Effects of sodium bicarbonate on cell growth, lipid accumulation, and morphology of Chlorella vulgaris

Background: Low concentration NaHCO₃ (ca. 12 mM) had been demonstrated to be an excellent carbon source for industrially important green alga Chlorella vulgaris and high concentration NaHCO₃ (e.g. 160 mM) had been shown to be capable of controlling protozoa and stimulating lipid accumulation of another green alga, i.e., Neochloris oleoabundans. Furthermore, little was known about the mechanisms of the effects of NaHCO₃ on microalgae. Thorough studies on the effects of high NaHCO₃ on C. vulgaris and their mechanisms were therefore warranted.

Methods: We systematically compared the cell growth, lipid production, and cell morphology of the industrially important C. vulgaris in 160 mM NaHCO₃ or 160 mM NaCl media at different pH levels. These data allowed us to analyze the effects of total dissolved inorganic carbon (DIC) and individual DIC species on C. vulgaris. Cell growth of C. vulgaris at a range of concentrations at 160 mM or lower was also studied.

Results: Cellular lipid cell content of 494 mg g^-1 and lipid productivity of 44.5 mg L^-1 day^-1 were obtained at 160 mM NaHCO₃ and pH 9.5. High concentration NaHCO₃ (e.g. 160 mM) was inhibitive to cell growth but stimulating to lipid accumulation and caused unicellular C. vulgaris to transfer to colonial cells. Increasing pH in the range of 7.5-9.5 caused increasing inhibition to cell growth in 160 mM NaCl. Whereas the optimal pH for cell growth was 8.5 for 160 mM NaHCO₃ cultures. Comparative experiments with 0-160 mM NaHCO₃ indicate that 10 mM was the optimal concentration and increasing NaHCO₃ from 10 to 160 mM caused increasing inhibition to cell growth.

Conclusions: High concentration DIC was inhibitor to cell growth but stimulator to lipid accumulation of C. vulgaris. It caused unicellular C. vulgaris to transform to colonial cells. Results suggest that high concentration of a particular DIC species, i.e., dCO₂, was the primary stress responsible for cell growth inhibition. Where CO₃^2- was likely the DIC species responsible for lipid stimulation of C. vulgaris. Furthermore, we propose that the colony formation at high DIC conditions was employed by C. vulgaris to mitigate the stress by minimizing cell exposure to unfavorable environment.