"This Paper Should Not Have Been Published"
Scientists see fatal flaws in the NASA study of arsenic-based life.
On Thursday, Dec. 2, Rosie Redfield sat down to read a new paper called "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus." Despite its innocuous title, the paper had great ambitions. Every living thing that scientists have ever studied uses phosphorus to build the backbone of its DNA. In the new paper, NASA-funded scientists described a microbe that could use arsenic instead. If the authors of the paper were right, we would have to expand our notions of what forms life can take.
Redfield, a microbiology professor at the University of British Columbia, had been hearing rumors about the papers for days beforehand. On Monday, NASA released a Sphinxlike press release: "NASA will hold a news conference at 2 p.m. EST on Thursday, Dec. 2, to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life." Like a virulent strain of bacteria, speculation exploded over the next three days. "Did NASA Discover Life on One of Saturn's Moons?" asked Gawker, a Web site that does not often ask questions about astrobiology.
The truth was revealed on Thursday. At NASA's press conference, the scientists described their research, which was just then being posted on the Web site of the journal Science. They had not found life on one of Saturn's moons; instead, they had gone to the arsenic-laced waters of Mono Lake in California and isolated a strain of bacteria they dubbed GFAJ-1.
Back at the lab, they grew the bacteria in a broth of nutrients. When they gradually reduced the supply of phosphate (a molecule composed of one phosphorus atom and four oxygen atoms) and replaced it with arsenate (one arsenic and four oxygen atoms), the bacteria still managed to grow. The scientists examined the DNA of these hardy microorganisms and inferred that it contained arsenic.
As soon as Redfield started to read the paper, she was shocked. "I was outraged at how bad the science was," she told me.
Redfield blogged a scathing attack on Saturday. Over the weekend, a few other scientists took to the Internet as well. Was this merely a case of a few isolated cranks? To find out, I reached out to a dozen experts on Monday. Almost unanimously, they think the NASA scientists have failed to make their case. "It would be really cool if such a bug existed," said San Diego State University's Forest Rohwer, a microbiologist who looks for new species of bacteria and viruses in coral reefs. But, he added, "none of the arguments are very convincing on their own." That was about as positive as the critics could get. "This paper should not have been published," said Shelley Copley of the University of Colorado.
None of the scientists I spoke to ruled out the possibility that such weird bacteria might exist. Indeed, some of them were co-authors of a 2007 report for the National Academies of Sciences on alien life that called for research into, among other things, arsenic-based biology. But almost to a person, they felt that the NASA team had failed to take some basic precautions to avoid misleading results.
When the NASA scientists took the DNA out of the bacteria, for example, they ought to have taken extra steps to wash away any other kinds of molecules. Without these precautions, arsenic could have simply glommed to the DNA, like gum on a shoe. "It is pretty trivial to do a much better job," said Rohwer.
In fact, says Harvard microbiologist Alex Bradley, the NASA scientists unknowingly demonstrated the flaws in their own experiment. They immersed the DNA in water as they analyzed it, he points out. Arsenic compounds fall apart quickly in water, so if it really was in the microbe's genes, it should have broken into fragments, Bradley wrote Sunday in a guest post on the blog We, Beasties. But the DNA remained in large chunks—presumably because it was made of durable phosphate. Bradley got his Ph.D. under MIT professor Roger Summons, who co-authored the 2007 weird-life report. Summons backs his former student's critique.
But how could the bacteria be using phosphate when they weren't getting any in the lab? That was the point of the experiment, after all. It turns out the NASA scientists were feeding the bacteria salts which they freely admit were contaminated with a tiny amount of phosphate. It's possible, the critics argue, that the bacteria eked out a living on that scarce supply. As Bradley notes, the Sargasso Sea supports plenty of microbes while containing 300 times less phosphate than was present in the lab cultures.