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

Microalgae used for CO biofixation can effectively relieve CO emissions and produce high-value biomass to achieve "waste-to-treasure" bioconversion. However, the low CO fixation efficiency and the restricted application of biomass are currently bottlenecks, limiting the economic viability of CO biofixation by microalgae. To achieve high-efficient CO fixation and high-protein biomass production, the oleaginous microalga () was cultivated autotrophically through optimizing inorganic carbon and nitrogen supply. 0.42 g L NaHCO supplemented with 2% CO as a hybrid carbon source resulted in high biomass concentration (3.89 g L) and productivity (318.33) with CO fixation rate 544.21 mg L d 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 fixation rate (1043.95 mg L d) with protein content (58.48% DW) in batch 2 of repeated fed-batch cultures were achieved under 2.5 g L 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.