Uptake of CO2 and bicarbonate by intact cells and chloroplasts of Tetraedron minimum and Chlamydomonas noctigama

Using a mass-spectrometric disequilibrium technique, net uptake of HCO3– and CO2 during steady-state photosynthesis was studied in whole cells and chloroplasts from the green algae Tetraedron minimum and Chlamydomonas noctigama, grown in air enriched with 5% (v/v) CO2 (high-CO2 cells) or in air [0.035% (v/v) CO2; low-CO2 cells]. High- and low-CO2 cells of both species were able to take up CO2 and HCO3–, with maximum rates being largely unaffected by the growth conditions. High- and low-CO2 cells of T. minimum showed a pronounced preference for HCO3– while the rates of net HCO3– and CO2 uptake were similar in C. noctigama. The most significant differences between high- and low-CO2 cells of the two species were the 5- to 6-fold increase in the apparent affinities of net HCO3– uptake and CO2 uptake after acclimation to air. The high-affinity uptake systems for inorganic carbon were almost completely induced within 4 h in both algae. Photosynthetically active chloroplasts isolated from both species were also able to take up CO2 and HCO3–. As in whole cells, HCO3– was the dominant carbon species taken up by chloroplasts from T. minimum while CO2 and HCO3– were taken up at similar rates in plastids from C. noctigama. In addition, high-affinity uptake systems for CO2 and HCO3– were detected in chloroplasts preparations after acclimation of the parent cells to air. Isolation of ribulose-1,5-bisphosphate carboxylase/oxygenase revealed Km values of 13 and 42 µM CO2 for the enzymes from T. minimum and C. noctigama, respectively. These results are consistent with the presence of inducible and energy-dependent high-affinity HCO3– and CO2 uptake systems associated with chloroplasts, indicating that these organelles play an important role in the CO2-concentrating mechanism.