UPTAKE OF HCO3- AND CO2 IN CELLS AND CHLOROPLASTS FROM THE MICROALGAECHLAMYDOMONAS-REINHARDTII AND DUNALIELLA-TERTIOLECTA

Mass-spectrometric disequilibrium analysis was applied to investigate CO2 uptake and HCO3- transport in cells and chloroplasts of the microa lgae 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). High-and low-Ci cells of both species had the capa city to transport CO2 and HCO3-, with maximum rates being largely unaf fected by the growth conditions. In high-and low-Ci cells of D. tertio lecta, HCO3- was the dominant inorganic C species taken up, whereas HC O3- and CO2 were used at similar rates by C. reinhardtii. The apparent affinities of HCO3- transport and CO2 uptake increased 3- to 9-fold i n both species upon acclimation to air. Photosynthetically active chlo roplasts isolated from both species were able to transport CO2 and HCO 3-. For chloroplasts from C. reinhardtii, the concentrations of HCO3- and CO2 required for half-maximal activity declined from 446 to 33 mu M and 6.8 to 0.6 mu M, respectively, after acclimation of the parent c ells to air; the corresponding values for chloroplasts from D. tertiol ecta decreased from 203 to 58 mu M and 5.8 to 0.5 mu M, respectively. These results indicate the presence of inducible high-affinity HCO3- a nd CO2 transporters at the chloroplast envelope membrane.