Ironically, although the BP oil spill threatens ecological communities in and around the Gulf of Mexico, some of those communities are themselves based on natural oil and gas seeps. As today’s New York Times reports, there may be thousands of these ecosystems in the deep sea (up to 1.5 miles down), all based on chemosynthetic bacteria feeding on methane and hydrogen sulfide associated with gas seeps. In this sense they resemble hydrothermal vent communities, except that the temperature of the Gulf communities is lower and the bacteria feed on petrochemical products rather than dissolved minerals.
The Times article has a good slide show with photographs of some of the Gulf communities, which include molluscs, corals, and tubeworms:
Black coral in a cold-water seep: