Variation of photoautotrophic fatty acid production from a highly CO₂ tolerant alga, Chlorococcum littorale, with inorganic carbon over narrow ranges of pH

Photoautotrophic fatty acid production of a highly CO₂-tolerant green alga Chlorococcum littorale in the presence of inorganic carbon at 295 K and light intensity of 170 μmol-photon m^-2 s^-1 was investigated. CO₂ concentration in the bubbling gas was adjusted by mixing pure gas components of CO₂ and N₂ to avoid photorespiration and β-oxidation of fatty acids under O₂ surrounding conditions. Maximum content of total fatty acid showed pH-dependence after nitrate depletion of the culture media and increased with the corresponding inorganic carbon ratio. Namely, [HCO₃^-]/([CO₂]+n[ CO32-]) ratio in the culture media was found to be a controlling factor for photoautotrophic fatty acid production after the nitrate limitation. At a CO₂ concentration of 5% (vol/vol) and a pH of 6.7, the fatty acid content was 47.8 wt % (dry basis) at its maximum that is comparable with land plant seed oils.