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.Abbreviation Hepes 4-(2-hydroxyethyl)-1-piperazineethane-sulfonic acid