INDUCTION OF CO2 AND BICARBONATE TRANSPORT IN THE GREEN-ALGA CHLORELLA-ELLIPSOIDEA .1. TIME-COURSE OF INDUCTION OF THE 2 SYSTEMS

Changes in the physiological properties of the green alga Chlorella el lipsoidea (UTEX 20) were determined during adaptation from high CO2 to air. Cells of C. ellipsoidea, grown in high CO2, had an extremely low affinity for dissolved inorganic carbon (DIC). However, high-affinity DIC transport was induced rapidly after switching to air, which cause d a massive decrease in the DIC concentration in the medium. Rates of O-2 evolution without added carbonic anhydrase (CA) were compared with calculated rates of uncatalyzed CO2 formation in the medium as a meas ure of active HCO3- uptake. Cells were found to be able to use HCO3- a fter 5 h of adaptation and this capacity increased during the next 17 h. The stimulation of O-2 evolution upon CA addition was used as a mea surement of active CO2 transport: such stimulation occurred 2 h after transfer and increased during the next 5 h. Increases in O-2 evolution rates were correlated closely with an increasing capacity to accumula te intracellular pools of acid-labile DIC and with decreases in K-1/2( CO2) and CO2-compensation point of the cells. Treatment of cells with cycloheximide (5 mu g mL(-1)) during adaptation completely inhibited D IC transport induction, whereas treatment with chloramphenicol (400 mu g mL(-1)) had no effect, indicating the requirement for cytoplasmic p rotein synthesis in the induction. These results suggest that both CO2 and HCO3- transport are induced upon transfer of cells from high CO2 to air and that there is a temporal separation between the induction o f the two systems.