Effect of inorganic carbon on photoautotrophic growth of microalga Chlorococcum littorale

The growth rate of a highly CO₂-tolerant green alga, Chlorococcum littorale, was investigated in semi-batch cultures at a temperature of 22°C, a light intensity of 170 μmol-photon m^-2 s ^-1 and CO₂ concentrations ranging from 1 to 50% (v/v) at atmospheric pressure. In the experiments, solutions were bubbled with CO ₂ and N₂ gas mixtures to adjust CO₂ concentrations to minimize the influence of O₂. Growth rate, which was defined in terms of a specific growth rate μ, decreased with increasing CO₂ concentration at the conditions studied. The inhibition of growth by CO₂ gas could be attributed to the concentration of inorganic carbon in the culture medium. A growth model is proposed where key assumptions are the formation of bicarbonate ion HCO₃^- as substrate for algal growth and equilibrium between CO₂ inhibitor. The proposed growth model based on the Monod equation agreed with the experimental data to within 5% and provides better correlation than the conventional inhibition model, especially in the high CO₂ concentration region.