Methane-munching bacteria digested nearly all the gas released by the Deepwater Horizon blowout in the Gulf of Mexico within just four months, according to scientists, who say they were surprised by how quickly the bacteria worked.

The findings, published Thursday by a team of U.S. researchers in the peer-reviewed journal Science, are good news because they show the naturally occurring bacteria's digestion of methane did not lead to dangerously low oxygen levels, as was first feared.

The methane and an estimated five million barrels of oil were released into the Gulf starting on April 20 until the well was effectively sealed on July 15.

The methane-eating bacteria used up some oceanic oxygen while breaking down the gas, and again when they died and decomposed once the buffet was over.

Texas A&M University oceanographer John Kessler sampling water as part of the research into what happened to the methane gas released by the Deepwater Horizon blowout in April 2010. (Credit: Elizabeth Crap, NOAA)

But the oxygen depletion — a three per cent decrease from normal levels — was not significant enough to harm marine life, said Texas A&M University oceanographer John Kessler, the lead co-author of the study.

"Normally, you need to remove about 67 per cent of the oxygen to get to dangerous levels," he said. "Fortunately, the bacterial response to the spilled methane took approximately two months to get started, which allowed enough time for the methane to disperse enough so that no hypoxic or anoxic areas occurred as a result of the methane consumption."

But once the bacteria got to work, they appear to have devoured methane faster than anyone anticipated, even though methane concentrations around the well were higher than the team had seen anywhere else in the ocean.

Methane levels around the site of the disaster ranged from 10,000 to 100,000 times the ocean's normal background levels, Kessler said.

Methane naturally seeps from the ocean floor, but even measurements taken near methane vents did not come close to levels measured in the water around the Deepwater Horizon rig after it exploded last April 20.

Previous measurements of the rate at which bacteria decompose methane in the ocean suggested it would take a long time for the gas from the well to break down.

"It's just a slow process," Kessler said. "We suspected the methane would be around for years." The scientists think the difference in this case is that massive amounts of methane led to an equally massive spike in bacteria.

The results suggest that large-scale natural releases of methane in the deep ocean are likely to be met by a similarly rapid bacterial response, the researchers say.

While the amount of methane — a highly potent greenhouse gas — emitted by the well was huge, it was not enough to have a significant impact on atmospheric levels, Kessler said. And in any case, methane that might have been released into the atmosphere was largely trapped under heavy salt water long enough for the bacteria to do their work and keep it from dispersing into the air.

Bacteria that went after the oil released in the Deepwater blowout also used up oxygen, but Kessler said other studies showed the steepest recorded drop in oxygen levels in the water was about 40 per cent — again, not enough to harm marine wildlife.

The methane study did not examine the fate of the Deepwater Horizon oil, but in November the U.S. government released a peer-reviewed "technical document" that estimated 17 per cent was directly recovered from the well, 16 per cent was chemically dispersed, five per cent was burned and three per cent skimmed. The document also estimated 24 per cent evaporated or dissolved.

Another U.S. government report prepared for Congress and released in December says there is still much uncertainty about the oil's fate.

"Even assuming that approximately half of the oil has been removed from the Gulf ecosystem through direct recovery, burning, skimming or evaporation, the fate of the remaining oil is unknown," the Congressional Research Service report states.

Kessler says oil is difficult to track because it is a much more complex substance than methane. It is made up of thousands of different molecules, some of which float, while others sink or dissolve.

The Congressional report suggests authorities may never know what happened to the oil.

"It is debatable whether the fate of the remaining oil will ever be established conclusively, because multiple challenges hinder this objective: the complexity of the Gulf system; resources required to collect data; and varied interpretations over the results and observations."