Impact of Acetate and Optimized Nitrate Levels on Mixotrophic Growth and Protein Dynamics in Chlorella Sorokiniana

Microalgae are well-known for their role as sustainable bio-factories, offering a promising solution to the global food and nutrition crisis. To clarify the potential of Chlorella sorokiniana UTEX 1230 for food applications, particularly as an alternative protein source, the study employed a mixotrophic cultivation mode with sodium acetate (NaAc) as a cost-effective organic carbon (NaAc-C) source. Varying levels of NaAc-C and nitrate-sourced nitrogen were investigated, optimizing the effect of metabolic characteristics of the microalgal growth. The designed heterotrophic cultivation confirmed the ability of C. sorokiniana UTEX 1230 to grow on NaAc-C, and then the mixotrophic cultures, when supported by both NaAc-C and CO2, exhibited superior growth performance, achieving double the biomass concentration compared to the autotrophic control. The addition of nitrogen (750 mg/L NaNO) facilitated the thorough metabolism of NaAc-C and enhanced photosynthetic activity indicated by a 196% increase in pigment levels, which resulted in a maximum biomass concentration of 2.82 g/L in the 150 mM NaAc-C group. A detailed analysis of nitrogen and protein concentrations over time revealed that higher nitrogen availability led to greater protein accumulation which was then degraded to support essential life activities under nitrogen starvation. Therefore, it is suggested that supplementing nitrate on the 3rd day and harvesting on the 4th day could be strategically implemented to increase protein yield from 0.17 g/L/d to 0.34 g/L/d. These findings offer theoretical guidance for further refining this microalgal strain for use as an alternative protein.