UC Riverside

As part of their unique life cycle, most Pacific salmonids transition from freshwater to saltwater, requiring various adjustments in physiology. However, molecular mechanisms underlying this transition are largely unknown. Additionally, acclimation to hypersaline conditions enhances the acute toxicity of certain thioether organophosphate and carbamate pesticides in some species of euryhaline fish, yet sublethal impacts have been far less studied. The current study aimed to determine underlying molecular mechanisms of Pacific salmonid smoltification, as well as determine how hypersaline acclimation impacts acute and sublethal toxicity of a common organophosphate pesticide, chlorpyrifos (CPF). A transcriptomics approach was used to assess differential gene expression in coho salmon (Oncorhynchus kisutch) liver, gills, and olfactory rosettes after salinity acclimation and found that the majority of the altered genes were tissue and salinity concentration dependent. From the few shared genes, a potential osmosenor was identified. Osmotic signal transduction cascades were also impacted in the three tissues. Salinity acclimation was then coupled with CPF to determine impacts on acute toxicity. Time to death of rainbow trout (Oncorhynchus mykiss) by CPF was more rapid in freshwater than in hypersaline water (16 ppth). Salinity acclimation did not impact metabolism, precipitation, or acetylcholinesterase inhibition of CPF. In contrast, mRNA expression of certain neurological targets was upregulated in saltwater acclimated fish, consistent with diminished neuronal signaling which may protect fish from cholinergic overload associated with acetylcholinesterase inhibition. Sublethal experiments, which are more environmentally relevant, were conducted to determine impacts on olfaction. Combined acclimation and exposure to CPF impacted rainbow trout olfaction at the molecular, physiological, and behavioral levels. Concurrent exposure to hypersalinity and 0.5 µg/L CPF upregulated four genes that inhibit olfactory signal transduction. At the physiological level, hypersalinity and chlorpyrifos caused a decrease in sensory response to the amino acid L-serine and the bile salt taurocholic acid. Combined acclimation and exposure also negatively impacted behavior and reduced the avoidance of a predator cue (L-serine). Overall, these results will be very useful in risk assessment strategies evaluating compounds of this nature in estuarine environments and freshwater environments that may be altered by hypersaline stress or rising sea levels.