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Industrial sludge is characterized by high moisture content, complex composition, and significant organic toxicity. This research aimed to improve the performance of Chlorococcum humicola in removing hydroquinone-domesticated sludge toxicity through the addition of indole-3-acetic acid (IAA). The maximum cell density and dry weight of C. humicola were attained when the IAA concentration was 5.0 × 10−6 mol/L, at 6.79 × 107 cells/mL and 2.60 g/L, respectively. The toxicity of the sludge degradation rate reached its peak level of 59.70% on the 20th day. The toxicity reduction was consistent with the decrease of RNA degradation-related proteins such as A5FN12, which can be hydrolyzed and absorbed by C. humicola via active transport. In the presence of IAA, C. humicola increased its energy production processes, such as glycolysis and oxidative phosphorylation, to facilitate active transport. Moreover, the efficiency of signal transduction was improved, leading to enhanced cellular metabolism. The suppression of biosynthesis of glutathione and arginine at the molecular level may contribute to this observed trend. Overall, this study provides new insights into the mechanism by which IAA promotes the degradation of sludge toxicity by C. humicola , offering a novel method and theoretical basis for the safe and resource-based utilization of industrial sludge. [Display omitted] • IAA can promote the growth of C. humicola. • Sludge toxicity correlates with RNA degradation proteins. • C. humicola enhanced ATP production processes with the addition of IAA. • Signaling pathway enhanced and the biosynthesis of glutathione and arginine declined. Minimal research exists on biological detoxifying treatment of industrial residual sludge. This research reveals the mechanism on the removal of sludge toxicity by algae with exogenous IAA. [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]

The activated sludge method is widely used for the treatment of phenol-containing wastewater, which gives rise to the problem of toxic residual sludge accumulation. Indole-3-acetic acid (IAA), a typical phytohormone, facilitates the microalgal resistance to toxic inhibition while promoting biomass accumulation. In this study, Chlorococcum humicola (C. humicola) was cultured in toxic sludge extract and different concentrations of IAA were used to regulate its physiological properties and enrichment of high value-added products. Ultimately, proteomics analysis was used to reveal the response mechanism of C. humicola to exogenous IAA. The results showed that the IAA concentration of 5 × 10−6 mol/L (M) was most beneficial for C. humicola to cope with the toxic stress in the sludge extract medium, to promote the activity of rubisco enzyme, to enhance the efficiency of photosynthesis, and, finally, to accumulate protein as a percentage of specific dry weight 1.57 times more than that of the control group. Exogenous IAA altered the relative abundance of various amino acids in C. humicola cells, and proteomic analyses showed that exogenous IAA stimulated the algal cells to produce more indole-3-glycerol phosphate (IGP), indole, and serine by up-regulating the enzymes. These precursors are converted to tryptophan under the regulation of tryptophan synthase (A0A383V983), and tryptophan can be metabolized to endogenous IAA to promote the growth of C. humicola. These findings have important implications for the treatment of toxic residual sludge while enriching for high-value amino acids. [Display omitted] • Appropriate IAA addition allowed C. humicola to thrive in the sludge extract medium. • Appropriate concentrations of IAA promote protein accumulation in algal cell. • Exogenous IAA can change the proportion of various amino acids in algal cell. • Up-regulation of A0A383V983 promoted tryptophan enrichment. • Exogenous IAA stimulated endogenous IAA synthesis. [ABSTRACT FROM AUTHOR]

Microbe inhibition produces intermediate metabolites and toxic substances that accumulate significantly in residual sludge toxicity after continuous treatment of coal gasification wastewater with activated sludge. To address this issue, Chlorococcum humicola was cultured with sludge extract. Proteomics analysis was used to investigate the mechanism of stress response in microalgae to sludge toxicity. Upon completion of growth, microalgae treated with sludge extract produced 0.725 ± 0.036 g/L of biomass. The total organic carbon concentration and toxicity removal rates on microalgae were 75.1 ± 3.8% and 47.4 ± 2.1%, respectively. The proteomics analysis revealed that under stress conditions, microalgae altered cellular components such as the nucleus and plasma membranes, affecting the photosynthetic and respiratory processes. The logarithmic phase was characterized by a down-regulation of oxidative phosphorylation and an increase in energy synthesis regulated by the isopropyl phosphate isomerase. The results demonstrate a proteomic basis for the reduction and utilization of waste sludge, as well as provide a molecular mechanism reference for optimizing the model of cultivating microalgae using sludge. [Display omitted] • Sludge extract from hydroquinone treatment was utilized to cultivate microalgae. • The optimal cultivation ratio would increase biomass production and TOC disposal. • Oxidative phosphorylation was controlled downwards in the logarithmic phase. • Energy synthesis and biosynthesis have been adjusted under toxic stress. Synopsis Sludge extract was used to reduce sludge toxicity and increase biomass accumulation in Chlorococcum humicola. Stress altered the nucleus and plasma membranes of microalgae. [ABSTRACT FROM AUTHOR]