Comparative toxicity of selected naphthenic acids, oil sands processed water and surface waters in rainbow trout hepatocytes: A gene expression study

Abstract

SOJ Biochemistry Open Access Research article deposits and is considered one of the world's largest oil reserves with an estimated yield of 27 billion cubic meters of crude oil [1]. OS consist of about 10% bitumen and 5% water, with the remainder being sand and clay. The extraction process involves adding hot caustic water to OS under aeration to separate bitumen from the OS (Clark extraction process). The crude oil fraction, which partitions at the surface, is then further extracted with the addition of solvents (naphtha, toluene) to yield a more concentrated non-polar fraction. The remaining water and sediment deposits constitute the OSPW, which is released into large tailing ponds covering many square kilometers. Numerous contaminants are found in OSPW, including a number of alkali-extractable compounds such as Naphthenic Acids (NAs), polyaromatic and aliphatic hydrocarbons and various inorganic elements such as sodium, vanadium, strontium, calcium, nickel and sulfates [2-4].Recent evidence has shown that OS mining activity was associated with increased releases of dissolved PAHs and heavy metals, which raised concerns about the ecotoxicological consequences of these activities [5-6]. NAs are cyclic aliphatic hydrocarbons that can reach concentrations as high as 50 mg/ L in tailing ponds [7]. NAs follow the C n H 2n-z O x rule which comprise at least one carboxylic acid and they are suspected to be a major component of OSPW toxicity. However, the presence of NAs has been observed in runoff waters (leachates) from OS storage sites, indicating that OSPW is not the only source of NA in the environment [8]. This highlights the difficulty of identifying specific markers of OSPW in the environment at low levels, although the highest concentrations of NAs are found in OSPW. The toxicity of OSPW to aquatic organisms has received increasing attention over the last decade. Toxicity has been examined at various levels, including endocrine disruption, DNA damage, immune competence, biotransformation and reproduction. Goldfish exposed to OSPW for 12 weeks showed elevated expression of pro inflammatorygenes such as interleukin-1β and tumor necrosis factor 2 in the spleen and kidneys [9]. This Abstract This study examined the cytotoxic properties of selected naphthenic Acids (NA), commercial mixtures of NA in rainbow trout hepatocytes and compares the responses with Oil Sand Processed Water (OSPW) and river water extracts. Hepatocytes were exposed to increasing concentrations of selected NAs (z = 0, 2, 4, 6, 8 and 10), 2 commercial mixtures of NA, 2 OSPW, and surface waters upstream and downstream from the Athabasca River at an OS development area (Alberta, Canada) for 48 h at 15°C. Cell viability, total RNA levels and the expression of 15 gene transcripts involved in biotransformation (CYP1A1, CYP3A4, GST and MDR), oxidative stress (SOD, CAT, GST), DNA damage/repair (UNG, APEX, LIG, GADD45, OGG), estrogenicity (VTG and ER2), cell growth (PCNA, GADD45), and glycolysis (GAPDH) were determined. Individual NAs induced the expression most of the genes, except for GST, whose expression was reduced. For NA mixtures, the most sensitive genes were those involved in DNA repair (APEX, LIG, GADD45, and OGG) and biotransformation (CYP1A1 and 3A4). The same pattern was observed for OSPW, except that MDR was the most sensitive gene, and GST gene expression was decreased as occurred with NAs. The responses for river water samples were generally lower than for OSPW, NAs and NA mixtures, and involved biotransformation (GST and MDR) genes, DNA repair (OGG, LIG) genes and potential endocrine disruption (ER2). Canonical analysis of gene expression data and cell viability revealed that genes involved in Xenobiotic metabolism, oxidative stress and DNA damage (repair) were strongly correlated with cytotoxic effects. Decision tree analysis revealed that these compounds separated into 4 distinct clusters (OSPW, upstream and downstream river water, NA mixtures), whereas the individual NAs were generally found in the OSPW cluster. OSPW and NA mixtures showed distinct properties from surface waters collected downstream from the OS development area suggesting that OSPW did not overly contaminated river waters.