The active uptake of carbon dioxide by the unicellular green algae Chlorella saccharophila and C. ellipsoidea

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Abstract
Mass spectrometry has been used to measure the rates of CO2 uptake of acid‐ and alkali‐grown cells of the green algae Chlorella ellipsoidea (UTEX 20) and C. saccharophila (UTEX 27). The time course of CO2 formation on addition of 100mmol m−3 K2CO3 to cells in the dark was used as an assay for external carbonic anhydrase (CA). No external CA was detected in acid‐grown cells of either species or in alkali‐grown cells of C. ellipsoidea but was present in alkali‐grown C. saccharophila. In the absence of external CA, or when it was inhibited by 5mmol m−3 acetazolamide, cells of both species, on illumination, rapidly depleted the free CO2 in the medium at pH 7.5 to near zero concentrations before maximum photosynthetic O2 evolution rates were established. Addition of bovine CA rapidly restored the equilibrium CO2 concentration in the medium, indicating that the cells were selectively taking up CO2. Transfer of cells to the dark caused a rapid increase in the CO2 concentration in the medium largely due to the efflux of inorganic carbon from the cells as CO2. This rapid light‐dependent CO2 uptake takes place against pH and concentration gradients and, thus, has the characteristics of active transport.