The Biological Effect And Anaerobic Removal Of Bensulfuron Methyl In Paddy Fields

Bensulfuron methyl(BSM),a typical representative of sulfonylurea herbicides,is mainly used for the removal of broad-leaved weeds in paddy fields.It has become one of the most commonly used herbicides in China because of the advantages of low toxicity,broad spectrum,strong selectivity and low frequency of application.Due to its excellent ability to control weed,BSM has made a great contribution to the increase of rice yield in China.With the development of demand for food,the production and use of BSM in China may increase further.However,BSM can not only play the role of weeding in rice fields,but also exist in the natural environment for a long time,and studies have found that BSM can cause toxic effects on plankton,algae and other microorganisms.Paddy fields,as the main application object of BSM,are not only rich in algae microorganisms,but also important sources of artificial methane.It is of great significance to explore the effect of BSM on microorganisms in paddy fields for maintaining a good paddy ecosystem.Because there exist both aerobic and anaerobic areas in rice fields,according to the chemical properties and distribution of BSM in rice fields,the biological effects of BSM on Microcystis aeruginosa and Methanosarcina acetivorans were studied.And the removal and transformation mechanism of BSM under anaerobic conditions was also analyzed.The results of this paper have a certain practical significance to guide the rational application of BSM,provide some guidance for the control of microcystis aeruginosa bloom,contribute to reducing greenhouse gas emissions and improving climate conditions,and provide theoretical basis for controlling and improving the ecosystem in rice fields.The main contents and conclusions of this paper are as follows:(1)The influence mechanism of BSM on M.aeruginosa and the anaerobic transformation process of microcystins(MCs)were analyzed.It was found that BSM had a concentration dependent effect on the growth of M.aeruginosa,that is,low concentration(<20 ?g/L)of BSM could promote the growth of M.aeruginosa,while high concentration(>50?g/L)of BSM could inhibit the growth of Microcystis aeruginosa.BSM might induce certain stress effect reaction on M.aeruginosa,which caused significant increase of content of MDA,GSH and POD activities.BSM,as an exogenous pollutant,could promote the production of M.aeruginosa MCs,and increase the release of MCs by increasing the cell membrane permeability.It was found that BSM could significantly down regulate the expression levels of rbcL,psaB and psbD,which were related to photosynthesis of M.aeruginosa,while the expression levels of mcyB genes related to MCs synthesis did not change significantly.BSM could affect the growth of M aeruginosa from the gene level.Microcystins,produced by M.aeruginosa,might have adverse effects on rice and paddy ecosystems.The removal possibility and degradation efficiency of MC-LR under anaerobic conditions were analyzed.The results showed that the degradation rate of MC-LR by stable anaerobic culture system was more than 80%.According to the results of liquid chromatography tandem mass spectrometry and the secondary mass spectrometry information,4 transformation products of MC-LR under anaerobic conditions were identified.The transformation mechanism of MC-LR under anaerobic conditions was proposed on the basis of the structure of transformation products.The results suggested that MC-LR could undergo the same degradation process under anaerobic condition as that under aerobic condition,that is,the linear hydrolysate of MC-LR was formed by the cracking of Adda-Arg bond.In addition,it was found for the first time that MC-LR could hydrolyze Ala-Mdha bond under anaerobic condition to form a novel hydrolytic intermediate,and then mediate the transformation process of MC-LR.Combined with the analysis of MC-LR degradation efficiency and microbial diversity in stable anaerobic culture system,the results of microbial community analysis showed that Candidatus Closidamon Cloacamonas acidaminovorans str.may play an important role in MC-LR biotransformation under anaerobic conditions.(2)The effects and influence mechanism of BSM on M.acetivorans C2A were revealed.Through the analysis of the biomass of M.C2A,it was found that after short-term exposure to BSM,the consumption of substrate was increased,which mediated the accelerated growth of M.acetivorans C2A.And the methanogenic activity of M.C2A was significantly up-regulated by BSM.Atomic force microscopy(AFM)was used to analyze the mechanical properties of the cell surface before and after BSM treatment.It was found that BSM could reduce the cell rigidity,which might accelerate the material exchange inside and outside the cell Quantitative analysis of the expression of methane related gene showed that BSM could significantly up regulate the gene mcrA,which may be the molecular mechanism of BSM improving methanogenic activity of M.acetivorans C2A.Non-targeted metabonomics analysis showed that BSM could promote some metabolic activities of M.acetivorans C2A,including tricarboxylic acid cycle,urea cycle,gluconeogenesis,amino acid synthesis,etc.The enhancement of metabolic activities may be one of the reasons that BSM promoted the growth and methanogenic activity of M.acetivorans C2A.(3)The mechanism of anaerobic removal and transformation of BSM was elucidated.BSM,in anaerobic conditions,can be effectively removed by stable anaerobic culture system,and the removal process includes hydrolysis,adsorption and degradation.In the adsorption process,EPS in anaerobic culture system may play an important role.The hydrolysis process and degradation process of BSM were accelerated with the increase of temperature.The hydrolysis and degradation products of BSM under anaerobic conditions were identified by liquid chromatography tandem high-resolution mass spectrometry.According to the structure of the degradation products,it was proposed that the degradation of BSM under anaerobic conditions was mainly through the break of sulfonylurea bridge and amide bond,and the degradation products would be further converted into methane under the action of stable anaerobic culture system.