The Gulf of Mexico ecosystem was ready and waiting for something like the Deepwater Horizon blowout, and seems to have made the most of it, a new scientific study suggests.

Petroleum-eating bacteria — which had dined for eons on oil seeping naturally through the sea floor — 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 the amount of oil 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 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 findings point to a different conclusion from that drawn by many 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 study published Tuesday also suggests indirectly that dispersants used to break the wellhead stream of oil into a mass of sub-microscopic particles may 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.

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 cloud of oil that drifted for months 4,000 feet below the surface underwater seems to have disappeared in the six weeks since the well was plugged.

One thing that many scientists feared — severe depletion of oxygen as microbes consumed the oil — apparently hasn’t happened. The Woods Hole study published last week found no decrease in oxygen in the oil plume, and the new study found only a slight one.

“Both of our papers suggest it is unlikely that the microbes would draw down oxygen to such a degree that it would affect fisheries and create dead zones,” said Richard Camilli, the Woods Hole researcher.

The research team is continuing to collect deepwater samples. It could still find an oil plume two weeks after the well was plugged. No plume could be found in the last three weeks, however. The oil that remains appears to be too diluted out to be detected.