December 02, 2010
"Life as we know it requires particular chemical elements and excludes others," explains co-author Ariel Anbar, a biogeochemist and astrobiologist who directs the astrobiology program at Arizona State University. "But are those the only options? How different could life be?"
The alien-like landscape of Mono Lake, California. Image © 2010 Henry Bortman.
"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," explains Anbar.
The concept that arsenic could replace phosphorous was first proposed by Wolf-Simon in a paper published last year in the International Journal of Astrobiology. Then she turned to the lab to prove it.
"We not only hypothesized that biochemical systems analogous to those known today could utilize arsenate in the equivalent biological role as phosphate," says Wolfe-Simon "but also that such organisms could have evolved on the ancient Earth and might persist in unusual environments today."
Wolfe-Simon in the field collecting samples at Mono Lake. Here she has just pulled up a push core of sediments (mud) from the bottom of the lake. This core will provide starters for cultures to which researchers will add high doses of arsenic. Image © 2010 Henry Bortman.
After collecting microorganisms from the lake, Wolf-Simon and colleagues grew the bacteria, dubbed strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria, in Petri dishes in the laboratory. First they provided their microscopic charges with all the phosphate salt they could need, but slowly the researchers added arsenic to the mix and took away the phosphate until the bacteria was thriving entirely off arsenic. Still, the scientists had to confirm that the bacteria had completely switched out the need for phosphorous with arsenic. With high-tech radio-tracers the scientists followed the arsenic into the bacteria’s cells, until they found that the arsenic replaced phosphate so fully as to incorporate itself into the bacteria's DNA. While the discovery gives 'adaptation' a new name, the researchers say life could go further yet.
"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," says co-author Paul Davies.
Wolf-Simon concludes that "this story isn't about arsenic or Mono Lake". Instead, she 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."
On her website www.ironlisa.org, Wolf-Simon hints: "this is only the beginning". No doubt the world will be watching closely.
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