Water oxidation in photosystem II ( PSII) is still insufficiently understood and is assumed to involve HCO3-. A Chlamydomonas mutant lacking a carbonic anhydrase associated with the PSII donor side shows impaired O-2 evolution in the absence of HCO3-. The O-2 evolution for saturating, continuous illumination (R-O2) was slower than in the wild type, but was elevated by HCO3- and increased further by Cah3. The R-O2 limitation in the absence of Cah3/HCO3- was amplified by H2O/D2O exchange, but relieved by an amphiphilic proton carrier, suggesting a role of Cah3/HCO3- in proton translocation. Chlorophyll fluorescence indicates a Cah3/HCO3- effect at the donor side of PSII. Time-resolved delayed fluorescence and O-2-release measurements suggest specific effects on proton-release steps but not on electron transfer. We propose that Cah3 promotes proton removal from the Mn complex by locally providing HCO3-, which may function as proton carrier. Without Cah3, proton removal could become rate limiting during O-2 formation and thus, limit water oxidation under high light. Our results underlie the general importance of proton release at the donor side of PSII during water oxidation.