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Light-induced acidification by the cyanobacterium Anabaena variabilis is biphasic (a fast phase I and slow phase II) and shown to be sodium-dependent with an optimum concentration of 40 to 60 millimolar Na(+). Cells grown under low CO(2) concentrations at pH 9 (i.e. mainly HCO(3) (-) present in the medium) exhibited the slow phase II of proton efflux only, while cells grown under low CO(2) concentrations at pH 6.3 (i.e. CO(2) and HCO(3) (-) present) exhibited both phases. Light-induced proton release of phase I was dependent on inorganic carbon available in the bathing medium with an apparent K(m) for CO(2) of 20 to 70 micromolar. As was concluded from the CO(2) dependence of acidification measured at different pH of the bathing medium, bicarbonate inhibited phase-I acidification noncompetetively. Acidification was inhibited by acetazolamide, an inhibitor of carbonic anhydrase. Apparently, acidification of phase I is due to a light-dependent uptake of CO(2) being converted to HCO(3) (-) by a carbonic anhydrase-like function of the HCO(3) (-)-transport system (M Volokita, D Zenvirth, A Kaplan, L Reinhold 1984 Plant Physiol 76: 599-602) before or during entering the cell, thus releasing one proton per CO(2) converted to HCO(3) (-).

Light-Induced Proton Release by the Cyanobacterium Anabaena variabilis