But two new studies of the bacterium, GFAJ-1, reported that it could not grow without the presence of phosphorus. The papers also challenged the original finding that small amounts of arsenic compounds had replaced phosphorus compounds in some DNA, membranes and other biologically central parts of the organism.
“Contrary to an original report, the new research clearly shows that the bacterium, GFAJ-1, cannot substitute arsenic for phosphorus to survive,” the journal concluded in a formal statement.
“The new research shows that GFAJ-1 does not break the long-held rules of life, contrary to how [lead author Felisa] Wolfe-Simon had interpreted her group’s data.”
Nonetheless, Science wrote that it would look with interest at further research regarding the bacterium, which it called “an extraordinarily resistant organism that should be of interest for further study, particularly related to arsenic-tolerance mechanisms.”
Wolfe-Simon, now on a NASA fellowship at the Lawrence Berkeley National Laboratory, is collaborating with senior scientist John A. Tainer on wide-ranging studies of the bacterium. In an interview Saturday, Wolfe-Simon and Tainer said that they had produced tentative results in the Berkeley lab almost identical to the original results at a U.S. Geological Survey laboratory, and that they were busy finishing the research and preparing another paper.
Tainer said the two new studies in Science may have come to different results than theirs because of the methodologies used, the precision used to detect arsenates and the provenance of the cells. He said the authors of the two new papers “may well regret some of their statements” in the future.
“There are many reasons not to find things — I don’t find my keys some mornings,” he said. “That doesn’t mean they don’t exist. The absence of a finding is not definitive.”
Wolfe-Simon and her numerous collaborators had made samples of GFAJ-1 broadly available after her initial results caused a storm of controversy, but she and Tainer said they may have been contaminated or modified in transit.
She said that all the researchers agreed that the bacterium survived in extraordinarily high levels of usually toxic arsenic compounds but that they disagreed about whether the organism used the arsenic compound to grow and whether it had incorporated the arsenic into its biology.
“I think it’s unclear whether this is the last word,” Wolfe-Simon said. “They’re not finding something that could be there in a minor amount.”
One of the new studies in Science was conducted by a team centered at Princeton University that included Rosemary Redfield of the University of British Columbia. She was one of the first and most vocal critics of the original Wolfe-Simon paper, and she said Sunday she was satisfied with how the process has played out.
“A very flawed paper was published and received an inordinate amount of publicity,” she wrote in an e-mail. “But other researchers responded very quickly. . . . Now refutations of the work by two independent research groups are appearing in the same high-profile journal, and the refutations are being well publicized. This is how science is supposed to work.”
The new study Redfield was part of did not find any microbial growth when arsenates were provided to the bacteria without phosphates. Wolfe-Simon had initially reported that the bacterium grew when phosphorus compounds were withheld but arsenic compounds were provided. The new study also found no biologically mediated arsenic in the microbe’s DNA, as WolfeSimon had reported.
The paper concludes that the bacterium is an extreme life form but one that has adapted to its environment in a manner similar to many others that live in conditions long thought to be unsuitable for life.
The second new study in Science came from a research group in Switzerland. That group also found no growth in the bacteria in a medium with arsenic compounds but no phosphorus. The paper suggested that WolfeSimon’s initial finding may have missed the presence of extremely small amounts of phosphorus in the arsenic medium, which then allowed the bacterium to grow.
The paper reported that the GFAJ-1 bacteria survived in a culture that had a ratio of arsenate to phosphate of 10,000 to 1, while other known arsenic-resistant microbes had ratios that were much lower. As a result, they concluded, the bacteria was a good candidate for further study.