Carbon-to-nitrogen ratio affects the biomass composition and the fatty acid profile of heterotrophically grown Chlorella sp. TISTR 8990 for biodiesel production

• In the dark, the carbon-to-nitrogen ratio (C/N) affects productivity of algae.
• The optimal C/N ratios are different for production of the biomass and the oil.
• In Chlorella sp., biotin does not affect the oil content of the biomass.
• Oil productivity is greater in heterotrophic growth relative to photosynthetic growth.

Chlorella sp. TISTR 8990 was cultivated heterotrophically in media with various initial carbon-to-nitrogen ratios (C/N ratio) and at different agitation speeds. The production of the biomass, its total fatty acid content and the composition of the fatty acids were affected by the C/N ratio, but not by agitation speed in the range examined. The biomass production was maximized at a C/N mass ratio of 29:1. At this C/N ratio, the biomass productivity was 0.68 g L−1 d−1, or nearly 1.6-fold the best attainable productivity in photoautotrophic growth. The biomass yield coefficient on glucose was 0.62 g g−1 during exponential growth. The total fatty acids (TFAs) in the freeze-dried biomass were maximum (459 mg g−1) at a C/N ratio of 95:1. Lower values of the C/N ratio reduced the fatty acid content of the biomass. The maximum productivity of TFAs (186 mg L−1 d−1) occurred at C/N ratios of 63:1 and higher. At these conditions, the fatty acids were mostly of the polyunsaturated type. Allowing the alga to remain in the stationary phase for a prolonged period after N-depletion, reduced the level of monounsaturated fatty acids and the level of polyunsaturated fatty acids increased. Biotin supplementation of the culture medium reduced the biomass productivity relative to biotin-free control, but had no effect on the total fatty acid content of the biomass.