Bottlenose dolphins living near the Deepwater Horizon oil spill in the Gulf of Mexico are showing striking signs of genetic changes related to a wide variety of bodily functions, according to a study published Wednesday.
The discovery underscores how scientists are unraveling the lasting effects associated with the unprecedented April 2010 disaster, which released an estimated 210 million gallons of crude oil off the coast of Louisiana and killed 11 people. It is also estimated to have killed more than 80,000 birds and nearly 26,000 marine mammals.
The study focused on dolphins in the highly polluted Barataria Bay near New Orleans and used blood tests to compare these dolphins to those living in the less polluted waters of Florida’s Sarasota Bay.
The researchers then discovered gene expression changes in the Barataria Bay dolphin population, including genes involved in immunity, inflammation, reproductive failure, lung problems and heart dysfunction. The results were published in the journal PLoS ONE.
These changes align with previously documented health effects, said co-author Sylvain De Guise, a professor in the department of pathobiology at the University of Connecticut. He also co-authored another study that found the dolphin population in Barataria Bay has declined by 45 percent since the disaster. In an assessment of dolphins that lived through the oil spill, De Guise and his colleagues found that nearly 80 percent were still experiencing some form of ill health, with lung disease the most common problem.
This new study leveraged data collected from dolphin health assessments conducted between 2013 and 2018. The team analyzed blood from 60 dolphins from Barataria Bay and 16 from Sarasota Bay and looked for molecular differences through a process called gene expression profiling. This method is a new way of understanding the health of an organism because it has the potential to allow early detection of disease and is easier to perform than traditional veterinary catch-and-release assessments.
The dual purpose of this study was to test and refine this method while trying to understand the underlying causes of the health consequences experienced by Barataria Bay dolphins. In the future, the study team hopes this method can help identify marine mammals at risk of disease.
“We can tell that dolphin populations are experiencing impacts, but we don’t really know what’s behind the disease and dysfunction that we’re seeing,” said first author Jeanine Morey, a research biologist working with the National Marine Mammal Foundation at the time . the study. “Through this molecular work, we are beginning to understand the root of the problem.”
Various stimuli can cause changes in gene expression, he said. Changes in gene expression, in turn, cause the body to respond. Because factors such as water temperature can also cause a change in gene expression, comparing Barataria Bay dolphins with Sarasota Bay dolphins helped the team identify exposure to oil pollution as a key difference between the two groups.
Additionally, because of previous studies assessing the health of Barataria Bay dolphins following the spill, the team could determine which dolphins in their cohort had been exposed to oil and which were born after the event. They found that the significant differences they observed came mostly from the dolphins that survived the disaster.
However, younger dolphins are not necessarily in the clear. Some of the most striking differences observed in the Barataria Bay group are linked to genes related to the immune system. A previous study found that these dolphins experienced problems with their immune systems as recently as 2018, and subsequent laboratory tests on dolphin cells and mice showed that these immune differences could be passed on to future generations. Changes in the immune system increase susceptibility to infectious diseases, which can also affect the dolphin’s reproductive success.
“The bottlenose dolphin population in Barataria Bay is not doing very well,” De Guise said. “If the recovery is underway, it will be in its infancy and not dependent on additional stressors.”
Stopping the stressors is unlikely. A study released in August found that traces of the Deepwater Horizon spill are still detectable, and new drilling and flood protection plans are expected to lead to the deaths of Barataria Bay dolphins if they go ahead as planned. Even smaller anthropogenic problems continue to interrupt the lives of dolphins.
“When we would go out on these health assessments, we would find dolphins entangled in lines and nets,” Morey said. “It’s very difficult to see these animals suffer.”