December 2, 2010
Earth-bound ‘Alien’ Life Found Thriving On Arsenic
Arizona State University scientists and their colleagues have found evidence that the toxic element arsenic can replace the essential nutrient phosphorus in biomolecules of a naturally occurring bacterium, which expands the scope of the search for life beyond Earth.
Scientists understand that all known life requires phosphorus, which is usually in the form of inorganic phosphate. However, astrobiologists Ariel Anbar and Paul Davies of Arizona State University have stepped up conversations about alternative forms of life."Life as we know it requires particular chemical elements and excludes others," says Anbar, a biogeochemist and astrobiologist who directs the astrobiology program at ASU. "But are those the only options? How different could life be?" Anbar and Wolfe-Simon are among a group of researchers who are testing the limits of life's chemical requirements.
"One of the guiding principles in the search for life on other planets, and of our astrobiology program, is that we should 'follow the elements,'" Anbar said in a press release. "Felisa's study teaches us that we ought to think harder about which elements to follow."
Felisa Wolfe-Simon, a former postdoctoral scientist in Anbar's research group and the lead author of the study, adds: "We took what we do know about the 'constants' in biology, specifically that life requires the six elements CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) in three components, namely DNA, proteins and fats, and used that as a basis to ask experimentally testable hypotheses even here on Earth."
She said that from this viewpoint, rather than highlighting the conventional view of the "diversity" of life, all life on Earth is essentially identical. However, the researchers have discovered a microbe that acts differently.
Davies has previously speculated that forms of life different from our own might exist side-by-side with known life on Earth in a "shadow biosphere."
Wolfe-Simon developed an idea with Davies and Anbar while at ASU that arsenic, which lies directly below phosphorus on the periodic table, might substitute for phosphorus in life on Earth.
"We not only hypothesized that biochemical systems analogous to those known today could utilize arsenate in the equivalent biological role as phosphate," notes Wolfe-Simon "but also that such organisms could have evolved on the ancient Earth and might persist in unusual environments today."
Wolfe-Simon, who is now a NASA astrobiology researcher in residence at the U.S. Geological survey, was one of the participants at a workshop entitled "Tree or Forest? Searching for Alternative Forms of Life on Earth."
"That's where it all began," Davies, a cosmologist, astrobiologist, theoretical physicist and director of the BEYOND Center, said in a press release.
"Felisa's talk was memorable for being a concrete proposal," Davies says. "Many of the talks at the workshop discussed searching for radically alternative forms of life with suggestions of the form 'maybe something roughly like this,' or 'maybe a bit like that.' But Felisa said, quite explicitly, 'this is what we go look for.' And, she did."
"The idea was provocative, but it made good sense," notes Anbar. "Arsenic is toxic mainly because its chemical behavior is so similar to that of phosphorus. As a result, organisms have a hard time telling these elements apart. But arsenic is different enough that it doesn't work as well as phosphorus, so it gets in there and sort of gums up the works of our biochemical machinery."
Wolfe-Simon began a collaboration with Ronald Oremland of the U.S. Geological Survey after leaving ASU in order to chase down the hypothesis.
She discovered "a bacterium that can grow by using arsenic instead of phosphorus."
The most recent discovery is a bacterium scooped up from sediments of eastern California's Mono Lake, which is extremely salty with naturally high levels of arsenic.
The researchers successfully grew microbes from the lake on a diet that was very lean on phosphorus, but also contained generous helpings of arsenic.
The researchers needed to address key issues like the levels of arsenic and phosphorus in the experiments and whether arsenic actually became incorporated into the organisms' DNA, proteins and cell membranes.
Davies said in the press release: "This organism has dual capability. It can grow with either phosphorous or arsenic. That makes it very peculiar, though it falls short of being some form of truly 'alien' life belonging to a different tree of life with a separate origin. However, GFAJ-1 may be a pointer to even weirder organisms. The holy grail would be a microbe that contained no phosphorus at all."
Davies predicts that the new organism "is surely the tip of a big iceberg, and so has the potential to open up a whole new domain of microbiology."
However, scientists are not the only ones who will be interested in this discovery.
"Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine," says Wolfe-Simon, adding that because microbes are major drivers of biogeochemical cycles and disease this study may open up a whole new chapter in biology textbooks.
"Yet, this story isn't about arsenic or Mono Lake," Wolfe-Simon says. "If something here on Earth can do something so unexpected, what else can life do that we haven't seen yet? Now is the time to find out."
Image 1: Image of Mono Lake Research area.
Image 2: Image of GFAJ-1 grown on arsenic.
Image 3: Image of GFAJ-1 grown on phosphorus.
On the Net:
- For more information about the finding and a complete list of researchers, visit: http://astrobiology.nasa.gov
- Arizona State University
- Science Express
- Video 1
- Video 2