خصوصیات یک سویه دارای قابلیت تضعیف یک مخلوط علف کش از کویسلورا و بن سولفرامنسی

A bacterial strain, designated as LS, was isolated from a contaminated soil and was found to be capable of utilizing quinclorac, bensulfuronmethyl, and a mixture of the two as carbon and energy sources for growth. Strain LS was identified as Ochrobactrum sp. based on its physiological-biochemical properties, 16S rDNA sequences, and phylogenetic analysis. The extent of degradation of quinclorac and bensulfuronmethyl at initial concentrations of 1.5 and 0.1 g L−1 was 90% and 67%, respectively, as measured by high performance liquid chromatography (HPLC). When a herbicide mixture of 0.34 g L−1 quinclorac and 0.02 g L−1 bensulfuronmethyl was applied as carbon sources, quinclorac and bensulfuronmethyl were degraded at 95.7% and 67.5%, respectively. It appears that quinclorac is utilized more easily in a mixture than in a single state. The optimal temperature for growth of strain LS was 37 °C. Strain LS grew well at pH 6 to 9 and had the highest degradation level for quinclorac and bensulfuronmethyl at an initial pH of 7 and 8, respectively. Addition of 0.25 g L−1 yeast extract could promote the growth and extent of degradation of quinclorac and bensulfuronmethyl by strain LS. Strain LS also showed the capability to degrade other aromatic compounds such as catechol, propisochlor, 4-chloro-2-methylphenoxyacetic acid sodium (MCPA-Na) and imazethapy. The isolate LS shows a huge potential to be used in bioremediation for treating complex herbicide residues.

Quinclorac, 3,7-dichloro-8-quinolinecarboxylic acid, belongs to a new class of highly selective, auxintype herbicides used on paddy rice to effectively control barnyard grass (Echinochloa crusgalli) (Grossmann, 1998). Bensulfuronmethyl, methyl-2-[(4,6-dimethoxy-pyrimidin-2-yl)aminocarbonylamino]sulfonyl- methyl benzoate, is a highly active sulfonylurea herbicide used to control most broad-leaved grasses and sedges in paddy rice (Brown, 1990). Mixtures of bensulfuronmethyl and quinclorac have been widely applied for their improved weed-control effect and wider weed spectrum, compared with bensulfuronmethyl or quinclorac individually, especially in a mixture of 0.02 g L−1 bensulfuronmethyl and 0.34 g L−1 quinclorac in water (Lu et al., 2005). Synthetic agrochemicals, such as quinclorac (L¨u et al., 2003b, 2004, 2006) and bensulfuronmethyl (Xie et al., 2004), can affect the microbial activity and cause an overall toxic effect on the environment. Therefore, their residues in the soil may act as potential environmental hazards and disturb the natural ecological equilibrium (Alexandre and Claudio, 2000). The widespread use of quinclorac and bensulfuronmethyl has led to controversy with respect to water and soil pollution. Kyung (1997) reported that a very small fraction of quinclorac was distributed in different parts of rice plants, whereas about 95% of the originally applied quinclorac remained in the 30-cmsurface layer of soil after harvest. Okamoto (1998) found that the concentration of bensulfuronmethyl in rivers and lakes of Japan was as high as 0.1–2.3 mg L−1. The concentration of bensulfuronmethyl reached 0.02 mg L−1 in groundwater near paddy rice fields in Italy (Wei et al., 1998). Bioremediation, based on certain species of microorganisms, is a cheap and effective way to decontaminate pesticide in contaminated soils and water. There are many successful examples of in-situ bioremediation of pesticide residues by microorganisms. At present, only one quinclorac degradation strain has been reported (L¨u et al., 2004), and the degradation of bensulfuronmethyl is also rarely reported (Brusa et al., 2001; Zhu et al., 2005). However, no information on the biodegradation of the mixture of quinclorac and bensulfuronmethyl has been published in the literature. Therefore, understanding the degraders of herbicide mixtures, as well as the dynamics of biodegradation, is of great importance. The purpose of the present work was to study the phylogenetic and degradation characterization of a pure bacterium capable of high degradation of the herbicide mixture.

Strain LS could not only use either quinclorac or bensulfuronmethyl as the sole carbon source but also degrade quinclorac and bensulfuronmethyl up to 95.7% and 67%, respectively, when a mixture of them was used as the carbon source. For strain LS, the optimal growth temperature was 37 ◦C, and theoptimal initial pH of medium for growing on quinclorac and bensulfuronmethyl was 7 and 8, respectively. Addition of yeast extract could stimulate the degradation of quinclorac and bensulfuronmethyl and the growth of strain LS. The results showed that strain LS could adapt to a wide pH range for growth and degradation and utilize the mixture of quinclorac and bensulfuronmethyl rapidly. This wide range of temperatures and pH values for growth suggested that strain LS might be well suited for bioremediation. Based on the morphology, Vitek system, Biolog GN, 16S rDNA, and phylogenetic characteristics, strain LS should be classified into the genus Ochrobactrum. From the analysis of Vitek, it was observed that the strain was almost similar to Ochrobactrum anthropi. Likewise, 16S rDNA sequence alignment also showed that the strain exhibited the highest similarity to Ochrobactrum anthropi type strain LMG 3331T (Holmes et al., 1988). Therefore, strain LS should be identified as Ochrobactrum anthropi. Among the known strains of Ochrobactrum anthropi and related strains, many of them can degrade xenobiotic compounds, such as atrazine (Laura et al., 1996), dichlorvos (Zhang et al., 2006), phenol, and several chlorophenols (Lechner et al., 1995; M¨uller et al., 1998; Favaloro et al., 2000). L¨u et al. (2003a) isolated a strain, Burkholderia cepacia WZ1, capable of degrading quinclorac from soil contaminated with pesticide. Brusa et al. (2001) demonstrated the biodegradation of bensulfuronmethyl in soil, whereas Zhu et al. (2005) isolated, characterized, and determined the degradative ability of the Brevibacterium sp. strain BH capable of degrading bensulfuronmethyl effectively from agricultural soils. However, the degradation ability was rather low in comparison with the results in this study. From this standpoint, strain LS might be more valuable for the synchronously disposal of quinclorac and bensulfuronmethyl residues. Further studies are required to determine the potential of this isolate for the removal of pesticide residues in soil.