Insights into the microbial degradation pathways of the ioxynil octanoate herbicide

Abstract

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.