At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na(+). The rate of photosynthesis greatly exceeded the CO(2) supply rate and indicated that HCO(3) (-) was taken up by a Na(+)-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na(+) and exceeded the CO(2) supply rate by 8 to 45 times. The apparent photosynthetic affinity (K((1/2))) for DIC was high (6-40 micromolar) and was not markedly affected by Na(+) concentration, whereas with air-grown cells K((1/2)) (DIC) decreased by more than an order of magnitude in the presence of Na(+). Lithium, which inhibited Na(+)-dependent HCO(3) (-) uptake in air-grown cells, had little effect on Na(+)-independent HCO(3) (-) uptake by standing culture cells. A component of total HCO(3) (-) uptake in standing culture cells was also Na(+)-dependent with a K((1/2)) (Na(+)) of 4.8 millimolar and was inhibited by lithium. Analysis of (14)C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na(+)-independent CO(2) transport system. The conversion from Na(+)-independent to Na(+)-dependent HCO(3) (-) uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO(3) (-) for subsequent photosynthetic fixation.