Study says old, weathered oil from spills is even more toxic to fish than fresh crude

Scientists still studying Deepwater Horizon spill impacts

Oil from BP's failed Deepwater Horizon drill rig and the Macondo well spread across the ocean in May 2010. PHOTO COURTESY NOAA.
Oil from BP’s failed Deepwater Horizon drill rig and the Macondo well spread across the Gulf of Mexico in May 2010. Photo courtesy NOAA.

Staff Report

It’s been six years since BP’s Deepwater Horizon drilling rig in the Gulf of Mexico failed disastrously, but scientists are still learning about how the oil affected ocean species and ecosystems.

In findings from new study released this week, researchers from the University of California, Riverside and the University of Miami Rosenstiel School of Marine and Atmospheric Science reported that old, weathered oil from the spill is even more toxic than fresh crude oil. Ultraviolet light changes changes the chemistry of the oil, the scientists said, further threatening numerous commercially and ecologically important fishes.

The Deepwater Horizon oil disaster was the worst on U.S. history. More than 3 million barrels of oil spilled into the Gulf, contaminating spawning habitats for many fishes. The oil also killed deep sea corals and had a devastating effect on dolphin reproduction.

“Ours is the first experiment evaluating the effects of DWH oil on the genetic responses of mahi-mahi embryos and larvae,” said Daniel Schlenk, a professor of aquatic ecotoxicology, who led the study published in Environmental Science and Technology. “It is also the first experiment of this nature on a lifestage and species that was likely exposed to the oil. We found that the weathering of oil had more significant changes in gene expression related to critical functions in the embryos and larvae than the un-weathered oil.”

In a lab setting, the researchers exposed fish embryos to the oils at key points in the reproductive cycle and studied how that exposure affected genetic development and expression. They also evaluated the toxicity and heart functions in animals using the embryos’ gene expression to predict biochemical, cellular, and tissue targets where the oil was causing an effect.

For their experiments, Schlenk and UM Rosenstiel School scientists caught the mahi-mahi off the coast of Miami, Fla., and exposed embryos to two types of oil: one set of embryos was exposed to slick oil (weathered) from the spill while another set was exposed to oil that came from the source of the spill. The researchers carried out the experiment this way because fish in the northern Gulf of Mexico had been exposed during the spill to both types of oil. Their study attempted to understand which of the two oils – slick oil or source oil – is worse for the fish and how oil affects development.

“In addition to impacts on heart development and function, the gene expression results illustrate the peripheral components of the nervous system involved in sensory function is impaired by oil exposure during early development,” said UM Rosenstiel School Professor Martin Grosell.

Sensory function is important for prey detection and predator avoidance.

“We found that the heart, eye and neurological function were affected,” Schlenk said. “In collaboration with other consortia members from the Universities of Miami, Texas, and North Texas, we are now following up on these results. Previous studies have shown that the heart is the primary target for oil. Our study shows that in addition to heart function, risk and recovery should also examine eye and neuronal function.”

The study was expedited by a unique software, On-RAMP, that the researchers used to identify the gene signatures from the fish.

“Normally, it can take months to annotate the genes and identify the regulatory directions of expression,” Schlenk said. “But by using On-RAMP, we could identify the genomic responses in a matter of weeks, allowing pathway analyses with sophisticated software normally only used for human/mice responses.”

Next, the research team will follow up with whole animal physiological and behavioral effects to see if the newly identified molecular responses can be linked to function.

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