The Methylation Process Of Mercury And Its Mechanism In The Sediments Of Thermokarst Lakes On The Qinghai-Tibet Plateau

Mercury is a global pollutant,and methylmercury is neurotoxic to living organisms and can accumulate and amplify through the food chain,thus posing a serious threat to human health.Due to global warming,the permafrost on the Qinghai-Tibet Plateau continues to degrade,and thermokarst lake,as typical karst landscapes,are the most visual representation of permafrost degradation,and their number and area are increasing.Thermokarst lake in the Arctic region have been shown to be hotspots for mercury methylation,and the pond sediments have become the "source" of methylmercury.As the "water tower of Asia",the Qinghai-Tibet Plateau is an important water source for the surrounding and downstream areas,but the methylation process of mercury in thermokarst lake on the Qinghai-Tibet Plateau has been poorly studied.The study of Hg methylation processes and related mechanisms in the sediments of thermokarst lakes on the Qinghai-Tibet Plateau is essential to understand the impact of climate change on the production of methylmercury in the permafrost zone of the Qinghai-Tibet Plateau,and to provide a basic theoretical basis for evaluating the fragile ecosystems and the risk of human exposure to Hg pollution in the cryosphere and the environmental effects of cryosphere retreat.In this thesis,37 thermokarst lake along the Qinghai-Tibet Highway in the central part of the Qinghai-Tibet Plateau were studied for their mercury content,influencing factors and their methylation processes in the sediments of thermokarst lake by combining field monitoring and indoor anaerobic incubation experiments.Through field monitoring and laboratory analysis,the physicochemical properties of thermokarst lake and the mercury content of the sediments were clarified,and the influence of environmental factors on the mercury content was analyzed;through anaerobic incubation experiments and the analysis of the mercury methylation microbial community,the process of mercury methylation in the sediments of thermokarst lake and its mechanism were investigated.The main findings of this thesis are as follows:(1)The mercury content level in thermokarst lake sediments in the central QinghaiTibet Plateau is low,but the ratio of methylmercury to total mercury content(Me Hg/THg)is higher than that in soil and freshwater lake sediments in the Arctic and Qinghai-Tibet Plateau.Compared with perennial permafrost and thermokarst lake in the Arctic and other parts of the Qinghai-Tibet Plateau,the THg and Me Hg levels in thermokarst lake sediments in this study area are both low,with an average total mercury level of 4.96 ± 2.23 ng/g(n = 128),ranging from 1.11 to 11.47 ng/g,and an average Me Hg level of 58.93 ± 73.71 pg/g(n = 128),ranging from 3.20 to 376.38 pg/g.This may be due to the fact that atmospheric deposition of mercury in the Qinghai-Tibet Plateau is less than in the Arctic,less influenced by human activities and dilution in the aqueous environment.However,the high ratio of methylmercury to total mercury content is close to the Arctic collapse marsh level,showing the strong mercury methylation ability of thermokarst lake sediments in this study area.The total mercury content did not show significant fluctuations with month,and there were no significant differences in the total mercury content of the thermokarst lake sediments between vegetation types.However,the methylmercury content of sediments in different months showed significant differences,and the methylmercury content of sediments in October was significantly higher than that in June and September,probably because the thermal stratification phenomenon caused by the icing of lakes in October produced anoxic or low-oxygen environment,which was favorable to the methylation process of mercury.(2)The environmental influences on the total mercury content and methylmercury content are different.The main influencing factors of total mercury content in thermokarst lake sediments include total sediment nitrogen and soil organic carbon,showing the correlation between total mercury content and soil organic matter.The main influencing factors of methylmercury content in the sediments of thermokarst lake include total sediment mercury content,sediment organic carbon,total sediment nitrogen,dissolved organic carbon in the water column and methane flux,indicating that the methylmercury content in the sediments is mainly influenced by the total sediment mercury content,microbial activities and soil organic matter.(3)The mercury methylation potential and dominant mercury methylation microbial communities varied widely among the different thermokarst lakes.Thermokarst lake BLH1 and BLH17 had stronger mercury methylation potential,while lake WDL6 showed poor mercury methylation potential.After 12 days of incubation,the methylmercury content increased about 40-fold in the mercury-added group of lake BLH1 and about 7-fold in lake BLH1,while the methylmercury content in the sediment of lake WDL6 did not increase significantly,showing obvious differences in the mercury methylation capacity of different thermokarst lakes.The Hg methylation microorganisms in the sediments of lake BLH1 included Geobacteraceae,Syntrophaceae,SRB and methanogens;the results of the microbial inhibitor addition experiment showed that the inhibition of the activity of both SRB and methanogens would have an effect on the methylation process,so it was presumed that they were the main methylation microbial communities.The sequencing results of methylation microorganisms in the sediments shows that the most abundant microbial communities for Hg methylation were Geobacteraceae and Syntrophaceae.The Hg methylation microorganisms in the sediments of lake BLH17 included Geobacteraceae and SRB;the results of microbial inhibitor addition experiments showed that the inhibition of SRB activity significantly affected the methylation process,so it is presumed that SRB are one of the main methylating microorganisms,and the sequencing results of sediment methylating microorganisms also showed that SRB and Geobacteraceae were the most abundant mercury methylating microbial communities in relative terms.The Me Hg content did not change significantly during the incubation of each inhibitor addition group in the lake WDL6,and the sequencing results of sediment methylation microorganisms showed that Geobacteraceae and Methanoregulaceae were the most abundant mercury methylation microbial communities in relative terms.In this paper,we investigated the levels of total mercury and methylmercury in the sediments of thermokarst lake on the Qinghai-Tibet Plateau,analyzed the environmental indicators that significantly influenced the mercury content,and compared the mercury methylation potential of different sediments and their main mercury methylation microbial communities.