Effect of external pH on inorganic carbon assimilation in unicellular marine green algae

Abstract

SUMMARY Carbonic anhydrase (CA) induction has been studied in three marine green algae under acidic (pH 4.5) or alkaline (pH 8.0) conditions. An inhibition of the induction of the external CA in acidic conditions, similar to that observed in some freshwater green algae, could be observed in only Chlorella saccharophila. In the two other species, Chlorococcum littorale and Stichococcus bacillaris, no significant difference in CA induction was found under two pH conditions. The exact function of the external CA of C. saccharophila remains unclear, since cells grown under acidic conditions (under which this enzyme is repressed) possess the same abilities to use inorganic carbon (Ci) as alkaline-grown cells. Internal pH values were not modified by the pH of the medium used to cultivate C. saccharophila. Regardless of the growth conditions, activities related to carbon fixation, that is, photosynthetic oxygen evolution, Ci uptake and assimilation were enhanced when the measurements were performed at acidic pH. This indicates that this marine alga is able to use CO2 more efficiently than HCO3−. No evidence could be found for a specific Ci uptake and assimilation system in the acid-grown cells.