Inorganic carbon transport across cell compartments of the halotolerant alga Dunaliella salina

Abstract

The inorganic carbon (Ci) accumulation and the intracellular location of carbonic anhydrase (CA, EC 4.2.1.1) in the halotolerant unicellular alga Dunaliella salina have been investigated. The rate of HCO3 ‐dependent O2 evolution was determined by growth conditions. Algae grown under high CO2 conditions (5% CO2 in air, v/v; high Ci cells) had a very low affinity for HCO3− at pH 7.0 and 8.2, whereas algae grown under low CO2 conditions (0.03% CO2 in air; low Ci cells) showed a high affinity for HCO3− at both pH values and were sensitive to Dextran‐bound sulfonamide (DBS), an inhibitor of extracellular CA. The photosynthetic rate or HCO4− dependent O2 evolution was always higher at pH 7.0 than at pH 8.2. Ethoxyzolamide (EZ), an inhibitor of total (extacellular plus intracellular) CA activity, strongly inhibited photosynthesis at both pH values. During adaptation from high to low CO2 conditions CA activity increased in chloroplasts in a process dependent on the novo protein synthesis. Carbonic anhydrase activity was found in the supernatant and pellet fractions of chloroplast homogenates. The rate of photosynthesis of chloroplasts from low Ci cells was higher at pH 7.0 than at pH 8.2. The alkalinization of the growth medium, which took place only in the presence of Ci, was partially inhibited by DBS and completely by EZ. We suggest that in D. salina CO2 is the general form of Ci transported across the plasma membrane and the chloroplast envelope and that bicarbonate enters the cell mainly, although not entirely, by an ‘indirect’ mechanism after dehydration to CO2.