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

摘要

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.