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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]