A multiple endpoint analysis of the effects of chronic exposure to sediment contaminated with Deepwater Horizon oil on juvenile Southern flounder and their associated microbiomes

•Juvenile southern flounder were exposed to sediment mixed with different amount of oil from the Deepwater Horizon spill.•The exposure was performed for 32 days, with growth and survival assessed throughout.•After the termination of the experiment, the survivors were examined at multiple endpoints, including histopathology and microbiome analysis.•The results indicated that the flounder were adversely affected at each endpoint examined.•Growth and survival were significantly reduced.•Histopathology found effects on gills and livers of exposed fish.•The microbiomes of the exposed fish were significantly altered by the exposure to sediment-associated oil in both gills and intestines.

Exposure to oiled sediments can negatively impact the health of fish species. Here, we examine the effects of chronic exposure of juvenile southern flounder, Paralichthys lethostigma, to a sediment-oil mixture. Oil:sediment mixtures are persistent over time and can become bioavailable following sediment perturbation or resuspension. Juvenile flounder were exposed for 32 days under controlled laboratory conditions to five concentrations of naturally weathered Macondo MC252 oil mixed into uncontaminated, field-collected sediments. The percent composition of individual polycyclic aromatic hydrocarbons (PAHs) of the weathered oil did not change after mixing with the sediment. Spiked exposure sediments contained 0.04–395 mg/kg tPAH50 (sum of 50 individual PAH concentration measurements). Mortality increased with both exposure duration and concentration of sediment-associated PAHs, and flounder exposed to concentrations above 8 mg/kg tPAH50 showed significantly reduced growth over the course of the experiment. Evident histopathologic changes were observed in liver and gill tissues of fish exposed to more than 8 mg/kg tPAH50. All fish at these concentrations showed hepatic intravascular congestion, macrovesicular hepatic vacoulation, telangiectasia of secondary lamellae, and lamellar epithelial proliferation in gill tissues. Dose-dependent upregulation of Cyp1a expression in liver tissues was observed. Taxonomic analysis of gill and intestinal commensal bacterial assemblages showed that exposure to oiled sediments led to distinct shifts in commensal bacterial population structures. These data show that chronic exposure to environmentally-relevant concentrations of oiled sediments produces adverse effects in flounder at multiple biological levels.