Bioremediation Of Acid Mine Drainage Research Articles

High acidity notably influences acid mine drainage treatment performance in constructed wetlands packed with composite organic substrates by affecting both abiotic and biotic routes

The impact of influent pH value (2.0–5.0) on remediation performance of constructed wetlands (CWs) packed with composite organic solids consisting of walnut shells, biochar and plastic filters for acid mine drainage (AMD) was evaluated in this study. Reed straw broth was supplemented to drive the microbial remediation process. Influent pH had a notable impact on both abiotic and biotic remediation performance, with very low efficacy at pH 2.0 and the best performance achieved at pH 5.0. The adsorption capacity of CW substrates for heavy metals and the growth of functional microbes for AMD treatment were restrained at high acidity. Moreover, the removal of conventional pollutants, plant growth and metal deposition in the CWs were negatively impacted by the dosing of AMD with low pH. A low pH of 2.0 resulted in a distinctive microbial community in the CWs with almost no sulfate-reducing bacteria (SRB). In other CW systems, SRB (Desulfosporosinus and Desulfobulbus) and organic degrading bacteria (Cellulomonas and Geobacter) were identified as keystone taxa to carry out AMD bioremediation synergistically in phase Ⅱ. CWs filled with composite organic substrates are promising systems for AMD treatment, but the pH of AMD should be preneutralized to ≥ 3.

Bioremediation of acid mine drainage using sulfate-reducing wetland bioreactor: Filling substrates influence, sulfide oxidation and microbial community

Two sulfate-reducing wetland bioreactors (SRB-1 filled with lignocellulosic wastes and SRB-2 with river sand) were applied for synthetic acid mine drainage treatment with bio-waste fermentation liquid as electron donor, and the influence of filling substrates on sulfate reduction, sulfur transformation and microbial community was studied. The presence of lignocellulosic wastes (mixture of cow manure, bark, sawdust, peanut shell and straw) in SRB-1 promoted sulfate reduction efficiency (68.9%), sulfate reduction rate (42.1 ± 11 mg S/(L·d)), dissolved sulfide production rate (27.4 ± 7 mg S/(L·d)), and particularly caused high conversion ratio of sulfate reduction into dissolved sulfide (66.4%). In comparison, the relatively low sulfate reduction efficiency (42.9%), sulfate reduction rate (27.0 ± 10 mg S/(L·d)), dissolved sulfide production rate (5.6 ± 3 mg S/(L·d)) and low dissolved sulfide conversion efficiency (21.2%) occurred in SRB-2. Mixed organic substrates including easily assimilated electron donors (in manure) and lignocellulosic matter were effective to promote quick start and long-term microbial sulfate reduction. More than 98% of produced dissolved sulfide was oxidized dominantly by photoautotrophic green sulfur bacteria (genera Chlorobium and Chlorobaculum), of which 64.6% and 54.5% was converted into elemental sulfur for SRB-1 and SRB-2. The oxidation of sulfide into elemental sulfur for potential recovery rather than sulfate is preferred. Diverse sulfate reducing bacteria and sulfide oxidizing bacteria co-existed in the treatment system, which led to a sustainable sulfur transformation. High metal removal efficiency for Fe (99.6%, 92.5%), Cd (99.9%, 99.9%), Zn (99.4%, 98.5%), Cu (94.5%, 94.6%) except for Mn (9.3%, 3.6%) was achieved, and effluent pH increased to 6.5–7.7 and 6.7–7.7 for SRB-1 and SRB-2, respectively. Microbial community was regulated by filling substrates. Synergism between lignocellulosic decomposing bacteria and sulfate reducing bacteria played a vital role in lignocellulosic bioreactor treating AMD, in addition to fermentation liquid serving as effective electron donor.

Genome-resolved metagenomics reveals depth-related patterns of microbial community structure and functions in a highly stratified, AMD overlaying mine tailings

Acid mine drainage (AMD) is a worldwide environmental problem, yet bioremediation is hampered by a limited knowledge of the reductive microbial processes in the AMD ecosystem. Here, we generate extensive metagenome and geochemical datasets to investigate how microbial populations and metabolic capacities driving major element cycles are structured in a highly stratified, AMD overlaying tailings environment. The results demonstrated an explicit depth-dependent differentiation of microbial community composition and function profiles between the surface and deeper tailings layers, paralleling the dramatic shifts in major physical and geochemical properties. Specifically, key genes involved in sulfur and iron oxidation were significantly enriched in the surface tailings, whereas those associated with reductive nitrogen, sulfur, and iron processes were enriched in the deeper layers. Genome-resolved metagenomics retrieved 406 intermediate or high-quality genomes spanning 26 phyla, including major new groups (e.g., Patescibacteria and DPANN). Metabolic models involving nitrogen, sulfur, iron, and carbon cycles were proposed based on the functional potentials of the abundant microbial genomes, emphasizing syntrophy and the importance of lesser-known taxa in the degradation of complex carbon compounds. These results have implications for in situ AMD bioremediation.

Bioremediation of acid mine drainage contaminated soils using bioattenuation, wastewater and air-injection system

Acid mine drainage (AMD) poses a serious challenge to the ecosystem due to its disastrous effect on soil and water resources which contributes to the pollution and reduction of the accessibility of potable water and arable land for agricultural activities. The treatment of contaminated soils using physical, chemical, or thermal methods requires some improvements to meet the required remediation purpose. The purpose of this study is to investigate the application of bioattenuation, wastewater, and air-injection for the remediation of AMD polluted soils. A microcosm comprising 1 kg of polluted soil was amended with varying loadings of domestic and brewery wastewaters for biostimulation treatments, and the bioattenuation received no amendment while the bioventing treatment was supplied with wastewaters and atmospheric air at 3 L/min for 30 mins intervals every 48 h. The investigation conducted under the ambient condition for 28 days recorded average metal removal efficiencies of 12–31% (bioattenuation), 50–66% for all heavy metals (iron, aluminum, copper and zinc) except manganese (28%) (biostimulation) and 56–70% (bioventing) while 32%, 35% and 52% sulfate removal efficiencies were recorded for bioattenuation, bioventing and biostimulation respectively. The study revealed that wastewater and bioventing systems can be efficiently utilized as an eco-friendly approach for the treatment of acid mine contaminated soils and pollution mitigation in the mining environments. In addition, from a comparative point of view, the results of the study further showed that the amendment of wastewater is more effective for sulfate removal while air injection enhanced metal removal from acid mine drainage contaminated soils. The synergetic effects have also been observed; the current study demonstrates that the combined application of biostimulation and bioventing system requires constant monitoring to ensure the provision of appropriate anoxic conditions for biosulfidogenic activity by enhancing sulfate removal while promoting an oxic environment for the oxidation process to facilitate metal removal for the effective treatment of acid mine drainage contaminated soil.

Construction and carbon source optimization of a microbial-plant coupled reactor for treating acid mine drainage.

Acid mine drainage (AMD) is recognized as one of the most serious contamination sources in the nonferrous metal mining industry. In this study, aerobic strains VCZ02 and VCZ09, which were identified as Leclercia adecarboxylata and Klebsiella aerogenes, were screened from 11 strains of copper-zinc-resistant bacteria in the soil of the Dexing copper mine with Cu2+/Zn2+ removal rates of 46.32%/41.03% and 57.96%/67.05%, respectively. The composition of extracellular polymers plays an important role in the removal of heavy metals by these two strains. A mixed community consisting of VCZ02 and VCZ09 was coupled with Sagittaria trifolia L.var.sinensis (Sims) Mak to construct a microbial-plant coupled reactor to remediate AMD. Under the optimal condition of sodium acetate as carbon source, the pH of AMD increased from less than 5 to above 6.5, showing Cu2+/Zn2+ removal rates of 70-80% and above 30%, respectively. SEM-EDS results showed that VZC02 and VZC09 in the coupled reactor also helped with resisting the toxicity of heavy metals to plants by forming biofilms on the root surface and increasing the content of heavy metals on the surface of roots, thus improving the treatment effect of plants. This study provides a theoretical basis for the bioremediation of AMD and its application.

Bioremediasi air asam tambang batubara dengan pengayaan bakteri pereduksi sulfat dan penambahan substrat organik

Bioremediasi air asam tambang batubara dengan pengayaan bakteri pereduksi sulfat dan penambahan substrat organik

Heavy metal removal and acid mine drainage neutralization with bioremediation approach

Mining Industry can improve the national economic situation; however, it also can damage the environment, mainly because of its wastewater that contains heavy metal and acidic solid compounds. When exposed to free air, sulfide minerals can be naturally oxidized and create acid mine drainage (AMD), a highly acidic waste that can mobilize heavy metals towards the environment. This literature study will discuss practical and sustainable biological processing to remove AAT. Sulfate Reducing Bacteria (SRB) were isolated from AMD polluted soil and grown inside an AMD-containing batch reactor. The environmental conditions (temperature, AMD concentration, SRB concentration, and contact time) were controlled during this research. The implementation of pH sampling was conducted every day, and the heavy metal final result was measured with an Inductive Coupled Plasma Optical Spectrophotometry or ICP-OES. SRB produced Hbiogenic2S that reacts with heavy metal and creates metal sulfide sediment. The remediation process by SRB will create biogenic alkalinity as an SRB side product that plays a significant role in neutralizing acidic water. Remediation is also influenced by organic compounds such as animal waste, rice, hay, or coconut husks. In this research, SRB plays a significant role as biosorbent that utilizes organic compounds as electron sources. The iron removal efficiency in AMD reached 96% and occurred on a contact time of 144 hours. To reach similar efficiency removal on a pilot scale, we planned AMD bioremediation on a tube-shaped reactor with 7.3m3 with 3.5 m height and 0.88 of each reactor radiuses. This bioremediation study has provided an alternative solution for environmental management quality due to AAT pollution in water and groundwater around mining areas.

Metagenomic Analysis of Biochemical Passive Reactors During Acid Mine Drainage Bioremediation Reveals Key Co-selected Metabolic Functions.

Acid mine drainage (AMD) is the major pollutant generated by the mining industry, and it is characterized by low pH and high concentration of metals and sulfate. The use of biochemical passive reactors (BPRs) is a promising strategy for its bioremediation. To date, there are various studies describing the taxonomical composition of BPR microbial communities, generally consisting of an assemblage of sulfate-reducing organisms inside Deltaproteobacteria, and a diverse set of anaerobic (ligno)cellulolytic bacteria; however, insights about its functional metagenomic content are still scarce. In previous studies, a laboratory-scale AMD bioremediation using biochemical passive reactors was designed and performed, tracking operation parameters, chemical composition, and changes, together with taxonomic composition of the microbiomes harbored in these systems. In order to reveal the main functional content of these communities, we used shotgun metagenomics analyses to explore genes of higher relative frequencies and their inferred functions during the AMD bioremediation from three BPRs representing the main microbiome compositions detected in the system. Remarkably, genes encoding for two-component regulatory systems and ABC transporters related to metal and inorganic ions, cellulose degradation enzymes, dicarboxylic acid production, and sulfite reduction complex were all detected at increased frequency. Our results evidenced that higher taxonomic diversity of the microbiome was arising together with a functional redundancy of the specific metabolic roles, indicating its co-selection and suggesting that its enrichment on BPRs may be implicated in the cumulative efficiency of these systems.

Addition of organic acids to acid mine drainage polluted wetland sediment leads to microbial community structure and functional changes and improved water quality

Acid mine drainage (AMD) is a serious environmental problem worldwide that requires efficient and sustainable remediation technologies including the use of biological mechanisms. A key challenge for AMD bioremediation is to provide optimal conditions for microbial-mediated immobilisation of trace metals. Although organic carbon and oxygen can enhance treatment efficiency, the effect on microbial communities is unclear. In this study, surface sediments from a natural wetland with proven efficiency for AMD bioremediation were artificially exposed to oxygen (by aeration) and/or organic carbon (in the form of mixed organic acids) and incubated under laboratory conditions. In addition to measuring changes in water chemistry, a metagenomics approach was used to determine changes in sediment bacterial, archaeal and fungal community structure, and functional gene abundance. The addition of organic carbon produced major changes in the abundance of microorganisms related to iron and sulfur metabolism (including Geobacter and Pelobacter) and increased levels of particulate metals via sulfate reduction. Aeration resulted in an increase in Sideroxydans abundance but no significant changes in metal chemistry were observed. The study concludes that the utilisation of organic carbon by microorganisms is more important for achieving efficient AMD treatment than the availability of oxygen, yet the combination of oxygen with organic carbon addition did not inhibit the improvements to water quality.

Bioremediation of acid mine drainage in constructed wetlands: Aspect of vegetation (Typha latifolia), loading rate and metal recovery

Two constructed wetlands, unplanted (CCW) and planted (PCW) units were investigated to assess the role of plants (Typha latifolia) and the impact of hydraulic loading rates in the remediation of acid mine drainage (pH ≤ 2) and metal attenuation. Results revealed significant differences (p < 0.05) between metal removal efficiencies of CCW and PCW, except for Al, Mn and Cr. CCW (92–42%) exhibited higher sulfate removal efficiency than PCW (85–30%), indicating significant impairment of microbial sulfate reduction in the presence of plants. The overall activity decreased from 2.55 and 2.21 to 2.04 and 1.90 mg chemical oxygen demand (COD) removed mg (total volatile solids) TVS–1 day−1, whereas specific sulfidogenic activity decreased from 0.79 and 0.76 to 0.62 and 0.59 mg sulfate reduced mg TVS–1 day−1 in CCW and PCW, respectively. Wetland media retaining metals (>90% of the total metal mass removed) was further extracted, good metal extraction efficiencies (%) were achieved for Fe (57.7–97.6), Al (39.2–61.3), Zn (65.9–98.8), Co (70.8–77.3), Ni (78.3–89.2) and Cr (82.2–87.1) using different chelating acids. Following extraction, metals were recovered as 49.1–97.6% Fe, 29.7–47.9% Al, 57.4–94.9% Zn, 50.3–73.6% Co, 67.0–86.5% Ni and 68.3–83.8% Cr.

A Fixed Bed Pervious Concrete Anaerobic Bioreactor for Biological Sulphate Remediation of Acid Mine Drainage Using Simple Organic Matter

The development of low-operational-cost and low-operational-complexity active sulphate (SO4) reducing bioremediation for Acid Mine Drainage (AMD) is an ongoing pursuit towards sustainable mining. This study introduces a fixed bed pervious concrete anaerobic bioreactor as a second stage AMD remediation process. The study investigated the pH self-regulation capabilities, SO4 remediation capabilities and the rate limiting parameters of the bioreactor using glucose as an organic matter source. The AMD was pre-treated using a permeable reactive barrier. A 21-day trial comprised of an increase in the SO4 loading rate while reducing the organic loading rate was undertaken to identify performance limiting conditions. A daily average SO4 concentration reduction rate of 55.2% was achieved over the initial 13 days of the experiments. The study found that a COD to SO4 ratio and VFA to alkalinity ratio below 5:1 and 0.5:1 respectively were performance limiting. The bioreactor was capable of self-regulating pH within the neutral range of 6.5 and 7.5. The study findings indicate that the bioreactor design can reduce operational costs and operational complexity of active AMD bioremediation.

Selection of organic materials potentially used to enhance bioremediation of acid mine drainage

Acid mine drainage (AMD), produced when sulfide minerals are subjected to oxygen and water, is one of the major issues in mining industries. Without proper management, AMD's release to the environment would cause seriously prolonged environmental and health issues, such as increases soil acidity and reduces water quality due to extremely low pH, high sulphate concentration, and heavy metal solubility. AMD treatments are divided into two categories, i.e., active treatment, conducted by applying a chemical to the AMD to neutralize pH and precipitate heavy metals; and passive treatment, which relies on biological and biochemical processes. The active treatment may provide an immediate effect, but costly and yet sustainable; meanwhile, passive treatment takes time to establish and to generate an effect, but it is more economical, sustainable, and environmentally friendly. The wetland system is an example of passive treatment. Therefore, this review focuses on passive treatments, especially the selection of organic materials used in constructed AMD wetland treatment. Organic materials play a central role in the wetland system, i.e., to chelate metal ions, remove sulphate from the solution, increase pH, and growth media for microbes, especially sulphate reducing bacteria (SRB) and plants are grown in the system. Overall, organic materials determine the effectiveness of the wetland system to neutralize AMD passively and sustainably.

In-situ remediation of acid mine drainage from abandoned coal mine by filed pilot-scale passive treatment system: Performance and response of microbial communities to low pH and elevated Fe

A field pilot-scale passive treatment system was developed for in-situ bioremediation of acid mine drainage (AMD). The microbial community and its variation were analyzed. The data proved that 93.7% of total soluble Fe and 99% of soluble Fe(II) could be removed by the system. Principal coordinates analysis (PCoA) showed that a low pH and an elevated Fe concentration within the system created a unique microbial community that was dominated by acidophilic iron-oxidizing bacteria and iron-reducing bacteria. Canonical correlation analysis (CCA) indicated that the pH, iron content and total sulfur jointly determined the composition of the microbial communities. Species of Ferrovum, Delftia, Acinetobacter, Metallibacterium, Acidibacter and Acidiphilium were highly enriched, which promoted the removal of iron. Furthermore, the results revealed important data for the biogeochemical coupling of microbial communities and environmental parameters. These findings are beneficial for further application of in-situ field bioreactors to remediate AMD.

Effects of the inoculum source, electron donor, and immobilization on the microbial community of sulfidogenic bioreactors

This study evaluated the biological treatment efficiency and microbial communities of eight anaerobic sequencing batch reactors treating sulfate-rich wastewater using inocula of sewage and industrial origin, immobilized or not, operated in two phases, first using lactate, and then ethanol as an electron donor. Regardless of the inoculum origin and immobilization, the maximum percentage of sulfate removal in the lactate phase was 80 ± 2%; the ethanol phase was 84 ± 3%. A lag phase in the sulfate removal was observed at the beginning of the reactor operation (lactate phase) for both types of sludge-inoculated reactors. The maximum chemical oxygen demand removal in the lactate phase was 70 ± 4%; the ethanol phase was 65 ± 5%. Pyrosequencing analysis demonstrated an increased abundance of sulfate-reducing bacteria. Only the reactors inoculated with industrial sludge presented methanogenic Archaea abundance of 3.44 and 4.15% in the lactate and ethanol phases, respectively. The genus Desulfovibrio was found in an abundance of 25.21 and 34.06% with ethanol as a carbon source in the reactors with the immobilized inocula of industrial origin, demonstrating the ability of the microbial consortium to adapt to the substrate and microenvironment. Understanding the potential for the effects of electron donors, inoculum origin, and its immobilization in the sulfate reduction and development of the microbial community can facilitate the establishment of strategies for the bioremediation of acid mine drainage.

Evaluation of organic carbon and microbial inoculum for bioremediation of acid mine drainage

Acid mine drainage (AMD) from abandoned mines cause environmental harm and requires cost-effective, passive remediation techniques. Mining activities expose mineral surfaces to oxidation, which accelerates the release and mobilization of acidity, sulfate, and metal ions in AMD. Sulfate-reducing bioreactors (SRBRs) are an attractive means to treat AMD by reducing sulfate, increasing pH, and precipitating metals as metal sulfides. Twelve SRBRs were constructed and operated for 12 months to evaluate AMD bioremediation performance as a function of carbon source and inoculum. Several locally-sourced waste organic materials were characterized by carbon fractionation; two mixtures with the same carbon profile but originating from different sources were used as organic substrates. Microbial communities from two different soil types were used to inoculate the bioreactors. The use of cow manure, corncob, and pinewood as the carbon source and soil from a local pond shore as the inoculum resulted in the greatest bioremediation efficiency, with a pH increase from 2.85 to 6.80 and removals of >82% for total Fe2+, >86% for Cu2+, >88% for Zn2+, and >65% for SO42-. The source of inoculum was found to impact bioremediation startup, with sulfate reduction observed 20 days earlier in high-performing inocula. The carbon source was found to impact long-term bioremediation efficiency, with 24% more SO42− and 38% more Fe2+ removed by the best performing carbon sources. Altogether, the combination of manure as a carbon source and marina soil as an inoculum resulted in the highest bioremediation efficiency. The high metal and sulfate removals combined with pH increases to near neutral, validate the use of SRBRs as an effective method for long-term AMD bioremediation.

Sulfate Reduction for Bioremediation of AMD Facilitated by an Indigenous Acidand Metal-Tolerant Sulfate-Reducer.

Acid mine drainage (AMD) has been a serious environmental issue that threatens soil and aquatic ecosystems. In this study, an acid-tolerant sulfate-reducing bacterium, strain S4, was isolated from the mud of an AMD storage pond in Vietnam via enrichment in anoxic mineral medium at pH 5. Comparative analyses of sequences of the 16S rRNA gene and dsrB gene involved in sulfate reduction revealed that the isolate belonged to the genus Desulfovibrio, and is most closely related to Desulfovibrio oxamicus (with 99% homology in 16S rDNA sequence and 98% homology in dsrB gene sequence). Denaturing gradient gel electrophoresis (DGGE) analyses of dsrB gene showed that strain S4 represented one of the two most abundant groups developed in the enrichment culture. Notably, strain S4 was capable of reducing sulfate in low pH environments (from 2 and above), and resistance to extremely high concentration of heavy metals (Fe 3,000 mg/l, Zn 100 mg/l, Cu 100 mg/ l). In a batch incubation experiment in synthetic AMD with pH 3.5, strain S4 showed strong effects in facilitating growth of a neutrophilic, metal sensitive Desulfovibrio sp. strain SR4H, which was not capable of growing alone in such an environment. Thus, it is postulated that under extreme conditions such as an AMD environment, acid- and metal-tolerant sulfate-reducing bacteria (SRB)- like strain S4 would facilitate the growth of other widely distributed SRB by starting to reduce sulfate at low pH, thus increasing pH and lowering the metal concentration in the environment. Owing to such unique physiological characteristics, strain S4 shows great potential for application in sustainable remediation of AMD.

Bioremediation of acid mine drainage using Fischer-Tropsch waste water as a feedstock for dissimilatory sulfate reduction

The treatment of acid mine drainage (AMD) using dissimilatory sulfate reduction (DSR) utilises the ability of sulfate reducing bacteria (SRB) to reduce sulfate to sulfide using organic compounds as electron donors. Given that AMD contains relatively low levels of organic matter, the electron donors need to be supplied externally to facilitate the bioremediation process. In this research we recognise Fischer Tropsch waste water (FTWW) as a potential organic source because of its richness in organic acids and alcohols. The FTWW was combined with synthetic AMD at an initial COD/SO42− ratio of 1.8 and was maintained in batch mode for 800 h before being turned to a fed batch mode, with an aerobic reactor receiving the sulfide rich effluent from the sulfate reducing reactor. The purpose of the aerobic reactor was to remove hydrogen sulfide through biological oxidation. The influent oxygen was regulated to avoid complete conversion of hydrogen sulfide to sulfate. We obtained up to 92.63 % sulfate removal, and almost 100 % COD removal in the anaerobic sulfate reducing reactor. The pH in the anaerobic reactor was raised from approximately 2 to above neutral. The subsequent aerobic sulfur oxidising reactor was able to convert up to 96.82 % of the sulfide produced in the anaerobic sulfate reducing reactor to sulfur.

Bacterial response to sharp geochemical gradients caused by acid mine drainage intrusion in a terrace: Relevance of C, N, and S cycling and metal resistance

A unique terrace with sharp gradient of environmental conditions was selected to study the microbial response and survival strategies to the extreme environments introduced by acid mine drainage (AMD) contamination. A combination of geochemical analyses, metagenomic sequencing, ex-situ microcosm setups, and statistical analyses were used to investigate the environment-microbe interactions. The microbial communities and metabolic potentials along the terrace were studied by focusing on the genes associated with important biogeochemical processes (i.e., C, N, S cycling and metal resistance). Results show that the variations of geochemical parameters substantially shaped the indigenous microbial communities. Sharp environmental gradients also impacted the microbial metabolic potentials, especially for C, N, and S cycling. Although the relative abundances of carbon fixing genes did not significantly vary along the environmental gradients, the taxa for carbon fixation varied significantly in more contaminated fields versus less contaminated fields, indicating the effects of AMD contamination on the autotrophic microbial communities. AMD input also influenced the N cycling, especially for nitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA). In addition, ex situ experiments were undertaken to evaluate the effects of AMD contamination on nitrogen fixation rates. Random Forest (RF) analysis indicated that nitrate, pH, total N, TOC exhibited positive correlations with the rates of nitrogen fixation while total Fe, Fe(III), and sulfate showed negative effects. Two co-occurrence networks at taxonomic and genomic levels indicated that geochemical parameters such as pH, TOC, total N, total S, and total Fe substantially influenced the innate microbial communities and their metabolic potentials. The current study provides an understanding for microbial response to AMD contamination and lays the foundation for future potential AMD bioremediation.

Uncovering microbial responses to sharp geochemical gradients in a terrace contaminated by acid mine drainage

Acid mine drainage (AMD) is harmful to the environment and human health. Microorganisms-mineral interactions are responsible for AMD generation but can also remediate AMD contamination. Understanding the microbial response to AMD irrigation will reveal microbial survival strategies and provide approaches for AMD remediation. A terrace with sharp geochemical gradients caused by AMD flooding were selected to study the microbial response to changes in environmental parameters related to AMD contamination. AMD intrusion reduced soil microbial community diversity and further changed phylogenetic clustering patterns along the terrace gradient. We observed several genera seldom reported in AMD-related environments (i.e., Corynebacterium, Ochrobactrum, Natronomonas), suggesting flexible survival strategies such as nitrogen fixation, despite the poor nutritional environment. A co-occurrence network of heavily-contaminated fields was densely connected. The phyla Proteobacteria, Acidobacteria, Chloroflexi, and Euryarchaeota were all highly interconnected members, which may affect the formation of AMD. Detailed microbial response to different soil characterizations were highlighted by random forest model. Results revealed the top three parameters influencing the microbial diversity and interactions were pH, Fe(III), and sulfate. Various acidophilic Fe- and S-metabolizing bacteria were enriched in the lower fields, which were heavily contaminated by AMD, and more neutrophiles prevailed in the less-contaminated upper fields. Many indicator species in the lower fields were identified, including Desulfosporosinus, Thermogymnomonas, Corynebacterium, Shewanella, Acidiphilium, Ochrobactrum, Leptospirillum, and Allobaculum, representing acid-tolerant bacteria community in relevant environment. The detection of one known sulfate-reducing bacteria (i.e., Desulfosporosinus) suggested that biotic sulfate reduction may occur in acidic samples, which offers multiple advantages to AMD contamination treatment. Collectively, results suggested that the geochemical gradients substantially altered the soil microbiota and enriched the relevant microorganisms adapted to the different conditions. These findings provide mechanistic insights into the effects of contamination on the soil microbiota and establish a basis for in situ AMD bioremediation strategies.

Low-Abundance Members of the Firmicutes Facilitate Bioremediation of Soil Impacted by Highly Acidic Mine Drainage From the Malanjkhand Copper Project, India.

Sulfate- and iron-reducing heterotrophic bacteria represented minor proportion of the indigenous microbial community of highly acidic, oligotrophic acid mine drainage (AMD), but they can be successfully stimulated for in situ bioremediation of an AMD impacted soil (AIS). These anaerobic microorganisms although played central role in sulfate- and metal-removal, they remained inactive in the AIS due to the paucity of organic carbon and extreme acidity of the local environment. The present study investigated the scope for increasing the abundance and activity of inhabitant sulfate- and iron-reducing bacterial populations of an AIS from Malanjkhand Copper Project. An AIS of pH 3.5, high soluble SO42− (7838 mg/l) and Fe (179 mg/l) content was amended with nutrients (cysteine and lactate). Thorough geochemical analysis, 16S rRNA gene amplicon sequencing and qPCR highlighted the intrinsic metabolic abilities of native bacteria in AMD bioremediation. Following 180 days incubation, the nutrient amended AIS showed marked increase in pH (to 6.6) and reduction in soluble -SO42− (95%), -Fe (50%) and other heavy metals. Concomitant to physicochemical changes a vivid shift in microbial community composition was observed. Members of the Firmicutes present as a minor group (1.5% of total community) in AIS emerged as the single most abundant taxon (∼56%) following nutrient amendments. Organisms affiliated to Clostridiaceae, Peptococcaceae, Veillonellaceae, Christensenellaceae, Lachnospiraceae, Bacillaceae, etc. known for their fermentative, iron and sulfate reducing abilities were prevailed in the amended samples. qPCR data corroborated with this change and further revealed an increase in abundance of dissimilatory sulfite reductase gene (dsrB) and specific bacterial taxa. Involvement of these enhanced populations in reductive processes was validated by further enrichments and growth in sulfate- and iron-reducing media. Amplicon sequencing of these enrichments confirmed growth of Firmicutes members and proved their sulfate- and iron-reduction abilities. This study provided a better insight on ecological perspective of Firmicutes members within the AMD impacted sites, particularly their involvement in sulfate- and iron-reduction processes, in situ pH management and bioremediation.