Insights into the microbial degradation pathways of the ioxynil octanoate herbicide

This paper describes the biodegradation of the ioxynil octanoate herbicide by indigenous microorganisms isolated from herbicide impacted soil-enrichment cultures. Eleven positive hits out of twenty-nine microorganisms screened for nitrile hydratase, nitrilase and amidase activity were further evaluated based on their growth in microtiter plates containing liquid medium with increasing concentrations of herbicide (0.97-250 mM). Two strains were selected from this assay for biodegradation studies and were identified as Lysinibacillus boronitolerans MLH-31 and Bacillus cereus MLH-61. The bacterial degradation of ioxynil octanoate and its biodegradation products were monitored, identified and characterized by liquid chromatography tandem mass spectrometry (HPLC-MS/MS). In addition to 3,5-diiodo-4-hydroxybenzamide and 3,5-diiodo-4-hydroxybenzoic acid, which are commonly detected metabolites, two new metabolites were observed: mono-deiodinated compound 3-iodo-4-hydroxybenzoic acid and the product of Caromatic-CN cleaved 1,3-diiodophenol. The experimentally observed metabolites were correlated with the enzymatic systems involved, revealing the presence of esterases, nitrile hydratases, amidases, nitrilases, dehalogenases and carbon-carbon lyases during biodegradation. Lysinibacillus boronitolerans MLH-31 was found to degrade ioxynil octanoate at a rate of 97% over 7 days through a batch-resting cells experiment, while Bacillus cereus MLH-61 was found to do so at a rate of 75% under the same conditions. (AU)