It was reported at a news conference December 2, 2010 that NASA funded astrobiologist Felisa Wolfe-Simon has isolated a form of life that is capable of using arsenic instead of phosphorus as an elemental building block. These are microbes from the domain Archaea, which have an independent evolutionary history from Bacteria and Eukarya. The microbes were found living in an odd chemical stew in the sediment of Mono Lake in California. The lake is three times saltier than the ocean, highly alkaline, rich in carbonates, phosphorus, arsenic and sulfur.
ScienceNews describes how Wolfe-Simon isolated and cultured the GFAJ-1 strain of microbes, from the salt-loving Halomonadaceae family (extreme halophilic or salt-loving) in test tubes, then deprived them of phosphate. She added (the article says "force-fed") a radioactive isotope of arsenate instead, which allowed the element to be traced in the microbial cells. Control colonies were also maintained for which phosphate was not limited.
According to the data, microbes in the arsenate colonies appear to have substituted arsenate for the phosphate that binds sugars to form the backbone of the DNA double helix as well as replacing phosphate in other cellular processes. A good description of the details, methodology and the several tests confirming substitution can be found in an excellent on-line article in Astrobiology Magazine. The arsenic-based microbes are a bit smaller and slower-growing than the control colony microbes, but the arsenate atoms replacing phosphate appear to serve adequately in the molecules they are used to construct.
These Archaea normally exist in what may be the most arsenic-rich natural environment on our planet, so tolerance to arsenate is not surprising. What this experiment demonstrates is that the microbes are somehow able to substitute arsenic for phosphorus in stable molecular structures crucial for life to exist. Arsenic is chemically similar enough to phosphorus for life to readily absorb it into cells to use it as a substitute for phosphate atoms in the process of assembling proteins. But arsenic is more reactive than phosphorus, thus forms unstable molecules that do not function properly. This is the mechanism of arsenic's toxicity, which leads to cell death.
The GFAJ-1 Archaea appear to have the ability to stabilize the reactivity of arsenic in molecular bonds, though the precise mechanism of this ability is still to be identified. The exciting news is that this discovery is important evidence in favor of astrobiologists' hypotheses that life can exist based on elements other than the six essential to life as we know it - carbon, hydrogen, oxygen, phosphorus and sulfur.
Wolfe-Simon is now working to test whether GFA-1 uses its arsenic stabilizing ability in its natural environment of Mono Lake sediments. Further research is planned to re-examine other members of the Halomonas family known to be arsenic tolerant to see if they too can survive in a phosphate-free environment. While some scientists expressed skepticism about Wolfe-Simon's conclusions, if further tests confirm them the existence of a life form with arsenate DNA is a radically important discovery that just might result in a Nobel Prize!