Deep below the sea floor live massive colonies of primitive microbes.

Almost like one-celled zombies, these microbes use so little energy that it might be more accurate to call them undead rather than alive.

Yet scientists think that the species might provide a model for life on other planets. Even on this planet, such microbes might account for a whopping 10 percent of the Earth's biomass.

"In essence, these microbes are almost, practically dead by our normal standards," said Christopher House, a geosciences professor at Penn State University, and the lead author of the paper, in a release. "They metabolize a little, but not much."

The cold, lightless and energy-poor conditions under the seafloor provide a promising research analog for the harsh conditions in subsurface Martian soil or near hydrothermal vents on Europa, Jupiter's second moon.

"We do not expect the microbes in other places to be these microbes exactly," said House. "But, they could be living at a similar slow rate."

Subseafloor microbes, according to a metagenomic analysis to be published Thursday in Proceedings of the National Academy of Sciences, are genetically distinct from life on Earth's surface and oceans. The Archaea the Penn State researchers found might look like bacteria, but they don't eat or work like them. While E. coli

might double its numbers in 30 minutes, Archaea could take hundreds or even thousands of years to accomplish the same amount of growth.

The researchers conducted their work off the coast of South America in a region known as the Peru Margin. They sampled genetic material from the biomes at varying depths. Below 160 feet, the researcher said

Archaea account for 90 percent of the life present, and represent the most unique environment thus far revealed by metagenomic analysis.

The Archaea represent a thus-far untapped genetic repository for scientists looking for novel genes for changing metabolism, withstanding cold or synthesizing chemicals.

UCLA molecular biologist Jim

Lake called the results "very exciting." He also noted that more research into populations of isolated Archaea communities like the one described in the paper could do more than reveal the attributes of the microbial life.

"Our whole concept of microbial evolution is up for grabs," Lake said. "People are realizing there is lots of exchange and gene transfers between organisms, and I think the whole area is about to explode."

Lake noted that while many, like House and Biddle, think the Archaea are an ancient species, they could just be evolving very quickly because of their isolation, a bit like animals on the Galapagos islands.

The debate about how Archaea got so very different from other prokaryotes like bacteria highlights how little is known about them.

House's co-author, astrobiologist Jennifer Biddle, said that even the most basic questions about this kind of life remain unanswered.

"For example, how do they die?" asked Biddle.

Image: Close-up photographs from the drilling site, 1229. Courtesy of the Ocean Drilling Program.

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