High-efficient CO2-to-protein bioconversion by oleaginous Coccomyxa subellipsoidea through light quality shift and nitrogen supplementation strategy

Oleaginous microalgae are usually used in CO2-to-lipid bioprocess for biofuels production but still faced the low productivity of biomass and lipid with uncompetitive cost. A novel CO2-to-protein bioconversion pattern by oleaginous Coccomyxa subellipsoidea was implemented in the present work as high-value application of CO2. Results demonstrated that the highest CO2 fixation rate (RCO2, 0.74 g/L/d) and protein productivity (178 mg/L/d) with lower protein content (42.33% DW) were achieved under red light; but under blue light, the highest protein content (52.07% DW) were reached with lower productivity (156.94 mg/L/d) and RCO2 (0.52 g/L/d). The analysis of key genes transcription and enzymes activity revealed that red light boosted the photosynthetic activity and photoprotection contributing to the high-efficient CO2 fixation. Based on this, a tailored light quality shift and nitrogen supplementation in a two-phase culture (red light in phase 1; red: blue-1:1 with nitrate addition in phase 2) was developed, reaching the maximal biomass yield (6.12 g/L), RCO2 (0.90 g/L/d), protein content (52.00% DW) with protein productivity (265.83 mg/L/d) simultaneously, 2.0-fold, 1.2-fold, 2.9-fold and 2.3-fold higher than that under white light as the control. The highest contents of AAs (47.12% DW) and EAAs (18.35% DW) with essential amino acid index (1.2) were achieved, superior to FAO/WHO reference. Time-dependent gene transcription analysis revealed the up-regulated photosynthesis and nitrogen assimilation in the two-phase culture. This work demonstrated that the oleaginous C. subellipsoidea could be well used for protein production and CO2 fixation, providing a promising application approach in carbon neutrality and high-yield protein production.