Scientists skeptical about organism said to survive on arsenic
Last week, amid much fanfare, scientists reported they had found an organism that — unlike all previously observed life on Earth — was able to do without phosphorus and use the normally deadly element arsenic in its place.
This week, skeptical scientists expressed serious concerns about the discovery and the researchers’ interpretation of their experimental results.
“There must be a hundred things in that paper that have people going, ‘Hey, wait, that can’t be right,’” said Rosie Redfield, a microbiologist and professor of zoology at the University of British Columbia who kicked off the widespread criticism with a blog post last Saturday. “Each of those things is a little red flag.”
In a paper published in the prestigious journal Science, researchers affiliated with NASA’s Astrobiology Institute reported that the bacterium they found in California’s Mono Lake was able to survive in an arsenic-rich broth that included only trace amounts of phosphorus. The bacterium was able to incorporate arsenic into its DNA and in other parts of its cells — proof, they said, that it was subsisting on arsenic instead of phosphorus.
Redfield questioned the team’s assertion that there wasn’t enough phosphorus in the broth to sustain the bacteria. She raised doubts about the method the team used to prepare the microbes’ DNA for analysis, saying the bacteria could have become contaminated with outside arsenic during the process.
The next day, Harvard biogeochemist Alex Bradley wrote on the blog “We, Beasties” that the Science paper actually disproved its own result.
If there had been arsenic-based DNA in the cells, as the researchers asserted, it would have fallen apart quickly once it was extracted from the cells, Bradley wrote.
Previous experiments have shown that it takes just minutes for water to break down arsenate esters, the molecular parts that were ostensibly providing the structure for the bacterium’s DNA ladder. In contrast, phosphorus-based esters take millions of years to decay.
If there was DNA present to test, Bradley wrote, it had to be phosphorus-based.
“You never see a measurement that absolutely [proves] that it isn’t phosphorus-containing DNA,” said Steven Benner, an astrobiologist at the Foundation for Applied Molecular Evolution in Gainesville, Fla. He said the Science paper “also never rules out the obvious alternative explanation” — that these microbes were merely very good at tolerating arsenic, and were able to eke out an existence by scavenging tiny amounts of phosphorus.
Benner, who offered a skeptical take on the study during the NASA news conference, said that because these researchers challenged firmly established knowledge about chemistry, their burden of proof was very high.
“This is an exceptional claim, and exceptional claims require exceptional proof,” he said, paraphrasing the late scientist and writer Carl Sagan.
In an interview, Redfield said that the researchers’ eagerness to declare they had found a unique form of life might have led them to forego steps in the experiment that would have confirmed — or overturned — the result. “They overlooked what should have been simple opportunities to avoid contamination,” she said, adding that “scientists are human.”
She also said she “guessed” that the outside experts who vetted the study as part of the journal’s peer-review process may have gone too easy on the researchers. “They were unlucky that peer review missed problems with the paper,” she said.
Benner, who said he had seen the reviews, said the committee members were not biochemists and did not ask the right questions about the experiment.
The paper’s lead author, NASA astrobiology research fellow Felisa Wolfe-Simon, declined to discuss the controversy. But she responded to critics on her website Wednesday.
“We presented our data and results and drew our conclusions based on what we showed,” she wrote. “But we welcome lively debate since we recognize that scholarly discourse moves science forward.”
She said her team was “preparing a list of ‘frequently asked questions’ to help promote general understanding of our work.”
