Most toxic contaminants of Gulf oil spill are still out there

Bacteria ate only some toxins released during Deepwater Horizon oil spill in Gulf of Mexico, say two studies

By Snigdha Das
Published: Tuesday 05 August 2014


IN APRIL 2010 when the British Petroleum-operated Deepwater Horizon oil rig sank in the Gulf of Mexico following an explosion, it led to the largest oil spill in the US history. The damaged oil well spewed over 636 million litres of crude oil into the gulf in three months. More than 10,000 people, including military and volunteers, were engaged in the clean-up operation for nine months. But it turns out that some of the most toxic contaminants from the spill still remain within the gulf.

mapTwo studies show that while bacteria in the Gulf consumed many toxic components of the oil released, they did not soak up the most toxic contaminants, known as polycyclic aromatic hydrocarbons. PAHs are a group of semi-volatile organic compounds that are present in crude oil and can cause long-term health problems, such as cancer. Their persistence could have deleterious impacts on the sediment ecosystem.   

“Those PAHs could persist for a long time, particularly if they are buried in the ocean floor where lack of oxygen would slow PAH degradation by microorganisms,” said researcher Olivia Mason from Florida State University, US.

“They are going to persist in the environment and have deleterious effects on whatever is living in the sediment.”

To see what sort of contaminants might still be present, the researchers investigated the impact of oil deposition on microbial communities in sediment samples collected from 64 sites around the ruptured oil wellhead. In 14 samples, they sequenced the genetic material of microorganisms to understand their functional capacity to degrade oil. Seven of the 14 samples were contaminated with PAHs to the point where they exceeded Environment Protection Agency water quality benchmarks. The study was published online in The ISME journal on January 23, 2014. 

In another study published the July 7 issue of Frontiers in Microbiology, researchers found that a species of bacteria, Colwellia, that remained dominant during the spill can consume gaseous hydrocarbons and perhaps benzene, toluene, ethylbenzene and xylene compounds. But it did not consume PAH present in the water column plume or in the oil that settled on the seafloor.

“The microbial capacity to respond to and degrade hydrocarbons resulting from the oil spill has been primarily studied in the water column to date, but neglected in sediments,” the researchers say. Now evidence suggests that PAHs may have persisted in sediments, which has ecological consequences that have only begun to be determined. 

Studies have already shown the oil impacting diversity of meiofauna, animals smaller than 1 mm that live in the sediments and causing heart defects in tuna species


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