A dispersant used to speed up the biodegradation of crude oil following the BP oil spill in the Gulf of Mexico in 2010 may have actually had the opposite effect, according to a new study. Around 7 million liters (1.8 million gallons) of the chemical dispersants Corexit 9500 and 9527 were applied to the slick in an attempt to break up the 750 million liters (198 million gallons) of crude oil that spewed into the sea when the Deepwater Horizon oil rig sank, in the hope of stimulating biodegradation. However, the investigation, which was conducted by researchers at the University of Georgia, found that this may actually have negatively impacted natural populations of oil-degrading microbes.
As described in PNAS, the team recreated the conditions generated by the spill in a laboratory, taking water from a depth of 1,178 meters (3,865 feet) in the Gulf of Mexico, then adding crude oil and Corexit 9500 and measuring microbial activity. They discovered that, in the presence of oil and water only, numbers of a group of bacteria called Marinobacter swelled from two percent of the total microbe population in the water to 42 percent. This much was expected, as Marinobacter are known to degrade a number of hydrocarbons found in crude oil, and therefore tend to thrive when oil spills occur. However, when the dispersant was added to the mix, the relative abundance of Marinobacter dropped to less than five percent.
This is partly because, in the presence of Corexit 9500, members of a different group of bacteria called Colwellia – which degrade the dispersant rather than the oil – were found to flourish, reaching 26 to 43 percent of the overall microbe population. The researchers therefore suggest that, once the dispersant is added, Colwellia outcompetes Marinobacter, thereby preventing the biodegradation of crude oil. However, they also propose that this effect may be caused by some compounds in the dispersant itself directly affecting the Marinobacter, but suggest in a statement that more research is required to confirm this.
Summing up their findings, the team states that “the presence of dispersant selected against the most effective hydrocarbon degrading microorganisms,” and that Corexit 9500 therefore “did not stimulate biodegradation” – something which, if true, means that up to half of the oil released during the disaster may still be unaccounted for. According to lead researcher Samantha Joye, much of this could be lurking on the seabed, therefore posing a threat to marine life.
While these findings appear to provide evidence against the use of this particular dispersant due to its lack of efficacy, previous criticisms of Corexit have focused on its potentially damaging impact on marine ecosystems. For instance, in 2013 Robert McKee of the Gulf Oil Disaster Recovery Group explained that the “toxicity of the petroleum products is increased when it is dissolved into the water by dispersants.”
Main image credit: Deepwater Horizon Offshore Drilling Platform on Fire by Ideum - ideas + media via Flickr. CC BY-SA 2.0