HEXAZINONE- AND TEBUTHIURON-DEGRADING BACTERIA ISOLATED FROM TROPICAL SOILS

Authors: MOSTAFA, FADWA - OICD; Helling, Charles

Submitted to: International Union of Pure and Applied Chemistry
Publication Type: Abstract Only
Publication Acceptance Date: 4/30/2002
Publication Date: 8/4/2002
Citation: Mostafa, F.I., Helling, C.S. 2002. Hexazinone- and tebuthiuron-degrading bacteria isolated from tropical soils [abstract]. International Union of Pure and Applied Chemistry. Paper No. 2:5A.33, p. 47.

Interpretive Summary:

Technical Abstract: Hexazinone [3-cyclohexyl-6-dimethylamino-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] is a herbicide used for weed control in alfalfa, pineapple, sugarcane, coniferous plantings, and noncropped areas; tebuthiuron [1-(5-tert-butyl)-1,3,4-thiadiazol-2-yl)-1,3-dimethyl-urea] is used for brush control in rangeland, pastures, and rights-of-way. ARS-USDA research demonstrated that both herbicides are also highly effective for control of illicit Erythroxylum (coca) species. In related field research [1], hexazinone (Hex) persisted in tropical soils of Peru, Panama, and Hawaii (USA) with a half-life of ca. 3-5 weeks, and tebuthiuron (Teb), ca. 3-4 months, both significantly shorter than is typical for temperate-region areas. In the present research, bacterial strains with Hex- and Teb-degrading ability were isolated from oxisol soils of the Hawaii site and from a coca field in Colombia, South America. Enrichments from fresh soil were plated on N-free agar that contained only a basal salt medium (BSM) plus Hex or Teb. Surviving colonies were isolated and purified, then cultured in liquid broth (LB) medium for subsequent identification. Molecular characterization of the three predominant isolates (for each pesticide) was performed using the first 500 base pairs of the 16S ribosomal RNA gene. Based on 16S rDNA sequence analysis, the Hex-degrading organisms were Gram/+ bacteria Microbacterium foliorum and Paenibacillus illinoisensis, from Colombia soil, and Rhodococcus equi from Hawaii. For Teb, these organisms were: Methylobacterium organophilum (Gram/!) and P. pabuli (Gram/+) from Colombian soil; and Methylobacterium radiotolerans (Gram/!), from Hawaii. Biodegradation of Hex was tested after growing the purified cultures to their lag-2 growth phase in LB. Three-day-old cultures were then inoculated into BSM, BSM + N source, or BSM + C source and incubated aerobically at 26°C. Controls devoid of organisms, as well as the inoculated treatments, contained 10 ppm Hex. HPLC analysis showed loss of Hex accompanied by the increase of several unidentified hexazinone derivatives. About 90-95% loss of Hex occurred in 15 days from M. foliorum cultures, and slightly slower loss from the other two isolates. Hex losses seemed to be slightly retarded by the addition of a C-source to the BSM, but supplemental N did not affect degradation rate. Degradation in non-inoculated, sterile media indicated that chemical loss of Hex can occur in aqueous solution, with mineral salts. Similar research is underway with the Teb-degrading organisms. Hexazinone and tebuthiuron were degraded microbially, as sole C and N sources, by specific soil bacteria from tropical soils typical of those on which coca production occurs.