Analysis Of Microbial Community Structure Of Methane And Mercury Transformation In Lake Sediments

Freshwater sediments are very important material sinks,in where various substances migrate and transform,and microorganisms play a very important role.While microorganisms consume large amounts of organic matter to produce methane,a variety of microorganisms simultaneously conduct anaerobic oxidation of methane（AOM）,thereby reducing the natural emissions of methane to the atmosphere.Under human discharge and natural activities,a large number of hazardous materials enter the sediments,affecting the growth of microorganisms and the transformation of various substances.Mercury is one of the globally recognized hazardous materials,and a large number of emissions have also been generated due to the extensive use of mercury in human society.How does mercury enter the lake sediments affect the anaerobic oxidation of methane（AOM）,and how does it affect the structure of sediment community need to be further explored,which will have far-reaching significance for us to understand the geochemical cycle of methane and mercury in water and soil.In this study,the sediments of East Dongting Lake（EDL）,a natural lake,were collected.Through the determination of physical and chemical properties and 16S r RNA amplicon sequencing,the spatial distribution and biological process of microbial community structure related to methane and mercury transformation in the sediments of EDL were analyzed.The construction of sulfate-reduced methane anaerobic oxidation（SAMO）system was used to explore the influencing factors and microbiological characteristics of the migration and transformation of mercury and methylmercury by microorganisms in SAMO system,so as to obtain the cognition of mercury transformation in natural environment and simulated anaerobic methane oxidation system.The results show that:（1）The physical and chemical properties of EDL surface sediments have obvious differences at different depths.The concentrations of total mercury（THg）and methylmercury（Me Hg）reached the highest in sediments with water depth of 15–25 m,and the concentrations were 113.16μg/kg and 0.1966μg/kg,respectively.p H,TN and various forms of Hg significantly affected the microbial community structure of sediments at different depths in EDL.Carbon dioxide reduction and acetic acid decarboxylation are two main methanogen types in EDL,and the abundance of dominant methanogenic genera is the highest in sediments with water depth of 15-25 m.At the same time of methane production,anaerobic oxidation of methane is also the main methane oxidation pathway for methane degradation in sediments,and the dominant AOM microorganisms in different water depths are different.Although the abundance of methanogens is very low in the sediments with water depth of 25–35 m,the microorganisms using methane produced by methanogens for AOM are the most active.Bacillus has the highest abundance in sediments,but it has multiple metabolic functions in the natural environment,and the mercury methylation ability is not strong.Mercury-methylated functional genus Pseudomonas and Syntrophus are second only to Bacillus.（2）In the sediment system with methane as the sole carbon source,the higher the sulfate concentration,the lower the sulfate degradation rate,and sulfate promotes the AOM process by significantly affecting the microbial composition of the AOM system,especially the various microorganisms involved in the AOM.The0.1 Mpa high pressure environment had a significant inhibitory effect on the formation of sulfide and mercury methylation in SAMO system.Most of the inorganic mercury ions(Hg²⁺)in water are deposited in sediments and are methylated and demethylated by microorganisms.High concentrations of Hg²⁺inhibited the growth of most microorganisms in the system,inhibited CO₂ production and sulfate reduction,and weakened the microbial AOM process.Although abundant methylmercury（Me Hg）promotes CO₂ production,it also affects the growth of sulfate-reducing bacteria（SRB）involved in SAMO and weakens sulfate reduction in the system.The significant positive correlation between THg and CH₄,Me Hg and CO₂ indicated that mercury methylation and demethylation in this system might be related to SAMO.（3）High concentrations of sulfate,Hg²⁺or methylmercury in SAMO system resulted in significant changes in microbial community structure and dominated by microorganisms capable of utilizing mono-carbon（C-1）organic matter.Unclassified-f-Beijerinckiacease,a methylotrophic bacterium capable of metabolizing multiple C-1 substrates such as methylated sulfur compounds,has become the dominant genus with the highest abundance and the highest abundance in the group containing high concentration of Hg²⁺（15.45%）.This genus may promote mercury methylation by affecting the transfer of methyl groups and tetrahydrofolate and participating in the serine cycle.In this system,five known microorganisms involved in AOM and 16 anaerobic mercury methylation functional microorganisms were found.SRB Desulfobulbus became the most abundant microorganism involved in AOM;Desulfuromonas is the most abundant mercury methylating bacterium in each group.In SAMO system,a variety of SRB with sulfate reduction function become the most active microorganisms,whether AOM or mercury methylation and demethylation genera.