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 response to CO₂ concentration in the external medium hasbeen compared in two closely-related green algae, Chlorellaellipsoidea and Chlorella saccharophila. C. ellipsoidea, whengrown in high CO₂, had reduced activities of both CO₂ and transport and DIC transport activitieswere increased after the cells had acclimated to air. However,high CO₂-grown C. saccharophila had a comparable level of photosyntheticaffinity for DIC to that of air-grown C. ellipsoidea and thiswas accompanied by a capacity to accumulate high internal concentrationsof DIC. The high photosynthetic affinity and the high intracellularDIC accumulation did not change in cells grown in air exceptthat the occurrence of external carbonic anhydrase (CA) in air-grownC. saccharophila stimulated the intracellular DIC accumulationin the absence of added CA. These data indicate that activeDIC transport is constitutively expressed in C. saccharophila,presumably because this alga is insensitive to the repressiveeffect of high CO₂ on DIC transport. This strongly supportsthe existence of a direct sensing mechanism for external CO₂in Chlorella species, but also indicates that external CA isregulated independently of DIC transport in Chlorella species. Key words: Carbonic anhydrase, Chlorella, CO₂-insensitive, DIC transport, wild type