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.

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.

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.

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.

Genomic analysis of the mesophilic Thermotogae genus Mesotoga reveals phylogeographic structure and genomic determinants of its distinct metabolism.

The genus Mesotoga, the only described mesophilic Thermotogae lineage, is common in mesothermic anaerobic hydrocarbon-rich environments. Besides mesophily, Mesotoga displays lineage-specific phenotypes, such as no or little H2 production and dependence on sulfur-compound reduction, which may influence its ecological role. We used comparative genomics of 18 Mesotoga strains (pairwise 16S rRNA identity >99%) and a transcriptome of M. prima to investigate how life at moderate temperatures affects phylogeography and to interrogate the genomic features of its lineage-specific metabolism. We propose that Mesotoga accomplish H2 oxidation and thiosulfate reduction using a sulfide dehydrogenase and a hydrogenase-complex and that a pyruvate:ferredoxin oxidoreductase acquired from Clostridia is responsible for oxidizing acetate. Phylogenetic analysis revealed three distinct Mesotoga lineages (89.6%-99.9% average nucleotide identity [ANI] within lineages, 79.3%-87.6% ANI between lineages) having different geographic distribution patterns and high levels of intra-lineage recombination but little geneflow between lineages. Including data from metagenomes, phylogeographic patterns suggest that geographical separation historically has been more important for Mesotoga than hyperthermophilic Thermotoga and we hypothesize that distribution of Mesotoga is constrained by their anaerobic lifestyle. Our data also suggest that recent anthropogenic activities and environments (e.g., wastewater treatment, oil exploration) have expanded Mesotoga habitats and dispersal capabilities.

Vitamin and Amino Acid Auxotrophy in Anaerobic Consortia Operating under Methanogenic Conditions.

Syntrophy among Archaea and Bacteria facilitates the anaerobic degradation of organic compounds to CH4 and CO2. Particularly during aliphatic and aromatic hydrocarbon mineralization, as in the case of crude oil reservoirs and petroleum-contaminated sediments, metabolic interactions between obligate mutualistic microbial partners are of central importance. Using micromanipulation combined with shotgun metagenomic approaches, we describe the genomes of complex consortia within short-chain alkane-degrading cultures operating under methanogenic conditions. Metabolic reconstruction revealed that only a small fraction of genes in the metagenome-assembled genomes encode the capacity for fermentation of alkanes facilitated by energy conservation linked to H2 metabolism. Instead, the presence of inferred lifestyles based on scavenging anabolic products and intermediate fermentation products derived from detrital biomass was a common feature. Additionally, inferred auxotrophy for vitamins and amino acids suggests that the hydrocarbon-degrading microbial assemblages are structured and maintained by multiple interactions beyond the canonical H2-producing and syntrophic alkane degrader-methanogen partnership. Compared to previous work, our report points to a higher order of complexity in microbial consortia engaged in anaerobic hydrocarbon transformation. IMPORTANCE Microbial interactions between Archaea and Bacteria mediate many important chemical transformations in the biosphere from degrading abundant polymers to synthesis of toxic compounds. Two of the most pressing issues in microbial interactions are how consortia are established and how we can modulate these microbial communities to express desirable functions. Here, we propose that public goods (i.e., metabolites of high energy demand in biosynthesis) facilitate energy conservation for life under energy-limited conditions and determine the assembly and function of the consortia. Our report suggests that an understanding of public good dynamics could result in new ways to improve microbial pollutant degradation in anaerobic systems.

Genomic insights into temperature-dependent transcriptional responses of Kosmotoga olearia, a deep-biosphere bacterium that can grow from 20 to 79 °C.

Temperature is one of the defining parameters of an ecological niche. Most organisms thrive within a temperature range that rarely exceeds ~30 °C, but the deep subsurface bacterium Kosmotoga olearia can grow over a temperature range of 59 °C (20-79 °C). To identify genes correlated with this flexible phenotype, we compared transcriptomes of K. olearia cultures grown at its optimal 65 °C to those at 30, 40, and 77 °C. The temperature treatments affected expression of 573 of 2224 K. olearia genes. Notably, this transcriptional response elicits re-modeling of the cellular membrane and changes in metabolism, with increased expression of genes involved in energy and carbohydrate metabolism at high temperatures and up-regulation of amino acid metabolism at lower temperatures. At sub-optimal temperatures, many transcriptional changes were similar to those observed in mesophilic bacteria at physiologically low temperatures, including up-regulation of typical cold stress genes and ribosomal proteins. Comparative genomic analysis of additional Thermotogae genomes indicates that one of K. olearia's strategies for low-temperature growth is increased copy number of some typical cold response genes through duplication and/or lateral acquisition. At 77 °C one-third of the up-regulated genes are of hypothetical function, indicating that many features of high-temperature growth are unknown.

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.

Trace metal mobilization from oil sands froth treatment thickened tailings exhibiting acid rock drainage.

Froth treatment thickened tailings (TT) are a waste product of bitumen extraction from surface-mined oil sands ores. When incubated in a laboratory under simulated moist oxic environmental conditions for ~450d, two different types of TT (TT1 and TT2) exhibited the potential to generate acid rock drainage (ARD) by producing acid leachate after 250 and 50d, respectively. We report here the release of toxic metals from TT via ARD, which could pose an environmental threat if oil sands TT deposits are not properly managed. Trace metal concentrations in leachate samples collected periodically revealed that Mn and Sr were released immediately even before the onset of ARD. Spikes in Co and Ni concentrations were observed both pre-ARD and during active ARD, particularly in TT1. For most elements measured (Fe, Cr, V, As, Cu, Pb, Zn, Cd, and Se), leaching was associated with ARD production. Though equivalent acidification (pH2) was achieved in leachate from both TT types, greater metal release was observed from TT2 where concentrations reached 10,000ppb for Ni, 5000ppb for Co, 3000ppb for As, 2000ppb for V, and 1000ppb for Cr. Generally, metal concentrations decreased in leachate with time during ARD and became negligible by the end of incubation (~450d) despite appreciable metals remaining in the leached TT. These results suggest that using TT for land reclamation purposes or surface deposition for volume reduction may unfavorably impact the environment, and warrants application of appropriate strategies for management of pyrite-enriched oil sands tailings streams.

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.

Next-Generation Sequencing Assessment of Eukaryotic Diversity in Oil Sands Tailings Ponds Sediments and Surface Water.

Tailings ponds in the Athabasca oil sands (Canada) contain fluid wastes, generated by the extraction of bitumen from oil sands ores. Although the autochthonous prokaryotic communities have been relatively well characterized, almost nothing is known about microbial eukaryotes living in the anoxic soft sediments of tailings ponds or in the thin oxic layer of water that covers them. We carried out the first next-generation sequencing study of microbial eukaryotic diversity in oil sands tailings ponds. In metagenomes prepared from tailings sediment and surface water, we detected very low numbers of sequences encoding eukaryotic small subunit ribosomal RNA representing seven major taxonomic groups of protists. We also produced and analysed three amplicon-based 18S rRNA libraries prepared from sediment samples. These revealed a more diverse set of taxa, 169 different OTUs encompassing up to eleven higher order groups of eukaryotes, according to detailed classification using homology searching and phylogenetic methods. The 10 most abundant OTUs accounted for > 90% of the total of reads, vs. large numbers of rare OTUs (< 1% abundance). Despite the anoxic and hydrocarbon-enriched nature of the environment, the tailings ponds harbour complex communities of microbial eukaryotes indicating that these organisms should be taken into account when studying the microbiology of the oil sands.

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.

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

iso-Alkanes are major components of petroleum and have been considered recalcitrant to biodegradation under methanogenic conditions. However, indigenous microbes in oil sands tailings ponds exposed to solvents rich in 2-methylbutane, 2-methylpentane, 3-methylpentane, n-pentane, and n-hexane produce methane in situ. We incubated defined mixtures of iso- or n-alkanes with mature fine tailings from two tailings ponds of different ages historically exposed to different solvents: one, ~10 years old, receiving C5-C6 paraffins and the other, ~35 years old, receiving naphtha. A lengthy incubation (>6 years) revealed iso-alkane biodegradation after lag phases of 900-1800 and ~280 days, respectively, before the onset of methanogenesis, although lag phases were shorter with n-alkanes (~650-1675 and ~170 days, respectively). 2-Methylpentane and both n-alkanes were completely depleted during ~2400 days of incubation, whereas 2-methylbutane and 3-methylpentane were partially depleted only during active degradation of 2-methylpentane, suggesting co-metabolism. In both cases, pyrotag sequencing of 16S rRNA genes showed codominance of Peptococcaceae with acetoclastic (Methanosaeta) and hydrogenotrophic (Methanoregula and Methanolinea) methanogens. These observations are important for predicting long-term greenhouse-gas emissions from oil sands tailings ponds and extend the known range of hydrocarbons susceptible to methanogenic biodegradation in petroleum-impacted anaerobic environments.

Anaerobic alkane biodegradation by cultures enriched from oil sands tailings ponds involves multiple species capable of fumarate addition.

A methanogenic short-chain alkane-degrading culture (SCADC) was enriched from oil sands tailings and transferred several times with a mixture of C6, C7, C8 and C10 n-alkanes as the predominant organic carbon source, plus 2-methylpentane, 3-methylpentane and methylcyclopentane as minor components. Cultures produced ∼40% of the maximum theoretical methane during 18 months incubation while depleting the n-alkanes, 2-methylpentane and methylcyclopentane. Substrate depletion correlated with detection of metabolites characteristic of fumarate activation of 2-methylpentane and methylcyclopentane, but not n-alkane metabolites. During active methanogenesis with the mixed alkanes, reverse-transcription PCR confirmed the expression of functional genes (assA and bssA) associated with hydrocarbon addition to fumarate. Pyrosequencing of 16S rRNA genes amplified during active alkane degradation revealed enrichment of Clostridia (particularly Peptococcaceae) and methanogenic Archaea (Methanosaetaceae and Methanomicrobiaceae). Methanogenic cultures transferred into medium containing sulphate produced sulphide, depleted n-alkanes and produced the corresponding succinylated alkane metabolites, but were slow to degrade 2-methylpentane and methylcyclopentane; these cultures were enriched in Deltaproteobacteria rather than Clostridia. 3-Methylpentane was not degraded by any cultures. Thus, nominally methanogenic oil sands tailings harbour dynamic and versatile hydrocarbon-degrading fermentative syntrophs and sulphate reducers capable of degrading n-, iso- and cyclo-alkanes by addition to fumarate.

DNA stable-isotope probing of oil sands tailings pond enrichment cultures reveals different key players for toluene degradation under methanogenic and sulfidogenic conditions.

Oil sands tailings ponds are anaerobic repositories of fluid wastes produced by extraction of bitumen from oil sands ores. Diverse indigenous microbiota biodegrade hydrocarbons (including toluene) in situ, producing methane, carbon dioxide and/or hydrogen sulfide, depending on electron acceptor availability. Stable-isotope probing of cultures enriched from tailings associated specific taxa and functional genes to (13)C6- and (12)C7-toluene degradation under methanogenic and sulfate-reducing conditions. Total DNA was subjected to isopycnic ultracentrifugation followed by gradient fraction analysis using terminal restriction fragment length polymorphism (T-RFLP) and construction of 16S rRNA, benzylsuccinate synthase (bssA) and dissimilatory sulfite reductase (dsrB) gene clone libraries. T-RFLP analysis plus sequencing and in silico digestion of cloned taxonomic and functional genes revealed that Clostridiales, particularly Desulfosporosinus (136 bp T-RF) contained bssA genes and were key toluene degraders during methanogenesis dominated by Methanosaeta. Deltaproteobacterial Desulfobulbaceae (157 bp T-RF) became dominant under sulfidogenic conditions, likely because the Desulfosporosinus T-RF 136 apparently lacks dsrB and therefore, unlike its close relatives, is presumed incapable of dissimilatory sulfate reduction. We infer incomplete oxidation of toluene by Desulfosporosinus in syntrophic association with Methanosaeta under methanogenic conditions, and complete toluene oxidation by Desulfobulbaceae during sulfate reduction.

Comparative analysis of metagenomes from three methanogenic hydrocarbon-degrading enrichment cultures with 41 environmental samples.

Methanogenic hydrocarbon metabolism is a key process in subsurface oil reservoirs and hydrocarbon-contaminated environments and thus warrants greater understanding to improve current technologies for fossil fuel extraction and bioremediation. In this study, three hydrocarbon-degrading methanogenic cultures established from two geographically distinct environments and incubated with different hydrocarbon substrates (added as single hydrocarbons or as mixtures) were subjected to metagenomic and 16S rRNA gene pyrosequencing to test whether these differences affect the genetic potential and composition of the communities. Enrichment of different putative hydrocarbon-degrading bacteria in each culture appeared to be substrate dependent, though all cultures contained both acetate- and H2-utilizing methanogens. Despite differing hydrocarbon substrates and inoculum sources, all three cultures harbored genes for hydrocarbon activation by fumarate addition (bssA, assA, nmsA) and carboxylation (abcA, ancA), along with those for associated downstream pathways (bbs, bcr, bam), though the cultures incubated with hydrocarbon mixtures contained a broader diversity of fumarate addition genes. A comparative metagenomic analysis of the three cultures showed that they were functionally redundant despite their enrichment backgrounds, sharing multiple features associated with syntrophic hydrocarbon conversion to methane. In addition, a comparative analysis of the culture metagenomes with those of 41 environmental samples (containing varying proportions of methanogens) showed that the three cultures were functionally most similar to each other but distinct from other environments, including hydrocarbon-impacted environments (for example, oil sands tailings ponds and oil-affected marine sediments). This study provides a basis for understanding key functions and environmental selection in methanogenic hydrocarbon-associated communities.

Draft Genome Sequence of Uncultivated Desulfosporosinus sp. Strain Tol-M, Obtained by Stable Isotope Probing Using [13C6]Toluene.

A draft Desulfosporosinus genome was assembled from the metagenome of a methanogenic [(13)C6]toluene-degrading community. The Desulfosporosinus sp. strain Tol-M genome is distinguished from that of previously published Desulfosporosinus strain by containing bss, bbs, and bam genes encoding enzymes for anaerobic biodegradation of monoaromatic hydrocarbons and lacking dsrAB genes for dissimilatory sulfate reduction.

Draft Genome Sequence of Uncultivated Toluene-Degrading Desulfobulbaceae Bacterium Tol-SR, Obtained by Stable Isotope Probing Using [13C6]Toluene.

The draft genome of a member of the bacterial family Desulfobulbaceae (phylum Deltaproteobacteria) was assembled from the metagenome of a sulfidogenic [(13)C6]toluene-degrading enrichment culture. The "Desulfobulbaceae bacterium Tol-SR" genome is distinguished from related, previously sequenced genomes by suites of genes associated with anaerobic toluene metabolism, including bss, bbs, and bam.

Microbial metabolism alters pore water chemistry and increases consolidation of oil sands tailings.

Tailings produced during bitumen extraction from surface-mined oil sands ores (tar sands) comprise an aqueous suspension of clay particles that remain dispersed for decades in tailings ponds. Slow consolidation of the clays hinders water recovery for reuse and retards volume reduction, thereby increasing the environmental footprint of tailings ponds. We investigated mechanisms of tailings consolidation and revealed that indigenous anaerobic microorganisms altered porewater chemistry by producing CO and CH during metabolism of acetate added as a labile carbon amendment. Entrapped biogenic CO decreased tailings pH, thereby increasing calcium (Ca) and magnesium (Mg) cations and bicarbonate (HCO) concentrations in the porewater through dissolution of carbonate minerals. Soluble ions increased the porewater ionic strength, which, with higher exchangeable Ca and Mg, decreased the diffuse double layer of clays and increased consolidation of tailings compared with unamended tailings in which little microbial activity was observed. These results are relevant to effective tailings pond management strategies.