Journal of Experimental Botany, Vol. 51, No. 349, pp. 1341-1348,
August 2000
© 2000 Oxford University Press
Original Papers |
Active transport of CO2 and bicarbonate is induced in response to external CO2 concentration in the green alga Chlorella kessleri
1 Department of Biology, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3
2 Department of Chemistry, Kwansei-Gakuin University, 1–1-155 Uegahara, Nishinomiya, Japan 662
The time-course of induction of CO2 and 
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 was taken as an indicator of 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 (Pmax) 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 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% O2, which indicates that a change in the CO2/O2 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 µg 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 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 µM, 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.
Key words: Bicarbonate transport, Chlorella kessleri, CO2 transport, induction.