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Arsenic-eating bacteria may redefine 'life as we know it'
Posted: December 2, 2010

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Researchers have discovered a bacteria that can substitute toxic arsenic for phosphorous, the first organism ever identified that appears to thrive in the absence of an element thought to be critical to life as it has long been defined. The discovery may require a redefinition of the basic requirements for life while expanding the potential environments on Earth -- and across the cosmos -- where life might be possible.

An electron micrograph of the bacterial strain GFAJ-1. Credit: Science/AAAS
"The definition of life has just expanded," Ed Weiler, director of space science at NASA headquarters in Washington, said in a statement. "As we pursue our efforts to seek signs of life in the solar system, we have to think more broadly, more diversely and consider life as we do not know it."

In a paper published online Thursday in the journal Science, a team of NASA-funded researchers reported a bacterial strain known as GFAJ-1, found in mud taken from Mono Lake in California, that was able to thrive in the laboratory when phosphate was taken away, slowly replaced by increasing levels of arsenate. GFAJ-1 was able to use the poison in its normal biochemical processes to build DNA, RNA and other critical molecules, the researchers reported.

Arsenic is chemically similar to phosphorous but it is normally extremely toxic, forming unstable bonds with other elements. But Felisa Wolfe-Simon, a geomicrobiologist at the U.S. Geological Survey in Menlo Park, Calif., and her colleagues wondered if the poison could have played a role in the development of ancient organisms evolving in toxic environments.

To find out, Wolfe-Simon collected GFAJ-1 from Mono Lake, a salty desert lake that has high arsenic and phosphorous levels to begin with, and began laboratory tests in which increasing levels of arsenate were added to a growth medium. The process was continued until phosphate concentrations were so low the bacteria had to incorporate arsenate to survive.

To her surprise, GFAJ-1 appeared to do just that.

"Not only did this microbe cope, but it grew and it thrived," she said. "And that was amazing. Nothing should have grown."

Wolfe-Simon and her colleagues then carried out a series of sophisticated tests to make sure the poison was actually being incorporated in the organism's biochemistry and was not simply being "stored" in cellular vacuoles.

"All life that we know of requires carbon, hydrogen, nitrogen, oxygen, phosphorous and sulfur," she said at a news briefing. "And it uses those six elements in ... DNA and RNA, or the information technology of the cell, in proteins, which are the molecular machines, and the lipids, which separates you from everything else. We've discovered an organism that can substitute one element for another in these major biomolecules."

Describing her research, Wolfe-Simon said "what I've presented to you today is a microbe doing something different than life as we knew it."

"I was taught as a biochemist that all life on Earth, all life we know of, to harken back to the 'pale blue dot' ideas of Carl Sagan, all life we know of is here so far," she said. "And if there's an organism on Earth that's doing something different, we've cracked open the door to what's possible for life elsewhere in the universe. And that's profound. ... This microbe substitutes arsenic for phosphorous in its basic biomolecules. What else might we find? What else might we want to look for?"

The research appears to expand the range of possibly habitable environments on Earth and on other planets and moons. On a more fundamental level, the finding may indicate the very definition of what is required for life to exist needs to be broadened to encompass a wider range of possibilities.

"I find this result delightful because it makes me have to expand my notion of what environmental constituents might enable habitability," said Pamela Conrad, an astrobiologist at NASA's Goddard Space Flight Center. "The implication is we still don't know everything there is to know about what might make a habitable environment on another planet or a satellite of another planet. We have to increasingly broaden our perspective."