In northern Alberta, mining operations to obtain bitumen from the oil sands generates large volumes of tailings. These are a mixture of sand, clay, water, organic solvents and residual bitumen that are deposited into old open pits, creating tailings ponds, where they are allowed to settle with the final goal of land reclamation. To speed up the sedimentation process, the addition of gypsum (CaSO4 ∙ 2H2O) is currently a management approach used bysome companies. This creates a deep watery mud line with very low oxygen permeability and enough sulfate to support the growth of anaerobic microbial communities. In this thesis work, the microbial physiology and communities associated with oil sands tailings ponds were assessed. Chemical, physiological, and molecular biology approaches were used to determine the key microbial processes (methanogenesis, sulfate reduction/oxidation), identify key substrates, and determine the dominant microbial community members in the anaerobic and aerobic zones of tailings ponds. Microbial community analysis showed that in the anaerobic zone of tailings, the sulfate-reducing/disproportionating bacterium Desulfocapsa. and the sulfide oxidizer/iron reducer Thiobacillus sp. are among the most prevalent organisms when sulfate is present. After sulfate is depleted, methanogenic Archaea become predominantly active and Methanosaeta and Methanolinea in association with Syntrophus dominate in the ponds, presumably interacting to biodegrade the available organic compounds. The residual naphtha components that constitute part of the tailings composition are the preferred electron donors in anaerobic zones (in comparison to naphthenic acids) based on enrichment culture studies. In naphtha-amended laboratory cultures, a variety of methanogens in association with Thauera sp. and Desulfocapsa sp. became enriched as the dominant organisms. Overall, microbial community composition as a function of depth in tailings ponds paralleled key
microbial processes that were measured (sulfate reduction and methanogenesis). In the aerobic surface water, other microbes with known metabolic capabilities to degrade hydrocarbon-derived compounds such as naphthenic acids were found. The results of this work also showed that operational changes to tailings ponds shift the microbial community structure and functions. For example, pond closure resulted in a shift from a predominantly methanogenic and sulfate-reducing environment to one dominated by putative hydrocarbon degraders, indicating a positive management outcome in microbial activity associated with pond closure.