Oil rests on the floor of the Pacific Ocean.

Thousands of feet below the bottom of the sea, off the shores of Santa Barbara, single-celled organisms are feasting on oil.

Until now, nobody knew how many oily compounds were being devoured by the microscopic creatures, but new research led by David Valentine of UC Santa Barbara and Chris Reddy of Woods Hole Oceanographic Institution in Massachusetts has shed new light on just how extensive their diet can be.

In a report published in the journal Environmental Science & Technology, Valentine, Reddy, lead author George Wardlaw of UCSB, and three other co-authors detail how the microbes are dining on thousands of compounds that make up the oil seeping from the sea floor.

“It takes a special organism to live half a mile deep in the Earth and eat oil for a living,” said Valentine, an associate professor of earth science. “There’s this incredibly complex diet for organisms down there eating the oil. It’s like a buffet.”

And, the researchers found, there may be one other byproduct being produced by all of this munching on oil — natural gas. “They’re eating the oil, and probably making natural gas out of it,” Valentine said. “It’s actually a whole consortium of organisms — some that are eating the oil and producing intermediate products, and then those intermediate products are converted by another group to natural gas.”

Reddy, a marine chemist, said the research provides important new clues in the study of petroleum. “The biggest surprise was that microbes living without oxygen could eat so many compounds that compose crude oil,” he said. “Prior to this study, only a handful of compounds were shown, mostly in laboratory studies, to be degraded anaerobically. This is a major leap forward in understanding petroleum geochemistry and microbiology.”

The diet of the single-cell microbes is far more diverse than previously thought, Valentine said. “They ate around 1,000 of the 1,500 compounds we could trace, and presumably are eating many more.”

Research for this project began seven years ago and much of the testing was done at one of the planet’s best natural labs. “We have the world’s most prolific hydrocarbon seep field sitting right offshore of Santa Barbara, ” Valentine said. “We have something on the order of 100 barrels of oil a day coming up from the sea floor.”

The source of this oil seepage is near Platform Holly, but it’s not being caused by the drilling. “It’s just oil that is naturally oozing out, probably has been for thousands of years,” Valentine explained. “Holly just happens to be near some of these seepage areas, which is fortuitous because we were able to get samples from about a mile deep.”

Valentine noted that the microbes prefer the lighter compounds of oil, the gasoline part of the black goo. They tend to leave behind the heavily weathered residue, which is what makes its way to the surface and, sometimes, to the beaches in the form of tar.