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In photosynthesis, cyanobacteria, algae and plants fix carbon dioxide (CO(2)) into carbohydrates; this is necessary to support life on Earth. Over 50 years ago, Otto Heinrich Warburg discovered a unique stimulatory role of CO(2) in the Hill reaction (i.e., O(2) evolution accompanied by reduction of an artificial electron acceptor), which, obviously, does not include any carbon fixation pathway; Warburg used this discovery to support his idea that O(2) in photosynthesis originates in CO(2). During the 1960s, a large number of researchers attempted to decipher this unique phenomenon, with limited success. In the 1970s, Alan Stemler, in Govindjee's lab, perfected methods to get highly reproducible results, and observed, among other things, that the turnover of Photosystem II (PSII) was stimulated by bicarbonate ions (hydrogen carbonate): the effect would be on the donor or the acceptor, or both sides of PSII. In 1975, Thomas Wydrzynski, also in Govindjee's lab, discovered that there was a definite bicarbonate effect on the electron acceptor (the plastoquinone) side of PSII. The most recent 1.9Å crystal structure of PSII, unequivocally shows HCO(3)(-) bound to the non-heme iron that sits in-between the bound primary quinone electron acceptor, Q(A), and the secondary quinone electron acceptor Q(B). In this review, we focus on the historical development of our understanding of this unique bicarbonate effect on the electron acceptor side of PSII, and its mechanism as obtained by biochemical, biophysical and molecular biological approaches in many laboratories around the World. We suggest an atomic level model in which HCO(3)(-)/CO(3)(2-) plays a key role in the protonation of the reduced Q(B). In addition, we make comments on the role of bicarbonate on the donor side of PSII, as has been extensively studied in the labs of Alan Stemler (USA) and Vyacheslav Klimov (Russia). We end this review by discussing the uniqueness of bicarbonate's role in oxygenic photosynthesis and its role in the evolutionary development of O(2)-evolving PSII. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial., (Copyright © 2012 Elsevier B.V. All rights reserved.)

In this study we employ isotope ratio membrane-inlet mass spectrometry to probe the turnover efficiency of photosystem II (PSII) membrane fragments isolated from spinach at flash frequencies between 1Hz and 50Hz in the presence of the commonly used exogenous electron acceptors potassium ferricyanide(III) (FeCy), 2,5-dichloro-p-benzoquinone (DCBQ), and 2-phenyl-p-benzoquinone (PPBQ). The data obtained clearly indicate that among the tested acceptors PPBQ is the best at high flash frequencies. If present at high enough concentration, the PSII turnover efficiency is unaffected by flash frequency of up to 30Hz, and at 40Hz and 50Hz only a slight decrease by about 5-7% is observed. In contrast, drastic reductions of the O(2) yields by about 40% and 65% were found at 50Hz for DCBQ and FeCy, respectively. Comparison with literature data reveals that PPBQ accepts electrons from Q(A)(-) in PSII membrane fragments with similar efficiency as plastoquinone in intact cells. Our data also confirm that at high flashing rates O(2) evolution is limited by the reactions on the electron-acceptor side of PSII. The relevance of these data to the evolutionary development of the water-splitting complex in PSII and with regard to the potential of artificial water-splitting catalysts is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial., (Copyright © 2012 Elsevier B.V. All rights reserved.)

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.