Inorganic-carbon transport was investigated in the eukaryotic marine microalgaeStichococcus minor, Nannochloropsis oculata and aMonallantus sp. Photosynthetic O2 evolution at constant inorganic-carbon concentration but varying pH showed thatS. minor had a greater capacity for CO2 rather than HCO 3 (-) utilization but forN. oculata andMonallantus HCO 3 (-) was the preferred source of inorganic carbon. All three microalgae had a low affinity for CO2 as shown by the measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0. At pH 8.3, where HCO 3 (-) is the predominant form of inorganic carbon, the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K 0.5 (CO2)] was 53 μM forMonallantus sp. and 125 μM forN. oculata, values compatible with HCO 3 (-) transport. Neither extra- nor intracellular carbonic anhydrase was detected in these three microalgal species. It is concluded that these microalgae lack a specific transport system for CO2 but that HCO 3 (-) transport occurs inN. oculata andMonallantus, and in the absence of intracellular carbonic anhydrase the conversion of HCO 3 (-) to CO2 may be facilitated by the internal pH of the cell.
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.
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