Desulfosporosinus acidiphilus sp. nov.: a moderately acidophilic sulfate-reducing bacterium isolated from acid mining drainage sediments

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.

Anaerobic microbial metabolism in soil columns affected by highly alkaline pH: Implication for biogeochemistry near construction and demolition waste disposal sites

Construction and demolition wastes (CDWs) have become a significant environmental concern due to urbanization. CDWs in landfill sites can generate high-pH leachate and various constituents (e.g., acetate and sulfate) following the dissolution of cement material, which may affect subsurface biogeochemical properties. However, the impact of CDW leachate on microbial reactions and community compositions in subsurface environments remains unclear. Therefore, we created columns composed of layers of concrete debris containing-soil (CDS) and underlying CDW-free soil, and fed them artificial groundwater with or without acetate and/or sulfate. In all columns, the initial pH 5.6 of the underlying soil layer rapidly increased to 10.8 (without acetate and sulfate), 10.1 (with sulfate), 10.1 (with acetate), and 8.3 (with acetate and sulfate) within 35 days. Alkaliphilic or alkaline-resistant microbes including Hydrogenophaga, Silanimonas, Algoriphagus, and/or Dethiobacter were dominant throughout the incubation in all columns, and their relative abundance was highest in the column without acetate and sulfate (50.7-86.6%). Fe(III) and sulfate reduction did not occur in the underlying soil layer without acetate. However, in the column with acetate alone, pH was decreased to 9.9 after day 85 and Fe(II) was produced with an increase in the relative abundance of Fe(III)-reducing bacteria up to 9.1%, followed by an increase in the methanogenic archaea Methanosarcina, suggestive of methanogenesis. In the column with both acetate and sulfate, Fe(III) and sulfate reduction occurred along with an increase in both Fe(III)- and sulfate-reducing bacteria (19.1 and 17.7%, respectively), while Methanosarcina appeared later. The results demonstrate that microbial Fe(III)- and sulfate-reduction and acetoclastic methanogenesis can occur even in soils with highly alkaline pH resulting from the dissolution of concrete debris.

Sulphidogenic Bioprocesses for Acid Mine Water Treatment and Selective Recovery of Arsenic and Metals

Human communities need water and mineral resources, the supply of which requires the implementation of recycling and saving strategies. Both closed and active mining sites could beneficiate of the implementation of nature-based solutions, including bioreactors involving sulphate-reducing prokaryotes (SRP), in order to separate and recover arsenic (As) and metals from aqueous stream while producing clean water. Selective precipitation strategies can be designed based on the selection of microbial communities adapted to the pH conditions, generally acidic, and to available low-cost electron donors. Laboratory batch and continuous experiments must be implemented for each type of mine water in order to determine the optimal flow-sheet in which As could be precipitated as sulphides (orpiment or realgar), inside the bioreactor or offline, through stripping of biologically produced hydrogen sulphides (H2S). The respective concentrations and proportions of As and metals and the initial acid mine drainage pH are key parameters that will influence the feasibility of efficient selective precipitation. SRP-based bioreactors could be combined with complementary treatment steps in optimised mine water management solutions that will minimise the production of As-contaminated end-solid waste.

Harnessing the potential of the microbial sulfur cycle for environmental biotechnology

The sulfur cycle is a complex biogeochemical cycle characterized by the high variability in the oxidation states of sulfur. While sulfur is essential for life processes, certain sulfur compounds, such as hydrogen sulfide, are toxic to all life forms. Micro-organisms facilitate the sulfur cycle, playing a prominent role even in extreme environments, such as soda lakes, acid mine drainage sites, hot springs, and other harsh habitats. The activity of these micro-organisms presents unique opportunities for mitigating sulfur-based pollution and enhancing the recovery of sulfur and metals. This review highlights the application of sulfur-oxidizing and -reducing micro-organisms in environmental biotechnology through three illustrative examples. Additionally, it discusses the challenges, recent trends, and prospects associated with these applications.

Acidophilic heterotrophs: basic aspects and technological applications

Acidophiles comprise a group of microorganisms adapted to live in acidic environments. Despite acidophiles are usually associated with an autotrophic metabolism, more than 80 microorganisms capable of utilizing organic matter have been isolated from natural and man-made environments. The ability to reduce soluble and insoluble iron compounds has been described for many of these species and may be harnessed to develop new or improved mining processes when oxidative bioleaching is ineffective. Similarly, as these microorganisms grow in highly acidic media and the chances of contamination are reduced by the low pH, they may be employed to implement robust fermentation processes. By conducting an extensive literature review, this work presents an updated view of basic aspects and technological applications in biomining, bioremediation, fermentation processes aimed at biopolymers production, microbial electrochemical systems, and the potential use of extremozymes.

Sustained bacterial N2O reduction at acidic pH

Nitrous oxide (N2O) is a climate-active gas with emissions predicted to increase due to agricultural intensification. Microbial reduction of N2O to dinitrogen (N2) is the major consumption process but microbial N2O reduction under acidic conditions is considered negligible, albeit strongly acidic soils harbor nosZ genes encoding N2O reductase. Here, we study a co-culture derived from acidic tropical forest soil that reduces N2O at pH 4.5. The co-culture exhibits bimodal growth with a Serratia sp. fermenting pyruvate followed by hydrogenotrophic N2O reduction by a Desulfosporosinus sp. Integrated omics and physiological characterization revealed interspecies nutritional interactions, with the pyruvate fermenting Serratia sp. supplying amino acids as essential growth factors to the N2O-reducing Desulfosporosinus sp. Thus, we demonstrate growth-linked N2O reduction between pH 4.5 and 6, highlighting microbial N2O reduction potential in acidic soils.

Anaerobic dihydrogen consumption of nutrient-limited aquifer sediment microbial communities examined by stable isotope analysis

The biogeochemical consequences of dihydrogen (H2) underground storage in porous aquifers are poorly understood. Here, the effects of nutrient limitations on anaerobic H2 oxidation of an aquifer microbial community in sediment microcosms were determined in order to evaluate possible responses to high H2 partial pressures. Hydrogen isotope analyses of H2 yielded isotope depletion in all biotic setups indicating microbial H2 consumption. Carbon isotope analyses of carbon dioxide (CO2) showed isotope enrichment in all H2-supplemented biotic setups indicating H2-dependent consumption of CO2 by methanogens or homoacetogens. Homoacetogenesis was indicated by the detection of acetate and formate. Consumption of CO2 and H2 varied along the differently nutrient-amended setups, as did the onset of methane production. Plotting carbon against hydrogen isotope signatures of CH4 indicated that CH4 was produced hydrogenotrophically and fermentatively. The putative hydrogenotrophic Methanobacterium sp. was the dominant methanogen. Most abundant phylotypes belonged to typical ferric iron reducers, indicating that besides CO2, Fe(III) was an important electron acceptor. In summary, our study provides evidence for the adaptability of subsurface microbial communities under different nutrient-deficient conditions to elevated H2 partial pressures.

A novel mechanism for dissimilatory nitrate reduction to ammonium in Acididesulfobacillus acetoxydans

The biological route of nitrate reduction has important implications for the bioavailability of nitrogen within ecosystems. Nitrate reduction via nitrite, either to ammonium (ammonification) or to nitrous oxide or dinitrogen (denitrification), determines whether nitrogen is retained within the system or lost as a gas. The acidophilic sulfate-reducing bacterium (aSRB) Acididesulfobacillus acetoxydans can perform dissimilatory nitrate reduction to ammonium (DNRA). While encoding a Nar-type nitrate reductase, A. acetoxydans lacks recognized nitrite reductase genes. In this study, A. acetoxydans was cultivated under conditions conducive to DNRA. During cultivations, we monitored the production of potential nitrogen intermediates (nitrate, nitrite, nitric oxide, hydroxylamine, and ammonium). Resting cell experiments were performed with nitrate, nitrite, and hydroxylamine to confirm their reduction to ammonium, and formed intermediates were tracked. To identify the enzymes involved in DNRA, comparative transcriptomics and proteomics were performed with A. acetoxydans growing under nitrate- and sulfate-reducing conditions. Nitrite is likely reduced to ammonia by the previously undescribed nitrite reductase activity of the NADH-linked sulfite reductase AsrABC, or by a putatively ferredoxin-dependent homolog of the nitrite reductase NirA (DEACI_1836), or both. We identified enzymes and intermediates not previously associated with DNRA and nitrosative stress in aSRB. This increases our knowledge about the metabolism of this type of bacteria and helps the interpretation of (meta)genome data from various ecosystems on their DNRA potential and the nitrogen cycle.IMPORTANCENitrogen is crucial to any ecosystem, and its bioavailability depends on microbial nitrogen-transforming reactions. Over the recent years, various new nitrogen-transforming reactions and pathways have been identified, expanding our view on the nitrogen cycle and metabolic versatility. In this study, we elucidate a novel mechanism employed by Acididesulfobacillus acetoxydans, an acidophilic sulfate-reducing bacterium, to reduce nitrate to ammonium. This finding underscores the diverse physiological nature of dissimilatory reduction to ammonium (DNRA). A. acetoxydans was isolated from acid mine drainage, an extremely acidic environment where nitrogen metabolism is poorly studied. Our findings will contribute to understanding DNRA potential and variations in extremely acidic environments.

Nutrient structure dynamics and microbial communities at the water-sediment interface in an extremely acidic lake in northern Patagonia

Lake Caviahue (37° 50 'S and 71° 06' W; Patagonia, Argentina) is an extreme case of a glacial, naturally acidic, aquatic environment (pH ~ 3). Knowledge of the bacterial communities in the water column of this lake, is incipient, with a basal quantification of the bacterioplankton abundance distribution in the North and South Basins of Lake Caviahue, and the described the presence of sulfur and iron oxidizing bacteria in the lake sediments. The role that bacterioplankton plays in nutrient utilization and recycling in this environment, especially in the phosphorus cycle, has not been studied. In this work, we explore this aspect in further depth by assessing the diversity of pelagic, littoral and sediment bacteria, using state of the art molecular methods and identifying the differences and commonalties in the composition of the cognate communities. Also, we investigate the interactions between the sediments of Lake Caviahue and the microbial communities present in both sediments, pore water and the water column, to comprehend the ecological relationships driving nutrient structure and fluxes, with a special focus on carbon, nitrogen, and phosphorus. Two major environmental patterns were observed: (a) one distinguishing the surface water samples due to temperature, Fe2+, and electrical conductivity, and (b) another distinguishing winter and summer samples due to the high pH and increasing concentrations of N-NH4+, DOC and SO42-, from autumn and spring samples with high soluble reactive phosphorus (SRP) and iron concentrations. The largest bacterial abundance was found in autumn, alongside higher levels of dissolved phosphorus, iron forms, and increased conductivity. The highest values of bacterial biomass were found in the bottom strata of the lake, which is also where the greatest diversity in microbial communities was found. The experiments using continuous flow column microcosms showed that microbial growth over time, in both the test and control columns, was accompanied by a decrease in the concentration of dissolved nutrients (SRP and N-NH4+), providing proof that sediment microorganisms are active and contribute significantly to nutrient utilization/mobilization.

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.

In situ enrichment of sulphate-reducing microbial communities with different carbon sources stimulating the acid mine drainage sediments

The applications of sulphate-reducing microorganisms (SRMs) in acid mine drainage (AMD) treatment systems have received extensive attention due to their ability to reduce sulphate and stabilize metal(loid)s. Despite great phylogenetic diversity of SRMs, only a few have been used in AMD treatment bioreactors. In situ enrichment could be an efficient approach to select new effective SRMs for AMD treatment. Here, we performed in situ enrichment of SRMs in highly stratified AMD sediment cores using different kinds of carbon source mixture. The dsrAB (dissimilatory sulfite reductase) genes affiliated with nine phyla (two archaeal and seven bacterial phyla) and 26 genera were enriched. Remarkably, those genes affiliated with Aciduliprofundum and Vulcanisaeta were enriched in situ in AMD-related environments for the first time, and their relative abundances were negatively correlated with pH. Furthermore, 107 dsrAB-containing metagenome-assembled genomes (MAGs) were recovered from metagenomic datasets, with 14 phyla (two archaeal and 12 bacterial phyla) and 15 genera. The relative abundances of MAGs were positively correlated with total carbon and sulphate contents. Our findings expanded the diversity of SRMs that can be enriched in AMD sediment, and revealed the physiochemical properties that might affect the growth of SRMs, which provided guidance for AMD treatment bioreators.

Mobility of antimony in contrasting surface environments of a mine site: influence of redox conditions and microbial communities

Microbial processes can influence the complex geochemical behaviour of the toxic metalloid antimony (Sb) in mining environments. The present study is aimed to evaluate the influence of microbial communities on the mobility of Sb from solid phases to water in different compartments and redox conditions of a mining site in southwest (SW) Spain. Samples of surface materials presenting high Sb concentrations, from two weathered mining waste dumps, and an aquatic sediment were incubated in slurries comparing oxic and anoxic conditions. The initial microbial communities of the three materials strongly differed. Incubations induced an increase of microbial biomass and an evolution of the microbial communities' structures and compositions, which diverged in different redox conditions. The presence of active bacteria always influenced the mobility of Sb, except in the neutral pH waste incubated in oxic conditions. The effect of active microbial activities in oxic conditions was dependent on the material: Sb oxic release was biologically amplified with the acidic waste, but attenuated with the sediment. Different bacterial genera involved in Sb, Fe and S oxidation or reduction were present and/or grew during incubation of each material. The results highlighted the wide diversity of microbial communities and metabolisms at the small geographic scale of a mining site and their strong implication in Sb mobility.

Enrichment of rare methanogenic Archaea shows their important ecological role in natural high-CO2 terrestrial subsurface environments

Introduction

Long-term stability of underground CO₂ storage is partially affected by microbial activity but our knowledge of these effects is limited, mainly due to a lack of sites. A consistently high flux of mantle-derived CO₂ makes the Eger Rift in the Czech Republic a natural analogue to underground CO₂ storage. The Eger Rift is a seismically active region and H₂ is produced abiotically during earthquakes, providing energy to indigenous microbial communities.

Methods

To investigate the response of a microbial ecosystem to high levels of CO₂ and H₂, we enriched microorganisms from samples from a 239.5 m long drill core from the Eger Rift. Microbial abundance, diversity and community structure were assessed using qPCR and 16S rRNA gene sequencing. Enrichment cultures were set up with minimal mineral media and H₂/CO₂ headspace to simulate a seismically active period with elevated H₂.

Results and discussion

Methane headspace concentrations in the enrichments indicated that active methanogens were almost exclusively restricted to enrichment cultures from Miocene lacustrine deposits (50-60 m), for which we observed the most significant growth. Taxonomic assessment showed microbial communities in these enrichments to be less diverse than those with little or no growth. Active enrichments were especially abundant in methanogens of the taxa Methanobacterium and Methanosphaerula. Concurrent to the emergence of methanogenic archaea, we also observed sulfate reducers with the metabolic ability to utilize H₂ and CO₂, specifically the genus Desulfosporosinus, which were able to outcompete methanogens in several enrichments. Low microbial abundance and a diverse non-CO₂ driven microbial community, similar to that in drill core samples, also reflect the inactivity in these cultures. Significant growth of sulfate reducing and methanogenic microbial taxa, which make up only a small fraction of the total microbial community, emphasize the need to account for rare biosphere taxa when assessing the metabolic potential of microbial subsurface populations. The observation that CO₂ and H₂-utilizing microorganisms could only be enriched from a narrow depth interval suggests that factors such as sediment heterogeneity may also be important. This study provides new insight on subsurface microbes under the influence of high CO₂ concentrations, similar to those found in CCS sites.

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.

Sulfur-oxidizing bacteria (SOB) and sulfate-reducing bacteria (SRB) in oil reservoir and biological control of SRB: a review

Sulfur-oxidizing bacteria (SOB) and sulfate-reducing bacteria (SRB) inhabit oilfield production systems. Sulfur oxidation driven by SOB and dissimilatory sulfate reduction driven by SRB play important roles in sulfur cycle of oil reservoirs. More importantly, hydrogen sulfide produced by SRB is an acidic, flammable, and smelly toxic gas associated with reservoir souring, corrosion of oil-production facilities, and personnel safety. Effective control of SRB is urgently needed for the oil industry. This depends on an in-depth understanding of the microbial species that drive sulfur cycle and other related microorganisms in oil reservoir environments. Here, we identified SOB and SRB in produced brines of Qizhong block (Xinjiang Oilfield, China) from metagenome sequencing data based on reported SOB and SRB, reviewed metabolic pathways of sulfur oxidation and dissimilatory sulfate reduction, and ways for SRB control. The existing issues and future research of microbial sulfur cycle and SRB control are also discussed. Knowledge of the distribution of the microbial populations, their metabolic characteristics and interactions can help to develop an effective process to harness these microorganisms for oilfield production.

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.

Glycerol amendment enhances biosulfidogenesis in acid mine drainage-affected areas: An incubation column experiment

Microbial sulfate (SO₄²⁻) reduction in Acid Mine Drainage (AMD) environments can ameliorate the acidity and extreme metal concentrations by consumption of protons via the reduction of SO₄²⁻ to hydrogen sulfide (H₂S) and the concomitant precipitation of metals as metal sulfides. The activity of sulfate-reducing bacteria can be stimulated by the amendment of suitable organic carbon sources in these generally oligotrophic environments. Here, we used incubation columns (IC) as model systems to investigate the effect of glycerol amendment on the microbial community composition and its effect on the geochemistry of sediment and waters in AMD environments. The ICs were built with natural water and sediments from four distinct AMD-affected sites with different nutrient regimes: the oligotrophic Filón Centro and Guadiana acidic pit lakes, the Tintillo river (Huelva, Spain) and the eutrophic Brunita pit lake (Murcia, Spain). Physicochemical parameters were monitored during 18 months, and the microbial community composition was determined at the end of incubation through 16S rRNA gene amplicon sequencing. SEM-EDX analysis of sediments and suspended particulate matter was performed to investigate the microbially-induced mineral (neo)formation. Glycerol amendment strongly triggered biosulfidogenesis in all ICs, with pH increase and metal sulfide formation, but the effect was much more pronounced in the ICs from oligotrophic systems. Analysis of the microbial community composition at the end of the incubations showed that the SRB Desulfosporosinus was among the dominant taxa observed in all sulfidogenic columns, whereas the SRB Desulfurispora, Desulfovibrio and Acididesulfobacillus appeared to be more site-specific. Formation of Fe³⁺ and Al³⁺ (oxy)hydroxysulfates was observed during the initial phase of incubation together with increasing pH while formation of metal sulfides (predominantly, Zn, Fe and Cu sulfides) was observed after 1–5 months of incubation. Chemical analysis of the aqueous phase at the end of incubation showed almost complete removal of dissolved metals (Cu, Zn, Cd) in the amended ICs, while Fe and SO₄²⁻ increased towards the water-sediment interface, likely as a result of the reductive dissolution of Fe(III) minerals enhanced by Fe-reducing bacteria. The combined geochemical and microbiological analyses further establish the link between biosulfidogenesis and natural attenuation through metal sulfide formation and proton consumption.

Novel Microorganisms Contribute to Biosulfidogenesis in the Deep Layer of an Acidic Pit Lake

Cueva de la Mora is a permanently stratified acidic pit lake with extremely high concentrations of heavy metals at depth. In order to evaluate the potential for in situ sulfide production, we characterized the microbial community in the deep layer using metagenomics and metatranscriptomics. We retrieved 18 high quality metagenome-assembled genomes (MAGs) representing the most abundant populations. None of the MAGs were closely related to either cultured or non-cultured organisms from the Genome Taxonomy or NCBI databases (none with average nucleotide identity >95%). Despite oxygen concentrations that are consistently below detection in the deep layer, some archaeal and bacterial MAGs mapped transcripts of genes for sulfide oxidation coupled with oxygen reduction. Among these microaerophilic sulfide oxidizers, mixotrophic Thermoplasmatales archaea were the most numerous and represented 24% of the total community. Populations associated with the highest predicted in situ activity for sulfate reduction were affiliated with Actinobacteria, Chloroflexi, and Nitrospirae phyla, and together represented about 9% of the total community. These MAGs, in addition to a less abundant Proteobacteria MAG in the genus Desulfomonile, contained transcripts of genes in the Wood-Ljungdahl pathway. All MAGs had significant genetic potential for organic carbon oxidation. Our results indicate that novel acidophiles are contributing to biosulfidogenesis in the deep layer of Cueva de la Mora, and that in situ sulfide production is limited by organic carbon availability and sulfur oxidation.

Acetate Degradation at Low pH by the Moderately Acidophilic Sulfate Reducer Acididesulfobacillus acetoxydans gen. nov. sp. nov

In acid drainage environments, biosulfidogenesis by sulfate-reducing bacteria (SRB) attenuates the extreme conditions by enabling the precipitation of metals as their sulfides, and the neutralization of acidity through proton consumption. So far, only a handful of moderately acidophilic SRB species have been described, most of which are merely acidotolerant. Here, a novel species within a novel genus of moderately acidophilic SRB is described, Acididesulfobacillus acetoxydans gen. nov. sp. nov. strain INE, able to grow at pH 3.8. Bioreactor studies with strain INE at optimum (5.0) and low (3.9) pH for growth showed that strain INE alkalinized its environment, and that this was more pronounced at lower pH. These studies also showed the capacity of strain INE to completely oxidize organic acids to CO₂, which is uncommon among acidophilic SRB. Since organic acids are mainly in their protonated form at low pH, which increases their toxicity, their complete oxidation may be an acid stress resistance mechanism. Comparative proteogenomic and membrane lipid analysis further indicated that the presence of saturated ether-bound lipids in the membrane, and their relative increase at lower pH, was a protection mechanism against acid stress. Interestingly, other canonical acid stress resistance mechanisms, such as a Donnan potential and increased active charge transport, did not appear to be active.

Spatial and Temporal Constraints on the Composition of Microbial Communities in Subsurface Boreholes of the Edgar Experimental Mine

The deep biosphere hosts uniquely adapted microorganisms overcoming geochemical extremes at significant depths within the crust of the Earth. Attention is required to understand the near subsurface and its continuity with surface systems, where numerous novel microbial members with unique physiological modifications remain to be identified. This surface-subsurface relationship raises key questions about networking of surface hydrology, geochemistry affecting near-subsurface microbial composition, and resiliency of subsurface ecosystems. Here, we apply molecular and geochemical approaches to determine temporal microbial composition and environmental conditions of filtered borehole fluid from the Edgar Experimental Mine (∼150 m below the surface) in Idaho Springs, CO. Samples were collected over a 4-year collection period from expandable packers deployed to accumulate fluid in previously drilled boreholes located centimeters to meters apart, revealing temporal evolution of borehole microbiology. Meteoric groundwater feeding boreholes demonstrated variable recharge rates likely due to a complex and undefined fracture system within the host rock. 16S rRNA gene analysis determined that unique microbial communities occupy the four boreholes examined. Two boreholes yielded sequences revealing the presence of Desulfosporosinus, Candidatus Nitrotoga, and Chelatococcus associated with endemic subsurface communities. Two other boreholes presented sequences related to nonsubsurface-originating microbiota. High concentration of sulfate along with detected sulfur reducing and oxidizing microorganisms suggests that sulfur related metabolic strategies are prominent within these near-subsurface boreholes. Overall, results indicate that microbial community composition in the near-subsurface is highly dynamic at very fine spatial scales (<20 cm) within fluid-rock equilibrated boreholes, which additionally supports the role of a relationship for surface geochemical processes infiltrating and influencing subsurface environments. IMPORTANCE The Edgar Experimental Mine, Idaho Springs, CO, provides inexpensive and open access to borehole investigations for subsurface microbiology studies. Understanding how microbial processes in the near subsurface are connected to surface hydrological influences is lacking. Investigating microbial communities of subsurface mine boreholes provides evidence of how geochemical processes are linked to biogeochemical processes within each borehole and the geochemical connectedness and mobility of surface influences. This study details microbial community composition and fluid geochemistry over spatial and temporal scales from boreholes within the Edgar Mine. These findings are relevant to biogeochemistry of near-surface mines, caves, and other voids across planetary terrestrial systems. In addition, this work can lead to understanding how microbial communities relate to both fluid-rock equilibration, and geochemical influences may enhance our understanding of subsurface molecular biological tools that aid mining economic practices to reflect biological signals for lucrative veins in the near subsurface.

Comparative analysis of brewing wastewater and lactate as carbon sources for microbial community treating acid mine drainage in anaerobic MBBR systems

This study investigated the effect of carbon sources (n = 2) on the performance of a microbial community in an anaerobic moving-bed biofilm reactor (MBBR) treating acid mine drainage (AMD). The 1.5 L anaerobic MBBR was operated across a range of hydraulic retention times - HRT's (3-18 days), using different substrates, i.e. brewing wastewater and lactate as sole carbon sources and electron donors. Maximum sulphate reduction and chemical oxygen demand (COD) consumption rate was 21.94 and 24.28 mg SO42- L^-1 h^-1, and 0.473 and 0.697 mg COD L^-1 d^-1 for brewing wastewater and lactate supplemented bioreactors, respectively, at an HRT of 3 days. The maximum COD/SO42- ratio was found to be 2.564 in the bioreactor supplemented with brewing wastewater at an HRT of 15 days. The metal removal above 70% in the system supplemented with brewing wastewater followed the order; Be²⁺ > Fe²⁺ > Sr²⁺ > Pb²⁺ > Mg²⁺ > Cu²⁺ > Zn²⁺ > Li¹⁺ > Ca²⁺ in comparison to the system supplemented with lactate, Be²⁺ > Fe²⁺ > Sr²⁺ > Mg²⁺ > Cu²⁺ > Li¹⁺ > Zn²⁺ > Pb²⁺ after an HRT of 18 days. Complete removal of beryllium (II) was observed irrespective of the carbon source used. The results clearly showed that brewing wastewater can be deployed as a nutritional supplement in environmental remediation of AMD.

Microbiology of barrier component analogues of a deep geological repository

Canada is currently implementing a site selection process to identify a location for a deep geological repository (DGR) for the long-term storage of Canada's used nuclear fuel, wherein used nuclear fuel bundles will be sealed inside copper-coated carbon steel containers, encased in highly compacted bentonite clay buffer boxes, and sealed deep underground in a stable geosphere. Because a DGR must remain functional for a million years, it is important to examine ancient natural systems that serve as analogues for planned DGR components. Specifically, studying the microbiology of natural analogue components of a DGR is important for developing an understanding of the types of microorganisms that may be able to grow and influence the long-term stability of a DGR. This study explored the abundance, viability, and composition of microorganisms in several ancient natural analogues using a combination of cultivation and cultivation-independent approaches. Samples were obtained from the Tsukinuno bentonite deposit (Japan) that formed ∼10 mya, the Opalinus Clay formation (Switzerland) that formed ∼174 mya, and Canadian shield crystalline rock from Northern Ontario that formed ∼2.7 bya. Analysis of 16S rRNA gene amplicons revealed that three of the ten Tsukinuno bentonite samples analyzed were dominated by putative aerobic heterotrophs and fermenting bacteria from the phylum Actinobacteria, whereas five of the Tsukinuno bentonite samples were dominated by sequences associated with putative acidophilic chemolithoautotrophs capable of sulfur reduction. The remaining Tsukinuno bentonite samples, the Northern Ontario rock samples, and the Opalinus Clay samples generated inconsistent replicate 16S rRNA gene profiles and were associated primarily with contaminant sequences, suggesting that the microbial profiles detected were not sample-specific but spurious. Culturable aerobic heterotroph abundances were relatively low for all Tsukinuno bentonite samples, culturable anaerobic heterotrophs were only detected in half of the Tsukinuno samples, and sulfate-reducing bacteria (SRB) were only detected in one Tsukinuno sample by cultivation. Culture-specific 16S rRNA gene profiles from Tsukinuno clay samples demonstrated the presence of phyla Bacteroidetes, Proteobacteria, Actinobacteria, and Firmicutes among aerobic heterotroph cultures and additional bacteria from the phyla Actinobacteria and Firmicutes from anaerobic heterotroph plate incubations. Only one nucleic acid sequence detected from a culture was also associated with its corresponding clay sample profile, suggesting that nucleic acids from culturable bacteria were relatively rare within the clay samples. Sequencing of DNA extracted from the SRB culture revealed that the taxon present in the culture was affiliated with the genus Desulfosporosinus, which has been found in related bentonite clay analyses. Although the crystalline rock and Opalinus Clay samples were associated with inconsistent, likely spurious 16S rRNA gene profiles, we show evidence for viable and detectable microorganisms within several Tsukinuno natural analogue bentonite samples.

Desulfosporosinus metallidurans sp. nov., an acidophilic, metal-resistant sulfate-reducing bacterium from acid mine drainage

A novel, spore-forming, acidophilic and metal-resistant sulfate-reducing bacterium, strain OL^T, was isolated from a microbial mat in a tailing dam at a gold ore mining site. Cells were slightly curved immotile rods, 0.5 µm in diameter and 2.0-3.0 µm long. Cells were stained Gram-negative, despite the Gram-positive cell structure revealed by electron microscopy of ultrathin layers. OL^T grew at pH 4.0-7.0 with an optimum at 5.5. OL^T utilised H₂, lactate, pyruvate, malate, formate, propionate, ethanol, glycerol, glucose, fructose, sucrose, peptone and tryptone as electron donors for sulfate reduction. Sulfate, sulfite, thiosulfate, nitrate and fumarate were used as electron acceptors in the presence of lactate. Elemental sulfur, iron (III), and arsenate did not serve as electron acceptors. The major cellular fatty acids were C_16:1ω7c (39.0 %) and C_16 : 0 (12.1 %). The draft genome of OL^T was 5.29 Mb in size and contained 4909 protein-coding genes. The 16S rRNA gene sequence placed OL^T within the phylum Firmicutes, class Clostridia, family Peptococcaceae, genus Desulfosporosinus. Desulfosporosinus nitroreducens 59.4B^T was the closest relative with 97.6 % sequence similarity. On the basis of phenotypic and phylogenetic characteristics, strain OL^T represents a novel species within the genus Desulfosporosinus, for which we propose the name Desulfosporosinus metallidurans sp. nov. with the type strain OL^T (=DSM 104464^T=VKM В-3021^T).

Acetotrophic sulfate-reducing consortia develop active biofilms on zeolite and glass beads in batch cultures at initial pH 3

Sulfate-reducing microbial communities remain a suitable option for the remediation of acid mine drainage using several types of carrier materials and appropriate reactor configurations. However, acetate prevails as a product derived from the incomplete oxidation of most organic substrates by sulfate reducers, limiting the efficiency of the whole process. An established sulfate-reducing consortium, able to degrade acetate at initial acidic pH (3.0), was used to develop biofilms over granular activated carbon (GAC), glass beads, and zeolite as carrier materials. In batch assays using glycerol, biofilms successfully formed on zeolite, glass beads, and GAC with sulfide production rates of 0.32, 0.26, and 0.14 mmol H₂S/L·d, respectively, but only with glass beads and zeolite, acetate was degraded completely. The planktonic and biofilm communities were determined by the 16S rRNA gene analysis to evaluate the microbial selectivity of the carrier materials. In total, 46 OTUs (family level) composed the microbial communities. Ruminococcaceae and Clostridiaceae families were present in zeolite and glass beads, whereas Peptococcaceae was mostly enriched on zeolite and Desulfovibrionaceae on glass beads. The most abundant sulfate reducer in the biofilm of zeolite was Desulfotomaculum sp., while Desulfatirhabdium sp. abounded in the planktonic community. With glass beads, Desulfovibrio sp. dominated the biofilm and the planktonic communities. Our results indicate that both materials (glass beads and zeolite) selected different key sulfate-reducing microorganisms able to oxidize glycerol completely at initial acidic pH, which is relevant for a future application of the consortium in continuous bioreactors to treat acidic streams. KEY POINTS: • Complete consumption of glycerol and acetate at acidic pH by sulfate reduction. • Glass beads and zeolite are suitable materials to form sulfate-reducing biofilms. • Acetotrophic sulfate-reducing bacteria attached to zeolite preferably.

Biogeochemical dynamics and microbial community development under sulfate- and iron-reducing conditions based on electron shuttle amendment

Iron reduction and sulfate reduction are two of the major biogeochemical processes that occur in anoxic sediments. Microbes that catalyze these reactions are therefore some of the most abundant organisms in the subsurface, and some of the most important. Due to the variety of mechanisms that microbes employ to derive energy from these reactions, including the use of soluble electron shuttles, the dynamics between iron- and sulfate-reducing populations under changing biogeochemical conditions still elude complete characterization. Here, we amended experimental bioreactors comprised of freshwater aquifer sediment with ferric iron, sulfate, acetate, and the model electron shuttle AQDS (9,10-anthraquinone-2,6-disulfonate) and monitored both the changing redox conditions as well as changes in the microbial community over time. The addition of the electron shuttle AQDS did increase the initial rate of Fe^III reduction; however, it had little effect on the composition of the microbial community. Our results show that in both AQDS- and AQDS+ systems there was an initial dominance of organisms classified as Geobacter (a genus of dissimilatory Fe^III-reducing bacteria), after which sequences classified as Desulfosporosinus (a genus of dissimilatory sulfate-reducing bacteria) came to dominate both experimental systems. Furthermore, most of the ferric iron reduction occurred under this later, ostensibly “sulfate-reducing” phase of the experiment. This calls into question the usefulness of classifying subsurface sediments by the dominant microbial process alone because of their interrelated biogeochemical consequences. To better inform models of microbially-catalyzed subsurface processes, such interactions must be more thoroughly understood under a broad range of conditions.

Physiological, genomic, and sulfur isotopic characterization of methanol metabolism by Desulfovibrio carbinolicus

Methanol is often considered as a non-competitive substrate for methanogenic archaea, but an increasing number of sulfate-reducing microorganisms (SRMs) have been reported to be capable of respiring with methanol as an electron donor. A better understanding of the fate of methanol in natural or artificial anaerobic systems thus requires knowledge of the methanol dissimilation by SRMs. In this study, we describe the growth kinetics and sulfur isotope effects of Desulfovibrio carbinolicus, a methanol-oxidizing sulfate-reducing deltaproteobacterium, together with its genome sequence and annotation. D. carbinolicus can grow with a series of alcohols from methanol to butanol. Compared to longer-chain alcohols, however, specific growth and respiration rates decrease by several fold with methanol as an electron donor. Larger sulfur isotope fractionation accompanies slowed growth kinetics, indicating low chemical potential at terminal reductive steps of respiration. In a medium containing both ethanol and methanol, D. carbinolicus does not consume methanol even after the cessation of growth on ethanol. Among the two known methanol dissimilatory systems, the genome of D. carbinolicus contains the genes coding for alcohol dehydrogenase but lacks enzymes analogous to methanol methyltransferase. We analyzed the genomes of 52 additional species of sulfate-reducing bacteria that have been tested for methanol oxidation. There is no apparent relationship between phylogeny and methanol metabolizing capacity, but most gram-negative methanol oxidizers grow poorly, and none carry homologs for methyltransferase (mtaB). Although the amount of available data is limited, it is notable that more than half of the known gram-positive methanol oxidizers have both enzymatic systems, showing enhanced growth relative to the SRMs containing only alcohol dehydrogenase genes. Thus, physiological, genomic, and sulfur isotopic results suggest that D. carbinolicus and close relatives have the ability to metabolize methanol but likely play a limited role in methanol degradation in most natural environments.

Sulfate-reducing bacterial community shifts in response to acid mine drainage in the sediment of the Hengshi watershed, South China

Sulfate-reducing bacteria (SRB) are an attractive option for treating acid mine drainage (AMD) and are considered to be of great significance in the natural attenuation of AMD, but the available information regarding the highly diverse SRB community in AMD sites is not comprehensive. The Hengshi River, which is continually contaminated by AMD from upstream mining areas, was selected as a study site for investigation of the distribution, diversity, and abundance of SRB. Overall, high-throughput sequencing of the 16S rRNA and dsrB genes revealed the high diversity, richness, and OTU numbers of SRB communities, suggesting the existence of active sulfate reduction in the study area. Further analysis demonstrated that AMD contamination decreased the richness and diversity of the microbial community and SRB community, and led to spatiotemporal shifts in the overall composition and structure of sediment microbial and SRB communities along the Hengshi watershed. However, the sulfate reduction activity was high in the midstream, even though AMD pollution remained heavy in this area. Spatial distributions of SRB community indicated that species of Clostridia may be more tolerant of AMD contamination than other species, because of their predominance in the SRB communities. In addition, the results of CCA revealed that environmental parameters, such as pH, TS content, and Fe content, can significantly influence total microbial and SRB community structure, and dissolved organic carbon was another important factor structuring the SRB community. This study extends our knowledge of the distribution of indigenous SRB communities and their potential roles in natural AMD attenuation.

Optimising Brewery-Wastewater-Supported Acid Mine Drainage Treatment vis-à-vis Response Surface Methodology and Artificial Neural Network

This study investigated the use of brewing wastewater (BW) as the primary carbon source in the Postgate medium for the optimisation of sulphate reduction in acid mine drainage (AMD). The results showed that the sulphate-reducing bacteria (SRB) consortium was able to utilise BW for sulphate reduction. The response surface methodology (RSM)/Box–Behnken design optimum conditions found for sulphate reduction were a pH of 6.99, COD/SO42− of 2.87, and BW concentration of 200.24 mg/L with predicted sulphate reduction of 91.58%. Furthermore, by using an artificial neural network (ANN), a multilayer full feedforward (MFFF) connection with an incremental backpropagation network and hyperbolic tangent as the transfer function gave the best predictive model for sulphate reduction. The ANN optimum conditions were a pH of 6.99, COD/SO42− of 0.50, and BW concentration of 200.31 mg/L with predicted sulphate reduction of 89.56%. The coefficient of determination (R2) and absolute average deviation (AAD) were estimated as 0.97 and 0.046, respectively, for RSM and 0.99 and 0.011, respectively, for ANN. Consequently, ANN was a better predictor than RSM. This study revealed that the exclusive use of BW without supplementation with refined carbon sources in the Postgate medium is feasible and could ensure the economic sustainability of biological sulphate reduction in the South African environment, or in any semi-arid country with significant brewing activity and AMD challenges.

High Representation of Archaea Across All Depths in Oxic and Low-pH Sediment Layers Underlying an Acidic Stream

Parys Mountain or Mynydd Parys (Isle of Anglesey, United Kingdom) is a mine-impacted environment, which accommodates a variety of acidophilic organisms. Our previous research of water and sediments from one of the surface acidic streams showed a high proportion of archaea in the total microbial community. To understand the spatial distribution of archaea, we sampled cores (0-20 cm) of sediment and conducted chemical analyses and taxonomic profiling of microbiomes using 16S rRNA gene amplicon sequencing in different core layers. The taxonomic affiliation of sequencing reads indicated that archaea represented between 6.2 and 54% of the microbial community at all sediment depths. Majority of archaea were associated with the order Thermoplasmatales, with the most abundant group of sequences being clustered closely with the phylotype B_DKE, followed by "E-plasma," "A-plasma," other yet uncultured Thermoplasmatales with Ferroplasma and Cuniculiplasma spp. represented in minor proportions. Thermoplasmatales were found at all depths and in the whole range of chemical conditions with their abundance correlating with sediment Fe, As, Cr, and Mn contents. The bacterial microbiome component was largely composed in all layers of sediment by members of the phyla Proteobacteria, Actinobacteria, Nitrospirae, Firmicutes, uncultured Chloroflexi (AD3 group), and Acidobacteria. This study has revealed a high abundance of Thermoplasmatales in acid mine drainage-affected sediment layers and pointed at these organisms being the main contributors to carbon, and probably to iron and sulfur cycles in this ecosystem.

Biosulfidogenesis Mediates Natural Attenuation in Acidic Mine Pit Lakes

Acidic pit lakes are abandoned open pit mines filled with acid mine drainage (AMD)-highly acidic, metalliferous waters that pose a severe threat to the environment and are rarely properly remediated. Here, we investigated two meromictic, oligotrophic acidic mine pit lakes in the Iberian Pyrite Belt (IPB), Filón Centro (Tharsis) (FC) and La Zarza (LZ). We observed a natural attenuation of acidity and toxic metal concentrations towards the lake bottom, which was more pronounced in FC. The detection of Cu and Zn sulfides in the monimolimnion of FC suggests precipitation of dissolved metals as metal sulfides, pointing to biogenic sulfide formation. This was supported by microbial diversity analysis via 16S rRNA gene amplicon sequencing of samples from the water column, which showed the presence of sulfidogenic microbial taxa in FC and LZ. In the monimolimnion of FC, sequences affiliated with the putative sulfate-reducing genus Desulfomonile were dominant (58%), whereas in the more acidic and metal-enriched LZ, elemental sulfur-reducing Acidianus and Thermoplasma spp., and disproportionating Desulfocapsa spp. were more abundant. Furthermore, the detection of reads classified as methanogens and Desulfosporosinus spp., although at low relative abundance, represents one of the lowest pH values (2.9 in LZ) at which these taxa have been reported, to our knowledge. Analysis of potential biomarker lipids provided evidence that high levels of phosphocholine lipids with mixed acyl/ether glycerol core structures were associated with Desulfomonile, while ceramide lipids were characteristic of Microbacter in these environments. We propose that FC and LZ function as natural bioremediation reactors where metal sulfide precipitation is mediated by biosulfidogenesis starting from elemental sulfur reduction and disproportionation at an early stage (LZ), followed by sulfate reduction at a later stage (FC).

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.

Enrichment and isolation of acid-tolerant sulfate-reducing microorganisms in the anoxic, acidic hot spring sediments from Copahue volcano, Argentina

The geothermal Copahue-Caviahue (GCC) system (Argentina) is an extreme acidic environment, dominated by the activity of Copahue volcano. Environments characterised by low pH values, such as volcanic areas, are of particular interest for the search of acidophilic microorganisms with application in biotechnological processes. In this work, sulfate-reducing microorganisms were investigated in geothermal acidic, anaerobic zones from GCC system. Sediment samples from Agua del Limón (AL1), Las Máquinas (LMa2), Las Maquinitas (LMi) and Baño 9 (B9-2, B9-3) were found to be acidic (pH values 2.1-3.0) to moderate acidic (5.1-5.2), containing small total organic carbon values, and ferric iron precipitates. The organic electron donor added to the enrichment was completely oxidised to CO2. Bacteria related to 'Desulfobacillus acidavidus' strain CL4 were found to be dominant (67-83% of the total number of clones) in the enrichment cultures, and their presence was confirmed by their isolation on overlay plates. Other bacteria were also detected with lower abundance (6-20% of the total number of clones), with representatives of the genera Acidithiobacillus, Sulfobacillus, Alicyclobacillus and Athalassotoga/Mesoaciditoga. These enrichment and isolates found at low pH confirm the presence of anaerobic activities in the acidic sediments from the geothermal Copahue-Caviahue system.

Isolation, characterization and bioaugmentation of an acidotolerant 1,2-dichloroethane respiring Desulfitobacterium species from a low pH aquifer

A Desulfitobacterium sp. strain AusDCA of the Peptococcaceae family capable of respiring 1,2-dichloroethane (1,2-DCA) to ethene anaerobically with ethanol or hydrogen as electron donor at pH 5.0 with optimal range between pH 6.5-7.5 was isolated from an acidic aquifer near Sydney, Australia. Strain AusDCA is distant (94% nucleotide identity) from its nearest phylogenetic neighbor, D. metallireducens, and could represent a new species. Reference gene-based quantification of growth indicated a doubling time of 2 days in cultures buffered at pH 7.2, and a yield of 7.66 (± 4.0) × 106 cells µmol-1 of 1,2-DCA. A putative 1,2-DCA reductive dehalogenase was translated from a dcaAB locus and had high amino acid identity (97.3% for DcaA and 100% for DcaB) to RdhA1B1 of the 1,2-DCA respiring Dehalobacter strain WL. Proteomic analysis confirmed DcaA expression in the pure culture. Dehalogenation of 1,2-DCA (1.6 mM) was observed in batch cultures established from groundwater at pH 5.5 collected 38 days after in situ bioaugmentation but not in cultures established with groundwater collected at the same time from wells not receiving bioaugmentation. Overall, strain AusDCA can tolerate lower pH than previously characterized organohalide respiring bacteria and remained viable in groundwater at pH 5.5.

Reductive dissolution of jarosite by a sulfate reducing bacterial community: Secondary mineralization and microflora development

Jarosite is an iron-hydroxysulfate mineral commonly found in acid mine drainage (AMD). Given its strong adsorption capacity and its ability to co-precipitation with heavy metals, jarosite is considered a potent scavenger of contaminants in AMD-impacted environments. Sulfate-reducing bacteria (SRB) play an important role in the reductive dissolution of jarosite; however, the mechanism involved has yet to be elucidated. In this study, an indigenous SRB community enriched from the Dabaoshan mine area (Guangdong, China) was employed to explore the mechanism of the microbial reduction of jarosite. Different cultures, with or without dissolved sulfate and the physical separation of jarosite from bacteria by dialysis bags, were examined. Results indicate that the reduction of jarosite by SRB occurred via an indirect mechanism. In systems with dissolved sulfate, lactate was incompletely oxidized to acetate coupled with the reduction of SO₄^2- to S^2-, which subsequently reduced the Fe³⁺ in jarosite, forming secondary minerals including vivianite, mackinawite and pyrite. In systems without dissolved sulfate, jarosite dissolution occurred prior to reduction, and similar secondary minerals formed as well. Extracellular polymeric substances secreted by SRB appeared to facilitate the release of sulfate from jarosite. Structural sulfate in the solid phase of jarosite may not be available for SRB respiration. Although direct contact between SRB and jarosite is not necessary for mineral reduction, wrapping jarosite into dialysis bags suppressed the reduction to a certain extent. Microbial community composition differed in direct contact treatments and physical separation treatments. Physical separation of the SRB community from jarosite mineral supported the growth of Citrobacter, while Desulfosporosinus dominated in direct contact treatments.

Dissimilatory reduction of sulfate and zero-valent sulfur at low pH and its significance for bioremediation and metal recovery

Redox transformations of sulfur, involving dissimilatory and assimilatory oxidation and reduction reactions, occurs in water bodies and terrestrial environments worldwide, leading to dynamic cycling of this element throughout the biosphere. In cases where zero-valent (elemental) sulfur, sulfate and other oxidized forms are used as electron acceptor in (primarily) anaerobic microbial metabolisms, the end product is hydrogen sulfide (HS^- or H₂S, dependent on pH). While neutrophilic and alkalophilic sulfidogenic prokaryotes have been known for many decades, acid-tolerant and acidophilic strains and species have been isolated and characterized only in the past twenty or so years, even though evidence for sulfide generation on these environments was previously well documented. This review outlines the background and current status of the biodiversity and metabolisms of sulfate- and sulfur-reducing prokaryotes that are metabolically active in low pH environments, and describes the developing technologies in which they are being used to remediate acidic waste waters (which are often metal-contaminated) and to recover metal resources.

Microbial Communities Associated With Passive Acidic Abandoned Coal Mine Remediation

Acid mine drainage (AMD) is an environmental issue that can be characterized by either acidic or circumneutral pH and high dissolved metal content in contaminated waters. It is estimated to affect roughly 3000 miles of waterways within the state of Pennsylvania, with half being acidic and half being circumneutral. To negate the harmful effects of AMD, ∼300 passive remediation systems have been constructed within the state of Pennsylvania. In this study, we evaluated the microbial community structure and functional capability associated with Middle Branch passive remediation system in central PA. Sediment and water samples were collected from each area within the passive remediation system and its receiving stream. Environmental parameters associated with the remediation system were found to explain a significant amount of variation in microbial community structure. This study revealed shifts in microbial community structure from acidophilic bacteria in raw AMD discharge to a more metabolically diverse set of taxa (i.e., Acidimicrobiales, Rhizobiales, Chthoniobacteraceae) toward the end of the system. Vertical flow ponds and the aerobic wetland showed strong metabolic capability for sulfur redox environments. These findings are integral to the understanding of designing effective passive remediation systems because it provides insight as to how certain bacteria [sulfate reducing bacteria (SRBs) and sulfur oxidizing bacteria (SOBs)] are potentially contributing to a microbially mediated AMD remediation process. This study further supports previous investigations that demonstrated the effectiveness of SRBs in the process of removing sulfate and heavy metals from contaminated water.

Chitin as a substrate for the biostimulation of sulfate-reducing bacteria in the treatment of mine-impacted water (MIW)

This study aims to know the basis of sulfate-reducing bacteria (SRB) and chitin source relationship for the development of a biotreatment system for mine-impacted water (MIW). The MIW consists of river water impacted by coal acid mine drainage (AMD), an extremely acid effluent, rich in sulfate and dissolved metal ions, with a high pollutant potential. Chitin was used as metal ion sorbent and biostimulant of SRB, whose anaerobic dissimilatory metabolism reduces sulfate to sulfide. Microcosms were built in an oxygen-free atmosphere using chitin from two different sources: commercial chitin and shrimp shell waste, which contains calcium carbonate, an acidity removal agent, in addition to chitin. The results indicate that the shrimp shell performs best in removing sulfate (99.75%), iron (99.04%), aluminum (98.47%), and manganese (100%) ions. The iron ion sorption kinetics of the sediments were also studied; pseudo-second order behavior was observed. High-throughput sequencing analysis revealed the present bacterial community and its abundance in the microcosms after 11 and 30 treatment days: SRB were detected but were not the majority. Thus, this research aims to contribute to the sustainable treatment MIW through the employment of an abundant and low-cost biomaterial.

Microbial Communities of Polymetallic Deposits' Acidic Ecosystems of Continental Climatic Zone With High Temperature Contrasts

Acid mine drainage (AMD) systems are globally widespread and are an important source of metal pollution in riverine and coastal systems. Microbial AMD communities have been extensively studied for their ability to thrive under extremely acidic conditions and for their immense contribution to the dissolution of metal ores. However, little is known on microbial inhabitants of AMD systems subjected to extremely contrasting continental seasonal temperature patterns as opposed to maritime climate zones, experiencing much weaker annual temperature variations. Here, we investigated three types of AMD sites in Eastern Transbaikalia (Russia). In this region, all surface water bodies undergo a deep and long (up to 6 months) freezing, with seasonal temperatures varying between -33 and +24°C, which starkly contrasts the common well-studied AMD environments. We sampled acidic pit lake (Sherlovaya Gora site) located in the area of a polymetallic deposit, acidic drainage water from Bugdaya gold-molybdenum-tungsten deposit and Ulan-Bulak natural acidic spring. These systems showed the abundance of bacteria-derived reads mostly affiliated with Actinobacteria, Acidobacteria, Alpha- and Gammaproteobacteria, chloroplasts, Chloroflexi, Bacteroidetes, and Firmicutes. Furthermore, candidate taxa "Ca. Saccharibacteria" (previously known as TM7), "Ca. Parcubacteria" (OD1) and WPS-2 were represented in substantial quantities (10-20%). Heterotrophy and iron redox cycling can be considered as central processes of carbon and energy flow for majority of detected bacterial taxa. Archaea were detected in low numbers, with Terrestrial Miscellaneous Euryarchaeal Group (TMEG), to be most abundant (3%) in acidic spring Ulan-Bulak. Composition of these communities was found to be typical in comparison to other AMD sites; however, certain groups (as Ignavibacteriae) could be specifically associated with this area. This study provides insight into the microbial diversity patterns in acidic ecosystems formed in areas of polymetallic deposits in extreme continental climate zone with contrasting temperature parameters.

Recent advances in dissimilatory sulfate reduction: From metabolic study to application

Sulfate-reducing bacteria (SRB) are a group of diverse anaerobic microorganisms omnipresent in natural habitats and engineered environments that use sulfur compounds as the electron acceptor for energy metabolism. Dissimilatory sulfate reduction (DSR)-based techniques mediated by SRB have been utilized in many sulfate-containing wastewater treatment systems worldwide, particularly for acid mine drainage, groundwater, sewage and industrial wastewater remediation. However, DSR processes are often operated suboptimally and disturbances are common in practical application. To improve the efficiency and robustness of SRB-based processes, it is necessary to study SRB metabolism and operational conditions. In this review, the mechanisms of DSR processes are reviewed and discussed focusing on intracellular and extracellular electron transfer with different electron donors (hydrogen, organics, methane and electrodes). Based on the understanding of the metabolism of SRB, responses of SRB to environmental stress (pH-, temperature-, and salinity-related stress) are summarized at the species and community levels. Application in these stressed conditions is discussed and future research is proposed. The feasibility of recovering energy and resources such as biohydrogen, hydrocarbons, polyhydroxyalkanoates, magnetite and metal sulfides through the use of SRB were investigated but some long-standing questions remain unanswered. Linking the existing scientific understanding and observations to practical application is the challenge as always for promotion of SRB-based techniques.

Long-Term Transcriptional Activity at Zero Growth of a Cosmopolitan Rare Biosphere Member

Microbial diversity in the environment is mainly concealed within the rare biosphere (all species with <0.1% relative abundance). While dormancy explains a low-abundance state very well, the mechanisms leading to rare but active microorganisms remain elusive. We used environmental systems biology to genomically and transcriptionally characterize "Candidatus Desulfosporosinus infrequens," a low-abundance sulfate-reducing microorganism cosmopolitan to freshwater wetlands, where it contributes to cryptic sulfur cycling. We obtained its near-complete genome by metagenomics of acidic peat soil. In addition, we analyzed anoxic peat soil incubated under in situ-like conditions for 50 days by Desulfosporosinus-targeted qPCR and metatranscriptomics. The Desulfosporosinus population stayed at a constant low abundance under all incubation conditions, averaging 1.2 × 106 16S rRNA gene copies per cm³ soil. In contrast, transcriptional activity of "Ca. Desulfosporosinus infrequens" increased at day 36 by 56- to 188-fold when minor amendments of acetate, propionate, lactate, or butyrate were provided with sulfate, compared to the no-substrate-control. Overall transcriptional activity was driven by expression of genes encoding ribosomal proteins, energy metabolism, and stress response but not by expression of genes encoding cell growth-associated processes. Since our results did not support growth of these highly active microorganisms in terms of biomass increase or cell division, they had to invest their sole energy for maintenance, most likely counterbalancing acidic pH conditions. This finding explains how a rare biosphere member can contribute to a biogeochemically relevant process while remaining in a zero-growth state over a period of 50 days.IMPORTANCE The microbial rare biosphere represents the largest pool of biodiversity on Earth and constitutes, in sum of all its members, a considerable part of a habitat's biomass. Dormancy or starvation is typically used to explain the persistence of low-abundance microorganisms in the environment. We show that a low-abundance microorganism can be highly transcriptionally active while remaining in a zero-growth state for at least 7 weeks. Our results provide evidence that this zero growth at a high cellular activity state is driven by maintenance requirements. We show that this is true for a microbial keystone species, in particular a cosmopolitan but permanently low-abundance sulfate-reducing microorganism in wetlands that is involved in counterbalancing greenhouse gas emissions. In summary, our results provide an important step forward in understanding time-resolved activities of rare biosphere members relevant for ecosystem functions.

Co-culture of a Novel Fermentative Bacterium, Lucifera butyrica gen. nov. sp. nov., With the Sulfur Reducer Desulfurella amilsii for Enhanced Sulfidogenesis

Biosulfidogenesis can be used to remediate low pH and high metal content waters such as acid mine drainage and recover the present metals. The selection of a cheap electron donor for the process is important for the economic viability. In this work we isolated a novel versatile acidotolerant fermentative bacterium (strain ALE^T) that is able to use a great variety of substrates including glycerol. Strain ALE^T is an obligate anaerobe, and cells are motile, rod-shaped, spore-forming, and stain Gram-positive. Growth occurred in a pH range from 3.5 to 7 (optimum 5.5), and temperature range from 25 to 40°C (optimum 37°C). It grows by fermentation of sugars, organic acids and glycerol. It has the ability to use thiosulfate, iron and DMSO as electron acceptors. Its genome is 4.7 Mb with 5122 protein-coding sequences, and a G+C content of 46.9 mol%. Based on 16S rRNA gene sequence analysis, the closest cultured species is Propionispora hippei (91.4% 16S rRNA gene identity) from the Sporomusaceae family (Selenomonadales order, Negativicutes class, Firmicutes phylum). Based on the distinctive physiological and phylogenetic characteristics of strain ALE^T, a new genus and species Lucifera butyrica gen. nov., sp. nov., is proposed. The type strain is ALE^T (=JCM 19373^T = DSM 27520^T). Strain ALE^T is an incomplete oxidizer and acetate, among other products, accumulates during glycerol conversion. Strain ALE^T was used to extend the substrate range for sulfur reduction by constructing co-cultures with the acetate oxidizer and sulfur reducer Desulfurella amilsii. The co-culture was tested with glycerol as substrate in batch and chemostat experiments. Acetate formed by fermentation of glycerol by strain ALE^T resulted in sulfur reduction by D. amilsii. The co-culture strategy offers good perspectives to use a wide range of cost-efficient substrates, including glycerol, to produce sulfide by specialized sulfur reducers. The recovery of heavy metals from metalliferous streams may become economically feasible by this approach.

Note: The locus tag for the genes encoded in Lucifera butyrica is LUCI_^∗. To avoid repetition of the prefix along the text, the locus tags are represented by the specific identifier.

Thermodesulfobium sp. strain 3baa, an acidophilic sulfate reducing bacterium forming biofilms triggered by mineral precipitation

Sulfate reducing prokaryotes are promising candidates for the remediation of acidic metal-rich waste waters. However, only few acidophilic species have been described to date. Chemolithoautotrophic strain 3baa was isolated from sediments of an acidic mine pit lake. Based on its 16S-rRNA gene sequence it belongs to the genus Thermodesulfobium. It was identified as an acidophile growing in artificial pore water medium in the range of pH 2.6-6.6. Though the highest sulfate reduction rates were obtained at the lower end of this range, elongated cells and extended lag phases demonstrated acid stress. Sulfate reduction at low pH was accompanied by the formation of mineral precipitates strongly adhering to solid surfaces. A structural investigation by laser scanning microscopy, electron microscopy and X-ray microanalysis revealed the formation of Al hydroxides and Fe sulfides which were densely populated by cells. Al hydroxides precipitated first, enabling initial cell attachment. Colonization of solid surfaces coincided with increased sulfate reducing activity indicating more favourable growth conditions within biofilms compared with free-living cells. These findings point out the importance of cell-mineral interaction for biofilm formation and contribute to our understanding how sulfate reducing prokaryotes thrive in both natural and engineered systems at low pH.

MEBS, a software platform to evaluate large (meta)genomic collections according to their metabolic machinery: unraveling the sulfur cycle

The increasing number of metagenomic and genomic sequences has dramatically improved our understanding of microbial diversity, yet our ability to infer metabolic capabilities in such datasets remains challenging. We describe the Multigenomic Entropy Based Score pipeline (MEBS), a software platform designed to evaluate, compare, and infer complex metabolic pathways in large “omic” datasets, including entire biogeochemical cycles. MEBS is open source and available through https://github.com/eead-csic-compbio/metagenome_Pfam_score. To demonstrate its use, we modeled the sulfur cycle by exhaustively curating the molecular and ecological elements involved (compounds, genes, metabolic pathways, and microbial taxa). This information was reduced to a collection of 112 characteristic Pfam protein domains and a list of complete-sequenced sulfur genomes. Using the mathematical framework of relative entropy (H^΄), we quantitatively measured the enrichment of these domains among sulfur genomes. The entropy of each domain was used both to build up a final score that indicates whether a (meta)genomic sample contains the metabolic machinery of interest and to propose marker domains in metagenomic sequences such as DsrC (PF04358). MEBS was benchmarked with a dataset of 2107 non-redundant microbial genomes from RefSeq and 935 metagenomes from MG-RAST. Its performance, reproducibility, and robustness were evaluated using several approaches, including random sampling, linear regression models, receiver operator characteristic plots, and the area under the curve metric (AUC). Our results support the broad applicability of this algorithm to accurately classify (AUC = 0.985) hard-to-culture genomes (e.g., Candidatus Desulforudis audaxviator), previously characterized ones, and metagenomic environments such as hydrothermal vents, or deep-sea sediment. Our benchmark indicates that an entropy-based score can capture the metabolic machinery of interest and can be used to efficiently classify large genomic and metagenomic datasets, including uncultivated/unexplored taxa.

Evaluation of Primers Targeting the Diazotroph Functional Gene and Development of NifMAP - A Bioinformatics Pipeline for Analyzing nifH Amplicon Data

Diazotrophic microorganisms introduce biologically available nitrogen (N) to the global N cycle through the activity of the nitrogenase enzyme. The genetically conserved dinitrogenase reductase (nifH) gene is phylogenetically distributed across four clusters (I-IV) and is widely used as a marker gene for N2 fixation, permitting investigators to study the genetic diversity of diazotrophs in nature and target potential participants in N2 fixation. To date there have been limited, standardized pipelines for analyzing the nifH functional gene, which is in stark contrast to the 16S rRNA gene. Here we present a bioinformatics pipeline for processing nifH amplicon datasets - NifMAP ("NifH MiSeq Illumina Amplicon Analysis Pipeline"), which as a novel aspect uses Hidden-Markov Models to filter out homologous genes to nifH. By using this pipeline, we evaluated the broadly inclusive primer pairs (Ueda19F-R6, IGK3-DVV, and F2-R6) that target the nifH gene. To evaluate any systematic biases, the nifH gene was amplified with the aforementioned primer pairs in a diverse collection of environmental samples (soils, rhizosphere and roots samples, biological soil crusts and estuarine samples), in addition to a nifH mock community consisting of six phylogenetically diverse members. We noted that all primer pairs co-amplified nifH homologs to varying degrees; up to 90% of the amplicons were nifH homologs with IGK3-DVV in some samples (rhizosphere and roots from tall oat-grass). In regards to specificity, we observed some degree of bias across the primer pairs. For example, primer pair F2-R6 discriminated against cyanobacteria (amongst others), yet captured many sequences from subclusters IIIE and IIIL-N. These aforementioned subclusters were largely missing by the primer pair IGK3-DVV, which also tended to discriminate against Alphaproteobacteria, but amplified sequences within clusters IIIC (affiliated with Clostridia) and clusters IVB and IVC. Primer pair Ueda19F-R6 exhibited the least bias and successfully captured diazotrophs in cluster I and subclusters IIIE, IIIL, IIIM, and IIIN, but tended to discriminate against Firmicutes and subcluster IIIC. Taken together, our newly established bioinformatics pipeline, NifMAP, along with our systematic evaluations of nifH primer pairs permit more robust, high-throughput investigations of diazotrophs in diverse environments.

Sulfate Transporters in Dissimilatory Sulfate Reducing Microorganisms: A Comparative Genomics Analysis

The first step in the sulfate reduction pathway is the transport of sulfate across the cell membrane. This uptake has a major effect on sulfate reduction rates. Much of the information available on sulfate transport was obtained by studies on assimilatory sulfate reduction, where sulfate transporters were identified among several types of protein families. Despite our growing knowledge on the physiology of dissimilatory sulfate-reducing microorganisms (SRM) there are no studies identifying the proteins involved in sulfate uptake in members of this ecologically important group of anaerobes. We surveyed the complete genomes of 44 sulfate-reducing bacteria and archaea across six phyla and identified putative sulfate transporter encoding genes from four out of the five surveyed protein families based on homology. We did not find evidence that ABC-type transporters (SulT) are involved in the uptake of sulfate in SRM. We speculate that members of the CysP sulfate transporters could play a key role in the uptake of sulfate in thermophilic SRM. Putative CysZ-type sulfate transporters were present in all genomes examined suggesting that this overlooked group of sulfate transporters might play a role in sulfate transport in dissimilatory sulfate reducers alongside SulP. Our in silico analysis highlights several targets for further molecular studies in order to understand this key step in the metabolism of SRMs.

Recent Developments for Remediating Acidic Mine Waters Using Sulfidogenic Bacteria

Acidic mine drainage (AMD) is regarded as a pollutant and considered as potential source of valuable metals. With diminishing metal resources and ever-increasing demand on industry, recovering AMD metals is a sustainable initiative, despite facing major challenges. AMD refers to effluents draining from abandoned mines and mine wastes usually highly acidic that contain a variety of dissolved metals (Fe, Mn, Cu, Ni, and Zn) in much greater concentration than what is found in natural water bodies. There are numerous remediation treatments including chemical (lime treatment) or biological methods (aerobic wetlands and compost bioreactors) used for metal precipitation and removal from AMD. However, controlled biomineralization and selective recovering of metals using sulfidogenic bacteria are advantageous, reducing costs and environmental risks of sludge disposal. The increased understanding of the microbiology of acid-tolerant sulfidogenic bacteria will lead to the development of novel approaches to AMD treatment. We present and discuss several important recent approaches using low sulfidogenic bioreactors to both remediate and selectively recover metal sulfides from AMD. This work also highlights the efficiency and drawbacks of these types of treatments for metal recovery and points to future research for enhancing the use of novel acidophilic and acid-tolerant sulfidogenic microorganisms in AMD treatment.