Assessing changes to the root biofilm microbial community on an engineered floating wetland upon exposure to a controlled diluted bitumen spill

Conventional oil spill recovery practices can damage sensitive habitats, like freshwater shorelines, and leave residual oil in the environment, causing chronic exposure to interacting biota. Non-invasive remediation efforts are required to enhance recovery of residual oil after primary recovery without further environmental damage. Engineered Floating Wetlands (EFWs), also known as Floating Treatment Wetlands, have been used around the world for phytoremediation and biodegradation of aquatic contaminants. EFWs have the potential to increase crude oil biodegradation by enhancing surface area for microbial colonization and interaction with contaminants in the water column. To assess changes in the prokaryotic and eukaryotic microbial communities associated with EFWs in conjunction with primary recovery of shoreline washing, a contained oil spill of diluted bitumen was conducted in a shoreline enclosure in an experimental lake at the International Institute for Sustainable Development Experimental Lakes Area, northwestern Ontario, Canada, in 2019. Total polycyclic aromatic compound (PAC) concentrations in the water column of the experimental enclosure peaked 20 days post spill, declining to near background conditions by day 66, and were dominated by 2-, 3-, and 4-ring alkylated PACs. Although total sediment PACs were highly variable and were influenced by pyrogenic sources (e.g., retene from wood combustion/forest fires) in all sites, concentrations in the experimental enclosure indicated influence from diluted bitumen. The EFW prokaryotic community was diverse and evenly distributed, while the eukaryotic community had lower richness and evenness, with a few dominant organisms. To our knowledge, this was the first in-lake experiment studying EFWs for oil spill remediation under natural environmental conditions, responding to scientific and industrial research needs. While we were unable to confirm whether community shifts were a result of diluted bitumen or seasonal changes, EFWs supported natural microbial diversity, with presence of amplicon sequence variants capable of degrading PACs without the need for bacterial inoculation. Further research should assess EFW microbial changes with other oil products commonly transported in Canada. As well, assessing target PAC degradation rates, EFW surface area requirements, and microbial activity will continue to advance collective knowledge in this field on the potential of EFWs as a secondary remediation strategy.