There have been no studies to date on the mechanisms of inorganic carbon 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 K0 . 5 (CO2) of photosynthesis in this organism was estimated to be 1.09 μM at pH 7.5. The internal concentration of DIC was approximately 4050 μM at an external [DIC] of 45 μM. At air- equilibration levels of inorganic carbon this would be sufficient for a ten-fold accumulation ratio of CO2. Cells of N. frigida are capable of carbon- dependent photosynthesis at rates that exceed 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 active carbon-concentrating mechanism, associated with the ability to utilize external HCO3/- for photosynthesis.