Uptake, efflux, and photosynthetic utilization of inorganic carbon by the marine eustigmatophyte Nannochloropsis sp.

Uptake, efflux and utilization of inorganic carbon were investigated in the marine eustigmatophyte Nannochloropsis sp. grown under an air lever of CO₂. Maximal photosynthetic rate was hardly affected by raising the pH from 5.0 to 9.0. The apparent photosynthetic affinity for dissolved inorganic carbon (DIC) was 35 μM DIC between pH 6.5 to 9.0, but increased approximately threefold at pH 5.0 suggesting that HCO₃^- was the main DIC species used from the medium. No external carbonic anhydrase (CA) activity could be detected by the pH drift method. However, application of ethoxyzolamide (an inhibitor of CA) resulted in a significant inhibition of photosynthetic O₂ evolution and carbon utilization, 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 leak of CO₂ from the cells to the external medium. When the cells were exposed to low DIC concentrations, the ratio of internal to external DIC concentration was about eight. On the other hand, in the presence of 2 mM DIC, conditions prevailing in the marine environment, the internal concentration of DIC was only 50 % higher than the external one.