Soon after the Deepwater Horizon rig began spewing oil into the Gulf of Mexico, Andrew Whitehead and his colleagues at Louisiana State University set out to determine how that oil would affect gene expression in fish along the Gulf Coast. His team took samples of killifish and water from a number of sites along the Gulf Coast before, during, and after oil from the Deepwater Horizon spill reached the shore.
"Being local, having the tools in place, knowing something about the habitat and ecology of this fish, and knowing something about its physiology positioned us fairly well to get out there and exploit this fish and exploit these tools as a model to try to understand what's going on, what are the potential impacts of the oil spill," Whitehead says.
Previously, Whitehead's team studied genes involved in stress tolerance in Atlantic killifish, with a particular focus on how the fish react to PCB exposure. PCBs, Whitehead says, are similar to PAHs, the toxic part of oil, and so they used a microarray they had previously developed to study how the oil affected killifish in the salt marshes of the Gulf Coast. Killifish are the most abundant fish in the ecosystem — they do not stray far from that area — and thus, he adds, should give a good indication of the effects of the local environment.
As the researchers reported online in PNAS in September, about 1,500 genes showed site-dependent changes in expression over time. Whitehead notes that most of those gene expression changes were seen in killifish samples from the Grand Terre, La., site, which was the only study site to be covered with oil. "Those PCB-responsive genes are really predicting a divergence of genome expression at Grand Terre, indicating that those genes that turn on PCBs are also really getting turned on at Grand Terre, coincident with oil," Whitehead says, adding that this "is what you'd predict since PCBs and PAHs are mechanistically related."
In particular, the researchers found that genes associated with egg and cardiovascular development are divergently expressed in the Grand Terre samples. Currently, LSU's Fernando Galvez is exposing killifish embryos to sediments from oiled sites, and is so far seeing differences in development rate and hatching success between the embryos. "We are finding some very dramatic effects in that the survivors, the larvae, are barely moving," he says. "They are much smaller compared to the ones that are reared in control sediments."
In all, Galvez says this may mean that the fish, though safe to eat, could have population-wide troubles. "What is problematic is some of these sub-lethal exposures [may lead] to potential population-level effects where we start to see reductions in the size of the population," Galvez says. "That may eventually have larger cascading effects in terms of the population size."