Abstract
Transport of diluted bitumen (dilbit) from Canada’s oil sands region poses risk for leaks and spills of petroleum-derived contaminants into the environment. Exposure of fish to dilbit is known to cause cardiotoxicity, developmental deformities, and impairment in swim performance. However, previous studies have examined the toxicity of dilbit in laboratory settings which does not account for environmental and biological food-web variables that may alter exposure and/or toxicity of dilbit. Moreover, most methods of assessing organism health following oil exposure require lethal sampling. This work is a part of a larger set of experiments where dilbit spills were simulated within enclosures on a lake; the present study assesses the impacts of residual levels of dilbit that may have entered the surrounding lake environment from the enclosures following model spill cleanup. In order to understand the impacts of residual dilbit in an ecosystem setting without use of lethal sampling, epidermal mucus was collected and sequenced from lake charr (Salvelinus namaycush) exposed to residual dilbit in a boreal lake. While concentrations reached a maximum of 2.29 μg/L total polycyclic aromatic compounds (ΣPAC) within surface waters, surface water ΣPAC concentrations generally remained below 1 μg/L. Results of RNA sequencing were compared to sequencing data from mucus collected prior to dilbit additions. Differential gene expression and pathway analyses indicated dysregulation of genes associated with intermediary and energy metabolism as well as a trend in upregulation of cyp1a3 in epidermal mucus following dilbit exposure. Thus, results of the present study suggest that lake charr undergo consistent biological responses after exposure to residual levels of dilbit following a model spill, and that mRNA-based analysis of mucus may be a viable method for non-lethal oil exposure assessment. Overall, the results provide insight on the response of wild fish to very dilute dilbit exposures after a model spill cleanup.
Highlights
The oil sands region of Alberta, Canada has one of the largest known reserves of bitumen, containing an estimated 50 billion cubic meters of the heavy type crude oil (Canada National Energy Board, 2006)
ΣPAC concentrations of 0.058 μg/L were detected in lake surface waters prior to dilbit additions into shoreline enclosures in June 2019 (Figure 1); these levels could be linked to residual dilbit in sediments following previous 2018 dilbit experiments, through atmospheric sources, or from background polycyclic aromatic compounds (PACs) from organic sediment inputs
Following the additions of dilbit into shoreline enclosures on June 22 (Day 0), there was an increase in PACs within surface waters, reaching a maximum concentration of 2.29 μg/ L ΣPAC on Day 5 (Figure 1)
Summary
The oil sands region of Alberta, Canada has one of the largest known reserves of bitumen, containing an estimated 50 billion cubic meters of the heavy type crude oil (Canada National Energy Board, 2006). The toxicity of dilbit in freshwater environments has been minimally studied and has only recently begun to receive more extensive research (Dupuis and Ucan-Marin, 2015; National Academies of Sciences Medicine and Engineering, 2016). Exposure of dusky splitfin (Goodea gracilis) to crude oil resulted in increased antioxidant defense response in the epidermal mucus (Dzul-Caamal et al, 2016) Results from these previous studies suggest that epidermal mucus may be a useful target for environmental monitoring following oil spills. Previous studies were laboratory-based and may not reflect changes in chemical properties of oil brought about by environmental conditions such as a evaporation, photooxidation, emulsification, dispersion, biodegradation, and sedimentation (National Academies of Sciences Medicine and Engineering, 2016). Results of the present study will identify molecular pathways altered by dilbit in a whole-lake setting and further contribute to the development of mucus as a nonlethal tool for oil exposure assessment. The present study provides insight on the response of wild fish to very dilute μg/L dilbit exposure after a model spill cleanup
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