Active transport of CO2 and bicarbonate is induced in response to externalCO2 concentration in the green alga Chlorella kessleri

The time-course of induction of CO2 and HCO3- transport has been investigat ed during the acclimation of high CO2-grown Chlorella kessleri cells to dis solved inorganic carbon (DIC)-limited conditions. The rate of photosynthesi s of the cells in excess of the uncatalysed supply rate of CO2 from HCO3- w as taken as an indicator of HCO3- transport, while a stimulation of photosy nthesis on the addition of bovine carbonic anhydrase was used as an indicat or 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 af finity for DIC and CO2 of high CO2-grown cells was found to be about 30-fol d 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 i n 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 acclimat ing medium does not trigger induction of DIC transport. No active DIC trans port 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. The se results indicate no involvement of photorespiration in triggering induct ion. Active DIC transport induction was inhibited in cells treated with 5 m ug ml(-1) cycloheximide, but was unaffected by chloramphenicol treatment, i ndicating that the induction process requires de novo cytoplasmic protein s ynthesis. The total DIC concentration eliciting the induction and repressio n of CO2 and HCO3- transport was higher at pH 7.5 than at pH 6.6. The conce ntrations of external CO2 required for the induction and repression of DIC transport were 0 and 120 muM, respectively, and was independent of the pH o f the acclimation medium. Prolonged exposure to a critical external CO2 con centration elicits the induction of DIC transport in C. kessleri.