Extreme Life is Found in Deepest Layer of Earth’s Crust Living on Methane and Benzene -Bacteria Thriving for One-Billion Years Oceans Dominant Life Form.

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A remote expedition to the deepest layer of the Earth's oceanic crust has revealed a new ecosystem living over a kilometer deep. Analysis of this new biosphere suggests life could exist lower still.

To facilitate the task, the Integrated Ocean Drilling Programme led by Stephen Giovannoni of Oregon State University drilled down to 1391 meters through sediment and a layer of basalt, to hit the gabbroic layer, which lies directly above the mantle on the Atlantis Massif. Tectonic activity beneath this submerged mountain in the central Atlantic Ocean has pushed the gabbroic layer within 70 metres of the sea floor, where temperatures reach 102 °C.


At the gabbroic layer the researchers found communities of bacteria that were sparse but widespread. The type of bacteria they found came as a surprise to Giovannoni, who has previously found micro-organisms living in the basalt layer. "We expected to find similar organisms in the deeper layer," he reported to New Scientist. "But actually it was very different."

One key finding was that many of the gabbroic bugs had evolved to feed off hydrocarbons like methane and benzene, similar to the bacteria found in oil reservoirs and contaminated soil, which could mean that the bacteria migrated down from shallower regions rather than evolving inside the crust.

In an earlier 2005 expedition, Giovannoni and colleagues discovered that the smallest free-living cell known also has the smallest genome, or genetic structure, of any independent cell – and yet it dominates life in the oceans, thrives where most other cells would die, and plays a huge role in the cycling of carbon on Earth. In nature, bigger is not always better.

In a publication in the journal Science, the scientists outlined the growing knowledge about SAR11, a group of bacteria so dominant that their combined weight exceeds that of all the fish in the world's oceans. In a marine environment that's low in nutrients and other resources, they are able to survive and replicate in extraordinary numbers – a milliliter of sea water off the Oregon coast, for instance, might contain 500,000 of these cells.

"The ocean is a very competitive environment, and these bacteria apparently won the race," said Giovannoni. "Our analysis of the SAR11 genome indicates that they became the dominant life form in the oceans largely by being the simplest."

The new study outlines how SAR11 has one of the most compact, streamlined genomes ever discovered, with only 1.3 million base pairs – the smallest ever found in a free-living organism and a number that's literally tiny compared to something like the human genome.

"SAR11 has almost no wasted DNA," Giovannoni said. "This organism is extremely small and efficient. Every genetic part serves a purpose, more so than any other genome we've studied."

The organism is able to survive as an unattached cell in a hostile environment, has a complete set of biosynthetic pathways, and can reproduce efficiently by consuming dissolved organic matter.

"By comparison, humans are mostly junk DNA, with large parts of the human genome having no important function," Giovannoni said.

This type of genome streamlining, researchers say, appears to be a major factor in the evolutionary success of SAR11, which they believe may have been thriving for a billion years or more. One scientific hypothesis holds that natural selection acts to reduce genome size because of the metabolic burden of replicating "junk" DNA with no adaptive value – SAR11 supports that theory.

About half of photosynthesis and the resulting oxygen on Earth are produced by algae in the ocean, and microbes like SAR11 recycle organic carbon – producing the nutrients needed for algal growth.

"Ultimately, SAR11 through its sheer abundance plays a major role in the Earth's carbon cycle," Giovannoni said. "Quite simply, this is something we need to know more about. SAR11 is a major consumer of the organic carbon in the oceans, which nearly equals the amount of carbon dioxide in the atmosphere. The carbon cycle affects all forms of plant and animal life, not to mention the atmosphere and fossil fuel formation."

SAR11 was first discovered at Oregon State University in 1990. Since then researchers have learned that populations of SAR11 increase during the summer and decrease during the winter, in a cycle that correlates to the ebb and flow of organic carbon in the ocean surface. Molecular probes, gene cloning, sequencing techniques and other tools have been used in this exploration.

Casey Kazan

Sources:

http://www.plosone.org/static/users.action;jsessionid=09FDCB9AA5D93B74113EA657B3627F3B.ambra01

http://www.newscientist.com/article/mg20827874.800-life-is-found-in-deepest-layer-of-earths-crust.html
http://news.mongabay.com/2005/0824a-osu.html

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