INORGANIC CARBON UPTAKE BY AN ANTARCTIC SEA-ICE DIATOM, NITZSCHIA-FRIGIDA

There have been no studies to date on the mechanisms of inorganic carb on acquisition by Antarctic microalgae. Consequently, we have examined inorganic carbon (DIC) use in Nitzschia frigida, a diatom typical of the Antarctic bottom-ice community. The K-0.5 (CO2) of photosynthesis in this organism was estimated to be 1.09 mu M at pH 7.5. The internal concentration of DIC was approximately 4050 mu M at an external [DIC] of 45 mu M At air-equilibration levels of inorganic carbon this would be sufficient for a ten-fold accumulation ratio of CO2. Cells of N. f rigida are capable of carbon-dependent photosynthesis at rates that ex ceed that expected from uncatalysed CO2 supply to the cell. About 25% of the total carbonic anhydrase activity appears to be associated with the cell surface in N. frigida. These results support the hypothesis that N. frigida, like many microalgae from temperate waters, has an ac tive carbon-concentrating mechanism, associated with the ability to ut ilize external HCO3- for photosynthesis.