Active transport of CO2 and bicarbonate is induced in response to external CO2 concentration in the green alga Chlorella kessleri

The time-course of induction of CO2 and HCO3- transport has been investigated during the acclimation of high CO2-grown Chlorella kessleri cells to dissolved inorganic carbon (DIC)-limited conditions. The rate of photosynthesis of the cells in excess of the uncatalysed supply rate of CO2 from HCO3- was taken as an indicator of HCO3- transport, while a stimulation of photosynthesis on the addition of bovine carbonic anhydrase was used as an indicator of CO2 transport. The maximum rate of photosynthesis (P-max) was similar for high CO2-grown and low CO2-grown cells, but the apparent whole cell affinity for DIC and CO2 of high CO2-grown cells was found to be about 30-fold greater than in air-grown cells, which indicates a lower affinity for DIC and CO2. It was found that HCO3- and CO2 transport were induced in 5.5 h in cells acclimating to air in the light and in the presence and absence of 21% O-2, which indicates that a change in the CO2/O-2 ratio in the acclimating medium does not trigger induction of DIC transport. No active DIC transport was detected in high CO2-grown cells maintained on high CO2 for 5.5 h in the presence of 5 mM aminooxyacetate, an aminotransferase inhibitor. These results indicate no involvement of photorespiration in triggering induction. Active DIC transport induction was inhibited in cells treated with 5 mug ml(-1) cycloheximide, but was unaffected by chloramphenicol treatment, indicating that the induction process requires de novo cytoplasmic protein synthesis. The total DIC concentration eliciting the induction and repression of CO2 and HCO3- transport was higher at pH 7.5 than at pH 6.6. The concentrations of external CO2 required for the induction and repression of DIC transport were 0 and 120 muM, respectively, and was independent of the pH of the acclimation medium. Prolonged exposure to a critical external CO2 concentration elicits the induction of DIC transport in C. kessleri.