Biotransformation of 3,5‐dibromo‐4‐hydroxybenzonitrile under denitrifying, Fe(III)‐reducing, sulfidogenic, and methanogenic conditions

Abstract

Bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) is a halogenated aromatic nitrile herbicide used on a variety of crops for the postemergence control of annual broad-leaved weeds. The anaerobic biodegradability of bromoxynil and its aerobic transformation product, 3,5-dibromo-4-hydroxybenzoate, were examined in enrichment cultures established with anaerobic sediment under denitrifying, Fe(III)-reducing, sulfidogenic, and methanogenic conditions. Bromoxynil (100 microM) was depleted in 20 to 30 d in the methanogenic, sulfidogenic, and Fe(IIi)-reducing enrichments but was stable under denitrifying conditions. The 3,5-dibromo-4-hydroxybenzoate (100 microM) was depleted within 20 to 35 d under all four anaerobic conditions. Both compounds were stable in sterile controls. Bromoxynil and 3,5-dibromo-4-hydroxybenzoate were readily utilized upon respiking of the cultures. During utilization of bromoxynil, stoichiometric release of bromide was observed with transient accumulation of metabolites identified as bromocyanophenol, cyanophenol, and phenol. Bromoxynil heptanoate and octanoate were rapidly hydrolyzed to bromoxynil, which was further degraded. These results indicate that bromoxynil and 3,5-dibromo-4-hydroxybenzoate are degraded under different anaerobic conditions. Anaerobic degradation of bromoxynil proceeds via reductive debromination to 4-cyanophenol, which is further transformed to phenol and can ultimately be degraded to carbon dioxide.

Isotope Fractionation (δ13C, δ15N) in the Microbial Degradation of Bromoxynil by Aerobic and Anaerobic Soil Enrichment Cultures

“…Dominance of the debrominated metabolites was observed with 3-bromo-4-hydroxybenzonitrile and 4-hydroxybenzonitrile accounting for ≥96% of all metabolites (metabolites RH3 and RH6 in Table S4), equaling approximately seven times the remaining bromoxynil pool, at f = 0.07. These debrominated metabolites were also reported by others for the anaerobic incubation of bromoxynil with Desulfitobacterium chlororespirans and with enrichment cultures . All other metabolites are minor and reported here for the first time for anaerobic incubation.…”

“…13 Bromoxynil may also be biodegraded under anaerobic conditions, where degradation involves the reductive dehalogenase family (RDase). It may be facilitated by reductive dehalogenase solely (route 4), resulting in debromination and the production of 4-cyanophenol as shown, for example, in Desulfitobacterium chlororespirans 15 and in enrichment cultures, 16 as well as in cyano group removal (route 4.1) to produce phenol. 16 The extent of the degradation and the governing degradation pathways are important factors in bromoxynil application.…”

Dehalogenation of the Herbicides Bromoxynil (3,5-Dibromo-4-Hydroxybenzonitrile) and Ioxynil (3,5-Diiodino-4-Hydroxybenzonitrile) by Desulfitobacterium chlororespirans

“…Previous research has focused primarily on aerobic bromoxynil degradation (15,27,43,45,48), with little information addressing bromoxynil's fate under anaerobic conditions. To our knowledge, there has been only one report (22) specifically addressing anaerobic bromoxynil degradation. In that investigation, the microorganisms responsible were not identified.…”

How Early Diagenesis Reveals in Situ Biodegradation of Herbicides in Sediment

“…Bromoxynil is aerobically degraded into 3,5-dibromo-4-hydroxybenzonitrile. Knight et al (2003) estimated the microbial degradation under denitrifying, Fe(III)-reducing, sulfidogenic while 140 days are not enough to observe a significant degradation under denitrifying conditions, bromoxynil is degraded much faster (40d) under more reduced conditions. Dicamba anaerobic metabolisation produces 3,6-dichlorosalicylate that is in turn transformed under methanogenic conditions by O-demethylation in 6-chlorosalycilate.…”