Abstract: The nitrile-degrading actinomycetes Streptomyces canus CGMCC 13662 was used to study the degradation of benzonitrile and benzoic acid. Growing S. canus cells degraded 0.5 g·L^-1 benzonitrile completely within 120 h, with the final metabolite being benzamide; the resting cells could completely degrade benzonitrile within 24 h and benzamide and benzoic acid were no longer present by the end of the assay. The actinomycetes simultaneously degraded catechol and protocatechuic acid, indicating that the metabolic pathway of benzoic acid was via the catechol and protocatechuic acid pathway. The amount of benzoic acid produced by CoCl₂-induced actinomycetes after 8 h of benzonitrile metabolism was 6-fold lower than the control without cobalt, which indicated that benzonitrile degraded via the NHase/amidase and nitrilase pathways and CoCl₂ could regulate the benzonitrile metabolic flux between them. The degradation rate of benzonitrile decreased under acidic conditions, while benzamide degradation was sensitive to the pH of the resting cell solutions and the maximum benzamide degradation was at pH 7.5. Malate was the optimal co-substrate for benzonitrile degradation, leading to a decrease in the half-life of benzonitrile degradation from 5.40 to 3.80 h compared with degradation without the co-substrate. Glucose and sucrose had no effect on benzonitrile degradation as co-substrates, while they inhibited the further degradation of benzamide. This study provides a theoretical basis for eliminating residues of benzonitrile and intermediate metabolic pollutants in the environment and exploring the metabolic mechanism of benzonitrile.

Key words: Streptomyces canus, benzonitrile, benzoic acid, microbial degradation