Skepticism over discovery
Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution in Gainesville, Fla., remains skeptical.
If you "replace all the phosphates by arsenates," in the backbone of DNA, he said, "every bond in that chain is going to hydrolyze [react with water and fall apart] with a half-life on the order of minutes, say 10 minutes."
So "if there is an arsenate equivalent of DNA in that bug, it has to be seriously stabilized" by some as-yet-unknown mechanism, Benner said.
Benner suggests that perhaps the trace contaminants in the growth medium Wolf-Simon uses in her lab cultures are sufficient to supply the phosphorus needed for the cells' DNA. He thinks it's more likely that arsenic is being used elsewhere in the cells, in lipids for example.
"Arsenate in lipids would be stable," said Benner, and would "not fall apart in water." What appears in Wolfe-Simon's gel-purified extraction to be arsenate DNA, he added, may actually be DNA containing a standard phosphate-based backbone, but with arsenate associated with it in some unidentified way.
The discovery of GFAJ-1's unusual abilities suggests a number of avenues for further research. One obvious one is to see whether any other organisms can perform similar biochemical tricks.
Wolfe-Simon "would be very unlikely to have just found the only arsenic life-form on Earth on the first try. So it's got to be the tip of a very large iceberg," Davies said.
And indeed, Wolfe-Simon said she is already growing "14 or so other isolates" from Mono Lake on a phosphorus-free diet high in arsenic. They may be the same organism she's already identified, they may not. "I don't know anything else about them, except that they grow under similar conditions."
Meanwhile, Wolfe-Simon has ordered stock cultures of several previously identified Halomonas organisms, close relatives of GFAJ-1 on the genetic tree, all known to be arsenic-tolerant. She plans to test whether they, too, can survive in a phosphorus-free environment.
She's also interested in finding out whether GFAJ-1 is actively employing its arsenic-incorporating ability in its natural state. "You want to know, is this biology being done in the environment or is it some very bizarre thing, like a hat trick [that it does only] in the lab."
And Davies suggests it would be interesting to search in "an environment that has very little phosphorus and lots of arsenic" for an organism that requires arsenic to survive, "for which phosphorus would be the poison." Mono Lake, he pointed out, "has phosphorus as well arsenic."
These and other investigations will help to clarify how extensive a role arsenic plays both within GFAJ-1 and in terrestrial biology as a whole.
But while some scientists may reserve final judgment about Wolfe-Simon's conclusions until further details can be clarified, even Benner concedes that "If that organism has arsenate DNA, that is a world-class discovery."
Wolfe-Simon's research is funded by the NASA Exobiology/Evolutionary Biology program.
http://www.csmonitor.com/Science/2010/1202/How-does-an-arsenic-based-life-form-work-exactly
