There was a lot of coverage yesterday of the discovery of microbes that can apparently use arsenic instead of phosphorous in their metabolism. Phosphorous is a key element for life: the backbone of DNA is made of phosphorous atoms, and it is a key component in cell membranes and energy-carrying molecules. Phosphorous can make up as much as 20 percent of the dry weight of a rapidly-growing bacterium, according to UC Davis microbiologist Doug Nelson.
Arsenic is one space down the Periodic Table from phosphorous, and is poisonous because it is so similar that it can insert itself into biomolecules in place of phosphorous and mess them up.
The research group lead by Felisa Wolfe-Simon, a NASA astrobiology research fellow at the U.S. Geological Survey in Menlo Park, Calif., isolated a microbe they called GFAJ-1 from Mono Lake, which is very salty and alkaline. In the lab, GFAJ-1 could grow in the presence of arsenic, and continued to grow when they removed sources of phosphorous, they report. The paper was published Dec. 3 in the journal Science.
NASA’s press release — and much of the media coverage — made bold claims about “expanding the definition of life.” If life can evolve based on a different set of elements and chemical reactions than it uses here on Earth, then the possibilities for life appearing elsewhere in the universe become wider.
But do these big claims hold up?
Steve Kowalczykowski, a microbiology professor in the UC Davis College of Biological Sciences, notes that “extraordinary claims require extraordinary evidence.” The authors, he says, have isolated an interesting bacterium that can not only tolerate arsenic but metabolize it, and that bears out the title of their paper. But the published paper does not support without doubt the claims that GFAJ-1 can replace arsenic with phosphorous in biological molecules, Kowalczykowski said in an email:
However, the claim that they make in the Abstract: “Our data show evidence for arsenate in macromolecules that normally contain phosphate, most notably nucleic acids and proteins”; and the title of the press release in “Science News This Week“: “Bacterium Uses Arsenic To Build DNA and Other Molecules” are extraordinary and exaggerated statements, respectively, that are not verified by extraordinarily impeccable data. Consequently, I as yet cannot accept the broad implications of the paper that there is life that does not use phosphorus in its biomolecules. I am, however, impressed, that they have discovered a life form that is not only tolerant of a potent toxin, but that can thrive on it.
Kowalczykowski goes on to note that a few years ago a different group of researchers claimed to have found evidence of fossil microbes in a meteorite from Mars — a claim that was never substantiated.
A comprehensive round up of media coverage can be found at the Knight Science Journalism Tracker blog.