Methylmercury Degradation Process And Mechanism In Sediments Of Thermokarst Lakes On The Qinghai-Tibet Plateau

Permafrost in the northern hemisphere is a huge reservoir of mercury.As global warming causes permafrost warming and degradation,a large amount of mercury stored in permafrost may be released again.Permafrost degradation causes land subsidence,collapse,erosion and other phenomena to form thermokarst characteristics represented by thermokarst lakes.thermokarst lake sediments contain large amounts of organic carbon,rich microbial communities and are in an anaerobic environment.These conditions are very conducive to the microbial methylation process of mercury.Some studies have confirmed that thermokarst wetlands and lakes in Arctic permafrost regions are hot spots for mercury methylation.At the same time,the net amount of methylmercury in the environment depends on the opposing processes of methylmercury formation（methylation）and degradation（demethylation）.Microbial degradation is the main pathway of methylmercury degradation in opaque sediment environment.Thermokarst lakes are widely distributed in the Qinghai-Tibet Plateau.With the continuous increase in the number and area of thermokarst lakes,the impact on the surrounding aquatic ecosystem is also increasing.However,there are few researches on mercury content,mercury methylation and methyl mercury degradation in thermokarst lakes in Qinghai-Tibet Plateau.The study of methylmercury degradation process and related mechanism in the sediments of thermokarst lakes on the Qinghai-Tibet Plateau is of great importance for understanding the influence of climate change on methylmercury accumulation in the permafrost region of the Qinghai-Tibet Plateau.It also provides basic theoretical basis for evaluating the fragile ecosystem of the cryosphere,the risk of human exposure to mercury pollution and the environmental effects of cryosphere retreat.In this study,37 sediments of thermokarst lakes were collected along the Qinghai-Tibet Railway line in June,July,September and October,2020.Through field monitoring,anaerobic culture in laboratory and characterization of microbial communities by 16S r RNA amplification gene sequencing,mercury content and microbial degradation process of methylmercury in thermokarst lake sediments were studied.The degradation mechanism of methylmercury was explored.The main research results of this paper are as follows:（1）Methylmercury（MeHg）concentrations in the sediments of thermokarst lakes on the Qinghai-Tibet Plateau ranged from 5.55 to 358.00 pg/g,with an average of 55.70±68.69 pg/g（n=76）,which was significantly lower than the global mean concentration（1700 pg/g,n=495）of MeHg in sediments.The range of total mercury（HgT）concentration was 1.11-11.17 ng/g,with an average value of 4.51±2.33 ng/g（n=76）,which was much lower than the background value of mercury concentration in Chinese soil（40 ng/g）and the soil mercury concentration in permafrost regions around the Arctic（10-450 ng/g）.There was no significant difference in HgT concentration between different months,but the MeHg concentration in October was significantly higher than that in June,July and September,which might be due to the thermal stratification of the thermokarst lake,which strengthened the microbial methylation of mercury,and inhibited the photochemical degradation in the dark depth of the lake bottom water.（2）There was a significant positive correlation between HgT concentration and organic carbon（SOC）and total nitrogen（TN）,indicating that the organic matter in the sediments had strong complexation and fixation effect on mercury,which was conducive to the deposition and accumulation of mercury.The concentration of MeHg in lake sediments is positively correlated with HgT,SOC and TN.HgT is the substrate of methylation,SOC and TN can provide rich carbon sources and nutrients for microorganisms,and can also be used as electron donors for their metabolism,thus enhancing the activity of mercury-methylated microorganisms and promoting the generation of MeHg.（3）The experimental results of anaerobic culture（10℃,30 d）of the thermokarst lake sediments showed that the microbial degradation process of MeHg existed in the four sediments of BLH1,BLH7,WYK2 and WYK3.By the end point of the culture experiment（30 days）,the degradation ratio of MeHg in the sediments of the four lakes reached 39.94%,78.08%,59.05%and 45.55%,respectively.The kinetic experiments showed that the degradation rate(k_de)of MeHg was different in different lake sediments,and the degradation rate of BLH7 sediments was the highest,which was 0.100±0.018d^-1.（4）Further inhibitor experiments showed that the degradation process of MeHg and the dominant microbial community of BLH7 and WYK2 types of lake sediments were different.For the sediment of BLH7 lake,the degradation rate of MeHg in the addition of sulfate reducing bacteria inhibitor(Mo O₄^2-)was significantly lower（53.77%）than that in the control group（78.08%）.However,methanogenic bacteria inhibitors（sodium 2-bromoethyl sulfonate,BES）had no significant effect on the degradation process,suggesting that sulfate-reducing bacteria in the sediment of the lake may be partially involved in the degradation of MeHg.However,for WYK2 lake sediments,there was no significant difference in the proportion of MeHg degradation with sulfate-reducing bacteria inhibitor（56.46%）and control group（55.57%）.However,the degradation of MeHg was significantly inhibited by the addition of methanogens inhibitors,and the degradation ratio was only 16.58%,indicating that methanogens were likely to dominate the degradation of MeHg in the sediment of the lake.However,almost no CH₄ was produced in the degradation products of WYK2 treatment groups in the lake,indicating low activity of methanogens,which should be further explained in combination with the microbial community.（5）Deltaproteobacteria and Bacteroidetes contributed 31%to the degradation of MeHg in the BLH7 sediments of the thermokarst lake,while Firmicutes contributed 69%.For WYK2,methanogens were not found in the sediment microbial community,The microbial communities that degrade MeHg mainly include Gammaproteobacteria,Alphaproteobacteria,Deltaproteobacteria.The addition of methanogenic inhibitors may inhibit the microbial activity of some proteobacteria.