ROLE OF CARBONIC-ANHYDRASE IN ASSIMILATION OF INORGANIC CARBON IN DUNALIELLA-SALINA

• We studied changes in the rate of photosynthetic assimilation of inorganic carbon (C(i)) in relation to CO2 concentration by Dunaliella salina cells adapted to high (5%) and low (0.03%) concentrations of CO2. It is demonstrated that the rate of C(i) assimilation was higher in algae adapted to 0.03% CO2 than in culture adapted to 0.03% CO2. The period of adaptation of D. salina to low CO2 concentrations lasted 5-6 h, during which time activity of carbonic anhydrase (CA) increased on the cell surface and inside cells. Soluble CA (sCA) was detected in D. salina, together with membrane-bound CA of cytoplasmic (cCA) and thylakoid (tCA) membranes. The effect of CA inhibitors - acetazolamide (AZA) and ethoxyzolamide (EZA) - on photosynthesis of D. salina cells was studied in the work. It turned out that EZA lowered the rate of O2 evolution in algae adapted to 5% CO2 and totally suppressed this process in algae adapted to 0.03% CO2. Acetazolamide lowered the photosynthetic rate only in algae adapted to 0.03% CO2. It is concluded that intracellular forms of CA are needed for functioning of the C1-concentration mechanism in algae adapted to conditions of carbon dioxide limitation of photosynthesis. The authors discuss the mechanisms of adaptation of D. salina to low CO2 concentrations and the role played by CA in this process.

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