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Arsenic microbe in Mono Lake may reshape hunt for extraterrestrial life

Scientists have found a microbe in Mono Lake, California, that uses arsenic as a fundamental building block, changing the definition of 'life as we know it' and the search for extraterrestrial life.

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That led her to ask if the same might not hold true for the six fundamental elements. It turns out that in many key traits, arsenic is similar to phosphorus, she and her colleagues explain. Mono Lake turned out to be a good place to hunt for organisms that might function with arsenic as a building block because among its other attributes, it contains relatively high concentrations of the element.

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The team prepared Mono Lake mud in the lab, adding all the nutrients needed to encourage bacteria to grow, but varying the relative abundance of phosphorus and arsenic until the naturally occurring phosphorus was gone. The researchers watched to see if microbes would leave any survivors as the shift progressed. They did. And they thrived, albeit their growth was slower than bacteria exposed to higher phosphate levels.

But steeped as she was in conventional widsom, Wolfe-Simon says she didn't believe her results initially.

"I'm a biochemist. I said: This isn't right. I must have made a mistake," she recalled during a briefing Thursday at which she discussed the results.

But the team put the microbes through a series of detailed tests that demonstrated that arsenic indeed had replaced phosphorus in some key roles – for instance, as the "backbone" for the bacteria's ladder-like DNA, the molecule that contains an organism's genetic instructions.

A note of skepticism

For all the buzz the results have generated, however, it's too early to draw broad conclusions about arsenic as a one-for-one swap with phosphorus, says Steven Benner, a researcher at the Foundation for Applied Molecular Evolution in Gainesville, Fla.

Certain qualities of arsenic could make it a less-than-ideal replacement, he says. Arsenic's bonds to other elements in molecules, especially in the presence of water and at room-like temperatures, is relatively weak. So the molecules it inhabits tend to break apart easily – not terribly helpful if the arsenic is trying to hold DNA together.

It's possible that organisms may develop ways to in effect reinforce those weak links, he adds, but that remains an open issue.

However, in other environments, arsenic might well be a suitable stand-in for phosphorus, he acknowledges. For example, a hypothetical microbe on Titan – a frigid moon of Saturn dotted with lakes of hydrocarbon – might find arsenic a better replacement for phosphorus.

Whether or not the microbe proves to be a rare exception to the general rule, he adds, the microbes represent "an excellent system to support questions about how arsenic is tolerated and phosphorus is limited in organisms that are placed under environmental stress."

The results of the study by Wolfe-Simon, who is affiliated with the NASA Astrobiology Institute at the NASA-Ames Research Center in Mountain View, Calif., and her colleagues appears on Science Express, an on-line adjunct to the journal Science.

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