Characterization And Biogeochemical Significance Of Methanogens And Methanotrophs In Arsenic-rich Groundwater

High-arsenic groundwater generally exists in reducing environment,with relatively high methane concentration.However,the distribution characteristics of methanogens,methane-oxidizing bacteria and their bio-geochemistry implications are still unclear.Hangjinhouqi,Hetao Basin,Inner Mongolia as a typical high arsenic groundwater distribution area,are selected as the research area.This dissertation analyze the hydrogeochemical distribution of groundwater,the microbial community structure and the distribution of functional genes,explores the influences of environmental factors（abiotic factors）and microbial activities（biotic factors）on methane emission and consumption,and the arsenic release,and revealed the roles and significances of functional genes related to As,C,N,P,S biogeochemical cycling.The main findings are listed as following:（1）Along with the groundwater flow,the concentrations of total arsenic,As（Ⅲ）and As（V）showed increasing trends,while CH₄decreased slightly after an initial increase,which had the relatively high concentration in HCO₃-Na and Cl-Na types of groundwater.There were significant differences between the two regions of the alluvial fan（zone I）and the flat plain area（zone II）in terms of the distribution of As,As（V）,Fe,Fe²⁺,HCO₃^-and TOC（p<0.05）.The concentrations of these components were lower in zone I than in zone II,which suggested that the cycle processes of C,Fe and As were different.（2）Bacteria Burkholderiaceae and archaea Candidatus Methanoperedens were dominant microorganisms in the groundwater.These microorganisms can engage in the metabolism processes of As,Fe,N,S and methane cycling.C cycling genes accounted for 49%of functional genes in C,N,P,S elements,and the absolute copy numbers of methane oxidation genes were higher than that of methane production genes.As（V）reduction genes were the dominant genes in all As-related genes.Microbial community structures and functions（C,N,P,S and As metabolism）were significantly different in zones I and II,and As,CH₄,NO₂^-and Fe had significant impacts on microbial community structure and microbial functional genes.It implied that microbial mediated C,N,As cycling was closely related to redox conditions and chemical properties.（3）The co-occurrence network analysis showed that microbial-mediated processes of C,N,P,S and As metabolism were significantly different between zones I and II.In zone I,with more oxidizing conditions,two processes may inhibit the mobilization of As:one was that the released As（Ⅲ）from microbial mediated methane oxidation coupled with As（V）reduction was oxidized or methylated through aox or ars M,and another was that As（Ⅲ）oxidation was coupled with denitrification process.However,in zone II,with reducing conditions,the release of arsenic may be induced by microbially mediated methane oxidation（mcr A）and Fe（Ⅲ）reduction processes and sulfate reduction（dsr A/B）.