Uptake of HCO3 2 and CO2 in Cells and Chloroplasts from the Microalgae Chlamydomonas reinhardtii and Dunaliella tertiolecta

Mass-spectrometric disequilibrium analysis was applied to investigate CO2 uptake and HCO3 2 transport in cells and chloroplasts of the microalgae Dunaliella tertiolecta and Chlamydomonas reinhardtii, which were grown in air enriched with 5% (v/v) CO2 (high-Ci cells) or in ambient air (low-Ci cells). Highand low-Ci cells of both species had the capacity to transport CO2 and HCO3 , with maximum rates being largely unaffected by the growth conditions. In highand low-Ci cells of D. tertiolecta, HCO3 2 was the dominant inorganic C species taken up, whereas HCO3 2 and CO2 were used at similar rates by C. reinhardtii. The apparent affinities of HCO3 2 transport and CO2 uptake increased 3to 9-fold in both species upon acclimation to air. Photosynthetically active chloroplasts isolated from both species were able to transport CO2 and HCO3 . For chloroplasts from C. reinhardtii, the concentrations of HCO3 2 and CO2 required for half-maximal activity declined from 446 to 33 mM and 6.8 to 0.6 mM, respectively, after acclimation of the parent cells to air; the corresponding values for chloroplasts from D. tertiolecta decreased from 203 to 58 mM and 5.8 to 0.5 mM, respectively. These results indicate the presence of inducible highaffinity HCO3 2 and CO2 transporters at the chloroplast envelope membrane.