Bicarbonate Ion as a Critical Factor in Photosynthetic Oxygen Evolution

Abstract

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 in-cubation and light pretreatment of chloroplasts in various con-centrations of bicarbonate, just prior to assay, indicate that bi-carbonate binds to chloroplasts in the dark and is released again as the Hill reaction proceeds in the light. It is suggested that bi-carbonate ions may play a critical role in the oxygen-evolving process in photosynthesis. The ability of CO. to stimulate the Hill reaction has been extensively studied but remains little understood. The discovery of this phenomenon was first claimed by Boyle (5), although his results were later questioned (1). The first unequivocal evi-dence was presented by Warburg and Krippahl (20; also see 19), and since then many workers have observed it (4, 6-8, 16, 17, 21). However, as was pointed out by Good (6), there is little agreement as to the conditions necessary for observing