Long-Term Incubation Reveals Methanogenic Biodegradation of C5 and C6 iso-Alkanes in Oil Sands Tailings

Uncovering Anaerobic Hydrocarbon Biodegradation Pathways in Oil Sands Tailings from Two Different Tailings Ponds via Metabolite and Functional Gene Analyses

Oil sands tailings, a slurry of alkaline water, silt, clay, unrecovered bitumen, and residual hydrocarbons generated during bitumen extraction, are contained in ponds. Indigenous microbes metabolize hydrocarbons and emit greenhouse gases from the tailings. Metabolism of hydrocarbons in tailings ponds of two operators, namely, Canadian Natural Upgrading Limited (CNUL) and Canadian Natural Resources Limited (CNRL), has not been comprehensively investigated. Previous reports have revealed sequential and preferential hydrocarbon degradation of alkanes in primary cultures established from CNUL and CNRL tailings amended separately with mixtures of hydrocarbons (n-alkanes, iso-alkanes, paraffinic solvent, or naphtha). In this study, activation pathway of hydrocarbon biodegradation in these primary cultures was investigated. The functional gene analysis revealed that fumarate addition was potentially the primary activation pathway of alkanes in all cultures. However, the metabolite analysis only detected transient succinylated 2-methylpentane and 2-methylbutane metabolites during initial methanogenic biodegradation of iso-alkanes and paraffinic solvent in all CNUL and CNRL cultures amended with iso-alkanes and paraffinic solvent. Under sulfidogenic conditions (prepared only with CNUL tailings amended with iso-alkanes), succinylated 2-methylpentane persisted throughout incubation period of ~ 1100 days, implying dead-end nature of the metabolite. Though no metabolite was detected in n-alkanes- and naphtha-amended cultures during incubation, assA/masD genes related to Peptococcaceae were amplified in all CNUL and CNRL primary cultures. The findings of this present study suggest that microbial communities in different tailings ponds can biodegrade hydrocarbons through fumarate addition as activation pathway under methanogenic and sulfidogenic conditions.

Aerobic biodegradation of cycloalkanes in non-aqueous extracted oil sands tailings

Management of growing volumes of fluid fine tailings (FFT) is a significant challenge for oil sands industry. A potential alternative non-aqueous solvent extraction (NAE) process uses cycloalkane solvent such as cyclohexane or cyclopentane with very little water and generates smaller volumes of 'dry' solids (NAES) with residual solvent. Here we investigate remediation of NAES in a simulated bench-scale upland reclamation scenario. In the first study, microcosms with nutrient medium plus FFT as inoculum were amended with cyclohexane and incubated for ∼1 year, monitoring for cyclohexane biodegradation under aerobic conditions. Biodegradation of cyclohexane occurred under aerobic conditions with no metabolic intermediates detected. A second study using NAES mixed with FFT spiked with cyclohexane and cyclopentane, with or without additional nutrients (nitrogen and phosphorus), showed complete and rapid aerobic biodegradation of both cycloalkanes in NAES inoculated with FFT and supplemented with nutrients. 16S rRNA gene sequencing revealed dominance of Rhodoferax and members of Burkholderiaceae during aerobic cyclohexane biodegradation in FFT, and Hydrogenophaga, Acidovorax, Defluviimonas and members of Porticoccaceae during aerobic biodegradation of cyclohexane and cyclopentane in NAES inoculated with FFT and supplemented with nutrients. The findings indicate that biodegradation of cycloalkanes from NAES is possible under aerobic condition, which will contribute to the successful reclamation of oil sands tailings for land closure.

Biodegradation of 2-methylpentane in fluid fine tailings amended with a mixture of iso-alkanes under sulfate-reducing conditions

Anaerobic microorganisms in Canada Natural Upgrading Limited (CNUL) fluid fine tailings (FFT) are sustained by residual solvent hydrocarbons. Although FFT are methanogenic in nature, sulfate-reducing microorganisms represent a significant portion of FFT bacterial community. In this study, we examined biodegradation of three iso-alkanes (2-methylbutane, 2-methylpentane, and 3-methylpentane), representing major iso-alkanes in paraffinic solvent, in CNUL FFT under sulfate-reducing conditions. During ∼1100 days of incubation, only 2-methylpentane was degraded partially, whereas 2-methylbutane and 3-methylpentane were not degraded. During active degradation of 2-methylpentane, the bacterial community was dominated by Anaerolineaceae followed by Syntrophaceae, Peptococcaceae, Desulfobacteraceae, and Desulfobulbaceae. The archaeal community was co-dominated by acetoclastic (Methanosaetaceae) and hydrogenotrophic (Methanobacteriaceae) methanogens. This study underlines the limited capability of the microbial community indigenous to CNUL FFT in degrading recalcitrant iso-alkanes under sulfate-reducing conditions.

Mitigating methane emission from oil sands tailings using enzymatic and lime treatments

Engineering strategies to reduce greenhouse gases (GHGs) emissions by inhibiting methanogenesis in oil sands tailings have rarely been examined. In this study, we explored the potential impact of chemical treatment (lime) and biological treatment using enzymes (lysozyme and protease) on inhibiting methane emissions from tailings. Overall, treatment with protease 3%, lysozyme 3%, and lime 5000 ppm reduced CH₄ production (by 52%, 28%, and 25%, respectively) and were weakly associated with the archaeal abundance. Enzymes treatment resulted in a higher reduction in CH₄ production compared with lime treatment. A 3% lysozyme treatment suppressed CH₄ production (the change in methane was 0.48 mmol) and reduced the degradation of hexane throughout the experiment. Similarly, 3% protease suppressed CH₄ production throughout the experiment (the change in methane was 0.78 mmol), which could be attributed to the pH reduction to pH 4.9 at week 23 resulting from the formation of volatile fatty acids. Another possible mechanism could be the formation of toxic compounds, such as high nitrogen content, after protease treatment that inhibited the microbial community. The toxicity effect to Vibrio fischeri was greater with lysozyme 3% and protease 3% treatment than with lime treatment (124 TU and 76 TU, respectively). Lime treatment resulted in the highest reduction in 16S rRNA gene copies from 5.7 × 10⁶ cells g^-1 (control) to 2.7 × 10⁵, 1.71 × 10⁵, and 1.4 × 10⁵ cells g^-1 for 1600, 3500, and 5000 ppm treatments, respectively. This study supports further work to examine and determine the optimum conditions (e.g., enzyme and lime dosages) for CH₄ inhibition.

Geochemical Stability of Oil Sands Tailings in Mine Closure Landforms

Oil sands surface mining in Alberta has generated over a billion cubic metres of waste, known as tailings, consisting of sands, silts, clays, and process-affected water that contains toxic organic compounds and chemical constituents. All of these tailings will eventually be reclaimed and integrated into one of two types of mine closure landforms: end pit lakes (EPLs) or terrestrial landforms with a wetland feature. In EPLs, tailings deposits are capped with several metres of water while in terrestrial landforms, tailings are capped with solid materials, such as sand or overburden. Because tailings landforms are relatively new, past research has heavily focused on the geotechnical and biogeochemical characteristics of tailings in temporary storage ponds, referred to as tailings ponds. As such, the geochemical stability of tailings landforms remains largely unknown. This review discusses five mechanisms of geochemical change expected in tailings landforms: consolidation, chemical mass loading via pore water fluxes, biogeochemical cycling, polymer degradation, and surface water and groundwater interactions. Key considerations and knowledge gaps with regard to the long-term geochemical stability of tailings landforms are identified, including salt fluxes and subsequent water quality, bioremediation and biogenic greenhouse gas emissions, and the biogeochemical implications of various tailings treatment methods meant to improve geotechnical properties of tailings, such as flocculant (polyacrylamide) and coagulant (gypsum) addition.

Methanogenic Biodegradation of iso-Alkanes by Indigenous Microbes from Two Different Oil Sands Tailings Ponds

iso-Alkanes, a major fraction of the solvents used in bitumen extraction from oil sand ores, are slow to biodegrade in anaerobic tailings ponds. We investigated methanogenic biodegradation of iso-alkane mixtures comprising either three (2-methylbutane, 2-methylpentane, 3-methylpentane) or five (2-methylbutane, 2-methylpentane, 2-methylhexane, 2-methylheptane, 2-methyloctane) iso-alkanes representing paraffinic and naphtha solvents, respectively. Mature fine tailings (MFT) collected from two tailings ponds, having different residual solvents (paraffinic solvent in Canadian Natural Upgrading Limited (CNUL) and naphtha in Canadian Natural Resources Limited (CNRL)), were amended separately with the two mixtures and incubated in microcosms for ~1600 d. The indigenous microbes in CNUL MFT produced methane from the three-iso-alkane mixture after a lag of ~200 d, completely depleting 2-methylpentane while partially depleting 2-methylbutane and 3-methylpentane. CNRL MFT exhibited a similar degradation pattern for the three iso-alkanes after a lag phase of ~700 d, but required 1200 d before beginning to produce methane from the five-iso-alkane mixture, preferentially depleting components in the order of decreasing carbon chain length. Peptococcaceae members were key iso-alkane-degraders in both CNUL and CNRL MFT but were associated with different archaeal partners. Co-dominance of acetoclastic (Methanosaeta) and hydrogenotrophic (Methanolinea and Methanoregula) methanogens was observed in CNUL MFT during biodegradation of three-iso-alkanes whereas CNRL MFT was enriched in Methanoregula during biodegradation of three-iso-alkanes and in Methanosaeta with five-iso-alkanes. This study highlights the different responses of indigenous methanogenic microbial communities in different oil sands tailings ponds to iso-alkanes.

Biofilms for Turbidity Mitigation in Oil Sands End Pit Lakes

End pit lakes (EPLs) have been proposed as a method of reclaiming oil sands fluid fine tailings (FFT), which consist primarily of process-affected water and clay- and silt-sized particles. Base Mine Lake (BML) is the first full-scale demonstration EPL and contains thick deposits of FFT capped with water. Because of the fine-grained nature of FFT, turbidity generation and mitigation in BML are issues that may be detrimental to the development of an aquatic ecosystem in the water cap. Laboratory mixing experiments were conducted to investigate the effect of mudline biofilms made up of microbial communities indigenous to FFT on mitigating turbidity in EPLs. Four mixing speeds were tested (80, 120, 160, and 200 rpm), all of which are above the threshold velocity required to initiate erosion of FFT in BML. These mixing speeds were selected to evaluate (i) the effectiveness of biofilms in mitigating turbidity and (ii) the mixing speed required to 'break' the biofilms. The impact of biofilm age (10 weeks versus 20 weeks old) on turbidity mitigation was also evaluated. Diverse microbial communities in the biofilms included photoautotrophs, namely cyanobacteria and Chlorophyta (green algae), as well as a number of heterotrophs such as Gammaproteobacteria, Desulfobulbia, and Anaerolineae. Biofilms reduced surface water turbidity by up to 99%, depending on the biofilm age and mixing speed. Lifting and layering in the older biofilms resulted in weaker attachment to the FFT; as such, younger biofilms performed better than older biofilms. However, older biofilms still reduced turbidity by 69% to 95%, depending on the mixing speed. These results indicate that biostabilization is a promising mechanism for turbidity mitigation in EPLs.

A Deep Look into the Microbiology and Chemistry of Froth Treatment Tailings: A Review

In Alberta’s Athabasca oil sands region (AOSR), over 1.25 billion m³ of tailings waste from the bitumen extraction process are stored in tailings ponds. Fugitive emissions associated with residual hydrocarbons in tailings ponds pose an environmental concern and include greenhouse gases (GHGs), reduced sulphur compounds (RSCs), and volatile organic compounds (VOCs). Froth treatment tailings (FTT) are a specific type of tailings waste stream from the bitumen froth treatment process that contains bioavailable diluent: either naphtha or paraffins. Tailings ponds that receive FTT are associated with the highest levels of biogenic gas production, as diverse microbial communities biodegrade the residual diluent. In this review, current literature regarding the composition, chemical analysis, and microbial degradation of FTT and its constituents is presented in order to provide a more complete understanding of the complex chemistry and biological processes related to fugitive emissions from tailings ponds receiving FTT. Characterizing the composition and biodegradation of FTT is important from an environmental perspective to better predict emissions from tailings ponds and guide tailings pond management decisions.

Gas ebullition from petroleum hydrocarbons in aquatic sediments: A review

Gas ebullition in sediment results from biogenic gas production by mixtures of bacteria and archaea. It often occurs in organic-rich sediments that have been impacted by petroleum hydrocarbon (PHC) and other anthropogenic pollution. Ebullition occurs under a relatively narrow set of biological, chemical, and sediment geomechanical conditions. This process occurs in three phases: I) biogenic production of primarily methane and dissolved phase transport of the gases in the pore water to a bubble nucleation site, II) bubble growth and sediment fracture, and III) bubble rise to the surface. The rate of biogenic gas production in phase I and the resistance of the sediment to gas fracture in phase II play the most significant roles in ebullition kinetics. What is less understood is the role that substrate structure plays in the rate of methanogenesis that drives gas ebullition. It is well established that methanogens have a very restricted set of compounds that can serve as substrates, so any complex organic molecule must first be broken down to fermentable compounds. Given that most ebullition-active sediments are completely anaerobic, the well-known difficulty in degrading PHCs under anaerobic conditions suggests potential limitations on PHC-derived gas ebullition. To date, there are no studies that conclusively demonstrate that weathered PHCs can alone drive gas ebullition. This review consists of an overview of the factors affecting gas ebullition and the biochemistry of anaerobic PHC biodegradation and methanogenesis in sediment systems. We next compile results from the scholarly literature on PHCs serving as a source of methanogenesis. We combine these results to assess the potential for PHC-driven gas ebullition using energetics, kinetics, and sediment geomechanics analyses. The results suggest that short chain <C₁₀ alkanes are the only PHC class that alone may have the potential to drive ebullition, and that PHC-derived methanogenesis likely plays a minor part in driving gas ebullition in contaminated sediments compared to natural organic matter.

Anaerobic Degradation of Paraffins by Thermophilic Actinobacteria under Methanogenic Conditions

Microbial anaerobic alkane degradation is a key process in subsurface oil reservoirs and anoxic environments contaminated with petroleum, with a major impact on global carbon cycling. However, the thermophiles capable of water-insoluble paraffins (>C₁₇) degradation under methanogenic conditions has remained understudied. Here, we established thermophilic (55 °C) n-paraffins-degrading (C₂₁-C₃₀) cultures from an oil reservoir. After over 900 days of incubation, the even-numbered n-paraffins were biodegraded to methane. The bacterial communities are dominated by a novel class-level lineage of actinobacteria, 'Candidatus Syntraliphaticia'. These 'Ca. Syntraliphaticia'-like metagenome-assembled genomes (MAGs) encode a complete alkylsuccinate synthases (ASS) gene operon, as well as hydrogenases and formate dehydrogenase, and several enzymes potentially involved in alkyl-CoA oxidation and the Wood-Ljungdahl pathway. Metatranscriptomic analysis suggests that n-paraffins are activated via fumarate addition reaction, and oxidized into carbon dioxide, hydrogen/formate and acetate by 'Ca. Syntraliphaticia', that could be further converted to methane by the abundant hydrogenotrophic and acetoclastic methanogens. We also found a divergent methyl-CoM reductase-like complex (MCR) and a canonical MCR in two MAGs representing 'Ca. Methanosuratus' (within candidate phylum Verstraetearchaeota), indicating the capability of methane and short-chain alkane metabolism in the oil reservoir. Ultimately, this result offers new insights into the degradability and the mechanisms of n-paraffins under methanogenic conditions at high temperatures.

Methanogenic biodegradation of iso-alkanes and cycloalkanes during long-term incubation with oil sands tailings

Microbes indigenous to oil sands tailings ponds methanogenically biodegrade certain hydrocarbons, including n-alkanes and monoaromatics, whereas other hydrocarbons such as iso- and cycloalkanes are more recalcitrant. We tested the susceptibility of iso- and cycloalkanes to methanogenic biodegradation by incubating them with mature fine tailings (MFT) collected from two depths (6 and 31 m below surface) of a tailings pond, representing different lengths of exposure to hydrocarbons. A mixture of five iso-alkanes and three cycloalkanes was incubated with MFT for 1700 d. Iso-alkanes were completely biodegraded in the order 3-methylhexane > 4-methylheptane > 2-methyloctane > 2-methylheptane, whereas 3-ethylhexane and ethylcyclopentane were only partially depleted and methylcyclohexane and ethylcyclohexane were not degraded during incubation. Pyrosequencing of 16S rRNA genes showed enrichment of Peptococcaceae (Desulfotomaculum) and Smithella in amended cultures with acetoclastic (Methanosaeta) and hydrogenotrophic methanogens (Methanoregula and Methanoculleus). Bioaugmentation of MFT by inoculation with MFT-derived enrichment cultures reduced the lag phase before onset of iso-alkane and cycloalkane degradation. However, the same enrichment culture incubated without MFT exhibited slower biodegradation kinetics and less CH₄ production, implying that the MFT solid phase (clay minerals) enhanced methanogenesis. These results help explain and predict continued emissions of CH₄ from oil sands tailings repositories in situ.

Methanogenic biodegradation of C9 to C12n-alkanes initiated by Smithella via fumarate addition mechanism

In the present study, a methanogenic alkane-degrading (a mixture of C₉ to C₁₂n-alkanes) culture enriched from production water of a low-temperature oil reservoir was established and assessed. Significant methane production was detected in the alkane-amended enrichment cultures compared with alkane-free controls over an incubation period of 1 year. At the end of the incubation, fumarate addition metabolites (C₉ to C₁₂ alkylsuccinates) and assA genes (encoding the alpha subunit of alkylsuccinate synthase) were detected only in the alkane-amended enrichment cultures. Microbial community analysis showed that putative syntrophic n-alkane degraders (Smithella) capable of initiating n-alkanes by fumarate addition mechanism were enriched in the alkane-amended enrichment cultures. In addition, both hydrogenotrophic (Methanocalculus) and acetoclastic (Methanothrix) methanogens were also observed. Our results provide further evidence that alkanes can be activated by addition to fumarate under methanogenic conditions.

Novel copper-containing membrane monooxygenases (CuMMOs) encoded by alkane-utilizing Betaproteobacteria

Copper-containing membrane monooxygenases (CuMMOs) are encoded by xmoCAB(D) gene clusters and catalyze the oxidation of methane, ammonia, or some short-chain alkanes and alkenes. In a metagenome constructed from an oilsands tailings pond we detected an xmoCABD gene cluster with <59% derived protein sequence identity to genes from known bacteria. Stable isotope probing experiments combined with a specific xmoA qPCR assay demonstrated that the bacteria possessing these genes were incapable of methane assimilation, but did grow on ethane and propane. Single-cell amplified genomes (SAGs) from propane-enriched samples were screened with the specific PCR assay to identify bacteria possessing the target gene cluster. Multiple SAGs of Betaproteobacteria belonging to the genera Rhodoferax and Polaromonas possessed homologues of the metagenomic xmoCABD gene cluster. Unexpectedly, each of these two genera also possessed other xmoCABD paralogs, representing two additional lineages in phylogenetic analyses. Metabolic reconstructions from SAGs predicted that neither bacterium encoded enzymes with the potential to support catabolic methane or ammonia oxidation, but that both were capable of higher n-alkane degradation. The involvement of the encoded CuMMOs in alkane oxidation was further suggested by reverse transcription PCR analyses, which detected elevated transcription of the xmoA genes upon enrichment of water samples with propane as the sole energy source. Enrichments, isotope incorporation studies, genome reconstructions, and gene expression studies therefore all agreed that the unknown xmoCABD operons did not encode methane or ammonia monooxygenases, but rather n-alkane monooxygenases. This study broadens the known diversity of CuMMOs and identifies these enzymes in non-nitrifying Betaproteobacteria.

Second-generation stoichiometric mathematical model to predict methane emissions from oil sands tailings

Microbial metabolism of fugitive hydrocarbons produces greenhouse gas (GHG) emissions from oil sands tailings ponds (OSTP) and end pit lakes (EPL) that retain fluid tailings from surface mining of oil sands ores. Predicting GHG production, particularly methane (CH₄), would help oil sands operators mitigate tailings emissions and may assist regulators evaluating the trajectory of reclamation scenarios. Using empirical datasets from laboratory incubation of OSTP sediments with pertinent hydrocarbons, we developed a stoichiometric model for CH₄ generation by indigenous microbes. This model improved on previous first-approximation models by considering long-term biodegradation kinetics for 18 relevant hydrocarbons from three different oil sands operations, lag times, nutrient limitations, and microbial growth and death rates. Laboratory measurements were used to estimate model parameter values and to validate the new model. Goodness of fit analysis showed that the stoichiometric model predicted CH₄ production well; normalized mean square error analysis revealed that it surpassed previous models. Comparison of model predictions with field measurements of CH₄ emissions further validated the new model. Importantly, the model also identified in-situ parameters that are currently lacking but are needed to enable future robust modeling of CH₄ production from OSTP and EPL in-situ.

Dewatering Oil Sands Tailings with Degradable Polymer Flocculants

We synthesized hydrolytically degradable cationic polymers by micellar radical polymerization of a short-chain polyester macromonomer, polycaprolactone choline iodide ester methacrylate (PCL₂ChMA) with two polyester units, and used them to flocculate oil sands mature fine tailings (MFT). We evaluated the flocculation performance of the homopolymer and copolymers with 30 mol % acrylamide (AM) by measuring initial settling rate (ISR), supernatant turbidity, and capillary suction time (CST) of the sediments. Flocculants made with trimethylaminoethyl methacrylate chloride (TMAEMC), the monomer corresponding to PCL_nChMA with n = 0, have improved performance over poly(PCL₂ChMA) at equivalent loadings due to their higher charge density per gram of polymer. However, MFT sediments flocculated using the PCL₂ChMA-based polymers are easier to dewater (up to an 85% reduction in CST) after accelerated hydrolytic degradation of the polyester side chains. This study demonstrates the potential of designing cationic polymers that effectively flocculate oil sands tailings ponds, and also further dewater the resulting solids through polymer degradation.

The microbiology of oil sands tailings: past, present, future

Surface mining of enormous oil sands deposits in northeastern Alberta, Canada since 1967 has contributed greatly to Canada's economy but has also received negative international attention due largely to environmental concerns and challenges. Not only have microbes profoundly affected the composition and behavior of this petroleum resource over geological time, they currently influence the management of semi-solid tailings in oil sands tailings ponds (OSTPs) and tailings reclamation. Historically, microbial impacts on OSTPs were generally discounted, but next-generation sequencing and biogeochemical studies have revealed unexpectedly diverse indigenous communities and expanded our fundamental understanding of anaerobic microbial functions. OSTPs that experienced different processing and management histories have developed distinct microbial communities that influence the behavior and reclamation of the tailings stored therein. In particular, the interactions of Deltaproteobacteria and Firmicutes with methanogenic archaea impact greenhouse gas emissions, sulfur cycling, pore water toxicity, sediment biogeochemistry and densification, water usage and the trajectory of long-term mine waste reclamation. This review summarizes historical data; synthesizes current understanding of microbial diversity and activities in situ and in vitro; predicts microbial effects on tailings remediation and reclamation; and highlights knowledge gaps for future research.

Acid-Base Polymeric Foams for the Adsorption of Micro-oil Droplets from Industrial Effluents

Separation of toxic organic pollutants from industrial effluents is a great environmental challenge. Herein, an acid-base engineered foam is employed for separation of micro-oil droplets from an aqueous solution. In acidic or basic environments, acid-base polymers acquire surface charge due to protonation or dissociation of surface active functional groups. This property is invoked to adsorb crude oil microdroplets from water using polyester polyurethane (PESPU) foam. The physicochemical surface properties of the foam were characterized using X-ray photoelectron spectroscopy, inverse gas chromatography, electrokinetic analysis, and micro-computed tomography. Using the surface charge of the foam and oil droplets, the solution pH (5.6) for maximum separation efficacy was predicted. This optimal pH was verified through underwater wetting behavior and adsorption experiments. The droplet adsorption onto the foam was governed by physisorption, and the driving forces were attributed to electrostatic attraction and Lifshitz-van der Waals forces. The foam was regenerated and reused multiple times by simple compression. The lowest trace oil content in the retentate was 3.6 mg L^-1, and all oil droplets larger than 140 nm were removed. This work lays the foundation for the development of a new class of engineered foam adsorbents with the potential to revolutionize water treatment technologies.

Methanogenic biodegradation of paraffinic solvent hydrocarbons in two different oil sands tailings

Microbial communities drive many biogeochemical processes in oil sands tailings and cause greenhouse gas emissions from tailings ponds. Paraffinic solvent (primarily C₅-C₆; n- and iso-alkanes) is used by some oil sands companies to aid bitumen extraction from oil sands ores. Residues of unrecovered solvent escape to tailings ponds during tailings deposition and sustain microbial metabolism. To investigate biodegradation of hydrocarbons in paraffinic solvent, mature fine tailings (MFT) collected from Albian and CNRL ponds were amended with paraffinic solvent at ~0.1wt% (final concentration: ~1000mgL^-1) and incubated under methanogenic conditions for ~1600d. Albian and CNRL MFTs exhibited ~400 and ~800d lag phases, respectively after which n-alkanes (n-pentane and n-hexane) in the solvent were preferentially metabolized to methane over iso-alkanes in both MFTs. Among iso-alkanes, only 2-methylpentane was completely biodegraded whereas 2-methylbutane and 3-methylpentane were partially biodegraded probably through cometabolism. 16S rRNA gene pyrosequencing showed dominance of Anaerolineaceae and Methanosaetaceae in Albian MFT and Peptococcaceae and co-domination of "Candidatus Methanoregula" and Methanosaetaceae in CNRL MFT bacterial and archaeal communities, respectively, during active biodegradation of paraffinic solvent. The results are important for developing future strategies for tailings reclamation and management of greenhouse gas emissions.

Sequential biodegradation of complex naphtha hydrocarbons under methanogenic conditions in two different oil sands tailings

Methane emissions in oil sands tailings ponds are sustained by anaerobic biodegradation of unrecovered hydrocarbons. Naphtha (primarily C₆-C₁₀; n- iso- and cycloalkanes) is commonly used as a solvent during bitumen extraction process and its residue escapes to tailings ponds during tailings deposition. To investigate biodegradability of hydrocarbons in naphtha, mature fine tailings (MFT) collected from Albian and CNRL tailings ponds were amended with CNRL naphtha at ∼0.2 wt% (∼2000 mg L^-1) and incubated under methanogenic conditions for ∼1600 d. Microbial communities in both MFTs started metabolizing naphtha after a lag phase of ∼100 d. Complete biodegradation/biotransformation of all n-alkanes (except partial biodegradation of n-octane in CNRL MFT) followed by major iso-alkanes (2-methylpentane, 3-methylhexane, 2- and 4-methylheptane, iso-nonanes and 2-methylnonane) and a few cycloalkanes (derivatives of cyclopentane and cyclohexane) was observed during the incubation. 16S rRNA gene pyrosequencing showed dominance of Peptococcaceae and Anaerolineaceae in Albian MFT and Anaerolineaceae and Syntrophaceae in CNRL MFT bacterial communities with co-domination of Methanosaetaceae and "Candidatus Methanoregula" in archaeal populations during active biodegradation of hydrocarbons. The findings extend the known range of hydrocarbons susceptible to methanogenic biodegradation in petroleum-impacted anaerobic environments and help refine existing kinetic model to predict greenhouse gas emissions from tailings ponds.

Methylotrophic methanogenesis discovered in the archaeal phylum Verstraetearchaeota

Methanogenesis is the primary biogenic source of methane in the atmosphere and a key contributor to climate change. The long-standing dogma that methanogenesis originated within the Euryarchaeota was recently challenged by the discovery of putative methane-metabolizing genes in members of the Bathyarchaeota, suggesting that methanogenesis may be more phylogenetically widespread than currently appreciated. Here, we present the discovery of divergent methyl-coenzyme M reductase genes in population genomes recovered from anoxic environments with high methane flux that belong to a new archaeal phylum, the Verstraetearchaeota. These archaea encode the genes required for methylotrophic methanogenesis, and may conserve energy using a mechanism similar to that proposed for the obligate H₂-dependent methylotrophic Methanomassiliicoccales and recently described Candidatus 'Methanofastidiosa'. Our findings indicate that we are only beginning to understand methanogen diversity and support an ancient origin for methane metabolism in the Archaea, which is changing our understanding of the global carbon cycle.

Co-occurrence of methanogenesis and N2 fixation in oil sands tailings

Oil sands tailings ponds in northern Alberta, Canada have been producing biogenic gases via microbial metabolism of hydrocarbons for decades. Persistent methanogenic activity in tailings ponds without any known replenishment of nutrients such as fixed nitrogen (N) persuaded us to investigate whether N2 fixation or polyacrylamide (PAM; used as a tailings flocculant) could serve as N sources. Cultures comprising mature fine tailings (MFT) plus methanogenic medium supplemented with or deficient in fixed N were incubated under an N2 headspace. Some cultures were further amended with citrate, which is used in oil sands processing, as a relevant carbon source, and/or with PAM. After an initial delay, N-deficient cultures with or without PAM produced methane (CH4) at the same rate as N-containing cultures, indicating a mechanism of overcoming apparent N-deficiency. Acetylene reduction and (15)N2 incorporation in all N-deficient cultures (with or without PAM) suggested active N2 fixation concurrently with methanogenesis but inability to use PAM as a N source. 16S rRNA gene pyrosequencing revealed little difference between archaeal populations regardless of N content. However, bacterial sequences in N-deficient cultures showed enrichment of Hyphomicrobiaceae and Clostridium members that might contain N2-fixing species. The results are important in understanding long-term production of biogenic greenhouse gases in oil sands tailings.

Preferential methanogenic biodegradation of short-chain n-alkanes by microbial communities from two different oil sands tailings ponds

Oil sands tailings ponds harbor diverse anaerobic microbial communities capable of methanogenic biodegradation of solvent hydrocarbons entrained in the tailings. Mature fine tailings (MFT) from two operators (Albian and CNRL) that use different extraction solvents were incubated with mixtures of either two (n-pentane and n-hexane) or four (n-pentane, n-hexane, n-octane and n-decane) n-alkanes under methanogenic conditions for ~600 d. Microbes in Albian MFT began methane production by ~80 d, achieving complete depletion of n-pentane and n-hexane in the two-alkane mixture and their preferential biodegradation in the four-alkane mixture. Microbes in CNRL MFT preferentially metabolized n-octane and n-decane in the four-alkane mixture after a ~80 d lag but exhibited a lag of ~360 d before commencing biodegradation of n-pentane and n-hexane in the two-alkane mixture. 16S rRNA gene pyrosequencing revealed Peptococcaceae members as key bacterial n-alkane degraders in all treatments except CNRL MFT amended with the four-alkane mixture, in which Anaerolineaceae, Desulfobacteraceae (Desulfobacterium) and Syntrophaceae (Smithella) dominated during n-octane and n-decane biodegradation. Anaerolineaceae sequences increased only in cultures amended with the four-alkane mixture and only during n-octane and n-decane biodegradation. The dominant methanogens were acetoclastic Methanosaetaceae. These results highlight preferential n-alkane biodegradation by microbes in oil sands tailings from different producers, with implications for tailings management and reclamation.