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Clams and worms fueled by gas?

A tenet of deep sea oceanography used to be that, in terms of life, the seafloor is a veritable desert at depths far beyond the touch of sunlight. But lately scientists have been finding the most exotic and lush biological communities thriving in some rather unlikely places -- from hydrothermal vents (SN: 1/12/80, p. 28) to a saline seep off of Florida (SN: 12/15/84, p. 374). Now a team of researchers from Texas A&M University in College Station, report the discovery of similar kinds of sea creatures living at oil and methane seeps about 150 miles south of the Louisiana coast.

"This report significantly expands the geographical area in which one would expect to find dense hydrothermal vent-type taxa in the deep ocean," write Mahlon C. Kennicutt Ii and co-workers in the Sept. 26 NATURE.

Among the animals that Kennitcutt's group collected last December were clams and tube worms. During another series of trawls in May in the same area, they recovered an even greater diversity of organisms including mussels. A number of new species were found, most of which appear to be related to those living near vents.

Like the hydrothermal vents and Florida seeps, the oil seeps, contain hydrogen sulfide (H.sub.2S) which is known to be used as an energy source by sulfur-oxidizing bacteria that live inside of animals or are eaten by them.

This process, called chemosynthesis, enables the communities to live with little or none of the sunlight required for phtosynthesis. But unlike these other environments, the Louisiana seeps also leak hydrocarbons which appear to contribute to the food chain as well. Kennicutt's group found that the concentration of carbon-13 isotopes (which can be traced through many food chains) in seep clams and tube worms reflected the range of values found in oil and methane. But because they also correspond to carbon-13 measurements of vent animals, it's not clear whether H.sub.2S or hydrocarbons are the dominant food source. Future studies using sulfur and carbon-14 isotopes should help. "Knowing organisms, they probably do a little of everything," says Kennicutt.

Another focus for study is how the animals tolerate the highly toxic substances at the seeps such as aromatic hydrocarbons. The researchers also plan to return to the seeps in a year with the Johnson Sealink I submersible to study the community structure of the animals. It is possible that most of the animals live at the periphery of the seeps, shielded by a zone of microbes that detoxify the chemicals, notes Kennicutt.

Since hydrocarbon seeps occur in many shelf areas, the researchers believe that seafloor biological communities may be much more prevalent than has been assumed. "Until sites other than the vents in the Pacific were found, people pretty much thought that this was probably an isolated evolutionary development," says Kennicutt. "The question now is whether all of these sites are linked together with common ancestry or have they all evolved by themselves?"
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Title Annotation:hydrocarbon seeps in ocean floor
Author:Weisburd, Stefi
Publication:Science News
Date:Oct 12, 1985
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