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This study characterized the toxicity and physiological effects of unweathered diluted bitumen (Access Western Blend dilbit; AWB) to fish. Embryos of Japanese medaka ( Oryzias latipes ) were exposed for 17 days to dilutions of physically-dispersed (water accommodated fraction; WAF) and chemically-dispersed (chemically-enhanced WAF; CEWAF) dilbit. AWB dilbit exposure was not lethal to medaka, but resulted in a high prevalence of blue sac disease (BSD), impaired development, and abnormal or un-inflated swim bladders at hatch. Physiological effects were indicated by the relative mRNA levels of key genes associated with, among others, cell cycling and the response to mutations ( p53 ), xenobiotic metabolism ( ahr , arnt2 ), phase I ( cyp1a ) and II processes associated with oxidative stress ( cat, g6pdh, hsp70, gst, gpx, gsr, nfe2, and sod ). AWB dilbit treatment increased p53 and cyp1a transcript levels (1.5-fold and >15-fold, respectively), with significant, but less pronounced changes in indicators of oxidative stress and metabolism. The exposure-related changes in embryotoxicity and mRNA synthesis were consistent with metabolism of polycyclic aromatic hydrocarbons (PAHs) to reactive and toxic metabolites. Medaka embryos responded similarly to WAF and CEWAF treatments, but CEWAF was about 100 times more efficient in delivering toxic concentrations of PAHs. The toxicity of chemically-dispersed nujol, a non-toxic mineral oil used as an experimental control, suggested that a portion of the observed effects of AWB could be attributed to excess dispersant in solution. This first study of the physiological effects of dilbit toxicity to fish embryos provides a baseline to compare toxicity between dilbit and conventional crude oils, and the groundwork for the development of molecular biomarkers of the sensitivity and level of risk of native Canadian fish species to dilbit exposure. [ABSTRACT FROM AUTHOR]

Abstract Canada has experienced a significant increase in the transport of diluted bitumen (dilbit), a predominant oil sands product that combines bitumen with diluents derived from oil-gas condensates and other proprietary compounds. The toxicity of dilbit to fish embryos, which are immobile and thus at a high risk of exposure to oil in the event of a spill, remains largely unknown for most species. This study assessed the toxicity of water accommodated fractions (WAF) and chemically enhanced water accommodated fractions (CEWAF) of two winter dilbit blends, Access Western Blend (AWB) and Cold Lake Blend (CLB), to fathead minnow (Pimephales promelas) embryos. The TPH-F EC50s for malformations were 834 and 1058 μg/L for AWB WAF and CEWAF, respectively, and 500 and 715 μg/L for CLB WAF and CEWAF, respectively. Levels of cyp1a mRNA increased up to 46- and 69-fold, respectively, reflecting increasing exposure to polycyclic aromatic compounds (PACs) in AWB and CLB. Similarly, levels of gst mRNA were elevated up to 3.8-fold and 2.7-fold with increasing total concentrations of PACs in AWB and CLB, respectively. However, there were no significant changes in mRNA levels of p53 , sod, cat , and gsr. These results suggest that the expression of cyp1a and gst may serve as biomarkers for dilbit exposure in fathead minnow, furthering our understanding of dilbit-responsive indicators of toxicity in fish species native to North America. This study is important as it utilizes the same exposure methodology to examine the toxicity of two commonly used Canadian dilbits, facilitating comparison of dilbit toxicity. [ABSTRACT FROM AUTHOR]