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Chlorella vulgaris (C. vulgaris) and Scenedesmus obliquus (S. obliquus) were compared to remove toxicity under conditions of sludge extract cultivation for 30 days. The toxicity of sludge extract, the growth characteristics, photosynthetic pigment, superoxide dismutase (SOD) enzyme and catalase (CAT) enzyme activities of the two microalgae were studied by contrast. The results showed that small molecular organic matter (<500 Da) was more easily utilized by microalgae. The toxicity in the toxic group of C. vulgaris and S. obliquus on the 30th day decreased to 56.8 ± 1.2% and 60.7 ± 2.8%, respectively. In the toxic group, the maximal SOD enzyme activity of C. vulgaris and S. obliquus were 2.02 U/mg proteins and 8.21 U/mg proteins, respectively, demonstrating that toxicity caused more oxidative damage to S. obliquus than to C. vulgaris. Proteomics analysis revealed that C. vulgaris mainly regulates energy synthesis and distribution primarily through sugar metabolism, and biomass synthesis primarily through carbon metabolism, whereas S. obliquus mainly regulates energy synthesis and distribution primarily through sugar metabolism and oxidative phosphorylation, resulting in sludge toxicity stress regulation. [Display omitted] • C. vulgaris ' ability to adapt to toxicity was stronger than S. obliquus. • Both C. vulgaris and S. obliquus decreased the toxicity to less than 50%. • Toxicity caused more oxidative damage to S. obliquus than to C. vulgaris. • C. vulgaris regulates energy and biomass through sugar and carbon metabolism. • S. obliquus regulates energy through sugar metabolism and oxidative phosphorylation. [ABSTRACT FROM AUTHOR]

Algae have great application prospects in excess sludge reclamation and recovery of high-value biomass. Chlorococcum humicola was cultivated in this research, using sludge extract (mixed with SE medium) with additions of 10%, 20%, and 30% CO 2 (v/v). Results showed that under 20% CO 2 , the dry weight and polysaccharide yield reached 1.389 ± 0.070 g/L and 313.49 ± 10.77 mg/L, respectively. 10% and 20% CO 2 promoted the production of cellular antioxidant molecules to resist the toxic stress and the toxicity of 20% CO 2 group decreased from 62.16 ± 3.11% to 33.02 ± 3.76%. 10% and 20% CO 2 accelerated the electron transfer, enhanced carbon assimilation, and promoted the photosynthetic efficiency, while 30% CO 2 led to photosystem damage and disorder of antioxidant system. Proteomic analysis showed that 20% CO 2 mainly affected energy metabolism and the oxidative stress level on the early stage (10 d), while affected photosynthesis and organic substance metabolism on the stable stage (30 d). The up-regulation of PSII photosynthetic protein subunit 8 (PsbA, PsbO), A0A383W1S5 and A0A383VRI4 promoted the efficiency of PSII and chlorophyll synthesis, and the up-regulation of A0A383WH74 and A0A2Z4THB7 led to the accumulation of polysaccharides. The up-regulation of A0A383VDH1, A0A383VX37 and A0A383VA86 promoted respiration. Collectively, this work discloses the regulatory mechanism of high-concentration CO 2 on Chlorococcum humicola to overcome toxicity and accumulate polysaccharides. • The optimal CO 2 was 20% (v/v) for Chlorococcum humicola cultured in sludge extract. • The toxicity of 20% CO 2 group decreased from 62.16 ± 3.11% to 33.02 ± 3.76%. • Up-regulation of PsbA, PsbO, A0A383W1S5 and A0A383VRI4 enhanced photosynthesis. • Up-regulation of A0A2Z4THB7 and A0A383WH74 promoted polysaccharide accumulation. • Up-regulation of A0A383VDH1, A0A383VX37 and A0A383VA86 enhanced respiration. Synopsis statementThis research is of great significance in wastewater treatment, excess sludge reclamation, CO 2 biological fixation and resource cultivation of algae. [ABSTRACT FROM AUTHOR]