Research Of The Influence By Iron-reducing Bacteria On Health Risk Of Rice Methylmercury In Paddy Soils

Previous studies have shown that seafood,such as fish,is the major source of dietary exposure to mercury for human beings.However,recent studies reveal that rice paddy soil is a hotspot of the mercury(Hg),where the neurotoxin methylmercury(Me Hg)is microbially produced,accumulated by rice plants and poses health risk,especially for the people who regard rice as the staple food.For instance in China,rice consumption could even contribute to 96% of the dietary exposure to Me Hg.Therefore,more attention has been paid to Hg contamination and Hg methylation process driven by microorganisms in paddy soils.The exploration of factors which influence the methylation in paddy soils,especially the microbial methylators,as well as the prediction of rice Me Hg risk is crucial for remediating the pollution of Hg in paddy soils.However,scientific uncertainties in understanding microbial Hg methylators in paddy soils would challenge our ability to predict Hg methylation and thus the health risk of Me Hg in rice: Meanwhile,accuracy of rice paddy biogeochemical models is normally restricted by the lack of data on microbial Hg methylators and the ignorance of relationship between microbial methylators and Hg methylation,particularly at higher spatial resolution,adding to uncertainties in predicting health risk of Me Hg associated with rice consumption.In this study,we included 67 paddy soils from 12 major rice-producing provinces across China,tring to figure out the key microbial Hg methylator and identify its role in Hg methylation by a series of experiments(in-situ experiment,inhibition experiment and batch experiment).Besides,we established a rice paddy biogeochemical model of Hg,connecting the methylation potential with the key microbial Hg methylator firstly,to predict the rice Me Hg risk in the future.(1)In China,the Hg concentration in the paddy soils are generally low,ranging from 23.8～446.1 μg/kg,which is lower than the Hg pollution risk screening value of0.5 mg/kg in the agricultural land soil,stipulated by the national standard.Me Hg concentrations range from 0.08 to 1.6 μg/kg.(2)To identify the key microbial Hg methylator in paddy soils,copies of hgc A methylation gene,as well as the relative abundances of different hgc A-containing methylating microbes were quantified in 67 paddy soils,by using the recently developed high-throughput sequencing technologies and broad-range primer pairs of hgc A.The analysis of hgc A amount indicates that the abundance is high,and the hgc A in archaea is 1.15 times higher than Deltaproteobacteria.However,there is no relationship between hgc A amount and Hg methylation potential(p > 0.05).This indicates that the amount of hgc A could not influence the Hg methylation in paddy soils while the community of Hg microbial methylators is crucial.(3)The results of high-throughput sequencing show that the iron-reducing bacteria(Fe RB)is the most abundant Hg microbial methylator in paddy soils,which averagely occupies 49.9%,while methanogens occupy 13.6% and sulfate-reducing bacteria(SRB)only occupies 6.7%.This indicates the absolute predominance of Fe RB in amount.Besides,the correlation analysis suggestes that the Fe RB is the most important one to influence the Hg methylation potential in paddy soils(r = 0.572,p < 0.05).And in the inhibition experiment,there was no change in the production of Me Hg while the SRB and methanogens were inhibited.This further proves the importance of Fe RB in Hg methylation.In summary,Fe RB is the most abundant and important Hg microbial methylator in paddy soils.(4)Based on the importance of Fe RB in Hg methylation,we found the distribution coupling of hgc A-containing Fe RB and methylation potential(Me Hg%)in paddy soils among China.Both the Fe RB and Me Hg% decrease from the northeast to southwest in China.This suggestes the controlling capability of Fe RB in Hg methylation and thus the production of Me Hg in paddy soils.Besides,our study also suggests that the C/N、Se、temperature and precipitation could influence the activity of Fe RB and thus the geographical distribution of Hg methylation potential.(5)Besides the identification of the importance of Fe RB in lab experiment,this study also explored controls of the identified key microbial methylator on the health risk of Me Hg in rice,by establishing and evaluating a rice paddy biogeochemical model of Hg.Monte carlo analysis of this model suggestes the relative importance of Fe RB,the controlling function of Fe RB is 1.7 times higher than Hg deposition and 1.25 times higher than Hg irrigation.The sensitivity analysis of parameters(SA＿10%)further proves the importance of Fe RB in Hg methylation.At national scale,the health risk of Me Hg(HQ)would decrease 7.53% in response to 10% decrease in the relative abundances of hgc A-containing Fe RB.Meaniwhile,only 1.97% or 8.03% decrease in HQ could be observed when the Hg input from atmospheric deposition or irrigation increases by 10%,respectively.This suggests the comparable role of Fe RB to irrigation,which is more important than the Hg deposition.Through the large-scale analysis in paddy soils,this study could not only reveal the importance of iron-reducing bacteria for Hg methylation,but also provide a theoretical and technical basis for selecting soil remediation methods to reduce Me Hg risk in paddy soils.(6)By revealing the key microbial Hg methylator in the world’s-largest soil-rice system,this study would help predict the Me Hg production in real-world environment in the future.As an example,under the situation of global warming in 2100,the risk of rice Me Hg,represented as HQ,would decrease 1.38%、2.22% and 3.87% in RCP 2.6、RCP 4.5 and RCP 8.5 when compared to the HQ in 2017,respectively.