The inhibition of the carbon concentrating mechanism of the green alga Chlorella saccharophila by acetazolamide

The effects of the carbonic anhydrase (CA) inhibitors acetazolamide (AZ) an d dextran-bound sulfonamide (DBS) on HCO3- -dependent O-2 evolution in Chlo rella saccharophila were evaluated, Addition of 4 muM AZ or 0.4 mg ml(-1) D BS to photosynthesizing cells reduced the O-2 evolution rate at low dissolv ed inorganic carbon (DIC) concentration, decreased the size of the intracel lular acid-labile carbon pool, and decreased the apparent affinity of the c ells for DIG. Measurement of the whole-cell affinity of cells for CO2 and H CO3- in the presence and absence of inhibitors indicated that active HCO3- transport was inhibited by AZ and DBS, The inhibition of HCO3- transport wa s 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.