Inorganic-carbon transport in some marine eukaryotic microalgae

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.