthesis of biomass. Although these vital processes are functionally and structurally well separated in photosynthetic organisms, there is a long-debated role for CO2/ in water oxidation. Using membrane-inlet mass spectrometry we demonstrate that acts as a mobile proton acceptor that helps to transport the protons produced inside of photosystem II by water oxidation out into the chloroplast's lumen, resulting in a light-driven production of O2 and CO2. Depletion of from the media leads, in the absence of added buffers, to a reversible down-regulation of O2 production by about 20%. These findings add a previously unidentified component to the regulatory network of oxygenic photosynthesis and conclude the more than 50-y-long quest for the function of CO2/ in photosynthetic water oxidation.

Studying the oxidation of water to molecular oxygen in photosynthetic and artificial systems by time-resolved membrane-inlet mass spectrometry

Monitoring isotopic compositions of gaseous products (e.g., H2, O2, and CO2) by time-resolved isotope-ratio membrane-inlet mass spectrometry (TR-IR-MIMS) is widely used for kinetic and functional analyses in photosynthesis research. In particular, in combination with isotopic labeling, TR-MIMS became an essential and powerful research tool for the study of the mechanism of photosynthetic water-oxidation to molecular oxygen catalyzed by the water-oxidizing complex of photosystem II. Moreover, recently, the TR-MIMS and (18)O-labeling approach was successfully applied for testing newly developed catalysts for artificial water-splitting and provided important insight about the mechanism and pathways of O2 formation. In this mini-review we summarize these results and provide a brief introduction into key aspects of the TR-MIMS technique and its perspectives for future studies of the enigmatic water-splitting chemistry.

Efficiency of photosynthetic water oxidation at ambient and depleted levels of inorganic carbon

Over 40 years ago, Joliot et al. (Photochem Photobiol 10:309-329, 1969) designed and employed an elegant and highly sensitive electrochemical technique capable of measuring O2 evolved by photosystem II (PSII) in response to trains of single turn-over light flashes. The measurement and analysis of flash-induced oxygen evolution patterns (FIOPs) has since proven to be a powerful method for probing the turnover efficiency of PSII. Stemler et al. (Proc Natl Acad Sci USA 71(12):4679-4683, 1974), in Govindjee's lab, were the first to study the effect of "bicarbonate" on FIOPs by adding the competitive inhibitor acetate. Here, we extend this earlier work by performing FIOPs experiments at various, strictly controlled inorganic carbon (Ci) levels without addition of any inhibitors. For this, we placed a Joliot-type bare platinum electrode inside a N2-filled glove-box (containing 10-20 ppm CO2) and reduced the Ci concentration simply by washing the samples in Ci-depleted media. FIOPs of spinach thylakoids were recorded either at 20-times reduced levels of Ci or at ambient Ci conditions (390 ppm CO2). Numerical analysis of the FIOPs within an extended Kok model reveals that under Ci-depleted conditions the miss probability is discernibly larger (by 2-3 %) than at ambient conditions, and that the addition of 5 mM HCO3 (-) to the Ci-depleted thylakoids largely restores the original miss parameter. Since a "mild" Ci-depletion procedure was employed, we discuss our data with respect to a possible function of free or weakly bound HCO3 (-) at the water-splitting side of PSII.

Phosphorylation controls the localization and activation of the lumenal carbonic anhydrase in Chlamydomonas reinhardtii

Background

Cah3 is the only carbonic anhydrase (CA) isoform located in the thylakoid lumen of Chlamydomonas reinhardtii. Previous studies demonstrated its association with the donor side of the photosystem II (PSII) where it is required for the optimal function of the water oxidizing complex. However this enzyme has also been frequently proposed to perform a critical function in inorganic carbon acquisition and CO₂ fixation and all mutants lacking Cah3 exhibit very poor growth after transfer to low CO₂ conditions.

Results/Conclusions

In the present work we demonstrate that after transfer to low CO₂, Cah3 is phosphorylated and that phosphorylation is correlated to changes in its localization and its increase in activity. When C. reinhardtii wild-type cells were acclimated to limiting CO₂ conditions, the Cah3 activity increased about 5–6 fold. Under these conditions, there were no detectable changes in the level of the Cah3 polypeptide. The increase in activity was specifically inhibited in the presence of Staurosporine, a protein kinase inhibitor, suggesting that the Cah3 protein was post-translationally regulated via phosphorylation. Immunoprecipitation and in vitro dephosphorylation experiments confirm this hypothesis. In vivo phosphorylation analysis of thylakoid polypeptides indicates that there was a 3-fold increase in the phosphorylation signal of the Cah3 polypeptide within the first two hours after transfer to low CO₂ conditions. The increase in the phosphorylation signal was correlated with changes in the intracellular localization of the Cah3 protein. Under high CO₂ conditions, the Cah3 protein was only associated with the donor side of PSII in the stroma thylakoids. In contrast, in cells grown at limiting CO₂ the protein was partly concentrated in the thylakoids crossing the pyrenoid, which did not contain PSII and were surrounded by Rubisco molecules.

Significance

This is the first report of a CA being post-translationally regulated and describing phosphorylation events in the thylakoid lumen.

The unique structure of carbonic anhydrase αCA1 from Chlamydomonas reinhardtii

Chlamydomonas reinhardtii α-type carbonic anhydrase (Cr-αCA1) is a dimeric enzyme that catalyses the interconversion of carbon dioxide and carbonic acid. The precursor form of Cr-αCA1 undergoes post-translational cleavage and N-glycosylation. Comparison of the genomic sequences of precursor Cr-αCA1 and other αCAs shows that Cr-αCA1 contains a different N-terminal sequence and two insertion sequences. A 35-residue peptide in one of the insertion sequences is deleted from the precursor during maturation. The crystal structure of the mature form of Cr-αCA1 has been determined at 1.88 Å resolution. Each subunit is cleaved into the long and short peptides, but they are linked together by a disulfide bond. The two subunits are linked by a disulfide bond. N-Glycosylations occur at three asparagine residues and the attached N-glycans protrude into solvent regions. The subunits consist of a core β-sheet structure composed of nine β-strands. At the centre of the β-sheet is the catalytic site, which contains a Zn atom bound to three histidine residues. The amino-acid residues around the Zn atom are highly conserved in other monomeric and dimeric αCAs. The short peptide runs near the active site and forms a hydrogen bond to the zinc-coordinated residue in the long chain, suggesting an important role for the short peptide in Cr-αCA1 activity.

The carbonic anhydrase isoforms of Chlamydomonas reinhardtii: intracellular location, expression, and physiological roles

Aquatic photosynthetic organisms, such as the green alga Chlamydomonas reinhardtii, respond to low CO(2) conditions by inducing a CO(2) concentrating mechanism (CCM). Carbonic anhydrases (CAs) are important components of the CCM. CAs are zinc-containing metalloenzymes that catalyze the reversible interconversion of CO(2) and HCO(3)(-). In C. reinhardtii, there are at least 12 genes that encode CA isoforms, including three alpha, six beta, and three gamma or gamma-like CAs. The expression of the three alpha and six beta genes has been measured from cells grown on elevated CO(2) (having no active CCM) versus cells growing on low levels of CO(2) (with an active CCM) using northern blots, differential hybridization to DNA chips and quantitative RT-PCR. Recent RNA-seq profiles add to our knowledge of the expression of all of the CA genes. In addition, protein content for some of the CA isoforms was estimated using antibodies corresponding to the specific CA isoforms: CAH1/2, CAH3, CAH4/5, CAH6, and CAH7. The intracellular location of each of the CA isoforms was elucidated using immunolocalization and cell fractionation techniques. Combining these results with previous studies using CA mutant strains, we will discuss possible physiological roles of the CA isoforms concentrating on how these CAs might contribute to the acquisition and retention of CO(2) in C. reinhardtii.

The mechanism of anthracene interaction with photosynthetic apparatus: a study using intact cells, thylakoid membranes and PS II complexes isolated from Chlamydomonas reinhardtii

Intact cells of Chlamydomonas reinhardtii as well as isolated thylakoid membranes and photosystem II complexes were used to examine a possible mechanism of anthracene (ANT) interaction with the photosynthetic apparatus. Since ANT concentrations above 1 mM were required to significantly inhibit the rate of oxygen evolution in PS II membrane fragments it may indicate that the toxicant did not directly interact with this photosystem. On the other hand, stimulation of oxygen uptake by ANT-treated thylakoids suggested that ANT could either act as an artificial electron acceptor in the photosynthetic electron transport chain or function as an uncoupler. Electron transfer from excited chlorophyll to ANT is impossible due to the very low reduction potential of ANT and therefore we propose that toxic concentrations of ANT increase the thylakoid membrane permeability and thereby function as an uncoupler, enhancing electron transport in vitro. Hence, its unspecific interference with photosynthetic membranes in vitro suggests that the inhibitory effect observed on intact cell photosynthesis is caused by uncoupling of phosphorylation.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Solute transporters in plant thylakoid membranes: Key players during photosynthesis and light stress

Plants utilize sunlight to drive photosynthetic energy conversion in the chloroplast thylakoid membrane. Here are located four major photosynthetic complexes, about which we have great knowledge in terms of structure and function. However, much less we know about auxiliary proteins, such as transporters, ensuring an optimum function and turnover of these complexes. The most prominent thylakoid transporter is the proton-translocating ATP-synthase. Recently, four additional transporters have been identified in the thylakoid membrane of Arabidopsis thaliana, namely one copper-transporting P-ATPase, one chloride channel, one phosphate transporter, and one ATP/ADP carrier. Here, we review the current knowledge on the function and physiological role of these transporters during photosynthesis and light stress in plants. Subsequently, we make a survey on the outlook of thylakoid activities awaiting identification of responsible proteins. Such knowledge is necessary to understand the thylakoid network of transporters, and to design strategies for bioengineering crop plants in the future.

A carbonic anhydrase inhibitor induces bicarbonate-reversible suppression of electron transfer in pea photosystem 2 membrane fragments

The effects of suppression of the carbonic anhydrase (CA) activity by a CA-inhibitor, acetazolamide (AA), on the photosynthetic activities of photosystem II (PS II) particles from higher plants were investigated. AA along with CA-activity inhibits the PS II photosynthetic electron transfer and the AA-induced suppression is totally reversed by the addition of bicarbonate (3-5 mM). Similar effect of recovery in the PS II photosynthetic activity was also revealed upon the addition of known artificial electron donors (potassium ferrocyanide and TMPD). Significance and possible functions of CA for the PS II donor side are discussed.

Crystallographic study of wild-type carbonic anhydrase alpha CA1 from Chlamydomonas reinhardtii

Carbonic anhydrases (CAs) are ubiquitously distributed and are grouped into three structurally independent classes (alphaCA, betaCA and gammaCA). Most alphaCA enzymes are monomeric, but alphaCA1 from Chlamydomonas reinhardtii is a dimer that is uniquely stabilized by disulfide bonds. In addition, during maturation an internal peptide of 35 residues is removed and three asparagine residues are glycosylated. In order to obtain insight into the effects of these structural features on CA function, wild-type C. reinhardtii alphaCA1 has been crystallized in space group P6(5), with unit-cell parameters a=b=134.3, c=120.2 A. The crystal diffracted to 1.88 A resolution and a preliminary solution of its crystal structure has been obtained by the MAD method.

A pea mutant (costata) expressing higher activity in thylakoid membrane-bound carbonic anhydrase alters PSII downregulation mechanisms

The interaction between photosynthetic electron transport and the activities of the thylakoid associated carbonic anhydrase (tCA), estimated as combined tCA activity in pea plants (Pisum sativum L. Borek cv., WT) and mutant form (costata 2/125) that differ in chlorophyll content have been compared. Chlorophyll a fluorescence changes after the inhibition of tCA by ethoxyzolamide (EZ), estimating possible role of tCA in PSII downregulation were investigated. Costata expresses higher tCA activity and higher O2 evolution in comparison to WT. Inhibition of tCA by EZ decreased effective PSII photochemistry that coincided with an enhancement in thermal dissipation, while maximal PSII quantum yield (F(v)/F(m)) did not significantly change. Ethoxyzolamide induced changes in fluorescence parameters that were more strongly expressed in costata 2/125. The results show that tCA is involved in the regulation of the proton gradient across thylakoid membranes and thus limits PSII downregulation.

Manganese-dependent carboanhydrase activity of photosystem II proteins

Four sources of carbonic anhydrase (CA) activity in submembrane preparations of photosystem II (PS II) isolated from pea leaves were examined. Three of them belong to the hydrophilic proteins of the oxygen-evolving complex of PS II with molecular mass 33 kDa (protein PsbO), 24 kDa (protein PsbP), and 18 kDa (protein PsbQ). The fourth source of CA activity is associated with a pigment-protein complex of PS II after removing three hydrophilic proteins by salt treatment. Except for protein PsbQ, the CA activity of all these proteins depends on the presence of Mn2+: the purified protein PsbO did not show CA activity before adding Mn2+ into the medium (concentration of Mn2+ required for 50% effect, EC(50), was 670 microM); CA activity of protein mixture composed of PsbP and PsbQ increased more than 5-fold upon adding Mn2+ (EC(50) was 45 microM). CA activity of purified protein PsbP increased 2-fold in the presence of 200 microM Mn2+. As indicated for the mixture of two proteins (PsbP and PsbQ), Mg2+, Ca2+, and Zn2+, in contrast to Mn2+, suppressed CA activity (both initial and Mn2+-induced activity). Since the found sources of CA activity demonstrated properties different from ones of typical CA (need for Mn2+, insensitivity or low sensitivity to acetazolamide or ethoxyzolamide) and such CA activity was found only among PS II proteins, we cannot exclude that they belong to the type of Mn-dependent CA associated with PS II.

Intact anthracene inhibits photosynthesis in algal cells: a fluorescence induction study on Chlamydomonas reinhardtii cw92 strain

Short-term (24h) experiments were performed to examine the effect of anthracene (ANT) on Chlamydomonas reinhardtii cw92 grown in a batch culture system aerated with 2.5% CO(2). At concentrations ranging from 0.7 to 5.6 microM, ANT inhibited the growth of population in a concentration-dependent manner and EC(50) calculated amounted to 1.6 microM. At concentrations from 0.7 to 4.2 microM ANT stimulated respiration and inhibited the intensity of photosynthesis but did not affect chlorophyll content in the cells. ANT influenced chlorophyll a fluorescence parameters, measured by OJIP test (O, J, I and P are the different steps of fluorescence induction curve). ANT diminished the performance index (PI), the yield of primary photochemistry (phi(Po)), the yield of electron transport (phi(Epsilonomicron), the efficiency of moving the electron beyond Qa(-) (Psi(0)) and the fraction of active oxygen evolving complexes (OEC). The fraction of active PS II reaction centres in the treated samples dramatically dropped. The most pronounced changes in ANT-treated cells were observed in the stimulation of energy dissipation parameter (DI(0)/RC). The only OJIP parameter that was not influenced by ANT was energy absorption by photosynthetic antennae (ABS). The results lead to a conclusion that the inhibition of photosynthesis may be a consequence of unspecific ANT-membrane interaction, resulting from hydrophobic character of this hydrocarbon.

Heat stress: an overview of molecular responses in photosynthesis

The primary targets of thermal damage in plants are the oxygen evolving complex along with the associated cofactors in photosystem II (PSII), carbon fixation by Rubisco and the ATP generating system. Recent investigations on the combined action of moderate light intensity and heat stress suggest that moderately high temperatures do not cause serious PSII damage but inhibit the repair of PSII. The latter largely involves de novo synthesis of proteins, particularly the D1 protein of the photosynthetic machinery that is damaged due to generation of reactive oxygen species (ROS), resulting in the reduction of carbon fixation and oxygen evolution, as well as disruption of the linear electron flow. The attack of ROS during moderate heat stress principally affects the repair system of PSII, but not directly the PSII reaction center (RC). Heat stress additionally induces cleavage and aggregation of RC proteins; the mechanisms of such processes are as yet unclear. On the other hand, membrane linked sensors seem to trigger the accumulation of compatible solutes like glycinebetaine in the neighborhood of PSII membranes. They also induce the expression of stress proteins that alleviate the ROS-mediated inhibition of repair of the stress damaged photosynthetic machinery and are required for the acclimation process. In this review we summarize the recent progress in the studies of molecular mechanisms involved during moderate heat stress on the photosynthetic machinery, especially in PSII.

Redox interaction of Mn-bicarbonate complexes with reaction centres of purple bacteria

It is found that dark reduction of photooxidized primary electron donor P870+ in reaction centres from purple anoxygenic bacteria (two non-sulphur Fe-oxidizing Rhodovulum iodosum and Rhodovulum robiginosum, Rhodobacter sphaeroides R-26 and sulphur alkaliphilic Thiorhodospira sibirica) is accelerated upon the addition of Mn2+ jointly with bicarbonate (30-75 mM). The effect is not observed if Mn2+ and HCO3(-) have been replaced by Mg2+ and HCO2(-), respectively. The dependence of the effect on bicarbonate concentration suggests that formation of Mn2+-bicarbonate complexes, Mn(HCO3)+ and/or Mn(HCO3)2, is required for re-reduction of P870+ with Mn2+. The results are considered as experimental evidence for a hypothesis on possible participation of Mn-bicarbonate complexes in the evolutionary origin of oxygenic photosynthesis in the Archean era.

On the effect of heavy water (D2O) on carbon isotope fractionation in photosynthesis

Internal conductance to carbon dioxide is a key aspect of leaf photosynthesis although is still not well understood. It is thought that it comprises two components, namely, a gas phase component (diffusion from intercellular spaces to cell walls) and a liquid phase component (dissolution, diffusion in water, hydration equilibrium). Here we use heavy water (D₂O), which is known to slow down CO₂ hydration by a factor of nearly three. Using ¹²C/¹³C stable isotope techniques and Xanthium strumarium L. leaves, we show that the on-line carbon isotope discrimination (Δ¹³C, or Δ_obs) associated with photosynthesis is not significantly decreased by heavy water, and that the internal conductance, estimated with relationships involving the deviation of Δ¹³C, decreased by 8-40% in 21% O₂. It is concluded that in typical conditions, the CO₂-hydration equilibrium does not exert an effect on CO₂ assimilation larger than 9%. The carbon isotope discrimination associated with CO₂ addition to ribulose-1,5,bisphosphate by Rubisco is slightly decreased by heavy water. This effect is proposed to originate from the use of solvent-derived proton/deuteron during the last step of the catalytic cycle of the enzyme (hydration/cleavage).

Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II

Since the end of the 1950s hydrogencarbonate ('bicarbonate') is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn(4)O(x)Ca cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (~0.3) HCO(3)(-) per PSII upon addition of formate. The same amount of HCO(3)(-) release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO(3)(-) from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO(3)(-) is corroborated by showing that a reductive destruction of the Mn(4)O(x)Ca cluster inside the MIMS cell by NH(2)OH addition does not lead to any CO(2)/HCO(3)(-) release. We note that even after an essentially complete HCO(3)(-)/CO(2) removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain > or =75% of their initial flash-induced O(2)-evolving capacity. We therefore conclude that HCO(3)(-) has only 'indirect' effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.