Mobile hydrogen carbonate acts as proton acceptor in photosynthetic water oxidation

Cyanobacteria, algae, and plants oxidize water to the O-2 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/HCO3 in water oxidation. Using membrane-inlet mass spectrometry we demonstrate that HCO3 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 O-2 and CO2. Depletion of HCO3 from the media leads, in the absence of added buffers, to a reversible down-regulation of O-2 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/ HCO3 in photosynthetic water oxidation.