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

Abstract.

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 K _m 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.