Oleaginous Microalga Coccomyxa subellipsoidea as a Highly Effective Cell Factory for CO2 Fixation and High-Protein Biomass Production by Optimal Supply of Inorganic Carbon and Nitrogen

Microalgae used for CO 2 biofixation can effectively relieve CO 2 emissions and produce high-value biomass to achieve “waste-to-treasure” bioconversion. However, the low CO 2 fixation efficiency and the restricted application of biomass are currently bottlenecks, limiting the economic viability of CO 2 biofixation by microalgae. To achieve high-efficient CO 2 fixation and high-protein biomass production, the oleaginous microalga Coccomyxa subellipsoidea ( C. subellipsoidea ) was cultivated autotrophically through optimizing inorganic carbon and nitrogen supply. 0.42 g L −1 NaHCO 3 supplemented with 2% CO 2 as a hybrid carbon source resulted in high biomass concentration (3.89 g L −1 ) and productivity (318.33) with CO 2 fixation rate 544.21 mg L −1 d −1 in shake flasks. Then, used in a 5-L photo-fermenter, the maximal protein content (60.93% DW) in batch 1, and the highest CO 2 fixation rate (1043.95 mg L −1 d −1 ) with protein content (58.48% DW) in batch 2 of repeated fed-batch cultures were achieved under 2.5 g L −1 nitrate. The relative expression of key genes involved in photosynthesis, glycolysis, and protein synthesis showed significant upregulation. This study developed a promising approach for enhancing carbon allocation to protein synthesis in oleaginous microalga, facilitating the bioconversion of the fixed carbon into algal protein instead of oil in green manufacturing.