Tolerant Plants And Their Accumulation Mechanism Of Mercury In Typical Mercury Mining Areas

Mercury?Hg?is a hazardous heavy metal,is listed as one of the most concerning pollutants,?including Pb,Hg,Cd,Cr and As?in The Twelfth Five-Year Plan on Comprehensive Prevention and Control of Heavy Metal Pollution of China.Mercury resources are abundant in Southwest China Which locates in the Circle-Pacific mercury mineralization with large-scale mining and smelting activities in a long history.Wanshan mercury mine as the largest mercury industrial base in Guizhou province,a large number of waste rocks and slags were abandoned on land and become one of the important release sources of mercury,which leads to serious mercury pollution in surrounding environment,causes severe pollution in crops,such as rice,corn and vegetables and poses health risks on residents.Phytoremediation has been paid much attention to because of its green,environmental and economic characteristics,however,the selection and determination of mercury-enriched plants or hyperaccumulator is the first step.Due to the long term influence of high mercury concentration,different species of plants in the wasteland of mercury mine area have strong adaptability to the absorption,accumulation and tolerance of mercury pollution,which provides the possibility for the search for plants with high mercury tolerance,therefore,wasteland is the effective target area and favorable place for seeking mercury-enriched plants or hyperaccumulator.The study of mercury-contaminated through the absorption,transfer and accumulation of mercury in perennial plants is valuable to discover and determine new plants to restore the mercury polluted land,which has practical significance and theoretical value.At present,research on mercury in plants in Guizhou mercury mining area are mostly focused on edible crops?such as corn,vegetables and rice,etc.?,however,researches on the dominant species of tolerant plants and mercury-enriched or hyperaccumulator in high-mercury wasteland are not comprehensive and in-depth,especially on the methylmercury in naturally growing plants and the prominent problem of mercury-enriched plants lies in the unclear mechanism.Therefore,uptake capacities and spatial distributions of total mercury?THg?,methylmercury?MeHg?in native plants on wastelands of Wanshan mercury mining region were investigated,the ability of enrichment and transfer of different plants was assessed and screening of tolerant plants and identification of hyperaccumulators were conducted as well.Based on data obtained from field investigation on other mercury mining regions including Tongren,Songtao,Wuchuan and Kaiyang,Chatian,Xinhuang,and Xiushan,as well as experimental exposure results,the mercury tolerance and physiological and biochemical responses of the potential hyperaccumulator of Pteris vittata L.,which was identified in the present study,were discussed through physiological and biochemical,plant morphology,ultrastructural changes,cell and subcellular levels and so on.Besides,X-ray absorption near-edge structure analysis?XANES?technique was applied to explore the chemical binding form and distribution proportion of mercury in"potential mercury hyperaccumulator",and to analyze the mechanisms of mercury enrichment and detoxification.Main results were described as follows.1.There are many species of plants in the abandoned lands of Wanshan mercury mine in Guizhou,and the variation range of species dominance is large,Pteris vittata L.,Rumex acetosa,Equisetum ramosissimum,and Imperata cylindrical are dominant species of tolerant plants.Total of 57 species dominant wild plants,belonging to 52 genera and 29 families were investigated,including fern,monocotyledon,and dicotyledon,among which most speices plant were feverfew,accounting for 32%.Dominance degree among species varied between 1.0%and 25%.The top ten plants in dominance were Pteris vittata L.,Equisetum ramosissimum,Sedum emarginatum Migo,Brassica ampestris,Rumex acetosa,Imperata cylindrical,Oenothera glazioviana,Artemisia verlotorum Lamotte,Neyraudia reynaudiana and Aster ageratoides,the dominance degree ranged from 2.6 and 22.The last ten plants were Coriaria nepalensis Wall,Primula sikkimensis Hook,Discocleidion rufescens,Telosma cordata,Pyracantha fortuneana,Telosma cordata,Euphorbia esula Linn and Arthraxon hispidus,and the dominance degree ranged from 1.0 to 6.0.Among these plants,dominance degree of Pteris vittata L.,Rumex acetosa,Equisetum ramosissimum,and Imperata cylindrica was as high as 16%,12%,11%and 9.6%,respectively,and were identified as dominant species of tolerant plants in the present study.2.There were great differences in the accumulation and transfer capacity of total mercury among different plants,Pteris vittata L.was identified as potential mercury hyperaccumulatorin.The concentrations of THg in root and aboveground of 57 investigated species plants were 0.08-163 mg/kg and 0.15-106 mg/kg,respectively.Elevated levels of THg concentrations were observed in Arthraxon hispidus and Pteris vittata L.,with average of 70±39 mg/kg and 38±39 mg/kg in aboveground parts,and 130±9.4 mg/kg and 24±44 mg/kg in roots,respectively,showing high abilities of Hg accumulation.THg in rhizosphere soil significantly correlated to both in root?R=0.32,p<0.0001?and aboveground?R=0.29,p<0.0001?,suggesting that soil Hg was an important source of Hg in plants.The enrichment and transfer cofficient of THg varies greatly among different species,among of which Pteris vittata L.was the most capable of absorbing and transferring Hg,with the average of BCFs,BAFs,and TFs of 0.30±0.53,1.8±5.8 and 2.6±3.0,respectively.For an individual plant of Pteris vittata L.,the BCF,BAF,and TF was 1.9,21 and 11,it also present the higher concent of in other mercury mining areas,and the root and aboveground were as highe as 69 mg/kg and 73 mg/kg,enrichement coefficient and transfer coefficient reached 2.9 and 6.0,respectively,showing strong accumulation and transfer ability of mercury,and it was identified as potential Hg hyperaccumulator in this study.3.The content of methylmercury in roots and aboveground of the dominant tolerant wild plants varied greatly,Pteris vittata L.also showed strong ability of accumulation and transfer methylmercury.Smilar to THg,concentrations of MeHg in plants varied widely,variation rang with 0.19-876?g/kg in roots and 0.06-275?g/kg in aboveground parts.Methylmercury in species of Arthraxon hispidus,Pteris vittata L.and Fallopia multiflora were higher level than in other plants,with averages of 853±32?g/kg,127±239?g/kg and 130±145?g/kg in root,and 247±8.1?g/kg,64±95?g/kg and 98±119?g/kg in aboveground,respectively The potential hyperaccumulator of Pteris vittata L.also demonstrated high capability to uptake and transfer MeHg,with BCFs,BAFs and TFs average of 14±28,10±18,and 1.3±1.8,respectively.It is worthy to note that MeHg concentration of Arthraxon hispidus was approximately as high as 1000?g/kg MeHg,which was much higher than Pteris vittata L.,suggesting its strong ability of MeHg accumulation.4.The Pteris vittata L.has a strong ability of absorbing,transporting,and enriching mercury,root cell wall and aboveground cytoplasm are the main sites of mercury binding.Results for experimental HgCl₂ exposure to Pteris vittata showed that both root and leaf exhibited Hg concentrations greater than 100 mg/kg under the dose of 1000 mg/kg HgCl₂ expoure,ranged from 640 to 1001 mg/kg and179 to 326 mg/kg,respectively,showing significantly positive correlation?R=0.89,p<0.0001?between root and leaf,which indicated the high ability of Hg accumulation by Pteris vittata.Subcelluar analysis for THg showed that the dominant proportions were observed in cell wall in root and in cytosol in leaf,with average of 52±19%?varying between 19%and 78%?and of 98±1.6%?ranging between 93%and 99%?,respectively.High distribution portions Hg observed in cell wall and cytosol might be an important mechanism for Hg accumulation and its detoxification in Pteris vittata.Physiological effects indicated that POD?peroxisome?was stimulated radical to lower the level of superoxide by lower or dose of 100 mg/kg HgCl₂ exposure.The high dose of 1000 mg/kg increased MDA?malonaldehyde?and injured cell membrane.TEM?Transmission Electron Microscope?analysis of the damage caused by the dose of1000 mg/kg exposure indicated cell wall incrassation,widening of intercellular space,atrophy and collapse of endothelia cells in root,and the damage of myelocyte,disorders of chloroplast structures and vacuolization in leaf.5.The main forms Hg in the Pteris vittata L.growing in wastelands of mercury mining areas were composed of black cinnabar,glutathione binding state and red cinnabar.X-ray absorption near-edge structure analysis?XANES?technique showed that chemical forms of Hg in both root?THg:69-163 mg/kg?and aboveground?THg:73-83 mg/kg?of Pteris vittata that naturally grown on wastelands were?-HgS,Hg-?SG?₂,and?-HgS.The former two forms accounted for52.0%-60.5%and 37.7%-45.9%in aboveground THg,respectively,and for51.9%-63.4%and 31.2%-33.9%in root THg,respectively.The form of?-HgS in both root and aboveground were low,varying between 5.3±12.6%and 14.2±6.4%,and between 1.8±12.2%and 2.2±5.5%,respectively.