Enhancing biomass, lipid production, and nutrient utilization of the microalga Monoraphidium sp. QLZ-3 in walnut shell extracts supplemented with carbon dioxide

•Algae were grown in walnut shell extracts (WSE) supplemented with carbon dioxide.•CO2 significantly boosted the biomass and lipid productivities of algae under WSE cultivation.•A combined strategy was developed for WSE utilization and biofuel production. Microalgae are a promising biofuel resource, but their high cost and low productivity hinder their commercial applications. In the present study, Monoraphidium sp. QLZ-3 was cultivated in walnut shell extracts (WSE) supplemented with carbon dioxide (CO2). Biomass was enhanced from 0.40 g L−1 to 1.18 g L−1, and lipid content reached 49.54% in WSE-12% CO2 media. Biomass and lipid productivity reached 196.88 and 97.52 mg L−1 d−1, which were 1.33- and 1.57-fold higher than those of the control, respectively. The amount of carbohydrates increased, but the protein contents decreased. Furthermore, the application of CO2 promoted nutrient and polyphenol absorption and upregulated the expression levels of lipid biosynthetic genes of this WSE-cultivated alga. These results indicated that coupling WSE and CO2 could be an efficient strategy to enhance biofuel production by microalgae.