An Effective Carbendazim-Degrading Combined Bacterium And Its Application

Organic pesticides have become important means for ensuring agricultural production, due to their application in the control of crop diseases and pests. Carbendazim, as a potent and broad-spectrum fungicide, is widely used in agriculture. However, continuous use of carbendazim leads to its high residue in soil and agricultural products, which may pose detrimental effects on soil environment and food safety. Therefore, the urgent and heavy task of removing carbendazim pesticide pollution from our environment should be taken seriously. In this experiment, we got an efficient combined bacterium by compositing two carbendazim-degrading strains isolated by our laboratory, Alcaligenes sp. and Rhodococcus sp., and its degradation characteristics and application in soil were then investigated. The residual level of carbendazim was determined by ultraviolet spectrophotometry and HPLC. Main content and conclusions are as follows:(1) Two carbendazim-degrading strains, Alcaligenes sp. and Rhodococcus sp., respectively designated A and R, were mixed in different proportions to develop highly effective combined bacteria. The combined bacterium AR5, with the inoculation ratio of A to R being one to four, showed optimal degradation capability. Under the condition of 30℃, pH6.0, 150r/min and inoculation amount 5%, the degrading rate for 200mg/L carbendazim by AR5 within 48 hours reached 100%,superior to that of pure bacterial cultures. Microbial morphologies of strains A and R were studied with scanning electron microscopic. It was found that A was a bacillus, about 2μm×0.5μm in size, and R was a coccus, approximately 1μm in diameter. The result of colony counting showed the population ratio of A to R in AR5 was close to 1:1.(2) The compound bacterium AR5 had excellent tolerance and degradation ability against carbendazim of high concentrations. For example, carbendazim at initial concentration of 400~600mg/L reduced to about 10 mg/L within 72 hours. Complete degradation of 100mg/L and 300mg/L carbendazim took 24 hours and 72 hours, respectively. The result of an orthogonal optimization experiment showed that pH value had significant effect on the carbendazim degrading rate of AR5 while temperature and inoculation amount had no observed effect. The optimal degradation condition for AR5 was 30℃, initial pH 6.0 and inoculation volume 7%, under which the degradation rate of 200mg/L carbendazim reached 75.76% within 24 hours. The addition of 0.2% nitrogen sources like urea and yeast extract, could further promote the biodegradation of carbendazim by the combined bacterium, but supplement with carbon sources of the same concentration had little impact on that.(3) The result of substrate degradation spectrum test indicated that besides carbendazim, the high-effective combined bacterium AR5 could degrade other pesticides such as thiabendazole, thiophanate-methyl, metalaxyl, bifenthrin, and deltamethrin of 100mg/L, and their degrading rates were 84.7%, 90.2%, 39.2%, 53.5% and 57.6%, respectively.(4) The carbendazim degradation ability of AR5 in soil, its relationship with several plant pathogenic fungi, and its plant security were studied in laboratory conditions. The results showed that, when temperature was 30℃and inoculation amount was 5%, AR5 had great ability of degrading carbendazim at 50, 100, 200mg/kg in soil, with their degrading rates within 5 days being 100%, 100% and 95.2%, respectively; moisture content of soil was critical factors for degrading rate, with moisture content 25% better than 20% as well as 15%; carbendazim degradation rate in unsterilized soil was slightly higher than that of the sterilized soil, inferring that indiginious microorganisms?could cooperate with the combined bacterium to degrade carbendazim. AR5 had antagonistic relationships with Curvularia lunata, Alternaria alternate, Valsa mal, Fusarium solani, Alternaria solani, Phytophthora capsici, Fusarium bulbigenum, Botrytis cinerea, so it was predicted that as an effective carbendazim-degrading strain, this combined bacterium also had the potential to control plant pathogens. The combined bacterial culture (diluted) had no obvious effect on the germination of cabbage seeds, tomato seeds, cucumber seeds, and it showed no pathogenicity on the above three vegetables.