Mechanism of Inorganic Carbon Uptake in Chlorella saccharophila: The Lack of Involvement of Carbonic Anhydrase

The acid-tolerant green alga Chlorella saccharophila maintainedphotosynthesis and accumulated intracellular pools of inorganiccarbon over a a range of external pH from 4.0 to 7.5. This accumulationwas unaffected by treatment of cells with 10 mol m^–3 acetazolamide(AZA). Cells grown at alkaline pH had extracellular carbonicanhydrase (CA), but CA activity was repressed when cells weregrown at pH 5.0. Acid-grown cells retained a high affinity forCO₂, both at acid and alkaline pH, and the ability to accumulateinorganic carbon. Rates of photosynthesis of acid-grown cellsand alkaline-grown AZA-treated cells at pH 8.0 were 2.5-foldhigher than the rate of CO₂ supply from the uncatalysed dehydrationof , indicating that the cells can take up as a source of substrate for photosynthesis. Isotopic disequilibrium experiments with acid-grown cells maintainingsteady-state photosynthesis at pH 7.5 demonstrate that ¹⁴C from¹⁴CO₂ was taken up more rapidly than from H¹⁴. This uptake takes place against a concentration gradient. Theseresults demonstrate that C. saccharophila cells have activetransport systems for the uptake of both CO₂ and and both operate without the mediation of CA. Key words: Bicarbonate transport, carbon dioxide, carbonic anhydrase, Chlorella saccharophila, inorganic carbon accumulation