Browsing by Author "Taraban, Ronald H."
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- Biodegradability of atrazine, cyanazine, and dicamba in wetland soilsGu, Ji-Dong; Berry, Duane F.; Taraban, Ronald H.; Martens, David C.; Walker, H. Lynn; Edmonds, William J. (Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University, 1992-02)Small amounts of applied pesticide chemicals eventually can end up in nontarget areas such as wetlands, sediments, and groundwater where anaerobic conditions often predominate. Runoff and leaching are major means by which pesticides move away from application sites. Pesticides also can find their way into nontarget areas as a result of inappropriate disposal and accidental spills. We evaluated the biodegradability of atrazine, cyanazine, and dicamba in wetland soils under nitratereducing and methanogenic conditions. Wetland soil samples were collected from three different sites in the Chesapeake Bay watershed region. These sites represented both tidal (Lawnes and Levy soils) and nontidal (Myatt soil) wetlands. Tidal wetlands are water-saturated throughout the year, whereas nontidal wetlands are saturated only during certain times of the year. Herbicide fate studies were conducted in wetland soil microcosms consisting of serum bottles filled with soil slurry and containing either atrazine, cyanazine, or dicamba. Atrazine was extremely stable in wetland soil microcosms regardless of incubation temperature, redox status (nitrate-reducing versus methanogenic conditions), or soil type. Neither temperature nor redox status affected cyanazine stability in Myatt wetland soil microcosms. We observed a significant decrease in cyanazine concentration in Lawnes wetland microcosms incubated under methanogenic and nitratereducing conditions. Losses were more pronounced at 25° than at 15°C. Results from enrichment culture studies suggest that cyanazine was cometabolized (i.e., cyanazine could not be used as a carbon and energy source by the microorganisms) in Lawnes soil microcosms. Dicamba was readily biodegraded in the wetland soils tested, although total mineralization was not achieved.
- Isolation and characterization of carbofuran and dicamba degrading bacteriaTaraban, Ronald H. (Virginia Tech, 1993)This study was conducted to isolate and characterize bacteria that have the capacity to degrade both carbofuran and dicamba. The pathways of degradation for both pesticides were elucidated. An aerobic, carbofuran-degrading bacterium was isolated from a high concentration carbofuran bioreactor. The isolate degrades carbofuran at the upper limit of carbofuran solubility (approximately 700 mg L⁻¹), to carbofuran phenol. In aqueous mineral salts medium with carbofuran as Furadan 4F (6 g L⁻¹ a.i.), degradation of carbofuran to undetectable levels required approximately 100 days. Although carbofuran phenol was not completely degraded, the cells remained viable in the presence of unusually high concentrations of both surfactant and carbofuran phenol. Additional nutrient sources had little effect upon the rate of degradation of carbofuran in pure culture. A dicamba-degrading consortium enriched from wetland soil, using the batch culture method, was used to elucidate the pathway of dicamba degradation under anaerobic conditions. The consortium consisted of one sulfate reducing bacterium, one fermenter, and three methanogens. The sulfate reducing bacterium was isolated from the consortium using sulfate as a terminal electron acceptor and 2-bromoethanesulfonic acid was added to inhibit the growth of the methanogens. Since the fermenter is dependent upon the methanogens, elimination of these organisms caused the elimination of the fermenter. Three methanogens (Methanothrix, Methanosarcina and Methanospirillum sp) were isolated with acetate and headspace gas consisting of H₂-CO₂. Degradation of dicamba proceded through an initial demethylation reaction yielding 3,6-dichlorosalicylic acid, as determined by high performance liquid chromatography (HPLC) analysis of aqueous medium. This was followed by a reductive dehalogenation reaction at the meta position of 3,6-dichlorosalicylic acid forming 6-chlorosalicylic acid. The metabolites were isolated using thin layer chromatography. Confirmation of metabolite identity was achieved using HPLC, and mass spectrometry. It appears that the fermenter was responsible for mediating the demethylation reaction. The consortium was unable to mineralize the aromatic ring. The substrate specificity of the dicamba-degrading consortium was investigated. The consortium was found to have the capacity to mediate the reductive dehalogenation of both 3-chlorosalicylic acid and 2,5-dichlorobenzoic acid at the meta position. The consortium was unable to dehalogenate either 3-chlorobenzoic acid, 4-chlorosalicylic acid, 5-chlorosalicylic acid, or 2,5-dichlorophenol. Addition of the reducing agent cysteine (0.025% and 0.050%) to a yeast extract amended (0.04%) mineral salts medium containing 3-chlorosalicylic acid reduced the rate of dehalogenation compared to medium containing sodium sulfide as the reducing agent. Only limited dehalogenation of 3- chlorosalicylic acid and 2,5-dichlorobenzoic acid was observed when the sulfate reducing bacterium was cultured alone in a yeast extract amended medium, suggesting that the mutualistic efforts of a mixed population of anaerobes were necessary to efficiently mediate reductive dehalogenation.