Degradation of three benzonitrile herbicides by Aminobacter MSH1 versus soil microbial communities: Pathways and kinetics

BACKGROUND: The herbicide dichlobenil was banned in the European Union after its metabolite 2,6-dichlorobenzamide (BAM) was encountered in groundwater. Owing to structural similarities, bromoxynil and ioxynil might be converted to persistent metabolites in a similar manner. To examine this, we used an indigenous soil bacterium Aminobactersp. MSH1 which is capable of mineralizing dichlobenil via BAM and 2,6-dichlorobenzoic acid (2,6-DCBA). 

RESULTS: Strain MSH1 converted bromoxynil and ioxynil to the corresponding aromatic metabolites, 3,5-dibromo-4-hydroxybenzoic acid (BrAC) and 3,5-diiodo-4-hydroxybenzoic acid (IAC) following Michaelis-Menten kinetics (adjusted R² between 0.907 and 0.999). However, in contrast to 2,6-DCBA, degradation of these metabolites was not detected in the pure-culture studies, suggesting that they might pose an environmental risk if similar partial degradation occurred in soil. By contrast, experiments with natural soils indicated 20-30% mineralization of ioxynil and bromoxynil within the first week. 

CONCLUSION: The degradation pathway of the three benzonitriles is initially driven by similar enzymes, after which more specific enzymes are responsible for further degradation. Ioxynil and bromoxynil mineralization in soil is not dependent on previous benzonitrile exposure. The accumulation of dead-end metabolites, as seen for dichlobenil, is not a major problem.