The profiles of nitrate reductase and carbonic anhydrase activity in batch cultivation of the marine microalgae Tetraselmis gracilis growing under different aeration conditions.

Tetraselmis gracilis, a Prasinophycean alga found in estuaries and in the open ocean, was cultivated under different conditions of aeration, which resulted in variations of inorganic carbon in the medium. Relative growth rates, nitrate reductase and carbonic anhydrase activities were daily determined and correlated to the concentration of nitrate, nitrite, phosphate, inorganic and organic carbon in the media. Nitrate reductase catalyzes the reversible carbon dioxide hydration reaction. The activity profiles of both enzymes during 10 days of cultivation under aeration with air showed an inverse relationship: the maximum in the activity of nitrate reductase coincided with the minimum of carbonic anhydrase activity. An ionizable organic carbon species with pKa in the range of metabolites of the photorespiratory path was found parallel with the increase of carbonic anhydrase activity and the decrease of nitrate reductase activity. The onset of photorespiration is probably one of the factors involved in the simultaneous regulation of these enzymatic processes. Cultures aerated with air containing 5% CO2 showed different profiles for nitrate reductase activity and nitrate uptake.