Transformation of the herbicide 2,6-dichlorobenzonitrile to the persistent metabolite 2,6-dichlorobenzamide (BAM) by soil bacteria known to harbour nitrile hydratase or nitrilase

In soil the herbicide 2,6-dichlorobenzonitrile (dichlobenil) is degraded to the persistent metabolite 2,6-dichlorobenzamide (BAM) which has been detected in 19% of samples taken from Danish groundwater. We tested if common soil bacteria harbouring nitrile-degrading enzymes, nitrile hydratases or nitrilases, were able to degrade dichlobenil in vitro. We showed that several strains degraded dichlobenil stoichiometrically to BAM in 1.5-6.0 days; formation of the amide intermediate thus showed nitrile hydratase rather than nitrilase activity, which would result in formation of 2,6-dichlorobenzoic acid. The non-halogenated analogue benzonitrile was also degraded, but here the benzamide intermediate accumulated only transiently showing nitrile hydratase followed by amidase activity. We conclude that a potential for dichlobenil degradation to BAM is found commonly in soil bacteria, whereas further degradation of the BAM intermediate could not be demonstrated.