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.
