All known living things depend on 6 major elements: carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus. Although scientists have previously found many cases of other elements substituting for one another in living organisms, they hadn't yet found evidence of a major element substitution.
A team led by Dr. Felisa Wolfe-Simon, a NASA astrobiology research fellow in residence at the U.S. Geological Survey in Menlo Park, California, set out to see if they could isolate bacteria that can use arsenic instead of phosphorus. Arsenic lies directly below phosphorus on the periodic table. The 2 elements have a similar charge and atomic radius. Arsenic is so toxic becase the metabolic pathways that use phosphate—the most common biological form of phosphorus, in which it's bound to 4 oxygen atoms—often can't distinguish it from arsenate, in which arsenic is bound to 4 oxygen atoms. Arsenate substitution in these pathways can disable normal metabolic reactions.
The researchers turned to Mono Lake in eastern California, which has high levels of arsenic in its water. They used lake sediments to inoculate an artificial medium with everything normally required for growth except added phosphate. Gradually, the scientists added higher amounts of arsenate. Their work was supported by NASA, the U.S. Department of Energy and NIH's National Center for Research Resources (NCRR).
In the online edition of Science on December 2, 2010, the scientists reported the isolation of a bacterium that can live using arsenate. The new strain, identified as GFAJ-1, is a member of the Halomonadaceae family of Gammaproteobacteria. GFAJ-1 grew faster when the researchers added phosphate to the media, but the bacteria could also grow with arsenate instead.
When the scientists added radiolabelled arsenate, they found arsenic associated
with proteins, metabolites, lipids and nucleic acids. To locate arsenic
inside the cells, the researchers used a hair-thin X-ray beam. The
results suggested that arsenic had replaced phosphorus within the bacteria's
cellular material. Further investigation into the types and locations of specific
atoms were consistent with arsenate incorporation into biomolecules including
DNA and proteins.
"We know that some microbes can breathe arsenic, but what we've found is a microbe doing something new—building parts of itself out of arsenic," Wolfe-Simon says. "If something here on Earth can do something so unexpected, what else can life do that we haven't seen yet?"