Oil spills at sea aren't all bad, at least not the natural ones in which the oil seeps up through the ocean bottom.

According to a report at last week's American Chemical Society meeting in New Orleans, there are natural communities of animals that depend on oil seepage as the primary nutrient in a complex food chain. Bacteria and other microorganisms digest the crude oil and, in turn, become food for larger animals such as mussels, clams and tubeworms.

The oil-based communities were found in sediments on the floor of the Gulf of Mexico, a region where oil deposits are well known and where offshore drilling is common. The research was led by Mahlon Kennicutt II of Texas A&M University's Geochemical and Environmental Research Group.

"Instead of hindering the population of organisms on the sea floor, it appears that the oil seepage actually enhances it," Kennicutt said.

Although underwater seepage of oil is sometimes fast enough to float to the surface and make an oil slick, even there bacteria break down significant amounts of the oil. The quantity that washes ashore to foul beaches and marshes is usually much smaller than that of accidental oil spills from tankers.