Enhanced degradation of dicamba by an anaerobic sludge acclimated from river sediment

Abstract Dicamba is an important herbicide that is widely used in agriculture. However, its residue pollutes the environment and thus its catabolism raises great concern. Anaerobic degradation is the main dicamba degradation pathway in the environment, however, the whole degradation pathway and the microorganisms involved remain unknown. In this study, an anaerobic sludge capable of efficiently degrading high concentration of dicamba was enriched from a river sediment. The degradation capacity reached 1.74 μmol dgs−1 days−1 which was 47-fold higher than that of the inoculated sediment. The acclimated sludge could degrade dicamba, 3,6-dichlorosalicylic acid, 6-chlorosalicylic acid, 3-chlorophenol and phenol but not salicylic acid, 3-chlorosalicylic acid, 4-chlorosalicylic acid, 2-chlorophenol or benzoic acid. Based on the identification of 3,6-dichlorosalicylic acid, 6-chlorosalicylic acid as well as four new dicamba metabolites, 3-chlorohydroquinone, 3-chlorophenol, phenol and butyric acid, a novel anaerobic degradation pathway was proposed for dicamba. Firstly, dicamba was O-demethylated to 3,6-dichlorosalicylic acid and then dechlorinated to 6-chlorosalicylic acid. Next, 6-chlorosalicylic acid was decarboxylated to 3-chlorohydroquinone, then dehydroxylated to 3-chlorophenol, and dechlorinated to phenol, which was further converted to butyric acid. The microbial community of the acclimated sludge was dominated by bacteria belonging to the genera Desulfovibrio, Exilispira, Caldisericum, Aminicenantales_unclassified, Syntrophus, Dehalobacter, Romboutsia, Syntrophorhabdus, and Saccharimonadales_unclassified, as well as archaea belonging to the genera Methanosaeta, Methanolinea, Bathyarchaeia_unclassified, and Methanomethylicus, all of which might be involved in the degradation of dicamba. This study provides a basis for further elucidation of the anaerobic catabolism of dicamba and development of anaerobic technology to treat high concentration dicamba wastewater.