The huge quantities of methane gas that bubbled out of BP's broken well in the Gulf of Mexico were eaten up almost entirely by undersea microbes by the end of August, a new study reports today.
Other scientists cautioned that much oil remained on the ocean floor, where it has penetrated deep into the sediment, as well as in fragile marshlands. Oil is still turning up in tar balls on beaches and in fishermen's nets.
But the latest study, published in Science, offers an important new piece of information to help resolve the question of what happened to the underwater plume of oil and gas that stretched for miles from BP's broken well.
Yesterday, the White House oil commission, which was set up in June to look into the causes of the disaster, produced a preview of its report to be released next week. It said the spill was avoidable and caused in part by a series of cost-cutting decision made by BP and its partners.
Methane is thought to account for 30% by weight of the output from BP's blown-out well, and was a major component of a vast plume of oil and gas that formed about 1,000 metres deep.
However, contrary to the expectations of the lead researcher in the new study, John Kessler, an oceanographer at Texas A&M University, that the methane would linger for years, nearly all of the gas was consumed by microbes within 120 days of the blow-out.
By the time Kessler and his team returned for the second of their three research missions to the Gulf on 18 August, the methane had been scrubbed.
"All of that evidence had pointed to a much longer lifetime of methane in deepwater plumes with a lifespan possibly as long as years," he said. "It was quite surprising."
Readings on methane and oxygen levels at 207 stations indicated a massive "bloom" of methane-eating underwater bacteria sometime between the end of June and the beginning of August. "It likely occurred after affected waters had flowed away from the wellhead," the study said.
Kessler said the data suggested that the process of degrading the oil only stopped because the microbes had eaten their way through the methane. "It appeared that nothing stopped it except for the availability of methane."
He said the findings also offered good news on the broader concern of climate change. Methane release from the oceans is seen as potential driver of climate change. But the study say any large-scale release would likely trigger the same response from bacteria.
The blow-out on the Deepwater Horizon rig last April was caused by a huge bubble of methane gas shooting up the riser pipe from the well on the ocean floor.
A government study concluded that BP's blown-out well spewed 4.9m barrels of oil into the Gulf (plus or minus about 10%): the equivalent of twice the crude spilled in the Exxon Valdez accident.
BP eventually rigged up a system to siphon oil directly from the broken well to a tanker ship, capturing 820,000 barrels.
But even with those efforts, as well as burning and skimming and natural processes such as evaporation, more than 2m barrels of oil remained in the Gulf, according to a study by the Congressional Research Service. The study said the fate of the oil may never be known.
The Science paper could help resolve one question, however: what happens when the Gulf's well-studied community of oil-eating bacteria meets 4.9m barrels of crude?
"Apparently there are a number of bacteria in this particular ecosystem which are ready and waiting to take advantage of any hydrocarbon source that may be there," said Steve Murawski, who was until this month the chief scientist for NOAA, and had not seen the study. "It looks like the water column effects are going to be ephemeral."
Other scientists who had seen the paper ahead of release, however, expressed skepticism of the idea of a sudden spurt in the population of methane-eating microbes three months after the blow-out. The study should also not be construed as an all-clear for the Gulf, scientists warned.
A study due later this month from Samantha Joye, a University of Georgia oceanographer, provides evidence that much of the oil not recovered from the Gulf sank to the ocean floor.
Joye's team found heavy deposits of oil in the sediment in a 2,600 sq mile area from the well, including remnants of oil that had been burned on the surface.
Under a UV camera, which shows oil as a bright lime green, "it looks like someone has come with a paintgun and just splotched all over the bottom," she said. "You can visibly see: it lights up like a Christmas tree."
She also said she had seen dead corals and heavily oiled crabs on the ocean floor.
It is also unclear what effect the oil and gas had on marine life – such as fish eggs – in the deep water before it was consumed by the microbes. "The book is only partially written here in terms of the impact," Murawski said. "The real question is going to be: did it leave any lasting effects in the ecological population – the invertebrates, turtles, whales, fisheries?"
Several scientists have tried to flesh out the role played by the microbes in scrubbing the Gulf of the oil that never made it to the surface or the shore.
A team from the Woods Hole Oceanographic Institute published a paper in Science last August documenting a 22-mile underwater plume of tiny oil droplets. The team said the oil was very slow to degrade.
One week later, Science published a paper from another team, from Lawrence Berkeley labs, saying the plume had been eaten by undersea microbes.
By mid-September, the picture shifted once more. A new paper, also involving Kessler and his team, suggested that while ethane and propane were being quickly consumed by bacteria, methane was being consumed at a far slower rate.
"It really is a scientific puzzle," said Kessler. "We are just now beginning to get to a stage to see all the data that has been collected to date, to start to tell a comprehensive story."