No abstract

The biodegradation of a mixture of low molecular weight n-alkanes (C6, C7, C8, and C10) was assessed under methanogenic conditions using mature fine tailings (MFT) produced by the oil sands industry in Alberta, Canada. Microorganisms present in the MFT mineralized the added n-alkane mixture, producing 16.2 (+/- 0.3) or 20.5 (+/- 0.1) mmol of methane in the headspace of microcosms spiked with 0.2% or 0.5% w/v n-alkanes, respectively, during 29 weeks of incubation. The spiked n-alkanes biodegraded in the sequence C10 > C8 > C7 > C6. Degradation of 100% C10, 97% C8, 74% C7, and 44% C6 occurred in a mixture of n-alkanes in the MFT spiked at 0.2% after 25 weeks of incubation. The same pattern of biodegradation was also observed in the MFT spiked with 0.5% n-alkanes. Stoichiometric calculations confirmed the mineralization of the degraded n-alkanes to methane. This study showed that the short-chain n-alkanes, which comprise a significant portion of the unrecovered naphtha used in bitumen extraction and released into the settling basins, can be biodegraded into methane. These findings may influence decisions regarding extraction processes and long-term management of MFT, and they suggest that intrinsic, methanogenic metabolism of these n-alkanes may occur in other anoxic environments.

Extraction of bitumen from mined oil sands ores produces enormous volumes of tailings that are stored in settling basins (current inventory ≥ 840 million m(3)). Our previous studies revealed that certain hydrocarbons (short-chain n-alkanes [C(6)-C(10)] and monoaromatics [toluene, o-xylene, m-xylene]) in residual naphtha entrained in the tailings are biodegraded to CH(4) by a consortium of microorganisms. Here we show that higher molecular weight n-alkanes (C(14), C(16), and C(18)) are also degraded under methanogenic conditions in oil sands tailings, albeit after a lengthy lag (~180 d) before the onset of methanogenesis. Gas chromatographic analyses showed that the longer-chain n-alkanes each added at ~400 mg L(-1) were completely degraded by the resident microorganisms within ~440 d at ~20 °C. 16S rRNA gene sequence analysis of clone libraries implied that the predominant pathway of longer-chain n-alkane metabolism in tailings is through syntrophic oxidation of n-alkanes coupled with CO(2) reduction to CH(4). These studies demonstrating methanogenic biodegradation of longer-chain n-alkanes by microbes native to oil sands tailings may be important for effective management of tailings and greenhouse gas emissions from tailings ponds.

Land-use change is one of the most important factors influencing soil microbial communities, which play a pivotal role in most biogeochemical and ecological processes. Using agroforestry systems as a model, this study examined the effects of land uses and edaphic properties on bacterial communities in three agroforestry types covering a 270 km soil-climate gradient in Alberta, Canada. Our results demonstrate that land-use patterns exert stronger effects on soil bacterial communities than soil zones in these agroforestry systems. Plots with trees in agroforestry systems promoted greater bacterial abundance and to some extent species richness, which was associated with more nutrient-rich soil resources. While Acidobacteria, Actinobacteria and Alphaproteobacteria were the dominant bacterial phyla and subphyla across land uses, Arthrobacter, Acidobacteria_Gp16, Burkholderia, Rhodanobacter and Rhizobium were the keystone taxa in these agroforestry systems. Soil pH and carbon contents emerged as the major determinants of bacterial community characteristics. We found non-random co-occurrence and modular patterns of soil bacterial communities, and these patterns were controlled by edaphic factors and not their taxonomy. Overall, this study highlights the drivers and co-occurrence patterns of soil microbial communities in agroforestry systems.

Naphtha, comprising low molecular weight aliphatics and aromatics (C3-C14), is used as a diluent in processing of bitumen from oil sands. A small fraction (<1%) is lost to tailings waste and incorporated into mature fine tailings (MFT). BTEX (benzene, toluene, ethylbenzene, and xylenes) and whole naphtha were assessed for biodegradation under methanogenic conditions using MFT from an oil sands tailings settling basin. MFT spiked with 0.05-0.1% w/v of BTEX compounds produced up to 2.1 (+/-0.1) mmol of methane during 36 weeks of incubation. Metabolism of 0.5-1.0% w/v naphtha in MFT yielded up to 5.7 (+/-0.2) mmol of methane during 46 weeks of incubation. Gas chromatographic analyses showed that BTEX degraded in the sequence: toluene > o-xylene > m- plus p-xylene > ethylbenzene > benzene. Only 15-23% of whole naphtha, mainly n-alkanes (in the sequence: nonane > octane > heptane) and some BTEX compounds (toluene > o-xylene > m-xylene), was metabolized. Other naphtha constituents, such as iso-paraffins and naphthenes, remained unchanged during this period. These results suggest that the microbial communities in the MFT can readily utilize certain fractions of unrecovered naphtha in oil sands tailings and support methanogenesis in settling basins. Current study findings could influence extraction process, MFT management, and reclamation options.