Role of carbonic anhydrase in photosynthetic CO2 fixation in Chlorella1

Chlorella vulgaris 11h cells grown in air enriched with 4% CO2 (high-CO2 cells) had carbonic anhydrase (CA) activity which was 20 to 90 times lower than that of algal cells grown in ordinary air (containing 0.04% CO2, low-CO2 cells). The CO2 concentration during growth did not affect either ribulose 1,5-bisphosphate (RuBP) carboxylase activity or its Km for CO2. When high-CO2 cells were transferred to low CO2 conditions, CA activity increased without a lag period, and this increase was accompanied by an increase in the rate of photosynthetic 14CO2 fixation under 14CO2-limiting conditions. On the other hand, CA activity as well as the rate of photosynthetic 14CO2 fixation at low 14CO2 concentrations decreased when low-CO2 cells were transferred to high CO2 conditions. Diamox, an inhibitor of CA, at 0.1 mM did not affect photosynthesis of low-CO2 cells at high CO2 concentration (0.5%). Diamox inhibited photosynthesis only under low CO2 concentrations, and the lower the CO2 concentration, the greater was the inhibition. Consequently, the CO2 concentration at which the rate of photosynthesis attained one-half its maximum rate (Km) greatly increased in the presence of this inhibitor. When CO2 concentration was higher than 1%, the photosynthetic rate in low-CO2 cells decreased, while that in high-CO2 cells increased. Fractionation of the low-CO2 cells in non-aqueous medium by density showed that CA was fractionated in a manner similar to the distribution of chlorophyll and RuBP carboxylase. These observations indicate that CA enhances photosynthesis under CO2-limiting conditions, but inhibits it at CO2 concentrations higher than a certain level. The mechanism underlying the above regulatory functions of CA is discussed.