The cells of Ectothiorhodospira shaposhnikovii grown under the anaerobic conditions in the light and in the dark under the aerobic conditions in a medium with acetate can fix considerable amounts of CO2 only in the presence of light. Incorporation of 14C from the labeled acetate into the cells in the dark does not depend on the presence of bicarbonate and inorganic electron donors through assimilation of 14C-acetate in the light increases significantly in the presence of bicarbonate and sulfide or thiosulfate. Fluoroacetate (5.0 . 10(-3) M) completely inhibits assimilation of 14C-acetate by the cells and formation of 14CO2 from it in the dark under the aerobic conditions, regardless of the growth conditions and the presence of inorganic electron donors. In the light, fluoroacetate produces a similar effect on 14C-acetate assimilation by the cells only in the absence of bicarbonate and inorganic electron donors. Therefore, carbon metabolism in Ectothiorhodospira shaposhnikovii differs in the light and in the dark.