title: Degradation of three benzonitrile herbicides by Aminobacter MSH1 versus soil microbial communities: pathways and kinetics.
abstract: 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 Aminobacter sp. 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 R(2) 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Environmental Science, Aarhus University, Frederiksborgvej 399, 4000, Roskilde, Denmark.
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Frková
authors.forename: Nora
authors.initials: N
authors.lastname: Badawi
authors.forename: Anders
authors.initials: A
authors.lastname: Johansen
authors.forename: Nadja
authors.initials: N
authors.lastname: Schultz-Jensen
authors.forename: Kai
authors.initials: K
authors.lastname: Bester
authors.forename: Sebastian Reinhold
authors.initials: SR
authors.lastname: Sørensen
authors.forename: Ulrich Gosewinkel
authors.initials: UG
authors.lastname: Karlson
mesh_terms: D001673:Biodegradation, Environmental; D006540:Herbicides / Q000378:metabolism*; D007462:Iodobenzenes / Q000378:metabolism; D007700:Kinetics; D009570:Nitriles / Q000378:metabolism*; D059127:Phyllobacteriaceae / Q000378:metabolism*; D012988:Soil Microbiology
chemical_list: D006540:Herbicides; D007462:Iodobenzenes; D009570:Nitriles; C008161:ioxynil; C006826:bromoxynil
keywords: Aminobacter MSH1; bromoxynil; dichlobenil; herbicide biodegradation kinetics; ioxynil; soil microbial communities