Phytostabilization potential of Jatropha curcas L. in polymetallic acid mine tailings

Greenhouse pot experiments were conducted to determine the growth response, metal tolerance, and phytostabilization potential of Jatropha curcas L The plants were grown on different degrees of multi-metal contaminated acid mine soils (T0, control; T1, moderately and T2, highly contaminated soils) with or without limestone amendments. The order of metal accumulation in J. curcas was roots>stems>leaves. The higher tolerance index (>90%) with no phytotoxic symptoms and growth reduction in T1 showed that this plant has the ability to tolerate polymetallic acid mine tailings. Further, various enzymatic and non-enzymatic antioxidants also actively involved in metal defense mechanism in J. curcas. On the other hand, to alleviate the predominant phytoavailable toxic metals such as Al, Cu, and Pb, different rates (0.1, 0.25, 0.50, and 1%) of limestone amendments were added in both T1 and T2 soils. The growth performance of J. curcas was improved due to the increase in soil pH and decrease in phytoavailable soil A1 (95%), Zn (approximately 75%), and Cu (approximately 65%) contents at 0.50% of lime addition. Based on the inherent tolerance ability of J. curcas in existing adverse environmental conditions without liming, it could be used as a suitable candidate for phytostabilization in acid mine tailings.     

Lithium,Vanadium and Chromium Uptake Ability of Brassica juncea from Lithium Mine Tailings

The potential for phytoremediation and phytostabilization of lithium in lieu with vanadium and chromium on a formulated acidic heterogeneous growth media engineered around lithium mine tailings, was investigated in four phases: (1) overall efficiency of the removal of the three metals, (2) bioaccumulation ratios of the three metals, (3) overall relative growth rate, and (4) translocation index of the three metals in the physiology of the hyperaccumulator plant. A pot study was conducted to assess the suitability of Brassica juncea (Indian mustard) in a phytoremediation process whereby it was lingered for eighty-six days under homogeneous growth conditions and irrigated bidaily with organic fertilizer amended with LiCl. A post harvest data analysis was achieved through ashing and the implementation of cold digestion procedure in a concentrated hydrochloric acidic matrix. In physiological efficiency parameters, the hyperaccumulator plant was twice as able to phytostabilize chromium and four times was able to phytostabilize vanadium in comparison to lithium. Moreover, it was extremely efficient in translocating and accumulating lithium inside its upper physiological sites, more so than chromium and vanadium, thereby demonstrating Indian mustard, as a hyperaccumulator plant, for phytoextraction and phytostabilization in an acidic heterogeneous rhizosphere, with an extremely low relative growth rate.     

Abundance and Activity of 16S rRNA,AmoA and NifH Bacterial Genes During Assisted Phytostabilization of Mine Tailings

Mine tailings in semiarid regions are highly susceptible to erosion and are sources of dust pollution and potential avenues of human exposure to toxic metals. One constraint to revegetation of tailings by phytostabilization is the absence of microbial communities critical for biogeochemical cycling of plant nutrients. The objective of this study was to evaluate specific genes as in situ indicators of biological soil response during phytoremediation. The abundance and activity of 16S rRNA, nifH, and amoA were monitored during a nine month phytostabilization study using buffalo grass and quailbush grown in compost-amended, metalliferous tailings. The compost amendment provided a greater than 5-log increase in bacterial abundance, and survival of this compost-inoculum was more stable in planted treatments. Despite increased abundance, the activity of the introduced community was low, and significant increases were not detected until six and nine months in quailbush, and unplanted compost and buffalo grass treatments, respectively. In addition, increased abundances of nitrogen-fixation (nifH) and ammonia-oxidizing (amoA) genes were observed in rhizospheres of buffalo grass and quailbush, respectively. Thus, plant establishment facilitated the short term stabilization of introduced bacterial biomass and supported the growth of two key nitrogen-cycling populations in compost-amended tailings.     

Phytoremediation of mine tailings in temperate and arid environments

Phytoremediation is an emerging technology for the remediation of mine tailings, a global problem for which conventional remediation technologies are costly. There are two approaches to phytoremediation of mine tailings, phytoextraction and phytostabilization. Phytoextraction involves translocation of heavy metals from mine tailings to the plant shoot biomass followed by plant harvest, while phytostabilization focuses on establishing a vegetative cap that does not shoot accumulate metals but rather immobilizes metals within the tailings. Phytoextraction is currently limited by low rates of metal removal which is a combination of low biomass production and insufficiently high metal uptake into plant tissue. Phytostabilization is currently limited by a lack of knowledge of the minimum amendments required (e.g., compost, irrigation) to support long-term plant establishment. This review addresses both strategies within the context of two specific climate types: temperate and arid. In temperate environments, mine tailings are a source of metal leachates and acid mine drainage that contaminate nearby waterways. Mine tailings in arid regions are subject to eolian dispersion and water erosion. Examples of phytoremediation within each of these environments are discussed. Current research suggests that phytoextraction, due to high implementation costs and long time frames, will be limited to sites that have high land values and for which metal removal is required. Phytostabilization, due to lower costs and easier implementation, will be a more commonly used approach. Complete restoration of mining sites is an unlikely outcome for either approach.     

Arbuscular Mycorrhizal Fungi (AMF) and Biosolids Enhance the Growth of a Native Australian Grass on Sulphidic Gold Mine Tailings

We tested the effect of the addition of biosolids combined with a native mycorrhizal inoculum (Arbuscular Mycorrhizal Fungi [AMF]) on growth of a native Australian grass, and on trace element stabilization of sulphidic gold mine tailings. A glasshouse trial was established on four substrates: tailings (T); tailings with a layer of 5 cm topsoil (TS); tailings amended with 100 dry t ha^?1 biosolids (LB), and tailings amended with 500 dry t ha^?1 biosolids (HB). Pots of 1.2 L of capacity were established; some were inoculated with a mixture of Glomus sp. (WUM51–9227), Scutelospora aurigloba (WUM51–53), and Acaulospora levis (WUM46) culture mix, and others were uninoculated controls. Seeds of the native Australian grass, Bothriochloa macra were sown in the pots. Root infection, plant biomass production, nutrients and trace element concentrations in shoots were investigated. Addition of biosolids significantly increased AMF infection of roots compared to unamended substrates. No clear qualitative differences in colonization were detected. Addition of biosolids and AMF together clearly improved the establishment and growth of the native grass. Similar trends in nutritional status were shown for biosolids and inoculation with AMF treatments. Mycorrhizal inoculation increased plant biomass production and the effectiveness of nutrient uptake. The combined use of biosolids and mycorrhizal inoculation could be a reliable method for phytostabilization purposes in polluted substrates.     

Jatropha curcas and assisted phytoremediation of a mine tailing with biochar and a mycorrhizal fungus

Soil pollution is an important ecological problem worldwide. Phytoremediation is an environmental-friendly option for reducing metal pollution. A greenhouse experiment was conducted to determine the growth and physiological response, metal uptake, and the phytostabilization potential of a nontoxic Jatropha curcas L. genotype when grown in multimetal-polluted conditions. Plants were established on a mine residue (MR) amended or not amended with corn biochar (B) and inoculated or not inoculated with the mycorrhizal fungus Acaulospora sp. (arbuscular mycorrhizal fungus, AMF). J. curcas was highly capable of growing in an MR and showed no phytotoxic symptoms. After J. curcas growth (105 days), B produced high desorption of Cd and Pb from the MR; however, no increases in metal shoot concentrations were observed. Therefore, Jatropha may be useful for phytostabilization of metals in mine tailings. The use of B is recommended because improved MR chemical properties conduced to plant growth (cation-exchange capacity, organic matter content, essential nutrients, electrical conductivity, water-holding capacity) and plant growth development (higher biomass, nutritional and physiological performance). Inoculation with an AMF did not improve any plant growth or physiological plant characteristic. Only higher Zn shoot concentration was observed, but it was not phytotoxic. Future studies of B use and its long-term effect on MR remediation should be conducted under field conditions.     

铜尾矿自然定居白茅对体内氮磷的适时分配及叶片氮磷代谢调节酶活性动态

白茅(Imperata cylindrica)为铜尾矿废弃地的自然定居优势植物。以农田正常生长的白茅居群为对照, 通过对不同生长时期的白茅体内氮(N)、磷(P)营养浓度, 叶片硝酸还原酶和酸性磷酸酶活性变化等的研究, 探讨铜尾矿胁迫下白茅在营养利用上的生理适应特征。研究结果表明, 在不同的生长时期, 白茅各器官内N、P分配存在一定的差异。在萌芽期, 两居群白茅体内的N、P均主要集中于根状茎。在花蕾期、成熟期时, 两居群白茅体内的N、P均向成熟叶片中迁移, 叶片中N、P浓度均达到最高。但在衰败期时, 铜尾矿居群白茅体内的N主要迁移到根状茎中, P在根状茎中的浓度也达到生长期中的最高值, 而农田居群白茅成熟叶片内N、P浓度依然最高。铜尾矿白茅叶片N、P的再吸收效率分别为49.54%-65.22%和74.71%-98.71%, 而在农田系统中分别为18.18%-52.81%和71.39%-84.07%; 铜尾矿白茅衰老叶片中P达到完全再吸收的程度。铜尾矿白茅叶片硝酸还原酶活性在生长旺盛期显著高于农田居群(p < 0.05), 是白茅加强对自身氮养分代谢活动调节作用的表现; 同一生长时期白茅叶片酸性磷酸酶活性在两种生境间差异性并不明显(p > 0.05), 但随着生长期的延长, 白茅叶片酸性磷酸酶活性表现出不断升高的趋势, 这有利于生长后期衰老叶片中有机P的水解再吸收。可见, 铜尾矿中生长的白茅通过对N、P养分的适时分配, 提高营养成分的再吸收效率, 调节N、P代谢相关调节酶活性的变化方式来减轻铜尾矿生境的营养胁迫。     

Plant growth promoting rhizobacteria and endophytes accelerate phytoremediation of metalliferous soils

Technogenic activities (industrial—plastic, textiles, microelectronics, wood preservatives; mining—mine refuse, tailings, smelting; agrochemicals—chemical fertilizers, farm yard manure, pesticides; aerosols—pyrometallurgical and automobile exhausts; biosolids—sewage sludge, domestic waste; fly ash—coal combustion products) are the primary sources of heavy metal contamination and pollution in the environment in addition to geogenic sources. During the last two decades, bioremediation has emerged as a potential tool to clean up the metal-contaminated/polluted environment. Exclusively derived processes by plants alone (phytoremediation) are time-consuming. Further, high levels of pollutants pose toxicity to the remediating plants. This situation could be ameliorated and accelerated by exploring the partnership of plant-microbe, which would improve the plant growth by facilitating the sequestration of toxic heavy metals. Plants can bioconcentrate (phytoextraction) as well as bioimmobilize or inactivate (phytostabilization) toxic heavy metals through in situ rhizospheric processes. The mobility and bioavailability of heavy metal in the soil, particularly at the rhizosphere where root uptake or exclusion takes place, are critical factors that affect phytoextraction and phytostabilization. Developing new methods for either enhancing (phytoextraction) or reducing the bioavailability of metal contaminants in the rhizosphere (phytostabilization) as well as improving plant establishment, growth, and health could significantly speed up the process of bioremediation techniques. In this review, we have highlighted the role of plant growth promoting rhizo- and/or endophytic bacteria in accelerating phytoremediation derived benefits in extensive tables and elaborate schematic sketches.     

Effects of Chromium Ore Tailings on Growth and Physiological Activities of Mesua ferrea L.

A greenhouse pot experiment was conducted to evaluate the feasibility of using a native ornamental plant, Mesua ferrea L. as phytostabilizer for chromium ore tailings (COT) and to assess the metal accumulation capacity. Different ratios of soil and COT were taken in pots and sowed with seeds of M. ferrea. Plants were harvested at various intervals and separated into roots and shoots for analysis of metal concentrations and physiological characteristics of the plants. The study revealed that the plant has great tolerance and stronger ability to accumulate Cr. The results suggested an increase in growth, chlorophyll content, antioxidant activities, as well as metal accumulation capacity of M. ferrea with increasing proportion of COT in the soil. This indicates the plant's efficiency to overcome any stress generated due to excess of chromium as well as other heavy metals. The order of accumulation of heavy metals was observed to be Fe>Cr>Ni>Cd>Co. The accumulation of Cr was higher in root compared to that in shoot. M. ferrea has found to be potential as a native species candidate for phytostabilization of chromium mine tailings.     

丛枝菌根在植物修复重金属污染土壤中的作用

丛枝菌根（Arbuscular mycorrhizae,AM）是自然界中分布最广的一类菌根,AM真菌能与陆地上绝大多数的高等植物共生,常见于包括重金属污染土壤在内的各种生境中。在重金属污染条件下,AM真菌可以减轻重金属对植物的毒害,影响植物对重金属的吸收和转运,在重金属污染土壤的植物修复中显示出极大的应用潜力。重点介绍了AM真菌对植物重金属耐性的影响及其在植物提取和植物稳定中的应用等方面的进展,讨论了未来研究所面临的任务和挑战。     

Adaptation and detoxification mechanisms of Vetiver grass (Chrysopogon zizanioides) growing on gold mine tailings

Vetiver grass (Chrysopogon zizanioides) was investigated for its potential use in the rehabilitation of gold mine tailings, its ability to extract and accumulate toxic metals from the tailings and its metal tolerant strategies. Vetiver grass was grown on gold mine tailings soil, in a hothouse, and monitored for sixteen weeks. The mine tailings were highly acidic and had high electrical conductivity. Vetiver grass was able to grow and adapt well on gold mine tailings. The results showed that Vetiver grass accumulated large amounts of metals in the roots and restricted their translocation to the shoots. This was confirmed by the bioconcentration factor of Zn, Cu, and Ni of >1 and the translocation factor of <1 for all the metals. This study revealed the defense mechanisms employed by Vetiver grass against metal stress that include: chelation of toxic metals by phenolics, glutathione S-tranferase, and low molecular weight thiols; sequestration and accumulation of metals within the cell wall that was revealed by the scanning electron microscopy that showed closure of stomata and thickened cell wall and was confirmed by high content of cell wall bound phenolics. Metal induced reactive oxygen species are reduced or eliminated by catalase, superoxide dismutase and peroxidase dismutase.     

Bioleaching of Heavy Metals from Mine Tailings by Aspergillus fumigatus

The bioleaching experiment was conducted for the removal of heavy metals from mine tailings. A fungal strain was isolated from the gold mine tailings and it has been identified as Aspergillus fumigatus based on its 18S rDNA analysis. Bioleaching using A. fumigatus was carried out in bioleaching step processes (one-step and two-step) at various tailings concentrations (1%, 2%, 4%, and 8% [w/v]). In the one-step bioleaching process where fungi were cultivated in the presence of the tailings, concentration of oxalic acid was the highest among the organic acids produced. On the other hand, in the two-step bioleaching process where the metabolic products of fungal growth, which have been separated from its biomass, were used, citric acid was dominant. In the one-step process, the highest As (62%), Fe (58%), Mn (100%), and Zn (54%) removals were observed at the lowest tailings concentration (1%). The removal of Pb at 1% tailings concentration in the one-step process was 56%, whereas 88% removal was achieved in the two-step process where citric acid was dominant. In general, heavy metals removal efficiency decreased with increased tailings of the concentration in both bioleaching processes. This study shows the possibility of using A. fumigatus to bioleach hazardous heavy meals from gold mine tailings.     

Efficacy of lime, biosolids, and mycorrhiza for the phytostabilization of sulfidic copper tailings in Chile: a greenhouse experiment

Inadequate abandonment of copper mine tailings under semiarid Mediterranean climate type conditions has posed important environmental risks in Chile due to wind and rain erosion. There are cost-effective technologies for tailings stabilization such as phytostabilization. However, this technology has not been used in Chile yet. This study evaluated in a greenhouse assay the efficacy of biosolids, lime, and a commercial mycorrhiza to improve adverse conditions of oxidized Cu mine tailings for adequate establishment and grow of Lolium perenne L. var nui. Chemical characterization of experimental substrates and pore water samples were performed; plant density, biomass production, chlorophyll content, and metal content in shoots was evaluated in rye grass plants after an eight-week growth period. Results showed that neutralization of tailings and superficial application of biosolids increased both aerial biomass production and chlorophyll content of rye grass. Increased Cu solubilization and translocation to shoots occurred after biosolids application (mixed), particularly on unlimed tailings, due to formation of soluble organometallic complexes with dissolved organic carbon (DOC) which can be readily absorbed by plant roots. Positive effects of mycorrhizal inoculation on rye grass growth were restricted to treatments with superficial application of biosolids, probably due to Cu toxicity effects on commercial mycorrhiza used (Glomulus intraradices).     

The influences of arbuscular mycorrhizal fungus on phytostabilization of lead/zinc tailings using four plant species

A greenhouse experiment was conducted to investigate the effects of the arbuscular mycorrhizal fungus Funneliformis mosseae on three parameters: Pb, Zn, Cu and Cd accumulation, translocation and plant growth in perennial ryegrass (Lolium perenne), tall fescue (Festuca arundinacea), showy stonecrop (Hylotelephium spectabile) and Purple Heart (Tradescantia pallida). The purpose of this work is to enhance site-specific phytostabilization of lead/zinc mine tailings using native plant species. The results showed that mycorrhizal fungi inoculation significantly increased plant biomass of F. arundinacea, H. spectabile and T. pallida. The Pb, Zn, Cu and Cd concentrations in roots were higher than those in shoots both with and without mycorrhizae, with the exception of the Zn concentration in H. spectabile. Mycorrhizae generally increased metal concentrations in roots and decreased metal concentrations in shoots of L. perenne and F. arundinacea. In addition, it was found that the majority of the bioconcentration and translocation factors were lower than 1 and mycorrhizal fungi inoculation further reduced these values. These results suggest that appropriate plant species inoculated with mycorrhiza might be a potential approach to revegetating mine tailing sites and that H. spectabile is an appropriate plant for phytostabilization of Pb/Zn tailings in northern China due to its higher biomass production and lower metal accumulation in shoots.     

Artificial inoculation-perspectives in tailings phytostabilization

Intensive mining and processing activities worldwide resulted in the generation of huge amounts of waste (tailings), generally characterized as toxic, radioactive, and/or hazardous. The exposure potential and, hence, the risk posed by such wastes is enhanced by a general lack of vegetation. Phytostabilization has proven to be efficient in reducing this risk. However, establishing vegetation on tailing dumps may be expensive due to the intensive use of amendments and chemical fertilizers. In this article, investigations on artificial inoculation of mine tailings with bacterial strains as a means to improve the development of vegetative covers and reduce application cost by eliminating chemical fertilization are presented and discussed. The development of plants and microbial communities from tailings, as well as the impact of inoculation on metal uptake in plants, were studied. Experiments were carried out in greenhouse using two types of mine tailings (phosphogypsum and sulphidic tailings) from the Romanian Black Sea coast. Indigenous herbaceous plants were cultivated on tailings with the addition of chemical fertilizers versus bacterial inoculation. After a 6-month experimental period, excellent plant growth, which is associated with a rich microbial community, was observed in all inoculated treatments, in contrast with poor plant growth and microbiota from the chemical fertilization treatments alone. Additionally, artificial inoculation improved plant resistance to heavy metals by reducing the uptake of some toxic metals. Once a rich microbial community is established, inoculation may be discontinued. Based on these results, efficient and cost-effective phytostabilization schemes can be proposed.     

Comparative study on effects of four energy plants growth on chemical fractions of heavy metals and activity of soil enzymes in copper mine tailings

Four gramineous energy plants, Miscanthus sacchariflorus, M. floridulus, Phragmites australis, and Arundo donax were grown on copper tailings in the field for four years. Their phytoremediation potential was examined in terms of their effects on the fractions of heavy metals and soil enzyme activities. Results showed that plantation of these four gramineous plants has improved the proportion of organic material (OM)-binding fraction of heavy metals in copper tailings as a whole, and reduced the proportion of exchangeable and residual fractions. In particular, M. sacchariflorus growth improved significantly the proportion of the OM-binding fractions of Cu (1.73 times), Cd (1.71 times), Zn (1.18 times), and Pb (3.14 times) (P < 0.05) and reduced markedly the residual fractions of Cu (64.45%), Cd (82.38%), Zn (61.43%), and Pb (73.41%) (P < 0.05). Except for A. donax, the growth of other three energy plants improved the activity of phosphatase, urease and dehydrogenase in copper tailings to some extent. In particular, the activity of soil phosphatase and urease in planted tailings differed significantly from that of control (P < 0.05). The effect of M. sacchariflorus growth on soil enzyme was the highest, followed by P. australis, M. floridulus, and A. donax. The content of each heavy metal fraction in soil was correlated with soil enzyme activities, especially the content of OM-binding fraction, which correlated significantly with the activities of phosphatase, urease and dehydrogenase in soil. According to the effects of four gramineous plants growth on activity of soil enzymes and fractions of heavy metals, M. sacchariflorus had the optimal effects for phytoremediation. Therefore, M. sacchariflorus was a candidate plant with great potential for the revegetation of heavy metal tailings.     

Phytostabilization of gold mine tailings, New Zealand. Part 1: Plant establishment in alkaline saline substrate

Tailings from the Macraes mine, southern New Zealand, are prone to wind erosion. Use of a vegetation cover for physical stabilization is one potential solution to this environmental problem. This study used field trials contained in lysimeters to 1), test the ability of different plant species to grow in un/amended tailings and 2), provide background information on the nutrient and chemical content of waters in tailings. Barley (Hordeum vulgare), blue lupin (Lupinus angustifolius), and rye corn (Secale cereale) were trialed, using Superphosphate fertilizer and sewage sludge as amendments. Rye corn grew well in fertilizer-amended tailings, but poorly in unamended tailings; barley growth was similar in amended and unamended tailings; blue lupins grew poorly overall The tailings had alkaline pH (7-8.5) and water rapidly (< 1 mo) interacted with the tailings to become strongly saline. Minor acid generation was neutralized by calcite, with associated release of calcium and carbonate ions. Leachate waters were supersaturated with respect to calcite and dolomite. Dissolved sodium concentrations were up to 1000 mg L(-1), but elevated Ca2+ calcium and Mg2+ ensured that sodicity was lower than plant-toxic levels. Rye corn is a potentially useful plant for rapid phytostabilization of tailings, with only minor phosphate amendment required.     

Effect of Organic Manures on the Growth of Cymbopogon citratus and Chrysopogon zizanioides for the Phytoremediation of Chromite-Asbestos Mine Waste: A Pot Scale Experiment

The abandoned chromite-asbestos mines are located in the Roro hills, West Singhbhum, Jharkhand, India, where mining operation ceased in 1983, and since then these mines are causing environmental pollution. The present study was planned to phytoremediate these metalloid and metal contaminated mine waste by using two aromatic grasses, Cymbopogon citratus and Chrysopogon zizanioides by applying different proportions of amendments (chicken manure, farmyard manure and garden soil). Mine waste has neutral pH, low electrical conductivity and organic carbon with higher concentration of total metals (Cr and Ni) as compared to soil. Application of manures resulted significant improvements of mine waste characteristics and plant growth, reduction in the availability of total extractable toxic metals (Cr, Ni) and increase in Mn, Zn and Cu concentration in the substrate. The maximum growth and biomass production for C. citratus and C. zizanioides were found in T-IV combination comprising of mine waste (90%), chicken manure (2.5%), farmyard manure (2.5%) and garden soil (5%). Addition of T-IV combination also resulted in low Cr and Ni accumulation in roots and reduction in translocation to shoots. Study indicates that C. citratus and C. zizanioides can be used for phytostabilization of abandoned chromite-asbestos mine waste with amendments.     

pH Shaping the Composition of sqhC-Containing Bacterial Communities

Sediment and tailings samples were collected from sites with a contrasting physicochemical gradient to investigate microbial squalene-hopene cyclase (sqhC) composition and distribution in terrestrial environments. Acidobacteria (66%), Alphaproteobacteria (96%) and Gammaproteobacteria (55%) were found to dominate sqhC communities, respectively, at the acidic Dajiuhu Peatland, the alkaline Heshang Cave and strongly acidic tailings of Tongling copper mine in China. Statistical analysis confirmed that pH was the important factor impacting the geographical distribution of sqhC at phylum level. sqhC gene abundance is comparable at the three sites. However, the total amount of hopanoids in per gram total organic matter content (TOC) is 1.75 times higher in the acidic peatland than that in the alkaline Cave and it is below the detecting limit in tailings of Tongling copper mine, inferring the potential impact of pH in regulating the hopanoid production.     

Identification of sulfate-reducing bacteria in methylmercury-contaminated mine tailings by analysis of SSU rRNA genes

Sulfate-reducing bacteria (SRB) are often used in bioremediation of acid mine drainage because microbial sulfate reduction increases pH and produces sulfide that binds with metals. Mercury methylation has also been linked with sulfate reduction. Previous geochemical analysis indicated the occurrence of sulfate reduction in mine tailings, but no molecular characterization of the mine tailings-associated microbial community has determined which SRB are present. This study characterizes the bacterial communities of two geochemically contrasting, high-methylmercury mine tailing environments, with emphasis on SRB, by analyzing small subunit (SSU) rRNA genes present in the tailings sediments and in enrichment cultures inoculated with tailings. Novel Deltaproteobacteria and Firmicutes -related sequences were detected in both the pH-neutral gold mine tailings and the acidic high-sulfide base-metal tailings. At the subphylum level, the SRB communities differed between sites, suggesting that the community structure was dependent on local geochemistry. Clones obtained from the gold tailings and enrichment cultures were more similar to previously cultured isolates whereas clones from acidic tailings were more closely related to uncultured lineages identified from other acidic sediments worldwide. This study provides new insights into the novelty and diversity of bacteria colonizing mine tailings, and identifies specific organisms that warrant further investigation with regard to their roles in mercury methylation and sulfur cycling in these environments.