The rate of photoinhibition of photosystem II (PSII) activities (photoinduced change of chlorophyll fluorescence yield, deltaF, and photoreduction of 2,6-dichlorophenol-indophenol) in O2-evolving pea subchloroplast membrane fragments in medium depleted of CO2 was considerably decreased upon addition of 5 mM NaHCO3 before the light treatment. A similar effect was observed when 100 microM MnCl2 was added before the photoinhibition. In PSII membrane fragments depleted of Mn, the preillumination led to irreversible loss of the capability of PSII to be reactivated by Mn2+, and the rate of the photoinhibition was decreased by a factor of 2 or 5 if the pre-illumination was done in the presence of 0.2 microM MnCl2 (approximately 4 Mn per PSII reaction center) added alone or in combination with 5 mM NaHCO3, respectively. A similar protective effect of bicarbonate was also revealed in the dark, during thermoinactivation of O2-evolving PSII at 40 degrees C: the rate of thermoinactivation of deltaF was decreased by a factor of 3 if 5 mM NaHCO3 was added to the medium. The results are consistent with the idea that bicarbonate is an essential component of the Mn-containing water-oxidizing complex of PSII, which decreases its susceptibility to photoinhibition and thermoinactivation.