Uptake of HCO(3)(−) and CO(2) 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.