Seeing through the clip of a fin: comparing the salmonid liver and caudal fin transcriptome in demonstrating oil spill response

Date

2021-08-25

Authors

Imbery, Jacob

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Abstract

Effective tracking and management of an oil spill are crucial to mitigating deleterious effects on fish. Currently, the area covered by an oil spill is tracked primarily through satellite and aerial imaging of the top slick. However, this method of oil spill tracking fails to capture the dispersal of water-soluble compounds released from a spill, such as polycyclic aromatic hydrocarbons (PAH) and naphthenic acids (NA), that can be environmentally persistent. Transcriptomic biomonitoring of sentinel animals can be a powerful method for tracking and evaluating the impact of environmental toxicants, however, current sampling methods are lethal. Instead, the present thesis work evaluates the use of the caudal fin transcriptome as a nonlethal alternative to demonstrate salmon biological response to the water accommodated fraction (WAF) generated during an oil spill. Most deleterious effects of WAF exposure presented in the literature were observed in fish embryos exposed to crude oil WAF under freshwater and/or warm marine conditions. As temperature, salinity, and oil type significantly impact oil WAF composition and toxicity, it is important to investigate effects of WAF exposure generated in cold-water marine conditions and with a variety of oil types. Herein, oil WAFs were generated at nonlethal concentrations with 100-to-1000 mg/L of one of four different oil types under cold-water marine conditions and juvenile coho smolts were exposed for 96 h. Paired caudal fin and liver tissues were collected from genotypically-sexed smolts and transcriptomic responses queried through gene-targeted quantitative real-time polymerase chain reaction (qPCR) and RNA-Seq assays. WAFs were characterized by the sum of 50 PAHS (tPAH-50) and measured using gas chromatography/triple quadrupole mass spectrometry. WAFs generated with low sulfur marine diesel (LSMD) resulted in a significant and sex-biased change in gene transcript abundance relative to unexposed seawater controls in both the liver and caudal fin, demonstrating PAH exposure, general and oxidative stress, and estrogenic activity. In both female and male liver and caudal fin, cyp1a1 exhibited the greatest increase in transcript abundance after 96 hours of LSMD WAF exposure, demonstrating that cyp1a1 is a robust, sex-independent bioindicator of LSMD WAF exposure at this time point. Further evaluating transcript abundance of ahr and cyp1a1 in the caudal fin significantly demonstrated response to WAFs generated with high sulfur fuel oil (HSFO), diluted bitumen (dilbit), and Alaskan crude oil relative to seawater controls. In contrast, response to Alaskan crude WAF exposure was not detected in the liver. HSFO WAF exposure induced the greatest cyp1a1 response in both tissues, with the lowest response to Alaskan crude WAF despite containing the second highest tPAH50 concentration demonstrating a need for better PAH-responsive transcript bioindicators. RNA-Seq analysis of LSMD and HSFO WAF exposures revealed significant responses in the liver and caudal fin transcriptomes that were specific to oil type, tissue, and genetic sex. In addition to enrichment of transcripts involved in immunomodulation (LSMD WAF) and a cancer-like cell state (HSFO WAF), RNA-Seq analysis identified liver and caudal fin bioindicator candidates common across oil WAF types. Ultimately, this work demonstrates a robust response of the coho salmon caudal fin transcriptome to oil WAF exposure that is more consistent between sexes and different oil exposures than the conventional liver tissue. This improved consistency is ideal for biomonitoring assays and demonstrates promise for using the caudal fin transcriptome as a non-lethal alternative for assessing and tracking oil spill exposure in marine environments.

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Keywords

Oil spill, non-lethal sample, salmon, sublethal biological effects, gene expression, transcriptomics, polycyclic aromatic hydrocarbons, RNA-Seq

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