Research On The Abiotic Methylation Of Mercury And Its Effect On The Microbial Community In Mercury-Contaminated Soil

Mercury and its compounds have caused serious environmental pollution.Methylmercury can be accumulated and magnified through the food chain,resulting in the potential biotoxicity.Although the gene level of biotic methylation of mercury has been conducted,few researches on abiotic methylation of mercury are reported.This paper evaluated the ecological risk of mercury in contaminated areas by field sampling and laboratory determination,analyzed reaction conditions and mechanism of abiotic methylation of mercury by chemical experiments in solution,studied changes of soil microbial community diversity and structure under the stress of mercury and methylmercury by High Throughput Sequencing technology,and elucidated the response of characteristic microorganisms to mercury and methylmercury.The main results are as follows:?1?The soil from study area was seriously contaminated by mercury,and methylmercury in soil was mainly transferred from mercury.Geoaccumulation index(I_geo)showed that the pollution level of mercury and methylmercury in soil were high.Potential ecological risk index?RI?indicated that the soil was heavily polluted by mercury and methylmercury and had high ecological risk.?2?The linear fitting of absorption and desorption process of mercury in soil by Langmuir,Freundlich and Temkin isothermal absorption models was conducted,with significant correlation coefficient.Langmuir model has the best fitting effect on the isothermal adsorption of mercury in soil,with the maximum adsorption capacity of863.62mg/kg.Freundlich model could better fit the desorption process of mercury,with the maximum desorption capacity of 16.8407mg/kg.The adsorption ability of soil to mercury was strong,and desorption rate of mercury in soil was relatively low,with the maximum desorption rate of 3.65%.The adsorption kinetics of mercury in soil complied with the Elovich model and the two-constant model.Mercury migrated poorly in soil and was mainly distributed in the topsoil?0-15cm?.?3?Methylcobalamin has been proved to be the efficient methyl donor in abiotic methylation of mercury.The methylation rate of mercury in abiotic methylation process was in direct proportion to the initial concentration of mercury and methyl donor.The maximum quantity of methylmercury was produced under the pH of 5.The relative low temperature could promote the abiotic methylation process of mercury,while high temperature inhibited it.The methylation rate was highest under the temperature of35?.Illumination might accelerate the abiotic methylation process of mercury.Compared to the reaction in solution,abiotic methylation rate in soil was lower and the methylation process was slower resulted from the absorption of mercury by soil.The paper showed that pH had the greatest effect on abiotic methylation.The worst condition of abiotic methylation was initial mercury concentration of 500mg/L,pH of 5and temperature of 35?.The abiotic methylation can better conform to second-order reaction kinetics compared with pseudo first-order kinetic model.?4?The mechanism of abiotic methylation was carbon anion transfer by analyzing the electron distribution,atomic orbital hybridization theory and electronegativity theory.The Co-C bond in methylcobalamin was labile,generating methyl radicals under the condition of illumination and heating.Methyl radicals were very active and could easily react with other substances.Hg²⁺,lost two electrons of Hg atom in the 6s orbit,had very strong electrophilic properties and could combine methyl radicals to form methylmercury.The mechanism of abiotic methylation of mercury in nature should be simultaneous existence of both carbon anion transfer and methyl radicals reaction.?5?The input of mercury and methylmercury had obvious influences on the richness and homogeneity of soil microbial communities.Richness of microbial communities reduced gradually with time.Low and medium concentration of mercury and methylmercury had little effects on the homogeneity.High concentration?500mg/kg?of mercury reduced homogeneity gradually,while high concentration of methylmercury first decreased the homogeneity and then increased it.High concentration of mercury showed the worst effect on the richness and homogeneity of microbial communities.?6?From the phylum level analysis,the relative abundance of Proteobacteria increased due to the contamination of mercury and methylmercury.Acidobacteria,Planctomycetes and Chloroflexi were sensitive to mercury contamination,while Acidobacteria,Planctomycetes and Nitrospirae were sensitive to methylmercury contamination.From the genus level analysis,the relative abundance of Lysobacter and Luteimonas increased due to the contamination of mercury and methylmercury.Massilia,Rhodanobacter and Burkholderia-Paraburkholderia proved to be resistant genus to mercury,while Methylotenera was the resistant genus to methylmercury.?7?As it was seen in community structure difference analysis,mercury could change the microbial community structure after input into the soil.High concentration of mercury had the worst impact on the community,and the impact intensified along with time.Low and medium concentrations of mercury had little effects on the soil microbial community structure.The contamination of methylmercury showed the similar effect on the microbial community,and the resistance of microorganisms to methylmercury had gradually increased with time.?8?Omnitrophica and Ignavibacteriae were so sensitive to mercury that they died rapidly under the stress of mercury,while Omnitrophica and Spirochaetae were sensitive to methylmercury.The genuses,which had high relative abundance and affected distinctly by mercury and methylmercury,included Methylotenera,massilia and Burkholderia from beta phytogenomycetes,Lysobacter,Luteimonas and Rhodanobacter from gama phytogenomycetes,and Nitrospira from Nitrospira.They were all gram-negative bacteria with thin cytoderm and complex components,quickly reacting to pollution stress.