Inorganic Carbon Uptake by Chlamydomonas reinhardtii

The rates of CO(2)-dependent O(2) evolution by Chlamydomonas reinhardtii, grown with either air levels of CO(2) or air with 5% CO(2), were measured at varying external pH. Over a pH range of 4.5 to 8.5, the external concentration of CO(2) required for half-maximal rates of photosynthesis was constant, averaging 25 micromolar for cells grown with 5% CO(2). This is consistent with the hypothesis that these cells take up CO(2) but not HCO(3) (-) from the medium and that their CO(2) requirement for photosynthesis reflects the K(m)(CO(2)) of ribulose bisphosphate carboxylase. Over a pH range of 4.5 to 9.5, cells grown with air required an external CO(2) concentration of only 0.4 to 3 micromolar for half-maximal rates of photosynthesis, consistent with a mechanism to accumulate external inorganic carbon in these cells. Air-grown cells can utilize external inorganic carbon efficiently even at pH 4.5 where the HCO(3) (-) concentration is very low (40 nanomolar). However, at high external pH, where HCO(3) (-) predominates, these cells cannot accumulate inorganic carbon as efficiently and require higher concentrations of NaHCO(3) to maintain their photosynthetic activity. These results imply that, at the plasma membrane, CO(2) is the permeant inorganic carbon species in air-grown cells as well as in cells grown on 5% CO(2). If active HCO(3) (-) accumulation is a step in CO(2) concentration by air-grown Chlamydomonas, it probably takes place in internal compartments of the cell and not at the plasmalemma.