7.9 In the deep, a community of one

http://arstechnica.com/journals/science.ars/2008/10/09/in-the-deep-a-community-of-

By John Timmer | Published: October 09, 2008 - 04:08PM CT

As researchers probed a varity of environments that were once thought to be inhospitable to life, they were surprised to find large bacterial communities thriving in places like near-boiling hot springs and volcanic vents deep in the ocean. Faced with this evidence, it was fair to wonder just where the outer edges of survival might be. We may have a hint of that from samples taken from deep in South African mines, which show that life can make it nearly three kilometers down, but it's far from the thriving communities we find in other extreme environments. In fact, it looks like the bacterial "community" in the mine may be comprised of a single species.

The authors of a paper describing the organism, to be published today in Science, can't rule out the possibility that there are other microorganisms down in the mine, but their approach seems to make the possibility very unlikely. To start with, they filtered a total of 5,600 liters of mine water to get their sample, which gave other microbes plenty of opportunities to make themselves known. Of the DNA sequences obtained from this sample, over 99.9 percent were from this single species; over half of the remainder were obvious contaminants from their own lab. If anything else is there, it's a small minority of the life present.

The bacteria, which goes by the name Candidatus Desulforudis audaxviator, is pretty homogeneous itself. Of 2.3 million bases present in the genome, all but 32 appeared to be identical in all of the population of bacteria that was sequenced. That's a lower rate than the human population, and all the more striking given the amount of time that DNA has to pick up mutations; estimates of the nutrient availability (generated primarily from the energy given off during radioactive decay) indicate that it may take 100 to 1,000 years for a cell to divide.

With no other organisms present to engage in symbiosis with, the bacteria carry genes to do everything they need. They can make all the amino acids, extract useful carbon from carbon monoxide, and either fix nitrogen or obtain it from ammonia in the water. Oxygen is scarce in the environment, and the organism doesn't appear to make any proteins that could possibly protect them from it. To run its metabolism, it reduces SO₄. If it runs short of any of these, it has a flagella and chemosensory proteins that help it move off in search of more.

Extreme environments tend to be inhabited by archaea, so this bacteria is somewhat an exception to the rule. But the authors note that many of the key genes that enable it to survive in the hostile deeps of the mine actually originated in archaea, but were picked up by horizontal gene transfer. In this sense, it's more of an exception that proves the rule.

The fact that this organism requires such a diverse array of capabilities simply to survive, and is the only thing that manages to do so, suggests that this environment represents one of the outer edges of survivability for terrestrial life.

Science, 2008. DOI: 10.1126/science.1155495

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