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

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Abstract
Washed protoplasts of low CO2 grown cells of Chlamydomonas reinhardtii were used to further characterize the ability for active CO2 transport. The CO2 transport mechanism and the high affinity for dissolved inorganic carbon were completely induced within 4 h after transferring 5% CO2 grown cells to ambient air (0.035% CO2). Net O2 evolution and CO2 uptake were saturable processes showing saturation between 100 and 200 μM DIC (1.6–3.2 μM CO2) at pH 8.0. For both O2 evolution in whole cells and CO2 uptake in the protoplasts the concentration of dissolved inorganic carbon required for 50% of the maximal rates was about 12 μM (= 0.20 μM CO2). 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 CO2 transport was driven by cyclic or pseudocyclic ATP formation and oxidative phosphorylation was not involved. These studies also show that CO2 transport and CO2 fixation are distinct mechanisms and that active CO2 uptake may occur in the absence of CO2 fixation. Key words: Chlamydomonas reinhardtii; CO2–HCO3− concentrating mechanism, CO2 transport, cyclic photophosphorylation, pseudocyclic photophosphorylation.

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