Active uptake of inorganic carbon by <i>Chlamydomonas reinhardtii</i>: evidence for simultaneous transport of HCO₃⁻ and CO₂ and characterization of active CO₂ transport

Abstract

Washed protoplasts of low CO 2 grown cells of Chlamydomonas reinhardtii were used to further characterize the ability for active CO 2 transport. The CO 2 transport mechanism and the high affinity for dissolved inorganic carbon were completely induced within 4 h after transferring 5% CO 2 grown cells to ambient air (0.035% CO 2 ). Net O 2 evolution and CO 2 uptake were saturable processes showing saturation between 100 and 200 μM DIC (1.6–3.2 μM CO 2 ) at pH 8.0. For both O 2 evolution in whole cells and CO 2 uptake in the protoplasts the concentration of dissolved inorganic carbon required for 50% of the maximal rates was about 12 μM (= 0.20 μM CO 2 ). Studies with 3-(3,4-dichloro-phenyl)-1,1 dimethylurea, dibromo-thymoquinone, tetramethyl phenylenediamine and protoplasts of a cytochrome c oxidase deficient mutant of C. reinhardtii indicated the CO 2 transport was driven by cyclic or pseudocyclic ATP formation and oxidative phosphorylation was not involved. These studies also show that CO 2 transport and CO 2 fixation are distinct mechanisms and that active CO 2 uptake may occur in the absence of CO 2 fixation. Key words: Chlamydomonas reinhardtii; CO 2 –HCO 3 − concentrating mechanism, CO 2 transport, cyclic photophosphorylation, pseudocyclic photophosphorylation.