The Stoichiometry between CO2 and H+ Fluxes Involved in the Transport of Inorganic Carbon in Cyanobacteria 1

AUTOR(ES)

Ogawa, Teruo

RESUMO

The pH of the medium during CO2 uptake into the intracellular inorganic carbon (Ci) pool of a high CO2-requiring mutant (E1) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO2 fixation is inhibited. There was an acidification of the medium during CO2 uptake in the light and an alkalization during CO2 efflux after darkening. A one to one stoichiometry existed between the amounts of H+ appearing in the medium and CO2 taken up into the intracellular Ci pool, regardless of the carbon species transported. The results indicate that (a) CO2 is taken up simultaneously with an efflux of equimolar H+, probably produced as a result of CO2 hydration during transport and (b) HCO3− produced by hydration of CO2 in the medium was transported into the cells without accompanying net flux of H+ or OH−. The influx and efflux of Ci during Ci transport produced nonequilibrium between CO2 and HCO3− in the medium, with the concentration of HCO3− being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of Ci is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO3− occurs during uptake of Ci.

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