MECHANISMS OF ADAPTATION OF MICROALGAE TO CONDITIONS OF CARBON-DIOXIDE LIMITATION OF PHOTOSYNTHESIS - POSSIBLE ROLE OF CARBONIC-ANHYDRASE

We studied the rate of assimilation of inorganic carbon (C(in)) in relation to CO2 concentration in Dunaliella salina. Chlorella vulgaris, Scenedesmus obliguus, and Chlamydomonas reinhardtii WT grown at 5 and 0.03% CO2. It is demonstrated that K(m) (CO2) of photosynthesis reached 50-60-mu-M in algae grown at 5% CO2, but 2-5-mu-M at 0.03% CO2. Carbonic anhydrase (CA) activity of intact cells and in the homogenate was significantly higher in algae grown at 0.03% CO2 than at 5% CO2. Several forms of CA are present, viz., soluble (sCA), membrane-bound CA of cytoplasmic (cCA) and thylakoid (tCA) membranes, and CA of intact cells (iCA). Acetoazolamide lowered the rate of C(in) assimilation in algae grown at 0.03% CO2, whereas ethoxyzolamide totally suppressed the ability of the algae to adapt to low CO2 concentrations. The inhibitor of plasmalemma ATPase vanadate likewise suppressed the rate of C(in) assimilation in the algae. It is concluded that enzymatic reactions catalyzed by CA and plasmalemma ATPase are involved in adaptation of algae to low CO2 concentrations and in mechanisms of C(in) concentration. Mechanisms governing adaptation of algae to conditions of carbon dioxide limitation of photosynthesis are discussed in the paper.