Sulfate reduction at low pH to remediate acid mine drainage

Unlocking the bioremediation potential of adapted Desulfovibrio desulfuricans in acidic low-temperature U-contaminated groundwater

Addressing the global challenge of uranium (U)-contaminated groundwater requires innovative bioremediation strategies. This study investigates Desulfovibrio desulfuricans, a neutrophilic and mesophilic sulfate-reducing bacteria (SRB) strain optimized for low-temperature (15 °C) and acidic (initial pH 4) conditions, to validate its bioaugmentation potential for uranium decontamination in groundwater. Our research aimed to assess its efficacy in treating U-contaminated groundwater and elucidate the optimal growth conditions for this strain in acidic and sulfate-enriched environments. We found that D. desulfuricans was phylogenetically distinct from the native microbial community in acidic U-contaminated groundwater, while it maintained appreciable activity in sulfate reduction under contaminated groundwater conditions after accumulation. Acid-tolerant D. desulfuricans removed 75.87 % of uranium and 30.64 % of sulfate from acidic U-contaminated groundwater (pH 4.0) at 15 °C within 14 days. Furthermore, we explored the optimal sulfate concentration for bacterial growth, which was found to be 2000 mg/L, and an elevated Fe2+ concentration from 100 to 1000 mg/L increasingly stimulated sulfate-reducing activity. These findings provide a novel insight into the application of neutrophilic and mesophilic SRB in bioremediation of acidic and low-temperature groundwater after accumulation and underscore the feasibility of bioremediation by using exogenously pure SRB.

Study on the growth and decline patterns and environmental drivers of pathogens during the stabilization process of simulated landfilling municipal solid waste

Waste and leachate in landfills are substantial reservoirs of pathogens, however information about the risk of pathogen contamination during the stabilization process under different landfill conditions is very limited. In this study, dynamic changes of culturable pathogens, bacteria community, and human bacterial pathogens (HBPs) during the stabilization process under different landfill conditions were investigated, and the environmental drivers were explored. Results showed that total coliforms, Enterococcus, and Staphylococcus aureus were the dominant pathogens detected in waste and leachate samples. During the landfill stabilization process, the concentration of culturable pathogens peaked at the hydrolysis-acidification stage (3.6 × 105 CFU·g-1) in the anaerobic condition, fluctuated from 4.18 × 104 to 5.35 × 105 CFU·g-1 in the anaerobic leachate-recirculation condition, and kept rising (from 4.18 × 104 to 2.12 × 106 CFU·g-1) in the micro-aerobic condition. Moreover, HBPs abundance and diversity in the waste and leachate under micro-aerobic conditions were higher than those under the other two conditions, suggesting a higher risk of pathogen contamination. Sulfate and pH were significantly (p < 0.05) correlated with the composition of bacterial communities and HBPs, likely serving as the major environmental driving factors. Additionally, the interactions between HBPs and functional bacterial groups tended towards cooperative symbiotic relationships, with hydrolytic-acidogenic bacteria promoting the growth and proliferation of most pathogens. These findings will help to understand the changes and environmental drivers of pathogens during landfill stabilization, which will provide a theoretical basis for the risk prevention and control of pathogens in waste disposal.

Investigation of Suitable, Readily Available, Sources of Sulfate-Reducing Bacteria Inoculum, and Evaluation of Sulfate Reduction Rates Achieved at Different pHs

This study investigated the suitability of readily available and naturally occurring sources of microorganisms (inoculum) to use for the cultivation of sulphate-reducing bacteria (SRB) for acid mine drainage (AMD) remediation. The selected inocula included AMD water (AMD), mud (MUD) and reed-bed mud (RM) from the AMD surrounds, mealworms (MW), cow dung (CD) and raw sewage sludge (RS). The suitability of the different inoculum sources was evaluated by comparing the SO4 2- reduction and sulfide (S2-) production rates at three different pHs. Experimental results showed that the AMD, MW, MUD and CD inoculum did not produce appreciable reduction of SO4 2- to S2- and were unsuitable sources of SRB inoculum. The inoculum evaluated in pH 2 media did not achieve SO4 2- reduction. Of the inoculum assessed in pH 4 media, only the RM inoculum achieved SO4 2- reduction (40%) with S2- production (36 mg/L). In contrast, a notable S2- production, RS (114 mg/L) and RM (99 mg/L), accompanied the SO4 2- reduction achieved in the pH 7.5 RS (44%) and RM (30%) samples. The improved S2- produced/SO4 2- removed conversion ratios for samples pH 7.5 RS (0.14) and pH 7.5 RM (0.17) are indicative of increased SRB activity and the suitability of these inoculum as SRB sources.

Performance of an integrated system composed of native soil amended with ZVI/FeS nanoparticles-based technosols and a bioreactor for the treatment of acid rock drainage

We assessed combined, low-cost physical-chemical and biological treatment system for the remediation of acid rock drainage (ARD). Two columns composed of native soil amended with zero valent iron and iron sulfide (ZVI/FeS) nanoparticles-based Technosols were connected to a sulfate-reducing (SR) bioreactor; this integrated treatment system was fed with ARD obtained from mines in El Oro, Ecuador. A control sulfate-reducing bioreactor fed directly with ARD was run in parallel. The bioreactors were operated dynamically for four periods, and every 21 days, the fraction of ARD in the influent was increased stepwise from 25 to 100%. The integrated treatment system demonstrated superior removal efficiencies for Fe (99%), As (99.9%), Cd (99%), Cu (98%), Mn (91%), and Zn (99.9%), while both bioreactors achieved similar sulfate reduction (50-52%) and raised ARD pH from 2.75 to ~ 8. However, the ZVI/FeS nanoparticles-based Technosols significantly improved the removal of As (99.9% vs. 68%) and Mn (91% vs. 71%), compared to the control bioreactor. Molecular ecology analysis of the biomass in the bioreactors using 16S rRNA sequencing revealed that higher ARD concentrations enhanced sulfidogenesis, promoting beneficial interactions within communities in the sulfate-reducing niche. This study highlights the value of the ZVI/FeS nanoparticles-based Technosols in optimizing ARD bioremediation, offering a promising solution for mining effluent management in low-middle-income countries (LMIC).

Unlocking the anaerobic conversion of crop residues: Biological pretreatments and the role of sulfide pathway in lignin degradation

Research on the reutilization of crop residues has gained significant attention as a strategy for generating energy and high-value chemicals from renewable sources, while simultaneously reducing feedstock costs and mitigating environmental pollution. Crop residues have been effectively applied in lignocellulosic sulfate-reducing bioreactors (LSRBs) for the treatment of mining-influenced water. A comprehensive evaluation of the state-of-the-art in LSRBs reveals their potential for leveraging syntrophic aerobic-anaerobic interactions between sulfate-reducing bacteria and facultative species, alongside cellulolytic-fermentative microorganisms, to facilitate the pretreatment of lignocellulosic biomass for biorefinery applications. Key variables influencing the availability of enzymatic substrates and the activity of lignin-degrading enzymes are identified, along with strategies to enhance catalytic efficiency. Additionally, approaches to ensure the availability of trace elements and to control the production of toxic intermediates that may hinder treatment processes are elucidated. Prominent strategies include the application of microaeration and the use of co-substrates. An innovative aspect is the exploitation of metal sulfide precipitation to mitigate toxicity while preventing the sequestration of hydrogen peroxide - an essential substrate for enzymatic activity - by sulfides generated during the process. This review emphasizes the need for scientific advancements focused on optimizing the valorization of lignocellulosic residues. A particular focus is placed on advancing the understanding of lignin's anaerobic degradation mechanisms, especially in systems co-treating lignocellulosic waste and mining-influenced waters. Such advancements hold promise for enhancing the efficiency and sustainability of biorefinery operations.

Mineral Carbonation for Carbon Sequestration: A Case for MCP and MICP

Mineral carbonation is a prominent method for carbon sequestration. Atmospheric carbon dioxide (CO2) is trapped as mineral carbonate precipitates, which are geochemically, geologically, and thermodynamically stable. Carbonate rocks can originate from biogenic or abiogenic origin, whereby the former refers to the breakdown of biofragments and the latter precipitation out of water. Carbonates can also be formed through biologically controlled mechanisms (BCMs), biologically mediated mechanisms (BMMs), and biologically induced mechanisms (BIMs). Microbial carbonate precipitation (MCP) is a BMM occurring through the interaction of organics (extracellular polymeric substances (EPS), cell wall, etc.) and soluble cations facilitating indirect precipitation of carbonate minerals. Microbially induced carbonate precipitation (MICP) is a BIM occurring via different metabolic pathways. Enzyme-driven pathways (carbonic anhydrase (CA) and/or urease), specifically, are promising for the high conversion to calcium carbonate (CaCO3) precipitation, trapping large quantities of gaseous CO2. These carbonate precipitates can trap CO2 via mineral trapping, solubility trapping, and formation trapping and aid in CO2 leakage reduction in geologic carbon sequestration. Additional experimental research is required to assess the feasibility of MICP for carbon sequestration at large scale for long-term stability of precipitates. Laboratory-scale evaluation can provide preliminary data on preferable metabolic pathways for different materials and their capacity for carbonate precipitation via atmospheric CO2 versus injected CO2.

Programming Surface Motility and Modulating Physiological Behaviors of Bacteria via Biosurfactant-Mimetic Polyurethanes

Modulating microbial motility and physiology can enhance the production of bacterial macromolecules and small molecules. Herein, a platform of water-soluble and amphiphilic peptidomimetic polyurethanes is reported as a means of regulating bacterial surface behavior and the concomitant production of extracellular polymeric substances (EPS). It is demonstrated that carboxyl (-COOH)-containing polyurethanes exhibited 17-fold and 80-fold enhancements in Pseudomonas aeruginosa (P. aeruginosa) swarming and twitching areas, respectively. Conversely, an amine (-NH2)-functionalized polyurethane reduces the P. aeruginosa swarming area by 58%. Similar influences on the surface motility of Escherichia coli (E. coli) and a nonswarming P. aeruginosa mutant strain are also observed. Notably, -COOH polyurethanes completely wet the agar hydrogel surface and promote bacterial surface proliferation, resulting in enhanced EPS and rhamnolipid production. The programming of bacterial spatial migration into designed patterns is achieved by leveraging the opposing influences of -NH2 and -COOH polyurethanes. The results highlight the potential of this synthetic polyurethane platform and potentially other polymer systems as an exciting approach to control bacterial surface behaviors and influence the production of engineered living materials.

Selective separation of light and heavy rare earth elements from acidic mine waters by integration of chelating ion exchange and ligand impregnated resin

This study addresses the potential of sourcing Critical Raw Materials (CRMs) using Acidic Mine Waters (AMWs) as a secondary resource. AMWs, often viewed as waste, contain valuable metals like zinc and copper, as well as critical metals like magnesium and cobalt. Moreover, recent studies also reported the presence of Rare Earth Elements (REEs) at concentrations (mg/L) that make their extraction both technically and economically viable. The research focuses on a circular process to recover these metals from AMWs, specifically from the Aznalcóllar open-pit mine, which contains 216 mg/L of Al, 47 mg/L of Fe, 547 mg/L of Zn, and 18.56 mg/L of REEs. The proposed method involves pre-treating the AMW to remove Fe and Al, achieving removals of over 99.9 % and 90 %, respectively, at pH 4.5. Following this, transition metals like Zn, Cd, and Cu were removed as sulphides with a removal efficiency exceeding 99 %. This pre-treatment step reduced the concentration of competing metals in the ion-exchange process, thereby enhancing the recovery and purity of REEs. To separate heavy and light REEs, two types of resins in series were used: an impregnated resin (TP272) and a chelating resin (S930), which can be regenerated using sulphuric acid (H2SO4). The final recovery of REEs as oxalates was achieved using oxalic acid and ammonia at pH 1, with further optimization of the elution process to minimize ammonia consumption and undesired precipitation of other oxalates. Finally, REE oxalates with purities exceeding 90 % were obtained. This research demonstrates a sustainable method for efficiently recovering valuable REEs from AMWs, while also addressing environmental concerns related to hazardous sludge generation.

A novelty strategy for AMD prevention by biogas slurry: Acetate acid inhibition effect on chalcopyrite biooxidation and leachate

With the widespread application of anaerobic digestion technology, biogas slurry become the main source of organic amendments in practice. Comprehensive studies into the inhibitory effects of low molecular weight (LMW) organic acids, essential components in biogas slurry, on the sulfide minerals biooxidation and its bioleaching (AMD) have been lacking. In this study, acetic acid (AA) served as a representative of LMW organic acids in biogas slurry to investigate its impact on the inhibition of chalcopyrite biooxidation by Acidithiobacillus ferrooxidans (A. ferrooxidans). It was shown that AA could slow down the chalcopyrite biooxidation and inhibit the jarosite formation on the mineral surface. Compared with the control group (0 ppm AA), the sulfate increment in the leachate of the 50 ppm, 100 ppm, and 200 ppm AA-treated groups decreased by 36.4%, 66.8%, and 69.0%, respectively. AA treatment (≥50 ppm) could reduce the oxidation of ferrous ions in the leachate by one order of magnitude. At the same time, the bacterial concentration of the leachate in the 50 ppm, 100 ppm, and 200 ppm AA-treated groups decreased by 70%, 93%, and 94%, respectively. These findings provide a scientific basis for new strategies to utilize biogas slurry for mine remediation and contribute to an enhanced comprehension of organic amendments to prevent AMD in situ in mining soil remediation.

Enrichment of acid-tolerant sulfide-producing microbes from an acidic pit lake

High concentrations of harmful metal(loid)s and extreme acidity are persistent environmental concerns in acidic pit lakes. In this study, we examine Cueva de la Mora (CM), a meromictic pit lake in the Iberian Pyrite Belt, Spain, as a model system. Our research aims to explore potential bioremediation strategies to mitigate the impacts of metal(loid)s and acidity in such environments. The major strategy applied in this research is to biologically stimulate sulfate reduction (i.e., biosulfidogenesis) in the deep layer of the lake to promote the formation of low-solubility sulfide minerals. Previous omics-based studies of CM have shown that several sulfate-reducing bacteria (SRB) taxa are present in the deep layer. However, their activities are likely limited by the availability of electron donors for sulfide production. Therefore, different amendments (glycerol, elemental sulfur, and glycerol + elemental sulfur) were tested to promote sulfide production and enrich acid-tolerant sulfide-producing microbes. Our results showed that glycerol stimulated dissimilatory sulfate reduction much faster than elemental sulfur alone, suggesting that electron donor limitations control sulfide production. Furthermore, the combined addition of glycerol and elemental sulfur (S(0)) resulted in the highest level of sulfide production. This indicates that S(0) can play a significant role as an electron acceptor in further promoting sulfide production when a suitable electron donor is present. Microbial community analysis revealed that Desulfosporosinus acididurans, a previously discovered acid-tolerant SRB, was enriched and became the dominant species in incubations with glycerol only (~76-96% abundance) or the combination of glycerol and S(0) (~93-99% abundance).

Acidophilic sulphate-reducing bacteria: Diversity, ecophysiology, and applications

Acidophilic sulphate-reducing bacteria (aSRB) are widespread anaerobic microorganisms that perform dissimilatory sulphate reduction and have key adaptations to tolerate acidic environments (pH <5.0), such as proton impermeability and Donnan potential. This diverse prokaryotic group is of interest from physiological, ecological, and applicational viewpoints. In this review, we summarize the interactions between aSRB and other microbial guilds, such as syntrophy, and their roles in the biogeochemical cycling of sulphur, iron, carbon, and other elements. We discuss the biotechnological applications of aSRB in treating acid mine drainage (AMD, pH <3), focusing on their ability to produce biogenic sulphide and precipitate metals, particularly in the context of utilizing microbial consortia instead of pure isolates. Metal sulphide nanoparticles recovered after AMD treatment have multiple potential technological uses, including in electronics and biomedicine, contributing to a cost-effective circular economy. The products of aSRB metabolisms, such as biominerals and isotopes, could also serve as biosignatures to understand ancient and extant microbial life in the universe. Overall, aSRB are active components of the sulphur and carbon cycles under acidic conditions, with potential natural and technological implications for the world around us.

Biogenic Sulfide-Mediated Iron Reduction at Low pH

Acid mine drainage (AMD) pollutes natural waters, but some impacted systems show natural attenuation. We sought to identify the biogeochemical mechanisms responsible for the natural attenuation of AMD. We hypothesized that biogenic sulfide-mediated iron reduction is one mechanism and tested this in an experimental model system. We found sulfate reduction occurred under acidic conditions and identified a suite of sulfate-reducing bacteria (SRB) belonging to the groups Desulfotomaculum, Desulfobacter, Desulfovibrio, and Desulfobulbus. Iron reduction was not detected in microcosms when iron-reducing bacteria or SRB were selectively inhibited. SRB also did not reduce iron enzymatically. Rather, the biogenic sulfide produced by SRB was found to be responsible for the reduction of iron at low pH. Addition of organic substrates and nutrients stimulated iron reduction and increased the pH. X-ray diffraction and an electron microprobe analysis revealed that the polycrystalline, black precipitate from SRB bioactive samples exhibited a greater diversity of iron chalcogenide minerals with reduced iron oxidation states, and minerals incorporating multiple metals compared to abiotic controls. The implication of this study is that iron reduction mediated by biogenic sulfide may be more significant than previously thought in acidic environments. This study not only describes an additional mechanism by which SRB attenuate AMD, which has practical implications for AMD-impacted sites, but also provides a link between the biogeochemical cycling of iron and sulfur.

Efficient cadmium removal from industrial wastewater generated from smelter using chemical precipitation and oxidation assistance

The effective treatment of cadmium (Cd) in smelting wastewater is of great industrial importance. This study investigates the efficient removal of Cd from real industrial smelting wastewater via chemical precipitation using a series of experiments. In particular, the effects of different precipitants, agitation conditions, and the addition of NaOCl on Cd removal and pH variation are investigated. CaO (3.75 g/L), NaOH (3.50 g/L), and Ca(OH)2 (3.75 g/L) are found to be effective in elevating the wastewater pH and achieving high Cd removal rates (>99.9%), while the use of NaOH as a precipitant maintains a high Cd removal rate even at low agitation intensities. The properties of the produced sludge and supernatant are also determined using moisture content, particle size, and sludge leaching analyses due to the importance of economic and environmental sustainability in filtration, dewatering, and waste disposal processes. In addition, the addition of 2% NaOCl is tested, revealing that it can improve the Cd removal efficiency of Ca(OH)2, thus potentially reducing processing costs and enhancing the environmental benefits. Overall, these findings offer valuable insights into the removal of Cd from smelting wastewater, with potential implications for both environmental sustainability and economic viability. PRACTITIONER POINTS: CaO, NaOH, and Ca(OH)2 effectively remove Cd (>99.9%) from smelting wastewater. The use of NaOH leads to high Cd removal rates even at low agitation speeds. Adding 2% NaOCl can reduce the Ca(OH)2 dose for more economical Cd removal.

Enhanced efficiencies on purifying acid mine drainage in constructed wetlands based on synergistic adsorption of attapulgite-soda residue composites and microbial sulfate reduction

Constructed wetlands (CWs) are a promising approach for treating acid mine drainage (AMD). However, the extreme acidity and high loads of heavy metals in AMD can easily lead to the collapse of CWs without proper pre-treatment. Therefore, it is considered essential to maintain efficient and stable performance for AMD treatment in CWs. In this study, pre-prepared attapulgite-soda residue (ASR) composites were used to improve the substrate of CWs. Compared with CWs filled with gravel (CWs-G), the removal efficiencies of sulfate and Fe, Mn, Cu, Zn Cd and Pb in CWs filled with ASR composites (CWs-ASR) were increased by 30% and 10-70%, respectively. These metals were mainly retained in the substrate in stable forms, such as carbonate-, Fe/Mn (oxide)hydroxide-, and sulfide-bound forms. Additionally, higher levels of photosynthetic pigments and antioxidant enzyme activities in plants, along with a richer microbial community, were observed in CWs-ASR than in CWs-G. The application of ASR composites alleviated the adverse effects of AMD stresses on wetland plants and microorganisms. In return, the increased bacteria abundance, particularly SRB genera (e.g., Thermodesulfovibrionia and Desulfobacca), promoted the formation of metal sulfides, enabling the saturated ASR adsorbed with metals to regenerate and continuously capture heavy metals. The synergistic adsorption of ASR composites and microbial sulfate reduction maintained the stable and efficient operation of CWs. This study contributes to the resource utilization of industrial alkaline by-products and promotes the breakthrough of new techniques for low-cost and passive treatment systems such as CWs.

Research Enhancing Acidic Mine Wastewater Purification: Innovations in Red Mud-Loess

This study investigates the adsorption of cadmium (Cd) by red mud-loess mixed materials and assesses the influence of quartz sand content on permeability. Shear tests are conducted using various pore solutions to analyze shear strength parameters. The research validates solidification methods for cadmium-contaminated soils and utilizes SEM-EDS, FTIR, and XRD analysis to elucidate remediation mechanisms. The findings suggest that the quartz sand content crucially affects the permeability of fine-grained red mud-loess mixtures. The optimal proportion of quartz sand is over 80%, significantly enhancing permeability, reaching a coefficient of 6.7 × 10-4 cm/s. Insufficient quartz sand content of less than 80% fails to meet the barrier permeability standards, leading to a reduced service life of the engineered barrier. Adsorption tests were conducted using various pore solutions, including distilled water, acidic solutions, and solutions containing Cd, to evaluate the adsorption capacity and shear characteristics of the red mud-loess mixture. Additionally, the study examines the behavior of Cd-loaded red mud-loess mixtures in various pore solutions, revealing strain-hardening trends and alterations in cohesiveness and internal friction angle with increasing Cd concentrations. The analysis of cement-red mud-loess-solidified soil demonstrates enhancements in soil structure and strength over time, attributed to the formation of crystalline structures and mineral formations induced by the curing agent. These findings provide valuable insights into the remediation of cadmium-contaminated soils.

Prediction of heavy metal removal performance of sulfate-reducing bacteria using machine learning

A robust modeling approach for predicting heavy metal removal by sulfate-reducing bacteria (SRB) is currently missing. In this study, four machine learning models were constructed and compared to predict the removal of Cd, Cu, Pb, and Zn as individual ions by SRB. The CatBoost model exhibited the best predictive performance across the four subsets, achieving R2 values of 0.83, 0.91, 0.92, and 0.83 for the Cd, Cu, Pb, and Zn models, respectively. Feature analysis revealed that temperature, pH, sulfate concentration, and C/S (the mass ratio of chemical oxygen demand to sulfate) had significant impacts on the outcomes. These features exhibited the most effective metal removal at 35 °C and sulfate concentrations of 1000-1200 mg/L, with variations observed in pH and C/S ratios. This study introduced a new modeling approach for predicting the treatment of metal-containing wastewater by SRB, offering guidance for optimizing operational parameters in the biological sulfidogenic process.

Improved Formation of Biomethane by Enriched Microorganisms from Different Rank Coal Seams

The influence of enrichment of culturable microorganisms in in situ coal seams on biomethane production potential of other coal seams has been rarely studied. In this study, we enriched culturable microorganisms from three in situ coal seams with three coal ranks and conducted indoor anaerobic biomethane production experiments. Microbial community composition, gene functions, and metabolites in different culture units by 16S rRNA high-throughput sequencing combined with liquid chromatography-mass spectrometry-time-of-flight (LC-MS-TOF). The results showed that biomethane production in the bituminous coal group (BC)cc resulted in the highest methane yield of 243.3 μmol/g, which was 12.3 times higher than that in the control group (CK). Meanwhile, Methanosarcina was the dominant archaeal genus in the three experimental groups (37.42 ± 11.16-52.62 ± 2.10%), while its share in the CK was only 2.91 ± 0.48%. Based on the functional annotation, the relative abundance of functional genes in the three experimental groups was mainly related to the metabolism of nitrogen-containing heterocyclic compounds such as purines and pyrimidines. Metabolite analysis showed that enriched microorganisms promoted the degradation of a total of 778 organic substances in bituminous coal, including 55 significantly different metabolites (e.g., purines and pyrimidines). Based on genomic and metabolomic analyses, this paper reconstructed the heterocyclic compounds degradation coupled methane metabolism pathway and thereby preliminarily elucidated that enriched culturable bacteria from different coal-rank seams could promote the degradation of bituminous coal and intensify biogenic methane yields.

Acetic acid stress response of the acidophilic sulfate reducer Acididesulfobacillus acetoxydans

Acid mine drainage (AMD) waters are a severe environmental threat, due to their high metal content and low pH (pH <3). Current technologies treating AMD utilize neutrophilic sulfate-reducing microorganisms (SRMs), but acidophilic SRM could offer advantages. As AMDs are low in organics these processes require electron donor addition, which is often incompletely oxidized into organic acids (e.g., acetic acid). At low pH, acetic acid is undissociated and toxic to microorganisms. We investigated the stress response of the acetotrophic Acididesulfobacillus acetoxydans to acetic acid. A. acetoxydans was cultivated in bioreactors at pH 5.0 (optimum). For stress experiments, triplicate reactors were spiked until 7.5 mM of acetic acid and compared with (non-spiked) triplicate reactors for physiological, transcriptomic, and membrane lipid changes. After acetic acid spiking, the optical density initially dropped, followed by an adaptation phase during which growth resumed at a lower growth rate. Transcriptome analysis revealed a downregulation of genes involved in glutamate and aspartate synthesis following spiking. Membrane lipid analysis revealed a decrease in iso and anteiso fatty acid relative abundance; and an increase of acetyl-CoA as a fatty acid precursor. These adaptations allow A. acetoxydans to detoxify acetic acid, creating milder conditions for other microorganisms in AMD environments.

Experimental study on the treatment of AMD by SRB immobilized particles containing "active iron" system

The inhibition and toxicity of high acidity and heavy metals on sulfate-reducing bacteria in acid mine drainage (AMD) were targeted. Highly active SRB immobilized particles were prepared using SRB, warm sticker wastes (iron powders), corncobs, and Maifan stones as the main matrix materials, employing microbial immobilization technology. The repair ability and reusability of highly active immobilized particles for AMD were explored. The results indicate that the adaptability of immobilized particles to AMD varied under different initial conditions, such as pH, Mn2+, and SO42-. The adsorption process of immobilized particles on Mn2+ follows the quasi-second-order kinetic model, suggesting that it involves both physical and chemical adsorption. The maximum adsorption capacity of immobilized particles for Mn2+ is 3.878 mg/g at a concentration of 2.0 mg/L and pH 6. On the other hand, the reduction process of immobilized particles on SO42- adheres to the first-order reaction kinetics, indicating that the reduction of SO42- is primarily driven by the dissimilation reduction of SRB. The maximum reduction rate of SO42- by immobilized particles is 94.23% at a concentration of 800 mg/L and pH 6. A layered structure with a flocculent appearance formed on the surface of the immobilized particles. The structure's characteristics were found to be consistent with sulfate green rust (FeII4FeIII2(OH)12SO4·8H2O). The chemisorption, ion exchange, dissimilation reduction, and surface complexation occurring between the matrices in the immobilized particles can enhance the alkalinity of AMD and decrease the concentration of heavy metals and sulfates. These results are expected to offer novel insights and materials for the treatment of AMD using biological immobilization technology, as well as improve our understanding of the mechanisms behind biological and abiotic enhanced synergistic decontamination.

Elemental sulfur redox bioconversion for selective recovery of phosphorus from Fe/Al-bound phosphate-rich anaerobically digested sludge: Sulfur oxidation or sulfur reduction?

The biological oxidation of elemental sulfur (S0) to sulfate and the reduction of S0 to sulfide provide a potential route for extracting and reclaiming phosphorus (P) from anaerobically digested sludge (ADS). However, the treatment performance, stability, and cost-effectiveness of the two opposing bioprocesses based on S° for selective P recovery from ADS remain unclear. This study aimed to compare the roles of S0-oxidizing bacteria (S0OB) and S0-reducing bacteria (S0RB) in liberating insoluble P from ADS through single-batch and consecutive multibatch experiments. Changes in P speciation in the sludge during the biological extraction processes were analyzed by using complementary sequential extraction and P X-ray absorption near-edge spectroscopy. Results showed that S0OB treatment extracted more phosphate from the sludge compared with S0RB treatment, but it also released a considerable amount of metal cations (e.g., heavy metals, Mg2+, Al3+, Ca2+) and negatively affected sludge dewaterability due to intense sludge acidification and cell lysis. At pH 1.2, the S0OB treatment released 92.9% of P from the sludge, with the dissolution of HAP, Fe-PO4, Mg3(PO4)2, and P-fehrrihy contributing 26.8%, 22.1%, 12.8%, and 10.5%, respectively. The S0RB treatment released 63.6% of P from the sludge at pH 7.0, with negligible dissolution of metal cations, thereby avoiding costly purification of the extract and alkali neutralization for pH adjustment. This treatment involved the replacement of phosphates bounded with Fe-PO4 (FePO4 and P-fehrrihy) and Al-PO4 (P-Alumina and AlPO4) with biogenic sulfides, with contributions of 72.7%, and 20.9%, respectively. Consecutive bioprocesses for P extraction were achieved by recirculating the treated sludge. Both S0OB and S0RB treatments did not affect the extent of sludge dewatering but considerably weakened the dewatering rate. The S0OB-treated sludge exhibited prolonged filtration time (from 3010 s to 9150 s) and expressing time (from 795 s to 4690 s) during compression dewatering. After removing metal cations using cation exchange resin (CER) and neutralizing using NaOH, a vivianite product Fe3(PO4)2·8H2O (purity: 84%) was harvested from the S0OB-treated extract through precipitation with FeSO4·7H2O. By contrast, a vivianite product Fe3(PO4)2·8H2O (purity: 81%) was directly obtained from the S0RB-treated extract through precipitation with FeSO4·7H2O. Ultimately, 79.8 and 57.9wt% of P were recovered from ADS through S0OB extraction-CER purification-alkali neutralization-vivianite crystallization, and S0RB extraction-vivianite crystallization, respectively. Collectively, biological S0 reduction is more applicable than biological S0 oxidation for selectively reclaiming P from Fe/Al-associated phosphate-rich ADS due to better cost-effectiveness and process simplicity. These findings are of significance for developing sludge management strategies to improve P reclamation with minimal process inputs.

Global trends and future prospects of acid mine drainage research

The uncontrolled release of acid mine drainage (AMD) results in the ongoing deterioration of groundwater and surface water, along with harmful impacts on aquatic ecosystems and surrounding habitats. This study employed a bibliometric analysis to examine research activities and trends related to AMD from 1991 to 2021. The analysis demonstrated a consistent growth in AMD research over the years, with a notable surge in the number of publications starting from 2014. Applied Geochemistry and Science of the Total Environment emerged as the top two extensively published journals in the field of AMD research. The USA held a prominent position, achieving the highest h-index (96) and central value (0.36) among 111 countries/territories, with China and Spain following closely behind. The author keyword analysis provides an overview of the main focuses in AMD research. Furthermore, the co-citation reference analysis reveals four primary domains of AMD research. Moreover, the prevention and remediation of AMD, including source prevention and migration control, as well as the hazards posed by heavy metals/metalloids and the mechanisms and techniques employed for their removal, are discussed in detail.

Microbial functionalities and immobilization of environmental lead: Biogeochemical and molecular mechanisms and implications for bioremediation

The increasing environmental and human health concerns about lead in the environment have stimulated scientists to search for microbial processes as innovative bioremediation strategies for a suite of different contaminated media. In this paper, we provide a compressive synthesis of existing research on microbial mediated biogeochemical processes that transform lead into recalcitrant precipitates of phosphate, sulfide, and carbonate, in a genetic, metabolic, and systematics context as they relate to application in both laboratory and field immobilization of environmental lead. Specifically, we focus on microbial functionalities of phosphate solubilization, sulfate reduction, and carbonate synthesis related to their respective mechanisms that immobilize lead through biomineralization and biosorption. The contributions of specific microbes, both single isolates or consortia, to actual or potential applications in environmental remediation are discussed. While many of the approaches are successful under carefully controlled laboratory conditions, field application requires optimization for a host of variables, including microbial competitiveness, soil physical and chemical parameters, metal concentrations, and co-contaminants. This review challenges the reader to consider bioremediation approaches that maximize microbial competitiveness, metabolism, and the associated molecular mechanisms for future engineering applications. Ultimately, we outline important research directions to bridge future scientific research activities with practical applications for bioremediation of lead and other toxic metals in environmental systems.

Approaches to mitigation of hydrogen sulfide during anaerobic digestion process - A review

Anaerobic digestion (AD) is the primary technology for energy production from wet biomass under a limited oxygen supply. Various wastes rich in organic content have been renowned for enhancing the process of biogas production. However, several other intermediate unwanted products such as hydrogen sulfide, ammonia, carbon dioxide, siloxanes and halogens have been generated during the process, which tends to lower the quality and quantity of the harvested biogas. The removal of hydrogen sulfide from wastewater, a potential substrate for anaerobic digestion, using various technologies is covered in this study. It is recommended that microaeration would increase the higher removal efficiency of hydrogen sulfide based on a number of benefits for the specific method. The process is primarily accomplished by dosing smaller amounts of oxygen in the digester, which increases the system's oxidizing capacity by rendering the sulfate reducing bacteria responsible for converting sulfate ions to hydrogen sulfide inactive. This paper reviews physicochemical and biological methods that have been in place to eliminate the effects of hydrogen sulfide from wastewater treated anaerobically and future direction to remove hydrogen sulfide from biogas produced.

Relationship between different physiological processes of Tomato seedlings exposed to acid mine water Uncovered using correlation analysis

This study was conducted to assess the correlation between growth response, phytoaccumulation factor of different tissues, and elemental composition in tomato seedlings exposed to acid mine water (AMW). In pairwise correlation determinations values of plant height, stem diameter, seed germination indices (radicle length, final germination percentage (FGP), emergency rate index (ERI), vigour index (VI), germination percentage (G%) and germination rate index (GRI)) and the elemental compositions (Cd, Cr, Cu, Ni and Zn) in the different plant tissues, root (root accumulation factor = RAF), stem (stem translocation factor = STF) and leaves (leaf translocation factor = LTF) were selected for the relationship determinations. Pearson correlation coefficients were calculated and revealed the relationships between the paired parameters. The study concluded that the strongly correlated physiological parameters were jointly co-ordinated in tomato seedlings exposed to AMW.

Past, present and future trends in the remediation of heavy-metal contaminated soil - Remediation techniques applied in real soil-contamination events

Most worldwide policy frameworks, including the United Nations Sustainable Development Goals, highlight soil as a key non-renewable natural resource which should be rigorously preserved to achieve long-term global sustainability. Although some soil is naturally enriched with heavy metals (HMs), a series of anthropogenic activities are known to contribute to their redistribution, which may entail potentially harmful environmental and/or human health effects if certain concentrations are exceeded. If this occurs, the implementation of rehabilitation strategies is highly recommended. Although there are many publications dealing with the elimination of HMs using different methodologies, most of those works have been done in laboratories and there are not many comprehensive reviews about the results obtained under field conditions. Throughout this review, we examine the different methodologies that have been used in real scenarios and, based on representative case studies, we present the evolution and outcomes of the remediation strategies applied in real soil-contamination events where legacies of past metal mining activities or mine spills have posed a serious threat for soil conservation. So far, the best efficiencies at field-scale have been reported when using combined strategies such as physical containment and assisted-phytoremediation. We have also introduced the emerging problem of the heavy metal contamination of agricultural soils and the different strategies implemented to tackle this problem. Although remediation techniques used in real scenarios have not changed much in the last decades, there are also encouraging facts for the advances in this field. Thus, a growing number of mining companies publicise in their webpages their soil remediation strategies and efforts; moreover, the number of scientific publications about innovative highly-efficient and environmental-friendly methods is also increasing. In any case, better cooperation between scientists and other soil-related stakeholders is still required to improve remediation performance.

Transport and fate of Cu and Cd in contaminated paddy soil under acid mine drainage

Long-term heavy metals accumulation caused by acid mine drainage (AMD) irrigation in paddy soils poses a severe threat to environmental health. However, the soil adsorption mechanisms under AMD flooding remain unclear. This study provides key insights into the fate of heavy metals in soil, particularly the retention and mobility mechanisms of Cu (copper) and Cd (cadmium) after AMD flooding. The migration and fate of Cu and Cd in uncontaminated paddy soils treated by AMD in Dabaoshan Mining area was investigated via column leaching experiments in the laboratory. The maximum adsorption capacities of Cu (658.04 mg kg^-1) and Cd (335.20 mg kg^-1) cations were predicted and the breakthrough curves were fitted using the Thomas and Yoon-Nelson models. Our findings demonstrated that Cd was more mobile than Cu. Furthermore, the soil had a greater adsorption capacity for Cu than Cd. Tessier's five-step extraction method was used to determine Cu and Cd fractions in leached soils at different depths and times. After AMD leaching, the relative and absolute concentrations of the easily mobile forms were all increased at different soil depths, thus increasing the potential risk to the groundwater system. Mineralogical characterization of the soil indicated that AMD flooding leads to the formation of mackinawite. This study provides insights into the distribution and transportation processes of soil Cu and Cd and their ecological effects under AMD flooding, as well as a theoretical basis for the establishment of corresponding geochemical evolution models and environmental governance in mining areas.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Synergistic between autotrophic and heterotrophic microorganisms for denitrification using bio-S as electron donor

In recent years, biological sulfur (bio-S) was employed in sulfur autotrophic denitrification (SAD) in which autotrophic Thiobacillus denitrificans and heterotrophic Stenotrophomonas maltophilia played a key role. The growth pattern of T.denitrificans and S.maltophilia exhibited a linear relationship between OD₆₀₀ and CFU when OD₆₀₀ < 0.06 and <0.1, respectively. When S.maltophilia has applied alone, the NorBC and NosZ were undetected, and denitrification was incomplete. The DsrA of S.maltophilia could produce sulfide as an alternative electron donor for T.denitrificans. Even though T.denitrificans had complete denitrification genes, its efficiency was low when used alone. The interaction of T.denitrificans and S.maltophilia reduced nitrite accumulation, leading to complete denitrification. A sufficient quantity of S.maltophilia may trigger the autotrophic denitrification activity of T.denitrificans. When the colony-forming units (CFU) ratio of S.maltophilia to T.denitrificans was reached at 2:1, the highest denitrification performance was achieved at 2.56 and 12.59 times higher than applied alone. This research provides a good understanding of the optimal microbial matching for the future application of bio-S.

Impacts of phosphorus and nitrogen absence on microbial diversity and sulfate removal in anaerobic batch reactors

Sulfate-rich effluents have been successfully treated in anaerobic reactors using sulfate-reducing bacteria (SRB). Many authors have demonstrated that these systems require nitrogen and phosphorous supplementation to achieve high sulfate removal rates. However, the resource ratio theory assumes that some species can be dominant according to the nutritional relations used or even without external nutrient supplementation. Thus, this study evaluated the SRB communities in batch reactors without external nitrogen and phosphorus sources based on most probable number (MPN) quantification, denaturing gradient gel electrophoresis (DGGE) analyses and sequencing. The sulfate and chemical oxygen demand (COD) removal and kinetic parameters were also determined. After 100 days of operation, the sulfate and COD removal achieved 71.8 ± 10% and 86.5 ± 10%, respectively. The SRB population increased from 8.10⁶ to 4 × 10¹² MPN 100 mL^-1, and the richness of SRB bands was much higher at the end of the experiment compared to the inoculum. In addition, the sequenced bands from SRB-DGGE showed similarities to Desulfacinum infernum, Desulfobulbus sp, Syntrophobacter and Desulfomicrobium aestuarii-related sequences. Therefore, biological treatment of acid mine drainage wastewater was effective in the absence of nutrients, lowering costs and providing high sulfate removal efficiency.

Laboratory simulation of the swampy forest system for the passive treatment of acid mine drainage in coal mine reclamation areas

Coal mining that applies the open pit method has the potential to generate acid mine drainage. Acid mine drainage (AMD) treatments must include processes to mitigate significant challenges; these treatments include active treatment with high costs and process uncertainty and passive treatment with its limitations. The new concept of the swampy forest system involves the development of passive treatment for AMD, which lowers costs, increases capacity, and provides a natural process to mitigate the AMD that has been generated. A laboratory simulation experiment was carried out to obtain the basic data required for the swampy forest system treatment. The basic reference data determined in this study, including the total volume of water, the water debt flows into the swampy forest scale laboratory system and the retention time, were obtained to bring the parameter values that failed to meet the quality standards into compliance, according to the applicable regulations. The AMD swampy forest treatment design in the pilot project at the treatment field can apply a scaled-up version of the basic data from the simulation laboratory experiment results.

The key role of biogenic arsenic sulfides in the removal of soluble arsenic and propagation of arsenic mineralizing communities

Biogeochemical processes govern the transport and availability of arsenic in sediments. However, little is known about the transition from indigenous communities to cultivable consortia when exposed to high arsenic concentrations. Such cultivable communities could be exploited for arsenic bioremediation of waste streams and polluted sites. Thus, it is crucial to understand the dynamics and selective pressures that shape the communities during the development of customized bacterial consortia. First, from the arsenic partitioning of two sediments with high arsenic concentrations, we found that up to 55% of arsenic was bioavailable because it was associated with the soluble, carbonate, and ionically exchangeable fractions. Next, we prepared sediment enrichment cultures under arsenate- and sulfate-reducing conditions to precipitate arsenic sulfide biominerals and analyze the communities. The produced biominerals were used as the inoculum to develop bacterial consortia via successive transfers. Tracking of the 16S rRNA gene in the fresh sediments, sediment enrichments, biogenic minerals, and bacterial consortia revealed differences in the bacterial communities. Removing the sediment caused a substantial decrease in diversity and shifts toward the dominance of the Firmicutes phylum to the detriment of Proteobacteria. In agreement with the 16S rRNA gene results, the sequencing of the arrA gene confirmed the presence of phylotypes closely related to Desulfosporosinus sp. Y5 (100% similarity), highlighting the pivotal role of this genus in the removal of soluble arsenic. Here, we demonstrated for the first time that besides being important as arsenic sinks, the biogenic arsenic sulfide minerals are reservoirs of arsenic resistant/respiring bacteria and can be used to culture them.

Acidophilic microorganisms in remediation of contaminants present in extremely acidic conditions

Acidophiles are a group of microorganisms that thrive in acidic environments where pH level is far below the neutral value 7.0. They belong to a larger family called extremophiles, which is a group that thrives in various extreme environmental conditions which are normally inhospitable to other organisms. Several human activities such as mining, construction and other industrial processes release highly acidic effluents and wastes into the environment. Those acidic wastes and wastewaters contain different types of pollutants such as heavy metals, radioactive, and organic, whose have adverse effects on human being as well as on other living organisms. To protect the whole ecosystem, those pollutants containing effluents or wastes must be clean properly before releasing into environment. Physicochemical cleanup processes under extremely acidic conditions are not always successful due to high cost and release of toxic byproducts. While in case of biological methods, except acidophiles, no other microorganisms cannot survive in highly acidic conditions. Therefore, acidophiles can be a good choice for remediation of different types of contaminants present in acidic conditions. In this review article, various roles of acidophilic microorganisms responsible for removing heavy metals and radioactive pollutants from acidic environments were discussed. Bioremediation of various acidic organic pollutants by using acidophiles was also studied. Overall, this review could be helpful to extend our knowledge as well as to do further relevant novel studies in the field of acidic pollutants remediation by applying acidophilic microorganisms.

Iron‑sulfur geochemistry and acidity retention in hydrologically active macropores of boreal acid sulfate soils: Effects of mitigation suspensions of fine-grained calcite and peat

Acid sulfate soils discharge large amounts of sulfuric acid along with toxic metals, deteriorating water quality and ecosystem health of recipient waterbodies. There is thus an urgent need to develop cost-effective and sustainable measures to mitigate the negative effects of these soils. In this study, we flushed aseptically-prepared MQ water (reference) or mitigation suspensions containing calcite, peat or a combination of both through 15-cm-thick soil cores from an acid sulfate soil field in western Finland, and investigated the geochemistry of Fe and S on the surfaces of macropores and in the solid columnar blocks (interiors) of the soil columns. The macropore surfaces of all soil columns were strongly enriched in total and HCl-extractable Fe and S relative to the interiors, owing to the existence of abundant Fe oxyhydroxysulfates (schwertmannite and partly jarosite) as yellow-to-brownish surface-coatings. The dissolution/hydrolysis of Fe oxyhydroxysulfates (predominantly jarosite) on the macropore surfaces of the reference columns, although being constantly flushed, effectively buffered the permeates at pH close to 4. These results suggest that Fe oxyhydroxysulfates accumulated on the macropore surfaces of boreal acid sulfate soils can act as long-lasting acidification sources. The treatments with mitigation suspensions led to a (near-)complete conversion of jarosite to Fe hydroxides, causing a substantial loss of S. In contrast, we did not observe any recognizable evidence indicating transformation of schwertmannite. However, sulfate sorbed by this mineral might be partially lost through anion-exchange processes during the treatments with calcite. No Fe sulfides were found in the peat-treated columns. Since Fe sulfides can support renewed acidification events, the moderate mineralogical changes induced by peat are desirable. In addition, peat materials can act as toxic-metal scavengers. Thus, the peat materials used here, which is relatively cheap in the boreal zone, is ideal for remediating boreal acid sulfate soils and other similar jarosite-bearing soils.

Insights into the effects of operating parameters on sulfate reduction performance and microbial pathways in the anaerobic sequencing batch reactor

Sulfate-reducing bacteria (SRB)-based anaerobic process has aroused wide concern in the treatment of sulfate-containing wastewater. Chemical oxygen demand-to-sulfate ratio (COD/SO42-) and HRT are two key factors that affect not only the anaerobic treatment performance but also the activity of SRB. In this study, an anaerobic sequencing batch reactor was constructed, and the effects of different operating parameters (COD/SO42-, HRT) on the relationship of sulfate (SO42-) reduction performance, microbial communities, and metabolic pathways were comprehensively investigated. The results indicated that the SO42- removal rates could achieve above 95% under different operating parameters. Bioinformatics analysis revealed that microbial community changed with reactor operation. At the genus level, the enrichment of Propionicclava and Peptoclostridium contributed to the establishment of a homotrophic relationship with Desulfobulbus, the dominant SRB in the reactor, which indicated that they took vital part in maintaining the structural and functional stability of the bacterial community under different operating parameters. In particular, an increasing trend of the relative abundance of functional genes encoding dissimilatory sulfate reduction was detected with the increase of COD/SO42-, which indicated high SO42- reduction potentials. This knowledge will help to reveal the mechanism of the effect of operating parameters on the anaerobic sulfate removal process, thus providing effective guidance for the targeted regulation of anaerobic sequencing batch bioreactors treating SO42--containing wastewater.

Treatment of acid rock drainage using a sulphate-reducing bioreactor with a limestone precolumn

Sulphate reducing bacteria (SRB) offer promise for the treatment of mine waste due to their effectiveness removing toxic heavy metals as highly insoluble metal sulphides and their ability to generate alkalinity. The main objective of this study was to develop a treatment composed of a sulphate-reducing bioreactor with a limestone precolumn for the removal of Cu(II) from a synthetic ARD. The purpose of the limestone column was to increase the pH values and decrease the level of Cu in the effluent to prevent SRB inhibition. The system was fed with a pH-2.7 synthetic ARD containing Cu(II) (10-40 mg/L), sulphate (2000 mg/L) and acetate (2.5 g COD/L) for 150 days. Copper removal efficiencies in the two-stage system were very high (95-99%), with a final concentration of 0.53 mg/L Cu, and almost complete removal occurred in the limestone precolumn. In the same manner, the acidity of the synthetic ARD was effectively reduced in the limestone precolumn to 7.3 and the pH was raised in the bioreactor (7.3-8.0). COD consumption by methanogens was predominant from day 0-118, but SRB dominated at the end of the experiment (day 150) when the average COD removal and sulphide production were 74.8% and 61.7%, respectively. Study of the microbial taxonomic composition in the bioreactor revealed that Methanosarcina and Methanosaeta were the most prevalent methanogens while the genera Desulfotomaculum and Syntrophobacter were the dominant SRB. Among the SRB identified Desulfotomaculum intricatum (99% identity) and Desulfotomaculum acetoxidans (96%) were the most abundant sequences of bacteria capable of using acetate.

Comparison of two acidophilic sulfidogenic consortia for the treatment of acidic mine water

Sulfate-reducing bioreactors are a biotechnological alternative for the treatment of acid mine drainage (AMD). In this study, two separate bioreactors with pH and temperature-controlled (Bio I and II) were operated with two different acidophilic microbial consortia to determine their efficiencies in sulfate removal from a synthetic acidic mine water. The bioreactors were operated for 302 days in continuous flow mode under the same parameters: fed with a sulfate solution of ∼30 mM with a pH of 2.5, the temperature at 30°C, stirred gently at 40 rpm and using a continuous stream of nitrogen to help remove the H2S produced in the bioreactor. The glycerol consumption, acetate production, and sulfate removal were monitored throughout the course of the experiment. The community composition and potential metabolic functional groups were analyzed via 16S rRNA partial gene sequencing. Bio I consortium reduced the sulfate, achieving a range of sulfate concentration from 4.7 to 19 mM in the effluent liquor. The removal of sulfate in Bio II was between 5.6 and 18 mM. Both bioreactors’ communities showed the presence of the genus Desulfosporosinus as the main sulfate-reducing bacteria (SRB). Despite differences in microbial composition, both bioreactors have similar potential metabolism, with a higher percentage of microorganisms that can use sulfate in respiration. Overall, both bioreactors showed similar performance in treating acidic mine water containing mostly sulfate using two different acidophilic sulfidogenic consortia obtained from different global locations.

Arsenic and cadmium bioavailability to rice (Oryza sativa L.) plant in paddy soil: Influence of sulfate application

Arsenic (As) and cadmium (Cd) accumulate easily in rice grains that pose a non-negligible threat to human health worldwide. Sulfur fertilizer has been shown to affect the mobilization of As and Cd in paddy soil, but the effect of co-contamination by As and Cd has not been explored. This study selected three soils co-contaminated with As and Cd from Shangyu (SY), Tongling (TL) and Ma'anshan (MA). Incubation experiments and pot experiments were carried out to explore the effect of sulfate supply (100 mg kg^-1) on the bioavailability of As and Cd in soil and the rice growth. The results showed that the exogenous sulfate decreased As concentrations in porewater of SY and TL by 51.1% and 29.2% through forming arsenic-sulfide minerals. The exchangeable Cd in soil also declined by 25.6% and 18.6% and transformed into Fe and Mn oxides-bound Cd. The relative abundance of Desulfotomaculum, Desulfurispora and dsr gene increased remarkably indicated that sulfate addition stimulated the activity of sulfate-reducing bacteria. In MA soil, sulfate addition immobilized Cd but had little effect on As solubility, which was speculated to be related to the high sulfate background of the soil. Further pot experiments showed that sulfate application significantly increased rice tillers, biomass, chlorophyll content in shoots, and decreased electrolyte leakage in root. Finally, sulfate significantly reduced As and Cd in SY rice shoots by 60.2% and 40.8%, respectively, while As decreased by 39.6% in TL rice shoots and Cd decreased by 23.0% in MA rice shoots. These results indicate that the application of sulfate can reduce the bioavailability of As and Cd in the soil-rice system and promote rice growth, and it is possible to reduce the accumulation of As and Cd in rice plants simultaneously.

Use of lignocellulosic waste materials in the passive treatment of highly alkaline wastewater contaminated with sulfates and metals - From a laboratory study to pilot scale

Passive wastewater treatment systems are an alternative to costly and ineffective chemical wastewater treatment methods. Lignocellulosic waste materials (LWM) are often used in passive wastewater treatment systems as a cheap and accessible source of nutrients. LWM, such as spent mushroom compost and woodchips, have been implemented for the successful management of mildly alkaline effluents, which constitute a large fraction of industrial wastewater. The objective of the study was to provide an extensive study of the parameters in four types of commonly used LWM (raw and composted sawdust, spent mushroom compost and woodchips), which can be used in the planning of a passive wastewater treatment plant. LWM were shown to remove up to 90% Zn²⁺ and Pb²⁺ from a model solution and neutralize wastewater. Moreover, the LWM were inhabited by a physiologically diverse microbial consortium containing sulfate-reducing and cellulolytic microbes, which can influence the treatment process. Another purpose of this study was to construct a pilot wastewater treatment plant based on the use of LWM and gravel and to present its ability to effectively treat extremely alkaline flotation wastewater (pH = 12) originating from a lead and zinc mine located in Montenegro. The treated wastewater had a unique, but challenging chemical composition for passive treatment, as it was heavily contaminated with sulfates (∼1200 mg/L) and lead (∼1 g/L). The removal within the developed installation reached a rate of 66%, while the treated effluent, after initial neutralization, was maintained at a pH of approximately 7. Lead and zinc concentrations after treatment were also kept at levels required by Montenegrin law for wastewater disposal.

Effect of temperature change on the performance of the hybrid linear flow channel reactor and its implications on sulphate-reducing and sulphide-oxidising microbial community dynamics

Semi-passive bioremediation is a promising strategy to mitigate persistent low volume mine-impacted wastewater containing high sulphate concentrations. Building on the proof of concept demonstration of the hybrid linear flow channel reactor (LFCR), capable of simultaneous biological sulphate reduction and partial sulphide oxidation with elemental sulphur recovery, the impact of key operating parameters, such as temperature, on process performance is critical to real-world application. Temperature fluctuates seasonally and across the diurnal cycle, impacting biological sulphate reduction (BSR) and partial sulphide oxidation. The process is reliant on the metabolic activity and synergistic interactions between sulphate-reducing (SRB) and sulphide-oxidising (SOB) microbial communities that develop within discrete oxic and anoxic microenvironments within the hybrid LFCR. In this study, the impact of operating temperature on process performance was evaluated by decreasing temperature with time from 30 to 10°C in each of three laboratory-scaled hybrid LFCR units operating in pseudo-steady state at 1 g/L sulphate. Using lactate as a carbon source, two reactor sizes (2 and 8 L) were considered, while the impact of lactate vs. acetate as carbon source was evaluated in the 2 L reactors. On incremental decrease in temperature from 30 to 10°C, a decrease in volumetric sulphate reduction rate was observed: from 0.144 to 0.059 mmol/L.h in the 2 L lactate-fed reactor; from 0.128 to 0.042 mmol/L.h in the 8 L lactate-fed reactor; and from 0.127 to 0.010 mmol/L.h in the 2 L acetate-fed reactor. Similarly, sulphate conversion efficiency decreased (2 L lactate-fed: 66% to 27%; 8 L lactate-fed: 61% to 20%; 2 L acetate-fed: 61% to 5%). A decrease in temperature below the critical value (15°C) led to considerable loss in metabolic activity and overall BSR performance. Sessile and planktonic microbial communities were represented by bacterial phyla including Proteobacteria, Synergistetes, Bacteroidetes, and Firmicutes. A diverse group of putative SRB (Deltaproteobacteria) and SOB, including Alpha, Beta, Gamma, and Epsilonproteobacteria phylotypes, were prevalent and shifted in relative abundance and community composition in response to decreasing temperature. Specifically, the decrease in the relative abundance of Deltaproteobacteria with decreasing temperature below 15°C corresponded with a loss of BSR performance across all three reactors. This study demonstrated the impact of low temperature on the physiological selection and ecological differentiation of SRB and SOB communities within the hybrid LFCR and its implications for real-world process performance.

A novel steady-state model to quantitively assess the effect of pH elevation by dissimilatory sulfate reduction process in acidic waters in mining areas

Acidic waters such as groundwater, drainage and lakes in mining area contain high-strength acids and metal ions, posing serious threats to aquatic ecosystems and human health. Dissimilatory sulfate reduction (DSR)-based processes are attractive technologies for remediating acidic waters because it produces alkalinity and sulfide for metal precipitation and acid neutralization. However, the effects of pH elevation achieved by DSR-based processes are case-sensitive and difficult to be quantitively assessed, which limits the application of DSR process for acidic water remediation. Therefore, in this study, a Sulfidogenic Acid mine water Remediation Model (SARM) considering the DSR process, weak acids balance, metal sulfide and hydroxide precipitations, and gas-liquid exchanges of H₂S and CO₂, was developed to quantitatively assess the effects of various environmental factors on the pH elevation by a DSR process in acidic waters. A long-term trial of a DSR reactor was conducted to calibrate and validate the SARM. The experimental results revealed that the DSR-based process is effective to relieve acidity. The calibrated SARM demonstrated the excellent performance to predict the pH variation in the DSR reactor, under the varied conditions of influent pH and organic concentration. The calibrated SARM was further validated with data collected from literatures, and the results verified that the proposed model is capable to accurately assess the effect of DSR process on acid neutralization and metal removals under various conditions in steady state. The model was employed to systematically evaluate the impacts of environmental factors on acid remediation within a DSR-based process. The results revealed that the background alkalinity plays an important role in acid neutralization. However, with an increase in sulfate reduction, biogenic sulfide and carbonate become the dominant buffering substances to neutralize acidity. Furthermore, the SARM was used to evaluate the applicability of the DSR-based process for the remediation of acidic waters by evaluating the sulfide production thresholds for acid neutralization and metal removal. The simulation results demonstrated that, the DSR-based process is recommended for the remediation of acidic waters with low background alkalinity. Collectively, the SARM proposed in this study was found to be a useful and efficient tool for quantitatively assessing the potential of DSR-based processes for neutralizing acidic waters, which is vital for biogeochemistry and environmental engineering research.

Role of the substrate on Ni inhibition in biological sulfate reduction

In treating mine-impacted waters using sulfate-reducing bacteria (SRB), metal inhibition and substrate selection are important factors affecting the efficiency of the bioprocess. This work investigated the role of the substrate (i.e. lactate, formate, glycerol and glucose) on Ni inhibition to SRB with sulfate-reducing activity tests at initial pH 5, 7 and 9 and 100 mg/L of Ni. Results indicated that the type of substrate was a significant factor affecting Ni inhibition in SRB, which was the most negligible in the lactate system, followed by glycerol, glucose, and formate. Although less significant, Ni inhibition also varied with the pH, leading for instance, to a reduction of 77% in the sulfate reducing activity for the formate system, but only of 28% for lactate at pH 5. The added substrate also influenced the precipitation kinetics and the characteristics of the precipitates, reaching Ni precipitation extents above 95%, except for glucose (83.2%).

Comparison of different small molecular weight alcohols for sustaining sulfidogenic bioreactors maintained at moderately low pH

Sulfate-reducing bacteria (SRB) catalyse the dissimilatory reduction of sulfate to hydrogen sulfide using a wide range of small molecular weight organic compounds, and hydrogen, as electron donors. Here we report the effects of different combinations of small molecular weight alcohols on the performance and bacterial composition of a moderately low pH sulfidogenic bioreactor (pH 4.0–5.5) operated at 35°C in continuous flow mode. Ethanol alone and methanol or ethanol used in combination with glycerol were evaluated based on their equivalent amounts of carbon. Although evidenced that methanol was utilised as electron donor to fuel sulfidogenesis at pH 5.5, rates of sulfate reduction/sulfide production were negatively impacted when this alcohol was first introduced to the system, though these rates increased in subsequent phases as a result of adaptation of the microbial community. Further increased dosage of methanol again caused rates of sulfidogenesis to decrease. Methanol addition resulted in perturbations of the bioreactor microbial community, and species not previously detected were present in relatively large abundance, including the sulfate-reducer Desulfovibrio desulfuricans. Ethanol utilization was evidenced by the increase in rates of sulfidogenesis as the dosage of ethanol increased, with rates being highest when the bioreactor was fed with ethanol alone. Concentrations of acetate in the effluent liquor also increased (up to 8 mM) as a result of incomplete oxidation of ethanol. This alcohol continued to be used as the electron donor for sulfate reduction when the bioreactor pH was decreased incrementally (to pH 4.0), but rates of sulfidogenesis decreased. The relative abundance of Dv. desulfuricans diminished as the bioreactor pH was lowered, while that of the acidophilic Firmicute Desulfosporosinus acididurans increased. This study has shown that all three alcohols can be used to fuel microbial sulfidogenesis in moderately acidic liquors, though the cost-effectiveness, availability and toxicity to the microbial community will dictate the choice of substrate.

Sulfate-reducing bioreactors subjected to high sulfate loading rate or acidity: variations in microbial consortia

Sulfate-reducing bioreactors are used in e.g. the mining industry to remove sulfate and harmful metals from process waters. These bioreactors are expected to be run for extended periods of time and may experience variations in the influent quality, such as increasing sulfate loading rate and decrease in pH, while being expected to function optimally. In this study we followed the sulfate removal rate and variation in microbial communities over a period of up to 333 days in three different up-flow anaerobic sludge blanket (UASB) bioreactors being submitted to increasing sulfate loading rate or decreasing pH. Sodium lactate was used as the sole carbon source and electron donor. All three bioreactors contained highly diverse microbial communities containing archaea, fungi and bacteria. Sulfurospirillum and Desulfovibrio were the most prominent bacterial genera detected in the bioreactors receiving the highest sulfate loading rates, and the greatest relative abundance of methanogenic archaea and the fungal genus Cadophora coincided with the highest sulfate reduction rates. In contrast, Sulfuricurvum was dominant in the bioreactor receiving influent with alternating pH, but its relative abundance receded in response to low pH of the influent. All bioreactors showed excellent sulfate removal even under extreme conditions in addition to unique responses in the microbial communities under changing operational conditions. This shows that a high diversity in the microbial consortia in the bioreactors could make the sulfate removal process less sensitive to changing operational conditions, such as variations in influent sulfate loading rate and pH.

Influents with high sulfate loading rate or low pH were successfully treated.

Microbial consortia showed versatility and adaption to changing operation.

Unknown interactions among microbial groups makes interpretation challenging.

Oxytetracycline co-metabolism with denitrification/desulfurization in SRB mediated system

Aquaculture wastewater contained a high remnant of oxytetracycline (OTC) and nitrate. In this study, OTC co-metabolized with denitrification/desulfurization was investigated in terms of kinetic analysis, pathway, microbial communities and produces analysis in sulfate-reducing bacteria (SRB) mediated system. Long-term acclimatization with sulfate (300 mg-S/L) could markedly accelerate the removed rate of OTC from 0.9 to 1.4 mg/g-SS/d, with the kinetic constants increasing from 0.2760 to 0.5232 d^-1, mainly via enzymes including adenosine-5'-phos-phosulfate reductase and cytochrome P450, and non-enzymatic process related to intermediates (adenosine-5'-phos-phosulfate and S⁰). Furthermore, OTC was likely detoxified by SRB enriched sludge mainly via hydrolysis, dehydration, oxidation and reduction. The denitrification process would postpone the OTC degradation via outcompeting electron donors with the desulfurization process. Redundancy analysis suggested that sulfur-oxidizing bacteria (Acidithiobacillus, Ochrobactrum) were highly related to OTC degradation processes. This study provides deep insight and a new opportunity for the treatment of aquaculture wastewater containing OTC, sulfate and nitrate by SRB sludge.

High rate production of concentrated sulfides from metal bearing wastewater in an expanded bed hydrogenotrophic sulfate reducing bioreactor

Metallurgical wastewaters contain high concentrations of sulfate, up to 15 g L-1. Sulfate-reducing bioreactors are employed to treat these wastewaters, reducing sulfates to sulfides which subsequently co-precipitate metals. Sulfate loading and reduction rates are typically restricted by the total H2S concentration. Sulfide stripping, sulfide precipitation and dilution are the main strategies employed to minimize inhibition by H2S, but can be adversely compromised by suboptimal sulfate reduction, clogging and additional energy costs. Here, metallurgical wastewater was treated for over 250 days using two hydrogenotrophic granular activated carbon expanded bed bioreactors without additional removal of sulfides. H2S toxicity was minimized by operating at pH 8 ± 0.15, resulting in an average sulfate removal of 7.08 ± 0.08 g L-1, sulfide concentrations of 2.1 ± 0.2 g L-1 and peaks up to 2.3 ± 0.2 g L-1. A sulfate reduction rate of 20.6 ± 0.9 g L-1 d-1 was achieved, with maxima up to 27.2 g L-1 d-1, which is among the highest reported considering a literature review of 39 studies. The rates reported here are 6-8 times higher than those reported for other reactors without active sulfide removal and the only reported for expanded bed sulfate-reducing bioreactors using H2. By increasing the influent sulfate concentration and maintaining high sulfide concentrations, sulfate reducers were promoted while fermenters and methanogens were suppressed. Industrial wastewater containing 4.4 g L-1 sulfate, 0.036 g L-1 nitrate and various metals (As, Fe, Tl, Zn, Ni, Sb, Co and Cd) was successfully treated with all metal(loid)s, nitrates and sulfates removed below discharge limits.

Isolation and Genome Analysis of an Amoeba-Associated Bacterium Dyella terrae Strain Ely Copper Mine From Acid Rock Drainage in Vermont, United States

Protozoa play important roles in microbial communities, regulating populations via predation and contributing to nutrient cycling. While amoebae have been identified in acid rock drainage (ARD) systems, our understanding of their symbioses in these extreme environments is limited. Here, we report the first isolation of the amoeba Stemonitis from an ARD environment as well as the genome sequence and annotation of an associated bacterium, Dyella terrae strain Ely Copper Mine, from Ely Brook at the Ely Copper Mine Superfund site in Vershire, Vermont, United States. Fluorescent in situ hybridization analysis showed this bacterium colonizing cells of Stemonitis sp. in addition to being outside of amoebal cells. This amoeba-resistant bacterium is Gram-negative with a genome size of 5.36 Mbp and GC content of 62.5%. The genome of the D. terrae strain Ely Copper Mine encodes de novo biosynthetic pathways for amino acids, carbohydrates, nucleic acids, and lipids. Genes involved in nitrate (1) and sulfate (7) reduction, metal (229) and antibiotic resistance (37), and secondary metabolite production (6) were identified. Notably, 26 hydrolases were identified by RAST as well as other biomass degradation genes, suggesting roles in carbon and energy cycling within the microbial community. The genome also contains type IV secretion system genes involved in amoebae resistance, revealing how this bacterium likely survives predation from Stemonitis sp. This genome analysis and the association of D. terrae strain Ely Copper Mine with Stemonitis sp. provide insight into the functional roles of amoebae and bacteria within ARD environments.

Enhanced sulfur recovery and sulfate reduction using single-chamber bioelectrochemical system

The aim of this study was to investigate the feasibility of sulfate removal and elemental sulfur (S⁰) recovery in the single-chamber bioelectrochemical system (S-BES). The performance of S-BES was compared with that of dual-chamber bioelectrochemical system (D-BES). The S-BES was constructed with graphite felt as the anode and graphite brush as the cathode. The D-BES was constructed with proton exchange membrane as the separator between anode and cathode chambers. With an applied voltage of 1.0 V and 1 g/L acetate as the substrate, the S-BES and D-BES were tested by feeding with 480 mg/L SO₄^2- in the phosphate buffer. Results showed that the maximum current density of 37.6 ± 4.5 mA/m³ was reached in the S-BES, which was higher than that in the D-BES (i.e., 22.2 ± 2.6 mA/m³). The SO₄^2- removal was much higher in the S-BES than in the D-BES (99.5% vs. 57.2%). In the effluent and the electrodes of S-BES, S⁰ was identified with Raman and X- Ray diffraction analyses. The S⁰ recovery on the anode was 13.7 times of that on the cathode of S-BES, indicating that S⁰ was mainly produced on the anode. The measured total S⁰ recovery reached 67.5% in the S-BES. High relative abundance of Desulfurella (47.1%) and Geobacter (26.1%) dominated the community in the anode biofilm of S-BES. The excellent performance of S-BES may be attributed to the neutral pH in the solution and the synergistic reaction between the anode and cathode. Results from this study should be useful to enhance the S-BES applications in treating wastewater containing sulfate.

Dynamic experiments of acid mine drainage with Rhodopseudomonas spheroides activated lignite immobilized sulfate-reducing bacteria particles treatment

Aiming at the problem that the treatment of acid mine drainage (AMD) by sulfate-reducing bacteria (SRB) biological method is susceptible to pH, metal ions, sulfate and carbon source. Lignite immobilized SRB particles (SRB-LP) and Rhodopseudomonas spheroides (R. spheroides) activated lignite immobilized SRB particles (R-SRB-LP) were prepared using microbial immobilization technology with SRB, R. spheroides and lignite as the main substrates. The dynamic experimental columns 1# and 2# were constructed with SRB-LP and R-SRB-LP as fillers, respectively, to investigate the dynamic repair effect of SRB-LP and R-SRB-LP on AMD. The mechanism of AMD treated with R-L-SRB particles was analyzed by scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectrometer and low-temperature nitrogen adsorption. The result showed that the combination of R. spheroides and lignite could continuously provide carbon source for SRB, so that the highest removal rates of SO₄^2-, Cu²⁺ and Zn²⁺ in AMD by R-SRB-LP were 93.97%, 98.52% and 94.42%, respectively, and the highest pH value was 7.60. The dynamic repair effect of R-SRB-LP on AMD was significantly better than that of SRB-LP. The characterization results indicated that after R-SRB-LP reaction, the functional groups of -OH and large benzene ring structure in lignite were broken, the lignite structure was destroyed, and the specific surface area was 1.58 times larger than before reaction. It illustrated that R. spheroides provided carbon source for SRB by degrading lignite. The strong SRB activity in R-SRB-LP, SRB can co-treat AMD with lignite, so that the dynamic treatment effect of R-SRB-LP on AMD is significantly better than that of SRB-LP.

Diversity and biogenesis contribution of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits

Microbial sulfate reduction, a vital mechanism for microorganisms living in anaerobic, sulfate-rich environments, is an essential aspect of the sulfur biogeochemical cycle. However, there has been no detailed investigation of the diversity and biogenesis contribution of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits. To elucidate this issue, soil samples from representative abandoned realgar deposits were collected. Microcosm assays illustrated that all three samples (2-1, 2-2, and 2-3) displayed efficient sulfate and As(V)-respiring activities. Furthermore, a total of 28 novel sequence variants of dissimilatory sulfite reductase genes and 2 new families of dsrAB genes were successfully identified. A novel dissimilatory sulfate-reducing bacterium, Desulfotomaculum sp. JL1, was also isolated from soils, and can efficiently respiratory reduce As(V) and sulfate in 4 and 5 days, respectively. JL1 can promote the generation of yellow precipitates in the presence of multiple electron acceptors (both contain sulfate and As(V) in the cultures), which indicated the biogenesis contribution of sulfate-reducing bacteria to the realgar mine. Moreover, this area had unique microbial communities; the most abundant populations belonged to the phyla Proteobacteria, Chloroflexi, and Acidobacteriota, which were attributed to the unique geochemistry characteristics, such as total organic carbon, total As, NO₃^-, and SO₄^2-. The results of this study provide new insight into the diversity and biogenesis contributions of sulfate-reducing bacteria in arsenic-contaminated soils from realgar deposits.

Microbial Diversity and Sulfur Cycling in an Early Earth Analogue: From Ancient Novelty to Modern Commonality

Life emerged and diversified in the absence of molecular oxygen. The prevailing anoxia and unique sulfur chemistry in the Paleo-, Meso-, and Neoarchean and early Proterozoic eras may have supported microbial communities that differ from those currently thriving on the earth's surface. Zodletone spring in southwestern Oklahoma represents a unique habitat where spatial sampling could substitute for geological eras namely, from the anoxic, surficial light-exposed sediments simulating a preoxygenated earth to overlaid water column where air exposure simulates oxygen intrusion during the Neoproterozoic era. We document a remarkably diverse microbial community in the anoxic spring sediments, with 340/516 (65.89%) of genomes recovered in a metagenomic survey belonging to 200 bacterial and archaeal families that were either previously undescribed or that exhibit an extremely rare distribution on the current earth. Such diversity is underpinned by the widespread occurrence of sulfite, thiosulfate, tetrathionate, and sulfur reduction and the paucity of sulfate reduction machineries in these taxa. Hence, these processes greatly expand lineages mediating reductive sulfur-cycling processes in the tree of life. An analysis of the overlaying oxygenated water community demonstrated the development of a significantly less diverse community dominated by well-characterized lineages and a prevalence of oxidative sulfur-cycling processes. Such a transition from ancient novelty to modern commonality underscores the profound impact of the great oxygenation event on the earth's surficial anoxic community. It also suggests that novel and rare lineages encountered in current anaerobic habitats could represent taxa that once thrived in an anoxic earth but have failed to adapt to earth's progressive oxygenation. IMPORTANCE Life on earth evolved in an anoxic setting; however, the identity and fate of microorganisms that thrived in a preoxygenated earth are poorly understood. In Zodletone spring, the prevailing geochemical conditions are remarkably similar to conditions prevailing in surficial earth prior to oxygen buildup in the atmosphere. We identify hundreds of previously unknown microbial lineages in the spring and demonstrate that these lineages possess the metabolic machinery to mediate a wide range of reductive sulfur processes, with the capacity to respire sulfite, thiosulfate, sulfur, and tetrathionate, rather than sulfate, which is a reflection of the differences in sulfur-cycling chemistry in ancient versus modern times. Collectively, such patterns strongly suggest that microbial diversity and sulfur-cycling processes in a preoxygenated earth were drastically different from the currently observed patterns and that the Great Oxygenation Event has precipitated the near extinction of a wide range of oxygen-sensitive lineages and significantly altered the microbial reductive sulfur-cycling community on earth.