Uptake of HCO3− and CO2 in Cells and Chloroplasts from the Microalgae Chlamydomonas reinhardtii and Dunaliella tertiolecta

Mass-spectrometric disequilibrium analysis was applied to investigate CO 2 uptake and HCO 3 − transport in cells and chloroplasts of the microalgae Dunaliella tertiolecta and Chlamydomonas reinhardtii , which were grown in air enriched with 5% (v/v) CO 2 (high-Ci cells) or in ambient air (low-Ci cells). High- and low-Ci cells of both species had the capacity to transport CO 2 and HCO 3 − , with maximum rates being largely unaffected by the growth conditions. In high- and low-Ci cells of D. tertiolecta , HCO 3 − was the dominant inorganic C species taken up, whereas HCO 3 − and CO 2 were used at similar rates by C. reinhardtii . The apparent affinities of HCO 3 − transport and CO 2 uptake increased 3- to 9-fold in both species upon acclimation to air. Photosynthetically active chloroplasts isolated from both species were able to transport CO 2 and HCO 3 − . For chloroplasts from C. reinhardtii , the concentrations of HCO 3 − and CO 2 required for half-maximal activity declined from 446 to 33 μm and 6.8 to 0.6 μm, respectively, after acclimation of the parent cells to air; the corresponding values for chloroplasts from D. tertiolecta decreased from 203 to 58 μm and 5.8 to 0.5 μm, respectively. These results indicate the presence of inducible high-affinity HCO 3 − and CO 2 transporters at the chloroplast envelope membrane.