Cells of a high CO(2)-requiring mutant (E(1)) and wild type of Synechococcus PCC7942 were incubated with COS in the light, then suspended in COS-free medium and their CO(2) exchange was measured using an open gas-analysis system under the conditions where photosynthetic CO(2) fixation is inhibited. When the suspension of cells untreated with COS was illuminated, the rate of CO(2) uptake was high and addition of carbonic anhydrase during illumination released a large amount of CO(2) from the medium into the gas phase. The COS treatment in the light markedly reduced the rate of CO(2) uptake by the cells and the amount of CO(2) released by carbonic anhydrase. Incubation of cells with COS in the dark had no effect on the CO(2)-exchange profile. The COS concentration required for 50% inhibition of CO(2) uptake was about 25 micromolar when the concentration of inorganic carbon (C(i)) in the medium was 60 micromolar; higher C(i) concentrations reduced the inhibitory effect of COS. Measurement of C(i) uptake in E(1) cells by a silicone oil centrifugation method also indicated marked reduction of the activities of (14)CO(2) and H(14)CO(3) (-) uptake in the cells treated with COS in the light. The results demonstrated that COS is a potent inhibitor of C(i) transport.