Role of Carbonic Anhydrase in Photosynthesis of Blue-Green Alga (Cyanobacterium) Anabaena variabilis ATCC 29413

The affinity for NaHCO3 (CO2) in photosynthesis of Anabaena variabilis ATCC 29413 was much higher in the cells grown under ordinary air (low-CO2 cells) than in those grown in air enriched with 2–4% CO2 (high-CO2 cells) (pH 8.0, 25°C). Ethoxyzolamide (50 μM) increased the Km(NaHCO3 in low-CO2 cells about nine times (from 14.3 to 125), while the maximum rate of photosynthesis decreased about 20%. When high-CO2 cells were transferred to low-CO2 conditions, carbonic anhydrase (CA) activity increased, while Km(NaHCO3) in photosynthesis decreased from 140 to 30 μM within about 5 h. The addition of CA to the suspension of both high- and low-CO2 cells enhanced the rates of photosynthetic O2 evolution under CO2-limiting conditions. The rate of 14CO2 fixation was much faster than that of H14CO3− fixation. The former reaction was greatly suppressed, while the latter was enhanced by the addition of CA. These results indicate that the active species of inorganic carbon utilized for photosynthesis was free CO2 irrespective of the CO2 concentration given during growth. It is suggested that CA plays an active role in increasing the affinity for CO2 in photosynthesis of low-CO2 cells of this blue-green alga.