C and N isotope fractionation during biodegradation of the pesticide metabolite 2,6-dichlorobenzamide (BAM): Potential for environmental assessments

2,6-Dichlorobenzamide (BAM) is a metabolite of the herbicide 2,6-dichlorobenzonitrile (dichlobenil), and a prominent groundwater contaminant. Observable compound-specific isotope fractionation during BAM formation - through transformation of dichlobenil by Rhodococcus erythropolis DSM 9685 - was small. In contrast, isotope fractionation during BAM degradation - with Aminobacter sp. MSH1 and ASI1, the only known bacterial strains capable of mineralizing BAM - was large, with pronounced carbon (ε _C = -7.5% to -7.8%) and nitrogen (ε_N = -10.7% to -13.5%) isotopic enrichment factors. BAM isotope values in natural samples are therefore expected to be dominated by the effects of its degradation rather than formation. Dual isotope slopes Δ (= Δδ¹⁵N/Δδ¹³C ≈ ε_N/ε_C) showed only small differences for MSH1 (1.75 ± 0.03) and ASI1 (1.45 ± 0.03) suggesting similar transformation mechanisms of BAM hydrolysis. Observations are in agreement with either a tetrahedral intermediate promoted by OH^- or H₃O⁺ catalysis, or a concerted reaction mechanism. Therefore, owing to consistent carbon isotopic fractionation, isotope shifts of BAM can be linked to BAM biodegradation, and may even be used to quantify degradation of this persistent metabolite. In contrast, nitrogen isotope values may be rather indicative of different sources. Our results delineate a new approach to assessing the fate of BAM in the environment.