Bicarbonate ion as a critical factor in photosynthetic oxygen evolution

Bicarbonate ion, not dissolved CO2 gas, is shown to increase 4-to 5-fold the rate of dichlorophenol indophenol reduction by isolated maize (Zea mays) chloroplasts. Glutaraldehyde fixed chloroplasts continue to exhibit bicarbonate-dependent 2,6-dichlorophenol indophenol reduction. Bicarbonate is shown to act close to the oxygen-evolving site, i.e. prior to the electron donation site of diphenyl carbazide to photosystem II. Dark incubation and light pretreatment of chloroplasts in various concentrations of bicarbonate, just prior to assay, indicate that bicarbonate binds to chloroplasts in the dark and is released again as the Hill reaction proceeds in the light. It is suggested that bicarbonate ions may play a critical role in the oxygen-evolving process in photosynthesis.