Inorganic-carbon uptake by a small-celled strain of Stichococcus bacillaris

Air-grown cells of a marine, small-celled (2 μm diameter) strain of Stichococcus bacillaris contained appreciable carbonic-anhydrase activity but this was repressed when cells were grown on air enriched with 5% (v/v) CO2. Assay of carbonic-anhydrase activity using intact cells and cell extracts showed all activity was intracellular in this Stichococcus strain. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0, where CO2 is the predominant form of inorganic carbon, showed that the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K0.5(CO2)] was 4.0 μM for both air- and CO2-grown cells. At pH 8.3 the K0.5(CO2) was 0.3 mM for air-grown and 0.6 mM for CO2-grown cells. Sodium ions did not enhance bicarbonate utilization. Measurement of the internal inorganic-carbon pool (HCO 3 (-) +CO2) by the silicone-oil-layer centrifugal filtering technique showed that air- and CO2-grown cells were able to concentrate inorganic carbon up to 20-fold in relation to the external medium at pH 5.0 but not at pH 8.3. In this alga the high affinity for CO2 and inorganic-carbon accumulation in CO2- and air-grown cells results from active CO2 transport that is not dependent on carbonic-anhydrase activity.