Inorganic-carbon uptake by the marine diatom Phaeodactylum tricornutum.

Air-grown cells of the marine diatom Phaeodactylum tricornutum showed only 10% of the carbonic-anhydrase activity of air-grown Chlamydomonas reinhardtii. Measurement of carbonic-anhydrase activity using intact cells and cell extracts showed all activity was intracellular in Phaeodactylum. Photosynthetic oxygen evolution at constant inorganic-carbon concentration but varying pH showed that exogenous CO2 was poorly utilized by the cells. Sodium ions increased the affinity of Phaeodactylum for HCO 3 (-) and even at high HCO 3 (-) concentrations sodium ions enhanced HCO 3 (-) utilization. The internal inorganic-carbon pool (HCO 3 (-) +CO2] was measured using a silicone-oil-layer centrifugal filtering technique. The internal [HCO 3 (-) +CO2] concentration never exceeded 15% of the external [HCO 3 (-) +CO2] concentration even at the lowest external concentrations tested. It is concluded that an internal accumulation of inorganic carbon relative to the external medium does not occur in P. tricornutum.