The effects of the carbonic anhydrase (CA) inhibitors acetazolamide (AZ) and dextran-bound sulfonamide (DBS) on HCO3--dependent O2 evolution in Chlorella saccharophila were evaluated. Addition of 4 µM AZ or 0.4 mg ml-1 DBS to photosynthesizing cells reduced the O2 evolution rate at low dissolved inorganic carbon (DIC) concentration, decreased the size of the intracellular acid-labile carbon pool, and decreased the apparent affinity of the cells for DIC. Measurement of the whole-cell affinity of cells for CO2 and HCO3- in the presence and absence of inhibitors indicated that active HCO3- transport was inhibited by AZ and DBS. The inhibition of HCO3- transport was independent of the inhibition of external and internal CA. These results suggest that the active uptake of HCO3- occurs initially by the interaction of HCO3- and a CA-like transporter.