SUBLETHAL EFFECTS OF DIETARY SELENOMETHIONINE ON JUVENILE WHITE STURGEON (ACIPENSER TRANSMONTANUS)
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Sturgeon are an ancient family of fish comprised of 26 known species, most of which are threatened or endangered. Populations of white sturgeon (WS; Acipenser transmontanus), which are endemic to western North America, have been declining since the 1800’s due to overharvesting, habitat alteration and increased contaminant loads from industrial and agricultural activities. The longevity, benthic lifestyle and late sexual maturation might make WS particularly susceptible to bioaccumulative toxicants, such as selenium (Se). Although Se is an essential micronutrient, it can be toxic to aquatic organisms at low concentrations and it is prevalent in surface waters due to various sources, including natural weathering of rocks and soil, agricultural runoff, discharge from mining and milling operations, and fossil fuel combustion. One organic form of Se, selenomethionine (SeMet), is a particular environmental concern due to its ability to persist and bioaccumulate through the food chain. Selenomethionine is known to persist in some WS habitats and it has been detected in their tissues and prey at concentrations above dietary and tissue toxicity thresholds. Selenomethionine can adversely affect a range of physiological processes in fish, including the physiological stress response; however, studies assessing the sensitivity of WS to SeMet are limited and the effects of SeMet on their physiological stress response are unknown. While oxidative stress has been hypothesized as one of the primary mechanisms of SeMet toxicity in teleosts, the mechanisms leading to adverse effects in WS are poorly understood. Therefore, the goal of this research was to expand the understanding of WS sensitivity to SeMet by assessing sublethal effects of dietary SeMet exposure, including effects on their ability to mount a stress response. As well as to provide insight into potential pathways of SeMet toxicity in WS, by linking molecular responses to adverse whole organism effects observed in a parallel study. Juvenile WS were fed either a control diet containing 1.4 μg Se/g dry mass (dm), or a spiked diet containing 5.6, 22.4 and 104.4 μg Se/g dm, for 72 d. Effects of SeMet on the cortisol stress response were evaluated by subjecting fish to a 2 min handling stressor on the final exposure day, followed by quantifying concentrations of cortisol, glucose and lactate in blood plasma, and glycogen levels in muscle and liver tissue. Furthermore, abundances of transcripts of genes along the hypothalamus-pituitary-interrenal (HPI) axis were analyzed using qPCR. RNA- seq analysis was conducted on liver samples from control and medium dosed fish, collected on day 10 of the exposure, to assess transcriptional responses and identify physiological processes that might be adversely impacted by SeMet. Exposure to dietary SeMet for 72 d did not impact the ability of WS to mount a cortisol response to an acute handling stressor; however, this response could only be assessed in the low and medium dose fish due to high mortalities in the high dose group. There was some indication that the medium dose of SeMet altered the ability of WS to utilize muscle glycogen for energy, following exposure to the acute handling stressor. Basal levels of plasma cortisol, glucose, lactate, and liver glycogen were significantly altered in the high dose fish. RNA-seq analysis identified 178 and 147 transcripts that were significantly down- and up- regulated, respectively. Biological pathways associated with the differentially expressed transcripts were analyzed through ClueGO, a Cytoscape plug-in. Selenomethionine altered several physiological processes, but the majority were involved with the regulation of energy, cholesterol/lipid and protein metabolism. Alterations in these pathways appeared to be associated with reduced hepatic energy stores and growth, as well as increased edema and mortality observed in WS by the end of the exposure. Overall, the results of this research indicate that WS are sensitive to dietary SeMet at concentrations greater than or equal to 22.4 μg/g dm based on the biochemical and molecular endpoints assessed in this research. Effects on energy, cholesterol/lipid and protein metabolism likely play an important role in the development of SeMet toxicity in WS and should be investigated in future studies. WS also appear to respond differently to SeMet from other fishes, as seen by the lack of effects on their stress response at lower concentrations, the lower magnitude cortisol response, and the development of edema at low concentrations. These differences highlight that cross-species extrapolations in regard to the toxicity of SeMet in fishes, cannot always be made, and this should be considered in future environmental risk assessments for Se.
DegreeMaster of Science (M.Sc.)
CommitteeJanz, David; Wiseman, Steve; Pyle, Greg; Liber, Karsten
Copyright DateOctober 2018