Microbial degradation of acylanilide herbicides

David Torres Villarreal, Purdue University

Abstract

Acylanilide herbicides are used widely in the United States, and the fate of these synthetic organic compounds in the environment is of concern. In order to better understand the metabolism of these pesticides, microorganisms capable of the metabolism of propachlor were isolated from a pesticide disposal site. Complete degradation of propachlor occurred when two strains, Moraxella DAK3 and Xanthobacter MAB2, were inoculated into a mineral salts medium containing propachlor. When the Moraxella DAK3 alone was grown on propachlor, a metabolite (2-chloro-N-isopropylacetamide) was released into the medium. Strain MAB2 could grow on this metabolite. DAK3 can respire and grow on N-substituted acylanilides containing methyl, ethyl, or isopropyl substitutions but is incapable of respiration or growth on acetanilide, aniline, or the acylanilide herbicides alachlor and metolachlor. DAK3 appears to be using the aromatic C atoms of propachlor for growth, as suggested by the growth yield on propachlor and the induction of catechol 2,3-oxygenase activity in acylanilide-grown cells. DAK3 cells grown on the herbicide were simultaneously induced to respire catechol but not other common intermediates of aromatic catabolism. Inhibitors of catechol metabolism also inhibited the metabolism of the acylanilides. Herbicide-cleaving ability was lost when DAK3 cells were disrupted by freezing, French-press, or lysozyme treatment. A parallel study examined the relationship between chemical structure and biodegradability of acylanilides by using a set of model compounds. Four bacterial isolates (two Gram-negative and two Gram-positive) that grew on acetanilide were used. These soil isolates cleaved the amide bond of acetanilide via an aryl acylamidase reaction, producing aniline and the organic acid acetate. A series of acetanilide analogs with alkyl substitutions on the nitrogen atom or the aromatic ring were tested for their ability to induce aryl acylamidase activity and act as substrates for the enzyme. Substrate range, in general, was limited to those analogs not di-substituted in the ortho position of the benzene ring or which contained an alkyl group on the nitrogen atom. These same N-substituted compounds did not induce enzyme activity either, whereas the ortho-substituted compounds could in some cases. The amidase activity appeared to be primarily in the soluble fraction of the cell for each isolate.

Degree

Ph.D.

Advisors

Konopka, Purdue University.

Subject Area

Microbiology|Environmental science

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