Field Notes 
Black water   Taking it slow in the deep sea

Black water breaking up

A large mass of black water has swept through Florida Bay over the past several months. Researchers are working to identify its source. This SeaWiFS image, taken on Feb. 4, 2002, at the height of the event, shows different colors of the water. In March, the area began breaking up into smaller pockets clustered along the north side of the 126-mile long Florida Keys. Recent evidence supports that the black water may be associated with a large algal bloom.

Lisa M. Pinsker

Taking it slow in the deep sea

Microorganisms living as deep as half a mile below the sea floor could comprise one-tenth to one-third of Earth’s living biomass. But scientists still do not know much about these extreme microbes. Three geoscientists from the University of Rhode Island have set out to measure how much life teems below the ocean floor. They say life there moves at a snail’s pace compared to life above ground.

Using data from the Ocean Drilling Program, Steven D’Hondt, Scott Rutherford and Arthur Spivak mapped the distribution of dissolved methane and sulfate in water within deep-sea sediments around the world. “These maps aren’t telling us anything new about the sediments themselves, but they do provide information about the dominant microbial processes that are occurring several meters to perhaps 100 or more meters below the sea floor,” Rutherford says.

As published in the March 18 Science, these global maps of methane and sulfate revealed two “provinces” of subsurface life. In the open-ocean areas, sulfate is abundant and metabolic activity is low. The ocean margins, however, are rich in methane and have the greatest metabolic activity. Overall, though, metabolic rates of life below the sea floor are orders of magnitude lower than those of life on Earth’s surface. “The activity may be as low as we estimate,” Rutherford says, or it could be higher if “most of the cells counted from deep-sea drill cores are really inactive or dead.”

In those places where the metabolic rate appears to be lowest, Rutherford says food is the key factor. As organic carbon falls through the water column to the sediments, it becomes oxidized and is sparsely available and difficult for microbes to break down and use. “So, microbes down here are adapted to a very slow pace of life and are probably living on the edge of starvation,” he says.

Researchers hope both to understand more about the maximum limits of life on Earth and to understand how the activity of sub-sea-floor microorganisms may directly affect the surface of Earth, Rutherford says. “Over geologic time, even microbes buried well below the sea floor can change ocean geochemistry, and, ultimately atmospheric carbon dioxide concentration.”

Lisa M. Pinsker

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