CO(2) entry into Synechococcus sp. PCC7942 cells was drastically inhibited by the water channel blocker p-chloromercuriphenylsulfonic acid suggesting that CO(2) uptake is, for the most part, passive via aquaporins with subsequent energy-dependent conversion to HCO3(-). Dependence of CO(2) uptake on photosynthetic electron transport via photosystem I (PSI) was confirmed by experiments with electron transport inhibitors, electron donors and acceptors, and a mutant lacking PSI activity. CO(2) uptake was drastically inhibited by the uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP) and ammonia but substantially less so by the inhibitors of ATP formation arsenate and N, N,-dicyclohexylcarbodiimide (DCCD). Thus a DeltamuH(+) generated by photosynthetic PSI electron transport apparently serves as the direct source of energy for CO(2) uptake. Under low light intensity, the rate of CO(2) uptake by a high-CO(2)-requiring mutant of Synechococcus sp. PCC7942, at a CO(2) concentration below its threshold for CO(2) fixation, was higher than that of the wild type. At saturating light intensity, net CO(2) uptake was similar in the wild type and in the mutant IL-3 suggesting common limitation by the rate of conversion of CO(2) to HCO3(-). These findings are consistent with a model postulating that electron transport-dependent formation of alkaline domains on the thylakoid membrane energizes intracellular conversion of CO(2) to HCO3(-).