UPTAKE, EFFLUX, AND PHOTOSYNTHETIC UTILIZATION OF INORGANIC CARBON BYTHE MARINE EUSTIGMATOPHYTE NANNOCHLOROPSIS SP

Uptake, efflux and utilization of inorganic carbon were investigated i n the marine eustigmatophyte Nannochloropsis sp. grown under an air le vel of CO2. Maximal photosynthetic rate was hardly affected by raising the pH from 5.0 to 9.0. The apparent photosynthetic affinity for diss olved inorganic carbon (DIC) was 35 mu M DIC between pH 6.5 to 9.0, bu t increased approximately threefold at pH 5.0 suggesting that HCO3- wa s the main DIC species used from the medium. No external carbonic anhy drase (CA) activity could be detected by the pH drift method. However, application of ethoxyzolamide (an inhibitor of CA) resulted in a sign ificant inhibition of photosynthetic O-2 evolution and carbon utilizat ion, suggesting involvement of internal CA or CA-like activity in DIC utilization. Under high light conditions, the rate of HCO3- uptake and its internal conversion to CO2 apparently exceeded the rate of carbon fixation, resulting in a large bak of CO2 from the cells to the exter nal medium. When the cells were exposed to low DIC concentrations, the ratio of internal to external DIC concentration was about eight. On t he other hand, in the presence of 2 mM DIC, conditions prevailing in t he marine environment, the internal concentration of DIC was only 50% higher than the external one.