Microbial community development in an oil sands pit lake

Surface mining and extraction of oil sands in Canada produces fluid tailings that contain several compounds of concern for the environment. One option for mine reclamation is the construction of Pit Lakes (PLs) to contain and remediate these tailings. Ultimately, PLs should support food webs typical of boreal lakes. From 2015 to 2021, we applied 16S/18S rRNA gene amplicon sequencing and metagenomics to monitor prokaryotic and eukaryotic microbes in the only full-scale PL of the oil sands industry (Base Mine Lake or BML), and compared it to two control environments: a freshwater reservoir unaffected by tailings, and active tailings ponds receiving regular industrial input. Microbial communities in BML were always intermediate to the two control environments based on alpha and beta diversity analyses. BML communities were highly variable with year, season, and water depth, and contained fewer core species than the freshwater reservoir. Several hydrocarbon degraders and sulfur cycling bacteria were identified as indicator species of tailings ponds, while several phototrophs were indicative of freshwater. However, all of these species were abundant in BML, suggesting that the PL supports food webs characteristic of each control environment. Over the 6-year study, the relative abundances of some common freshwater phytoplankton (Cryptomonas, Mychonastes, Trebouxiophyceae, Cyanobium) and heterotrophic bacteria (Sporichthyaceae, Ca. Fonsibacter, Ilumatobacteraceae, Microbacteriaceae, Ca. Planktophila) increased in BML. The results suggest that microbial communities and processes in BML represent an intermediate state between a tailings pond and a natural freshwater system, and did not stabilize within 10 years of its creation.