Microbial Biotransformation Products and Pathways of Dichloroacetamide Herbicide Safeners

Abstract

Dichloroacetamide safeners are common ingredients in commercial herbicide formulations. We previously investigated the environmental fate of dichloroacetamides via photolysis and hydrolysis, but other potentially important, environmentally relevant fate processes remain uncharacterized and may yield products of concern. Here, we examined microbial biotransformation of two dichloroacetamide safeners, benoxacor and dichlormid, to identify products and elucidate pathways. Using aerobic microcosms inoculated with river sediment, we demonstrated that microbial biotransformations of benoxacor and dichlormid proceed primarily, if not exclusively, via cometabolism. Benoxacor was transformed by both hydrolysis and microbial biotransformation processes; in most cases, biotransformation rates were faster than hydrolysis rates. We identified multiple novel products of benoxacor and dichlormid not previously observed for microbial processes, with several products similar to those reported for structurally related chloroacetamide herbicides, thus indicating potential for conserved biotransformation mechanisms across both chemical classes. Observed products include monochlorinated species such as the banned herbicide CDAA (from dichlormid), glutathione conjugates, and sulfur-containing species. We propose a transformation pathway wherein benoxacor and dichlormid are first dechlorinated, likely via microbial hydrolysis, and subsequently conjugated with glutathione. This is the first study reporting biological dechlorination of dichloroacetamides to yield monochlorinated products in aerobic environments.

Figure 1

(A) Measured concentrations (benoxacor and monochloro-benoxacor) and peak area (Benox-263) for the microbial biotransformation of benoxacor to yield two major microbial biotransformation products over 31 days. (B) Measured concentration (monochloro-benoxacor) and peak area (Benox-263) for the transformation of monochloro-benoxacor to yield the cysteine-related conjugate Benox-263. Error bars represent one standard deviation (n = 3; some error bars are obscured by the data points if very small). Retention times for each compound on the Obritrap mass spectrometer are as follows: benoxacor, 12.1 min; monochloro-benoxacor, 10.7 min; Benox-263, 12.9 min. Descriptions of the transformation products and their confidences in identification for the proposed structures are included in Table 1.

Figure 2

(A) Measured concentration and peak areas for the microbial biotransformation of dichlormid to yield two major monochlorinated microbial biotransformation products over the course of 31 days. The products were identified as the active herbicide CDAA and an isomer (based on data presented by Sivey and Roberts; Dich-173 is assumed to have the same molar absorptivity as CDAA). (B) Measured concentration and peak area for the transformation of CDAA to yield the proposed sulfur-containing metabolite CD-171 and a dimer (CD-308). Error bars representing one standard deviation (n = 3) are present but may be obscured by the data points. Retention times for each compound on the Obritrap mass spectrometer are as follows: dichlormid, 10.6 min; Dich-173, 8.0 min; CDAA, 8.8 min; CD-171, 7.4 min; CD-308, 10.5 min. Descriptions of the transformation products and their confidences in identification for the proposed structures are included in Table 1.