Anaerobic biodegradation of hydrocarbons in different oil sands tailings ponds: key microbial players and main activation pathway of hydrocarbon biodegradation

• Author / Creator

  Mohamad Shahimin, Mohd Faidz

• Anaerobic biodegradation of hydrocarbons is an important process in oil sands tailings ponds, which directly affects consolidation of tailings and extent of greenhouse gas emissions from tailings ponds. Since tailings ponds from different operators differ due to extraction process, age of tailings deposition in ponds, and tailings management techniques employed, biodegradability of entrained residual hydrocarbons and structure and activities of hydrocarbon-degrading microbial communities may differ from one tailings ponds to another. In this research, we examined methanogenic biodegradation of different extraction solvents (hydrocarbons used in bitumen extraction) and their major components (n-alkanes and iso-alkanes) in mature fine tailings (MFT) collected from two oil sands operators (Shell Albian Sands and CNRL). We examined biodegradation of recalcitrant hydrocarbons such as iso-alkanes under iron-, nitrate- and sulfate-reducing conditions in Albian MFT to determine if presence of higher reduction potential molecules/electron acceptors would accelerate biodegradation of these recalcitrant hydrocarbons. We characterized microbial communities and functional genes during the biodegradation process to identify key microbial players and main activation pathway of hydrocarbon biodegradation. The methanogenic cultures established from Albian and CNRL MFT exhibited unique hydrocarbon degradation patterns. Albian and CNRL MFT amended with paraffinic solvent and naphtha separately were incubated for ~1600 d. Albian and CNRL MFT exhibited ~400 and ~800 d lag phases, respectively, for paraffinic solvent (C5-C6) biodegradation after which n-alkanes were preferentially metabolized to CH4 over iso-alkane. A shorter lag phase (~100 d) was observed for naphtha (primarily ~C6-C10) biodegradation in both Albian and CNRL MFT. Both Albian and CNRL MFT sequentially biodegrade n-, iso- and cyclo-alkane compounds from naphtha during the incubation. MFT cultures amended with sole n-alkanes (C5-C10) and incubated for ~600 d revealed preferential biodegradation of shorter n-alkanes (C5-C8) in Albian MFT versus longer n-alkanes (C8-C10) in CNRL MFT though all n-alkanes were eventually biodegraded in both MFT. Interestingly when Albian and CNRL MFT were amended with shorter (mixture of three C5-C6 compounds) and longer (mixture of five C6-C9 compounds) iso-alkanes and incubated under methanogenic conditions, Albian MFT, after a lag period of ~200 d, biodegraded mixture of C6 iso-alkanes during ~1500 d of incubation. CNRL MFT also biodegraded C6 iso-alkanes after a lag phase of ~660 but it also displayed biodegradation of C6-C9 iso-alkanes after a lag phase of ~1200 d where iso-alkanes were sequentially biodegraded in the order of decreasing carbon-chain length. C6 iso-alkanes were studied under other reducing conditions. Only partial degradation of a C6 iso-alkane was observed under sulfate-reducing condition and no degradation was observed under iron- and nitrate-reducing conditions. These results suggest that iron- and nitrate-reducers indigenous to Albian MFT may not carry the appropriate genes for iso-alkanes degradation. The 16S rRNA gene pyrosequencing revealed that bacterial sequence reads related to Peptococcaceae were enriched during biodegradation of the hydrocarbons in all methanogenic cultures implicating the role of Peptococcaceae as the primary hydrocarbon-degraders in methanogenic MFT. Interestingly, the archaeal communities in all methanogenic cultures exhibited codominance of acetoclastic (Methanosaetaceae) and hydrogenotrophic (“Candidatus Methanoregula”), emphasizing important roles of both methanogens as acetate and hydrogen consumers, respectively, which renders the anaerobic degradation process in MFT a thermodynamically feasible reaction. The metabolite (intermediary biodegradation products) and functional gene analyses performed on all cultures during active degradation process revealed that fumarate addition might potentially be the primary anaerobic hydrocarbon activation pathway in both Albian and CNRL tailings ponds. The fumarate-added metabolites detected in methanogenic Albian and CNRL cultures were the transient products. However, under sulfate-reducing conditions, the fumarate-added metabolites persisted in the culture even after prolonged incubation, implying absence of microbial players capable of oxidizing these intermediary compounds. Subunit of alkylsuccinate/methylalkylsuccinate synthase (assA/masD) genes amplified in all the cultures further substantiates the postulation of fumarate addition as the primary anaerobic hydrocarbon activation pathway. The results demonstrate that microbial communities indigenous to tailings ponds have the potential to biodegrade structurally diverse hydrocarbons; however, the biodegradation pattern differs in different oil sands tailings ponds even though similar key bacterial and archaeal taxa were enriched during active biodegradation of the hydrocarbons. This work provides an insight into how microbial communities in oil sands tailings respond to the influx of complex hydrocarbons under different redox conditions, which could impact future tailings management and reclamation strategies.

• Subjects / Keywords

  • Hydrocarbons
  • Oil sands tailings
  • Anaerobic biodegradation
  • Methanogenesis

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3TT4FZ4Z

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.