Bacteria dining on the oily feast in the Gulf of Mexico enjoyed a first course made mainly of gases in the months after the Deepwater Horizon oil spill, according to a study published in Science1 today. This may have primed the microorganisms to digest some of the more complex hydrocarbons in oil, but raises questions about how much oil degradation has actually occurred.
A team of researchers studied four hydrocarbon plumes located at varying depths and in different directions, between 1 and 12.5 kilometres from the spill site. By analysing biodegradation both in the water and in lab experiments, they found that consumption of the gases most palatable to bacteria, such as ethane and propane, accounts for up to 70% of the observed oxygen decrease in the deepwater plumes.
Though the oxygen dips have not been extreme enough to cause oxygen-starved conditions in the deep water, researchers say the impacts on marine life are still an open question.
Most oil spills occur in surface waters, where gases escape quickly into the atmosphere. But in the deep waters of the Gulf, "natural gases are in many ways driving the show", says lead author David Valentine, a geomicrobiologist at the University of California, Santa Barbara.
In addition to the estimated 750 million litres of oil that gushed from the wellhead, Valentine estimates that 30% more hydrocarbons jetted out in the form of gases. Whereas a significant fraction of oil made it to the surface, gases are more soluble in water, so "almost all of the gas stayed deep", says Valentine.
Richard Camilli, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts, say that the study "provides clear evidence that microbial hydrocarbon degradation was mostly limited to natural gas".
The fact that gases such as ethane and propane break down quicker than other hydrocarbons is well established. But the surprise, says Camilli, was the "disproportionate role of these compounds".