The news made headlines worldwide including a New York Times story that ran in The Inquirer on Dec. 3.
On Thursday, the researchers issued a more modest claim. Instead of saying the microbes had completely substituted arsenic for phosphorus, a new statement says the arsenic replaced "a small percentage" of the phosphorus.
A number of biologists say they'll be surprised if even this stands the test of time.
The claims "do not follow from their results," said Simon Silver, a University of Illinois microbiologist who specializes in heavy-metal resistance in bacteria. "This conclusion is not merited from what they did and measured and I think it most likely is a mistake and should never have been claimed or published."
The findings were published in the journal Science, which also issued the researchers' latest statement. Most of its 16 pages were responses to critics.
At the original NASA news event, the team leader, Felisa Wolfe-Simon, had been vague about how much arsenic had substituted for phosphorus, but several times she implied that arsenic had replaced all the phosphorus in the bacterial DNA and other crucial biological molecules.
"All life requires carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur," she said. "I've shown here today that we've discovered a life-form that substitutes arsenic for phosphorus in its DNA. . . . It's solved the challenge of being alive in a very different way."
NASA reinforced this notion with an animated graphic of a DNA molecule, in which little orange balls representing phosphorus magically disappeared and were all replaced by green balls representing arsenic.
Following Wolfe-Simon at the news conference was a phosphate chemist who listed some of the possible applications of this new phosphorus-independent life-form, from fertilizer to biofuels.
Dialing down the excitement level Thursday was one of Wolfe-Simon's coauthors, Ronald Oremland of the U.S. Geological Survey. "This is not a new life-form," he said at a panel discussion held at a meeting of the American Geophysical Union. He emphasized that the bacteria grew much better when exposed to phosphorus.
He was baffled when asked why so many reporters used the word thrived to describe the bacteria's state in concentrated arsenic. The word can be traced back to Wolfe-Simon at the news conference. "Not only did these microbes cope, but they grew and thrived," she said, "and that was amazing."
Biologist Gerald Joyce of the Scripps Research Institute in La Jolla, Calif., said more measured claims were contained in "supplemental materials" accompanying the Science paper. That's where the authors say indirect tests suggest arsenic replaced about 4 percent of the phosphorus.
"There's 26 times more phosphate than arsenate," said Joyce, referring to the molecular forms of phosphorus - and possibly arsenic - that exist in DNA.
That would still be unprecedented, he said, if it can be replicated.
But Joyce anticipates follow-up studies will reduce that 4 percent arsenic in the DNA to somewhere between .1 percent and .01 percent, if they find it at all. "Hardly an arsenate life-form," he said.
That would make it harder to justify the idea that textbooks must be rewritten because life no longer needs phosphorus, as stated in one of NASA's press summaries: "Get your biology textbook . . . and an eraser!"
In the next few months, other researchers will likely conduct more direct tests to see how much arsenic is really embedded in the DNA of the team's organisms. Joyce is betting it's zero. So is Illinois' Silver.
Some have compared the hype surrounding the arsenic bacteria to a 1996 NASA announcement of possible fossilized Martian bacteria in a meteorite.
Silver says this latest claim is worse. To its credit, he said, the Mars meteorite team had been clearer about separating its evidence from its conclusions.
"Here they claim far-out conclusions without supporting results. It is more like Uri Geller spoon-bending and 'water with memory' and 'cold fusion,' " he said.
Members of the team were not available for additional comment Thursday. A NASA spokesman said it wasn't part of the agency's mission to evaluate peer-reviewed findings. "We funded it and by our charter when we have news, we have to release it to the public," said Dwayne Brown of NASA. "Our role was to tell the public about this finding, and that's what we did in the news conference."
The fact that the microbes can survive in concentrated arsenic is nothing new. In 1997, scientists published a paper in Nature Biotechnology showing they could grow E. coli in even more concentrated arsenic than Wolfe-Simon used, Scripps' Joyce said.
"It's relatively straightforward to get something that lives in a high concentration of arsenic," added Willem Stemmer, who did that study and is now CEO of the California biotech Amunix.
Others balked at NASA's insinuation that until Wolfe- Simon set them straight, they were stuck on the assumption that life elsewhere must use the exact same biochemistry as life here. Not only do biologists assume different chemistries are possible, they've already created some alternative forms of DNA - molecules dubbed PNA and TNA for example - that can also carry a genetic code.
What qualifies something as life is the ability to use some kind of molecule that will reproduce itself, carry information to the next generation, and evolve, said Christopher Switzer, a biochemist at the University of California, Riverside.
Or as Arizona State University geochemist Everett Shock put it, life is defined not by what it's made of but by what it does.
Scientists have created a number of new versions of DNA, Switzer said. They've changed both the backbones and the coding characters. "It's really crazy," he said of some of the novel variants his colleagues have created.
Contact staff writer Faye Flam
at 215-854-4977 or firstname.lastname@example.org.