Isolation,Characteristics And Metabolic Mechanism Of Chlorimuron-ethyl-degrading Strain And Its Remediation Effect

Chlorimuron-ethyl, developed by American E. I. Du Pont Company in the 1980s, is a kind of low toxicity, broad spectrum, selectively systematic sulfonylurea herbicide and mainly used to prevent or kill broadleaf and grassy weeds in soybean, corn and rice fields. However, the frequent use of chlorimuron-ethyl in large quantities to farmland, can lead to the weeds resistance, cause phytotoxicity to succession crops, contaminate the farmland including the water around fields and threaten public healthy eventually. In this study, a bacteria strain capable of degrading chlorimuron-ethyl was isolated from soils and used to examine degradation characteristics and metabolic mechanism. The results were summaried as follows:A bacteria strain D which utilized chlorimuron-ethyl as sole nitrogen source was isolated from chlorimuron-ethyl contaminated soils. The strain was identified as klebsiella sp. based on its morphological and physiological characteristics and the phylogenetic analysis of 16S rDNA gene sequence.The degradation characteristics of chlorimuron-ethyl by strain D mainly depends on substrate concentration, pH and temperature. The degradation kinectics of chlorimuron-ethyl by the isolate D could be described with the first-order kinetics equation. Under the condition of pH 7.0,30℃, the half-lives of chlorimuron-ethyl at the substrate concentration levels of 1,10 and 100 mg/L were 0.93、4.08 and 27.40 d, respectively; At 30℃,the degradation half-lives of 10 mg/L chlorimuron-ethyl were 2.99,3.91 and 6.20 d at pH 5.0,7.0 and 9.0, respectively; At pH 7.0,the half-lives of 10 mg/L chlorimuron-ethyl at 20,30 and 40℃ were measured to be 23.31,4.08 and 0.37 d, respectively. Taking all factors into consideration, the chlorimuron-ethyl degradation rates at different substrate concentrations, pH and temperatures varied with the order of 100 mg/L> 10 mg/L>1 mg/L, pH 7.0>pH 9.0>pH 5.0,30℃> 40℃>20℃, respectively.The analysis results of HPLC-MS/MS indicated that chlorimuron-ethyl was transformed into two dominant metabolites:o-benzoic sulfimide and 4-methoxy-6-chloro-2-amino-pyrimidin. The possible degradation pathway for chlorimuron-ethyl by the isolate D was proposed as follows:chlorimuron-ethyl was converted to 4-methoxy-6-chloro-2-amino-pyrimidine and ethyl-2-aminosulphonyl benzoate through the cleavage of the sulfonylurea bridge. 4-methoxy-6-chloro-2-amino-pyrimidine could be translated into carbon dioxide and water eventually. Ethyl-2-aminosulphonyl benzoate was converted to o-sulfamylbenzoic acid firstly, then to o-benzoic sulfimide by hydrolysis and cyclization. Meanwhile, ethyl-2-aminosulphonyl benzoate might be oxidated to ethyl 2-(isocyanatosulfonyl) benzoate.The results of bioassay indicated that strain D has efficient capacity of remediating the pollution caused by chlorimuron-ethyl. Based on the experiment, comparing with the corn in the same chlorimuron-ethyl concentration without strain D treatment, the inhibition ratios of plant height, fresh weight and dry weight of the corn in the water was decreased by 51.25%、36.83% and 23.40%, respectively. It showed that strain D could reduce the inhibition on the corn growth caused by chlorimuron-ethyl.