Effects Of Microorganisms And Environmental Factors On The Distribution Of Methylmercury In Coastal Wetland Soil

Mercury is a global pollutant.Mercury in the environment can be converted into methylmercury(MeHg)by microbial methylation under anaerobic conditions.MeHg is an organic mercury compound with high neurotoxicity,reproductive toxicity and developmental toxicity,which can enter the human body through the food chain and threaten human health.The wet and dry alternation caused by the fluctuation of tidal water in the coastal wetlands provides a good geochemical condition for improving the activity of anaerobic microorganisms in the soil,which easily leads to the accumulation of MeHg.Therefore,clarifying the content of total mercury(THg)and MeHg in coastal wetlands and their influencing factors is of great significance to better understand the migration and transformation mechanism of mercury in coastal areas,and at the same time can provide data for the formulation of relevant policies to reduce the risk of human MeHg exposure.This study spanned nearly 20 latitudes in China,and selected Dongzhai Port(DZG),Zhangjiangkou(ZJK),Minjiangkou(MJK),Hangzhou Bay(HZW),Yancheng(YC),Yellow River Delta(HHSJZ)Seven typical coastal wetlands in Liaohe Estuary(LHK),collected 138 soil samples in July-August 2018,using the variance analysis of linear fitting methods of random forests and structural equation model etc.,analyzed the soil THg and MeHg content of different regions,the community characteristics of mercury methylators,as well as the Relationship among mercury methylators,environmental factors and THg,MeHg.Based on the analysis results of this sampling,The study found:(1)The average THg content and MeHg content of coastal wetland soil were 33.816 ± 3.090ng/g and 0.346 ± 0.020 ng/g,respectively.The order of the average THg content in the 7 study areas was MJK(102.069 ng/g)> ZJK(45.504 ng/g)> HZW(32.720ng/g)> HHSJZ(21.787ng/g)> LHK(19.298 ng/g)> DZG(18.703ng/g)> YC(5.857 ng/g);the order of the average content of MeHg was HZW(0.550 ng/g)> HHSJZ(0.423 ng/g)> LHK(0.348 ng/g)> ZJK(0.343 ng/g)> MJK(0.338 ng/g)> YC(0.208 ng/g)> DZG(0.170 ng/g).When the THg content was less than 30ng/g,MeHg and THg are extremely significantly positively correlated(P < 0.01),and when the THg content is greater than 30ng/g,MeHg and THg have no significant correlation.When the THg content was low,its methylation ability was strong,so the area with low Hg content should also attract our attention.(2)Methylated microorganisms at mid-latitude region(HZW)had a high community diversity.The composition of methylated microbial communities in the north and south samples was obviously different.This difference was mainly caused by the annual mean temperature(MAT),annual precipitation(MAP),water content(MC),soil organic matter(SOM)and p H.The main phylums of methylated microorganisms in the wetland of the beach were proteobacteria(64.28%),Spirochaetes(9.52%),Euryarchaeota(9.06%),PVC＿group(5.63%),Firmicutes(3.66%),Nitrospirae(1.56%)and Cyanobacteria＿Melainabacteria＿group(1.27%),among which Deltproteobacteria of Proteobacteria had the absolute advantage;the main genera were Geobacter(32.73%),Desulfococcus(8.90%),Desulfuromonadales＿bacterium＿GWC2＿61＿20(5.91%),Treponema(5.25%)and Desulfobulbus(5.11%),etc.15 genera with relative abundance greater than 1%.By dividing the identified genera into functional groups,it was found that iron-reducing bacteria(Fe RB)were the predominant group of methylated microorganisms in the coastal mudflat wetland soils.(3)The THg content in the wetland of coastal wetland soil was affected by mercury source input and environmental factors.The THg in the MJK area was more than 1-10 times higher than other research areas,implying that there was a large anthropogenic mercury source input in this research area.The key environmental factors affecting THg content were MAT,MAP,total nitrogen(TN),TFe,and MC.Among them,TN and TFe had relatively large effects.In addition to considering the cumulative effects of N and TFe on soil Hg,they noted the homologous input of Hg and N and Fe or the same way input.(4)MeHg content was significantly affected by methylated microorganisms and environmental factors.The key microbial factors affecting MeHg content were the abundance of the deltaproteobacteria-HgcA gene and the relative abundance of the three groups of Desulfococcus,Desulfomonile and Lentisphaerae＿bacterium＿RIFOXYA12＿FULL＿48＿11.Among the above three groups,Desulfococcus had the greatest influence on MeHg content,and Desulfococcus is a typical SRB,indicating that SRB plays an important role in the actual mercury methylation process of coastal wetland soils,rather than the relatively higher abundance of Fe RB.The key environmental factors that affected the MeHg content were p H and TFe.In addition,MC could affect the MeHg content by affecting the abundance of the deltaproteobacteria-HgcA gene.TN,TFe,and SOM could affect the MeHg content by affecting the THg content.