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Petroleum-eating bacteria - which had dined for eons on oil seeping naturally through the seafloor - proliferated in the cloud of oil that drifted underwater for months after the April 20 accident. They not only outcompeted fellow microbes, they each ramped up their own internal metabolic machinery to digest the oil as efficiently as possible.

The result was a nature-made cleanup crew capable of reducing that reduced the amount of oil amounts in the undersea "plume" by half about every three days, according to research published online Tuesday by the journal Science.

The findings, by a team of scientists led by Terry C. Hazen of the Lawrence Berkeley National Laboratory in California, help explain one of the biggest mysteries a mystery of the disaster: Where has all the oil gone?

"What we know about the degradation rates fits with what we are seeing in the last three weeks," Hazen said. "We've gone out to the sites, and we don't find any oil, but we do find the bacteria."

The species dominating the digestion of the oil is a newly discovered one, Hazen said.

The findings point to a different conclusion from that drawn by readers of a study published last week, also in the journal Science. That research by scientists at Woods Hole Oceanographic Institute found no reduction in the oxygen content of the gigantic oil plume, suggesting that microbes were consuming the oil very slowly.

The Berkeley team study published Tuesday also indicates indirectly that dispersants used to break the wellhead stream of oil into a mass of submicroscopic particles might have speeded the cleanup. By increasing the surface area between oil and water, the dispersants seem to have provided the deep-sea microbes greater access to this unusual food source.

Alan Mearns, a senior staff scientist in the emergency response of the National Oceanic and Atmospheric Administration, called the Berkeley team study "critical to the understanding of the fate of what remains in the Gulf. This study shows that microbes are quickly degrading some components of subsurface oil found in the deep ocean without creating hazardous dead zones."

Some of the spill's 206 million gallons of oil has come ashore, some has sunk into bottom sediments, and a little is still a floating froth. But the mile-wide, 650-foot-high oil cloud of oil that for months drifted 4,000 feet underwater seems to have disappeared in the six weeks since the well was plugged.

The plume's whereabouts has been a contentious matter.

In tThe Woods Hole study, scientists described finding an undersea oil cloud June 23 to 27 similar to the one Hazen and his colleagues found between May 25 and June 2 - which was similar to one found soon after by people from the Monterey� Bay Aquarium Research Institute.