Effects of inorganic carbon and light on acetate assimilation by Nannochloropsis oceanica (Eustigmatophyceae) in mixotrophic cultivation

Microalgae are capable of accumulating biomass and high value-added products in mixotrophic culture. Carbon sources and light intensity have significant impacts on mixotrophic algal growth and metabolites accumulation. To investigate the underlying mechanism of interaction between metabolism of inorganic carbon and organic carbon, growth of Nannochloropsis oceanica with two different carbon sources (organic sodium acetate and inorganic sodium bicarbonate) and under three different light conditions (low, medium and high) was examined. A combination of sodium acetate and sodium bicarbonate significantly enhanced algal growth with higher acetate assimilation, when compared with single carbon sources. With increasing light intensity from low to high, the algal growth rates decreased in both autotrophic and mixotrophic cultures. Proteome and C-13 metabolic flux results showed that acetate utilization via the glyoxylate (GOX) cycle was enhanced, and both the tricarboxylic acid (TCA) cycle and putative C-4 pathway were stimulated in the presence of sodium acetate and sodium bicarbonate. These data suggest that sodium bicarbonate could facilitate acetate assimilation and enhance the putative C-4 pathway, GOX cycle and TCA cycle in N. oceanica under mixotrophic culture conditions.