Degradation and Mineralization of Nanomolar Concentrations of the Herbicide Dichlobenil and Its Persistent Metabolite 2,6-Dichlorobenzamide by Aminobacter spp. Isolated from Dichlobenil-Treated Soils

Abstract

2,6-Dichlorobenzamide (BAM), a persistent metabolite from the herbicide 2,6-dichlorobenzonitrile (dichlobenil), is the pesticide residue most frequently detected in Danish groundwater. A BAM-mineralizing bacterial community was enriched from dichlobenil-treated soil sampled from the courtyard of a former plant nursery. A BAM-mineralizing bacterium (designated strain MSH1) was cultivated and identified by 16S rRNA gene sequencing and fatty acid analysis as being closely related to members of the genus Aminobacter, including the only cultured BAM degrader, Aminobacter sp. strain ASI1. Strain MSH1 mineralized 15 to 64% of the added [ring-U-(14)C]BAM to (14)CO(2) with BAM at initial concentrations in the range of 7.9 nM to 263.1 muM provided as the sole carbon, nitrogen, and energy source. A quantitative enzyme-linked immunoassay analysis with antibodies against BAM revealed residue concentrations of 0.35 to 18.05 nM BAM following incubation for 10 days, corresponding to a BAM depletion of 95.6 to 99.9%. In contrast to the Aminobacter sp. strain ASI1, strain MSH1 also mineralized the herbicide itself along with several metabolites, including ortho-chlorobenzonitrile, ortho-chlorobenzoic acid, and benzonitrile, making it the first known dichlobenil-mineralizing bacterium. Aminobacter type strains not previously exposed to dichlobenil or BAM were capable of degrading nonchlorinated structural analogs. Combined, these results suggest that closely related Aminobacter strains may have a selective advantage in BAM-contaminated environments, since they are able to use this metabolite or structurally related compounds as a carbon and nitrogen source.