The localization of active inorganic carbon transport at the plasma membrane in Chlorella ellipsoidea

A study has been made of the capacity of whole cells and intact, photosynthetically active, isolated chloroplasts of Chlorella ellipsoidea (UTEX 20) to take up CO 2 by active transport. Assays for carbonic anhydrase activity and the monitoring of CO 2 uptake by cells and chloroplasts was carried out by mass spectrometry. No external carbonic anhydrase was detected in whole cells or on the outer surface of the isolated chloroplasts. Upon illumination, whole cells rapidly depleted CO 2 from the medium to a level below the equilibrium CO 2 concentration before maximum photosynthetic O 2 evolution rates were established. Addition of bovine carbonic anhydrase resulted in the reestablishment of equilibrium CO 2 concentrations, indicating that the cells were actively and selectively depleting the medium of CO 2 . This CO 2 uptake was inhibited by 10 μM diethylstilbestrol. No such rapid depletion of CO 2 was observed with isolated intact chloroplasts, although the organelles demonstrated rates of photosynthetic O 2 evolution of about 60% of the parent cells. Photosynthetic rates of chloroplast suspensions exceeded the rate of CO 2 supply only twofold, indicating that chloroplasts have a limited ability for HCO 3 − uptake. Intact cells, however, use HCO 3 − readily. These results indicate that the principal location of inorganic carbon transporters, both for CO 2 and HCO 3 − , is at the plasma membrane in this alga.