Washington, Aug 2 (IANS): More than a year after the world's biggest oil spill in the Gulf of Mexico, lingering questions about how it disappeared so quickly still exercise scientists. Bacterial microbes inside the slick degraded the oil at a very fast rate, researchers found.

The first ever study investigating the role of bugs in breaking down the slick has thrown up answers that represent surprisingly good news and a head-scratching mystery.

Studying samples from the surface slick and surrounding Gulf waters, scientists of the Woods Hole Oceanographic Institution (WHOI) found that bacterial microbes inside the slick degraded the oil at a rate five times faster than microbes outside the slick.

They accounted in large part for the disappearance of the slick some three weeks after Deepwater Horizon's Macondo well was shut off, the journal Environmental Research Letters.

At the same time, researchers observed no increase in the number of microbes inside the slick -- something that would be expected as a byproduct of increased consumption, or respiration, of the oil, according to a Woods Hole statement.

In this process, respiration combines food (oil in this case) and oxygen to create carbon dioxide and energy. "What did they do with the energy they gained from this increased respiration?" asked WHOI chemist Benjamin Van Mooy, senior study author. "They didn't use it to multiply. It's a real mystery," he said.

Van Mooy and his team were nearly equally taken aback by the ability of the microbes to chow down on the oil in the first place. Going into the study, he said, "We thought microbe respiration was going to be minimal."

This was because nutrients such as nitrogen and phosphorus -- usually essential to enable microbes to grow and make new cells -- were scarce in the water and oil in the slick. "We thought the microbes would not be able to respond," Van Mooy said.

But the WHOI researchers found, to the contrary, that the bacteria not only responded, but did so at a very high rate. They discovered this by using a special sensor called an oxygen optode to track the changing oxygen levels in water samples taken from the slick.

Bethanie Edwards, biochemist in Van Mooy's lab, who led the study, said she too was "very surprised" by the amount of oil consumption by the microbes. "It's not what we expected to see."