Active uptake of inorganic carbon byChlorella saccharophilais not repressed by growth in high CO₂

Regulation of transport of dissolved inorganic carbon (DIC) in response to CO₂ concentration in the external medium has been compared in two closely-related green algae, Chlorella ellipsoidea and Chlorella saccharophila. C. ellipsoidea, when grown in high CO₂, had reduced activities of both CO₂ and $HCO3-$ transport and DIC transport activities were increased after the cells had acclimated to air. However, high CO₂-grown C. saccharophila had a comparable level of photosynthetic affinity for DIC to that of air-grown C. ellipsoidea and this was accompanied by a capacity to accumulate high internal concentrations of DIC. The high photosynthetic affinity and the high intracellular DIC accumulation did not change in cells grown in air except that the occurrence of external carbonic anhydrase (CA) in air-grown C. saccharophila stimulated the intracellular DIC accumulation in the absence of added CA. These data indicate that active DIC transport is constitutively expressed in C. saccharophila, presumably because this alga is insensitive to the repressive effect of high CO₂ on DIC transport. This strongly supports the existence of a direct sensing mechanism for external CO₂ in Chlorella species, but also indicates that external CA is regulated independently of DIC transport in Chlorella species.