Inorganic carbon and pH effect on growth and lipid productivity of Tetraselmis suecica and Chlorella sp (Chlorophyta) grown outdoors in bag photobioreactors

Abstract

There has been considerable interest in cultivation of green microalgae (Chlorophyta) as a source of lipid that can alternatively be converted to biodiesel. However, almost all mass cultures of algae are carbon-limited. Therefore, to reach a high biomass and oil productivities, the ideal selected microalgae will most likely need a source of inorganic carbon. Here, growth and lipid productivities of Tetraselmis suecica CS-187 and Chlorella sp were tested under various ranges of pH and different sources of inorganic carbon (untreated flue gas from coal-fired power plant, pure industrial CO₂, pH-adjusted using HCl and sodium bicarbonate). Biomass and lipid productivities were highest at pH 7.5 (320 ± 29.9 mg biomass L⁻¹ day⁻¹and 92 ± 13.1 mg lipid L⁻¹ day⁻¹) and pH 7 (407 ± 5.5 mg biomass L⁻¹ day⁻¹ and 99 ± 17.2 mg lipid L⁻¹ day⁻¹) for T. suecica CS-187 and Chlorella sp, respectively. In general, biomass and lipid productivities were pH 7.5 > pH 7 > pH 8 > pH 6.5 and pH 7 > pH 7.5 = pH 8 > pH 6.5 > pH 6 > pH 5.5 for T. suecica CS-187 and Chlorella sp, respectively. The effect of various inorganic carbon on growth and productivities of T. suecica (regulated at pH = 7.5) and Chlorella sp (regulated at pH = 7) grown in bag photobioreactors was also examined outdoor at the International Power Hazelwood, Gippsland, Victoria, Australia. The highest biomass and lipid productivities of T. suecica (51.45 ± 2.67 mg biomass L⁻¹ day⁻¹ and 14.8 ± 2.46 mg lipid L⁻¹ day⁻¹) and Chlorella sp (60.00 ± 2.4 mg biomass L⁻¹ day⁻¹ and 13.70 ± 1.35 mg lipid L⁻¹ day⁻¹) were achieved when grown using CO₂ as inorganic carbon source. No significant differences were found between CO₂ and flue gas biomass and lipid productivities. While grown using CO₂ and flue gas, biomass productivities were 10, 13 and 18 %, and 7, 14 and 19 % higher than NaHCO₃, HCl and unregulated pH for T. suecica and Chlorella sp, respectively. Addition of inorganic carbon increased specific growth rate and lipid content but reduced biomass yield and cell weight of T. suecica. Addition of inorganic carbon increased yield but did not change specific growth rate, cell weight or content of the cell weight of Chlorella sp. Both strains showed significantly higher maximum quantum yield (F_v/F_m) when grown under optimum pH.