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• A genetically stable mutant strain KT16 was screened by UV-DES compound mutagenesis. • The enzyme activity of KT16 increased by 46.7% compared to the original strain DC8. • Mutagenesis enhanced the ability of S-TE, facilitated the recovery of protein in ES. • Whole genome resequencing analysis of KT16 was performed to detect the mutant sites. Excess sludge (ES), a resource-rich organic waste, can be solubilized by thermophilic enzymes to extract proteins for sludge reduction and resources recovery. To solve the problems of low hydrolysis effect of ES and low enzyme producing ability of wild thermophilic bacteria, ultraviolet and diethyl sulfate (UV-DES) were adopted to mutate thermophilic bacteria in this study. Mutation sites were detected and annotated by whole genome sequencing analysis. The results showed that UV-DES mutagenesis could effectively improve enzyme-producing capacity of thermophilic bacteria and promote the hydrolysis of ES. The protease activity of the mutant strain KT16 was 46.7 % higher than that of the original strain DC8. The protein extraction rate with enzyme produced by KT16 reached 83.3 %. The total content of proteins recycled through KT16 enzyme solution was 3539.6 mg·L-1, 18.4 % higher than that of DC8. This work provided a theoretical idea and technical guidance for the protein recovery from ES. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of acetate to butyrate ratio in electron acceptor (EA) on CCE was studied. • The higher content of butyrate in feedstock led to the higher production of caproate. • The maximum production of caproate was 3.7 ± 0.3 g·L−1, when butyrate was the only EA. • Butyrate would be selected as EA only when acetate was much lower than butyrate. • Metagenomic analysis proved that high butyrate content strengthened the CCE process. This study investigated the effect of electron acceptor (EA) distribution (acetate to butyrate ratio) on the carbon chain elongation (CCE) process. The results showed that the higher content of butyrate in the initial material led to the higher production of caproate. The maximum production of caproate was 3.74 ± 0.30 g·L−1, which was obtained when only butyrate was added as EA. Little caproate but much butyrate was produced where only acetate was added as EA. This indicated that CCE bacteria preferentially selected acetate as the EA to produce butyrate, and butyrate could be selected as EA to produce caproate only when the acetate content was much lower than butyrate. Unclassified_f_Dysgonomonadaceae , Massilibacterium , and Seramator were the predominant bacteria. Functional enzyme analysis showed that high butyrate content strengthened the fatty acid biosynthesis pathway and reverse β-oxidization pathway. The findings showed the importance of butyrate in CCE for caproate production. [ABSTRACT FROM AUTHOR]