Natural gas 'jump started' breakdown of Gulf oil
Bacteria breaking down oil from the Gulf of Mexico leak have been fuelled by natural gas in the water, a study suggests.
Tests suggest that natural gas released during the leak gave bacteria within the water a "jump start" in degrading contaminants.
The leak followed a blowout and fire on the Deepwater Horizon drilling rig in April which killed 11 people.
An estimated 4.9 million barrels of oil was released.
The results are published in the journal Science.
David Valentine, from the University of California Santa Barbara, and colleagues, took water samples from within the immediate vicinity of the well head in June.
They found four distinct "hydrocarbon plumes" of oil and gas, with gas making up about two thirds of the contaminants.
Results suggested the gas was being rapidly consumed by naturally occurring bacteria within the water, and that this process was also greatly depleting oxygen levels in the water.
"The propane, the butane and ethane in particular were being degraded very rapidly in and around the area of the well head," he told the BBC by phone from aboard the research vessel the "Pisces".
"It was happening very quickly after the emissions - at least within a week or two these gases were being largely consumed."
Having fed on the gases, the bacteria then appeared to turn to the oil.
"What we think is probably happening is these gases are serving to prime the population of bacteria and once the gases are gone, the bacteria begins to turn to select components of the oil," he says.
Bacteria were known to have played a role in removing oil released during the Gulf of Mexico leak.
At the time of an initial report commissioned by the US National Incident Command (NIC) and published in early August, a quarter of the oil released had been captured by burning, skimming and direct recovery from the wellhead.
Another quarter had naturally evaporated or dissolved, 8% had been dispersed chemically and 16% had been dispersed naturally through the biodegradation of oil droplets in the water.
This left 26% of the oil unaccounted for. How much oil remains in the water today is still unclear.
"We really don't have a handle on that yet," says Professor Valentine. "We know that there is still a very widespread signal of oil in the deep water in the Gulf and we don't have a real handle on exactly what within the oil has been consumed and what remains.
"However in the last 40 locations we've looked, in 20 of those we've seen tell-tale signatures of oil in the deep water so it seems it's not completely consumed, but its very likely that some components of it have been very heavily consumed.
"We think some of the more recalcitrant or less reactive oil components are still there in the water."