The Stoichiometry between CO(2) and H Fluxes Involved in the Transport of Inorganic Carbon in Cyanobacteria.

The pH of the medium during CO(2) uptake into the intracellular inorganic carbon (C(i)) pool of a high CO(2)-requiring mutant (E(1)) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO(2) fixation is inhibited. There was an acidification of the medium during CO(2) uptake in the light and an alkalization during CO(2) efflux after darkening. A one to one stoichiometry existed between the amounts of H(+) appearing in the medium and CO(2) taken up into the intracellular C(i) pool, regardless of the carbon species transported. The results indicate that (a) CO(2) is taken up simultaneously with an efflux of equimolar H(+), probably produced as a result of CO(2) hydration during transport and (b) HCO(3) (-) produced by hydration of CO(2) in the medium was transported into the cells without accompanying net flux of H(+) or OH(-). The influx and efflux of C(i) during C(i) transport produced nonequilibrium between CO(2) and HCO(3) (-) in the medium, with the concentration of HCO(3) (-) being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of C(i) is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO(3) (-) occurs during uptake of C(i).