Evidence against H⁺-HCO₃^- symport as the mechanism for HCO₃^- transport in the cyanobacterium Anabaena variabilis

The rate of inorganic carbon uptake and its steadystate accumulation ratio (intracellular/extracellular concentration) was determined in the cyanobacterium Anabaena variabilis as a function of extracellular pH. The free energy of protons ( {Mathematical expression}) across the plasmalemma was calculated from determinations of membrane potential, and intracellular pH, as a function of the extracellular pH. While inward proton motive force decreased with increasing extracellular pH from 6.5 to 9.5, rate of HCO₃^- influx and its accumulation ration increased. The latter is several times larger than would be expected should HCO₃^- influx be driven by {Mathematical expression}. It is concluded that HCO₃^- transport in cyanobacteria is not driven by the proton motive force.