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Buried rock, bacteria yield deep-sea feast.

It took seven long years, but oceanographers believe they have finally identified the kitchen, the raw ingredients and the cooks that prepare one of the more intriguing feasts on Earth.

Nearly every food web on the planet is supported by photosynthetic organisms, which use solar energy to produce carbohydrates. The 1984 discovery of groups of animals thriving at extreme depths in the Gulf of Mexico -- well beyond the reach of the sun's rays and far from any seafloor hot springs -- left scientists wondering what energy source sustained those communities. Analyses soon revealed that the animals exist in symbiosis with bacteria that produce carbohydrates by consuming energy-rich compounds. However, researchers could not tell where these compounds -- primarily methane and sulfide -- originated.

New work suggests that the compounds are produced in rock lying several kilometers beneath Florida's continental platform, report Christopher S. Martens and Charles K. Paul of the University of North Carolina in Chapel Hill and Jeffrey P. Chanton at Florida State University in Tallahassee.

During an expedition in the deep submersible Alvin in 1984, Martens and his colleagues found communities of mussels, tube worms, crabs, fish and other animals living on the cold seafloor at the base of the Florida escarpment--a 1,500-meter-high cliff that rises to the carbonate platform that holds Florida and the Bahamas. Paul says the cliff is about a tall as and steeper than a one-sided Grand Canyon. Studies by other researchers have suggested that the mussels derive their energy from bacteria that consume methane, while the tube worms draw their sustenance from bacteria that eat hydrogen sulfide.

Methane, the chief component of natural gas, can form through many processes, including the type of chemical reactions that produce petroleum. For this reason, scientists suspected that the methane on the seafloor might come from buried petroleum reservoirs. But Martens and his colleagues, who have examined the ratios of various carbon isotopes in the deep-sea methane, now suggest that it does not seep out of petroleum traps beneath the seafloor. Instead, they say, the methane has a biological source.

In the August GEOLOGY, the researchers propose that bacteria living in porous rock deep within the Florida platform produce the methane by consuming ancient organic matter stuck in the rock. Other bacteria in the rock convert seawater sulfate into the energy-rich sulfide, they suggest. Seawater circulating through the platform then transports the methane and sulfide to the hunry animals living by the foot of the cliff. In effect, the deep rock provides the kitchen and some of the raw materials, and the bacteria play the role of cooks.

These findings help scientists get to "the bottom of biology," Paul says. "We certainly know there are things that live in the ground, but how far down do they live?" The new evidence suggests for the first time that extremely active bacteria can exist beneath several kilometers of overlying rock, he says. --R. Monastersky
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Title Annotation:origin of compounds that support communities of animals and photosynthetic bacteria deep in the Gulf of Mexico
Author:Monastersky, Richard
Publication:Science News
Date:Aug 17, 1991
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