Effects of different concentrations of CO2 on Scenedesmus obliquus to overcome sludge extract toxicity and accumulate biomass

Large amounts of toxic excess sludge as well as high concentrations of carbon dioxide can be produced in coal-gasification industry. Microalgae has huge potential in the use of nutrients, the removal of toxic organic matter in excess sludge and CO2 fixation. At the same time, the cultivation of the microalgae and the accumulation of high-quality biomass are also the key problems of concern. In this study, the growth and biomass synthesis of Scenedesmus obliquus cultured in sludge extract under 0%–15% (v/v) CO2 were investigated. Results indicated that the highest microalgae biomass yield of 1.609 ± 0.012 g/L can be achieved under 15% CO2 on the 30th day. The maximal photochemical efficiency of PSⅡ (Fv/Fm) decreased in the first 12 h and then increased with the culture time, and the decline amplitude decreased with the increase of the CO2 concentration, indicating that CO2 slowed down the toxic inhibition of sludge extract to Scenedesmus obliquus, which was expressed as the down-regulation of p53 signaling pathway and protein A0A383WFI7. Proteomic analysis showed that under high-concentration CO2, the protein interaction network with the protein of photosystem II assembly (A0A383VSL5) as the core protein regulated the growth of Scenedesmus obliquus in terms of energy metabolism and material transportation. On the 4th day, Methyltransf_11 domain-containing protein (A0A383VH03) was up-regulated and promoted lipid synthesis, leading to the accumulation of lipids in Scenedesmus obliquus in the early stage and the increase of polysaccharides in the later stage. Collectively, this study revealed the regulation mechanism of CO2 on toxicity removal and carbon distribution of Scenedesmus obliquus. [Display omitted] •Scenedesmus obliquus cultured in toxic sludge extract could adapt to 0%–15% CO2.•CO2 enhanced the ability of S. obliquus to overcome toxicity of sludge extract.•The up-regulation of photosystem II assembly promoted the growth of S. obliquus.•Up-regulation of Methyltransf_11 domain-containing protein led to lipid synthesis.