ACTIVE UPTAKE OF INORGANIC CARBON BY CHLORELLA-SACCHAROPHILA IS NOT REPRESSED BY GROWTH IN HIGH CO2

Regulation of transport of dissolved inorganic carbon (DIC) in respons e to CO2 concentration in the external medium has been compared to two closely-related green algae, Chlorella ellipsoidea and Chlorella sacc harophila. C. ellipsoidea, when grown in high CO2, had reduced activit ies of both CO2 and HCO3- transport and DIC transport activities were increased after the cells had acclimated to air. However, high CO2-gro wn c. saccharophila had a comparable level of photosynthetic affinity for DIC to that of air-grown c. ellipsoidea and this was accompanied b y a capacity to accumulate high internal concentrations of DIC. The hi gh photosynthetic affinity and the high intracellular DIC accumulation did not change in cells grown in air except that the occurrence of ex ternal carbonic anhydrase (CA) in air-grown C. saccharophila stimulate d the intracellular DIC accumulation in the absence of added CA. These data indicate that active DIC transport is constitutively expressed i n C. saccharophila, presumable because this alga is insensitive to the repressive effect of high CO2 on DIC transport. This strongly support s the existence of a direct sensing mechanism for external CO2 in Chlo rella species, but also indicates that external CA is regulated indepe ndently of DIC transport in Chlorella species.