The Location of the Transporting System for Inorganic Carbon and the Nature of the Form Translocated in<i>Chlamydomonas reinhardtii</i>

Abstract

The permeability of the plasmalemma of Chlamydomonas reinhardtii cells was increased by treatment with poly-L-lysine or dimethylsulphoxide as indicated by 3-phosphoglyceric acid dependent O2 evolution. These treatments decreased the ability of the cells to accumulate inorganic carbon internally and hence their photosynthetic affinity for inorganic carbon in the medium. With saturating light and inorganic carbon, the photosynthetic rate was less affected by the poly-L-lysine and dimethylsulphoxide treatments. Thus the poly-L-lysine and dimethylsulphoxide did not alter the activity of the chloroplasts but rather made the intracellular inorganic carbon pool more freely exchangeable with the medium. It is concluded that the transporting system for inorganic carbon is located at the plasmalemma. Treatment with Diamox, an inhibitor of carbonic anhydrase, did not affect photosynthetic rate and accumulation of inorganic carbon when CO2 was supplied but strongly inhibited both parameters when HCO−3 was supplied. In a mutant of Chlamydomonas reinhardtii lacking a cell wall, carbonic anhydrase leaks to the medium and uptake of inorganic carbon is much faster when CO2 is supplied than when HCO−3 is supplied. These results suggest that CO2 rather than HCO−3 is the inorganic carbon species that is actively translocated across the plasmalemma.