Development of pH stable media for phototrophic and heterotrophic cultivation of Chlorella sorokiniana

The use of green microalgae Chlorella for the production of biopharmaceutics, nutraceuticals and biofuels requires high-density culture. A general concern for the cultivation of Chlorella sorokiniana is the growth limiting potential of extreme pH levels during phototrophic and heterotrophic culture. Described here is a media formulation strategy based on the chemical stability of tris-base as a substitute to tris-acetate in the tris-acetate phosphate (TAP) media. The tris-base concentration was further investigated on phototrophic cultivation of C. sorokiniana under different conditions of CO2 and light intensity. An optimal tris-base concentration of 0.2 M was found for biomass concentrations of ≥ 6.6 gL^−1 at 180 – 240 µmol m−2s^−1, while, at 20 mM and below, biomass concentration of 0.85 gL^−1 was produced. In all the experiments, pH variation remained within an acceptable range of 5.9 – 7.2. For the heterotrophic cultivation using glucose as a carbon source, pH value remained within the range specified and achieved biomass concentration of 5.5 gL^-1. Evaluation of fatty methyl ester (FAME) compositions showed no negative impact on selected FAME when compared with other studies which used different media. Ability to keep the broth pH within the required range showed good agreement between the media chemical compositions and biological responses of the cells. The use and optimisation of tris-base for other minimal media will be studied in the future for cultivation of other Chlorella strains.