New life form -- thriving on arsenic -- found on Earth
also choose arsenic?” in the International Journal of Astrobiology.
“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, now a NASA astrobiology research fellow in residence at the U.S. Geological Survey, was one of the participants, along with Anbar, at a workshop titled “Tree or Forest? Searching for Alternative Forms of Life on Earth,” that was organized in December 2006 by the BEYOND Center, a “cosmic think tank” at ASU.
“That’s where it all began,” says Davies, a cosmologist, astrobiologist, theoretical physicist and director of the BEYOND Center.
“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.”
After leaving ASU, Wolfe-Simon began a collaboration with Ronald Oremland of the U.S. Geological Survey to chase down the hypothesis. Oremland was a natural choice to bring into the project because he is a world expert in arsenic microbiology. What Wolfe-Simon discovered is presented in the Science Express paper titled “A bacterium that can grow by using arsenic instead of phosphorus.”
The latest discovery is all about a bacterium — strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria — scooped from sediments of eastern California’s Mono Lake, which is extremely salty with naturally high levels of arsenic.
In the laboratory, the researchers successfully grew microbes from the lake on a diet that was very lean on phosphorus, but included generous helpings of arsenic.
Key issues that the researchers needed to address were the levels of arsenic and phosphorus in the experiments and whether arsenic actually became incorporated into the organisms’ vital biochemical machinery, such as DNA, proteins and the cell membranes. A variety of sophisticated laboratory techniques was used to nail down where the arsenic went, including mass spectrometry measurements by Gordon at the W. M. Keck Foundation Laboratory for Environmental Biogeochemistry at ASU.
Commenting on the significance of the discovery, Davies says: “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. GFAJ-1, however, 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.”
It is not only scientists, however, 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, noting 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.”
