Carbonyl Sulfide: An Inhibitor of Inorganic Carbon Transport in Cyanobacteria

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Abstract
Cells of a high CO2-requiring mutant (E1) and wild type of Synechococcus PCC7942 were incubated with COS in the light, then suspended in COS-free medium and their CO2 exchange was measured using an open gas-analysis system under the conditions where photosynthetic CO2 fixation is inhibited. When the suspension of cells untreated with COS was illuminated, the rate of CO2 uptake was high and addition of carbonic anhydrase during illumination released a large amount of CO2 from the medium into the gas phase. The COS treatment in the light markedly reduced the rate of CO2 uptake by the cells and the amount of CO2 released by carbonic anhydrase. Incubation of cells with COS in the dark had no effect on the CO2-exchange profile. The COS concentration required for 50% inhibition of CO2 uptake was about 25 micromolar when the concentration of inorganic carbon (Ci) in the medium was 60 micromolar; higher Ci concentrations reduced the inhibitory effect of COS. Measurement of Ci uptake in E1 cells by a silicone oil centrifugation method also indicated marked reduction of the activities of 14CO2 and H14CO3− uptake in the cells treated with COS in the light. The results demonstrated that COS is a potent inhibitor of Ci transport.