Biomethylation Of Arsenic In The Shallow Groundwater System Of The Jianghan Plain

The health disaster caused by geogenic arsenic enrichment in groundwater is a worldwide environmental problem,it can lead to the endemic arsenic poisoning and threaten human health.As a key step in the geochemical cycle of arsenic,As methylation can reduce the environmental risk by transforming the highly toxic inorganic As to less toxic methylated As to minimize the damage to people.It can also provide an important and new way for regulating As pollution in groundwater.As methylation is common in nature.In present,most research about As biomethylation is focusing on paddy soil and wetland systems.It’s generally believed that microbes are crucial driving factor for As methylation.Identifying the relative abundance and diversity of microbial community related to As methylation in water and soil environment is the key point on revealing the relevant pathway.As methylation is controlled by multiple biogeochemical processes.The diversity of electron acceptor,including ferric iron and sulfate,and the complexity of the microbial community in aquifers all can control the As methylation and the demethylation of methylated arsenic.However,the research about the transformation between inorganic As and methylated As and the related mechanisms in groundwater system is limited.It’s urgent to reveal the enrichment mechanisms of methylated As by multiple methods.The shallow aquifers of the Jianghan Plain are the object of research.Based on the systematic groundwater chemical analysis and combined the fluorescence spectrum characteristic of the dissolved organic matter（DOM）and molecular characteristic,it’s favorable to identify the hydrochemistry environment contributing to methylated As enrichment and the controlling factor.Besides,based on the metagenomic sequencing and other microbiological approach,it’s conducive to recognize the dominant microbes related to As methylation and the relevant metabolic pathway.Combined the evidences of C-Fe isotopes and the results of incubation experiments of sediments in anaerobic condition,the present study reveal the biogeochemical processes controlling As methylation.Based on all the results above,the enrichment incubation experiments of functional microbes were conducted to identify the contribution of sulfate reduction in As methylation and the effects of methane cycle processes on As methylation and relevant mechanisms further.The primary research progress is listed as follows:1.Compared the methylated As concentrations in groundwater with other areas,the concentration of methylated As（mean（36.5μg/L）,maximum（444.3μg/L））in the Jianghan Plain was obviously higher.High concentration methylated As is mainly located in the old course and meanders of Yangtze River.Strong reducing condition and high concentration As（III）are both favorable for methylated As enrichment.The dissolved organic matter（DOM）dominated by microbial and terrestrial materials in the aquifers are key factors controlling As methylation.The molecular characteristics of DOM in groundwater by FT-ICR-MS analysis indicate that the main molecular composition in high concentration methylated As is CHOS and CHONS,which are highly unsaturated and polyphenols materials characterized with high NOSC value and larger molecular.Besides,the content of total DOM and DOM containing sulfur have positive relationship with Methylated As concentration.Under strong reducing condition,small molecular and highly active DOM can be degraded preferentially,which results in the reductive dissolution of iron oxides adsorbed As in aquifer sediments and lead to the enrichment of As further.2.Except for sulfate reducing bacteria（SRB）,microbes related to methane cycle also play a key role in high concentration methylated As groundwater.At genus and OTU level,the relative abundance of SRB,Desulfobacca and Desulfomaculum,Methanogen,Methanobacterium and Methanoregulaceae（family）,Methanotroph,Methylomonas and Candidatus Methanoperedence all increased along with the methylated As concentration.The results of metagenomic analysis indicate Desulfomaculum and Candidatus Methanoperedence contain Ars M（arsenite S-adenosylmethionine methyltransferase）,which have potentiality of As methylation.The obvious positive relationships between dsr B/ars M and methylated As concentration（R²=0.71,P<0.01;R²=0.69,P<0.01）confirm the key role of SRB on promoting As methylation.3.Processes related to sulfate reduction and methane cycle both can promote As methylation.In contrast,sulfate reduction is dominant in As methylation.According to the analysis ofδ¹³C_DIC/‰in groundwater,the concentration of methylated arsenic both increases obviously along with the decrease ofδ¹³C_DIC/‰value（-7.5--14.7）and Fe（II）concentration or increase ofδ¹³C_DIC/‰value（-6.8-3.2）and Fe（II）concentration.This shows the dominant role of sulfate reduction and methane cycle in promoting As methylation respectively.In addition,combined withδ⁵⁶Fe/‰evidence,sulfate reduction can promote As methylation obviously along with reductive dissolution of crystalline and amorphous iron oxides minerals.The methane oxidation process with reductive dissolution of crystalline iron oxides minerals also can promote arsenic methylation under methanogenesis.4.Besides SRB,other microbes under sulfate reducing condition also promote As enrichment in groundwater obviously.The As（III）concentration and the type of carbon source can both influence As methylation.In the anaerobic incubation experiments with in-situ sediments,the methylated As concentration is 17.2μg/L in the system with molybdate（Mo）amendment to inhibit sulfate reduction at the end of incubation.In addition,in the incubation experiments with YLW03 and B1-B SRB enrichment systems,the methylated As concentration is 8.4/7.5μg/L in the system with molybdate（Mo）amendment at the end of incubation.Besides SRB,other microbes with As（III）methylation potential,for example Geobacter,also play an important role under sulfate reduction.The efficiency of As methylation（7.8%）in the SRB enrichment system is higher compared to the system with in-situ sediments（4.9%）.High As（III）concentration can increase methylated As concentration.However,it can reduce the efficiency of As methylation.Besides,compared to acetate,lactate is more advantageous to microbial metabolic activity to promote As methylation.5.Methylotrophic methanogen can promote DMA demethylation.However,anaerobic methane oxidation process can promote As methylation.In the anaerobic incubation experiments with in-situ sediments,the methylated As concentration is 31.5μg/L under coupling occurrence between sulfate reduction and methanogenesis at the end of incubation.In contrast,the methylated As concentration is 74.7μg/L in the system with BES amendment to inhibit methanogenesis.Therefore,methanogenesis can restrain the occurrence of As methylation.The type of carbon source is a key factor that affects DMA demethylation.The DMA demethylation efficiency is 30.4%and 15.5%respectively in the systems with methanol and acetate amendment.It indicates methanol amendment is favorable for DMA demethylation.The dominant microbes include Methanosarcina.In the methanogen enrichment system,the results are similar.In comparison,under the condition with CH4 amendment to promote methane oxidation process,the methylated As concentration in the incubation systems with CH₄ amendment（25.1μg/L）is higher than the systems with no CH₄ amendment（2.2μg/L）.It indicates anaerobic methane oxidation process can promote As methylation.