It was hailed in 2010 as the most "alien" life-form yet: bacteria that reportedly, and unprecedentedly, had rewritten the recipe for DNA. And the secret ingredient was arsenic.
But now two new studies seem to have administered a final dose of poison to the already controversial finding.
Researchers led by then NASA astrobiologist Felisa Wolfe-Simon had found the organism, dubbed GFAJ-1, in arsenic-rich sediments of California's Mono Lake. They later reported in the journal Science that the bacterium thrived in arsenic-rich, phosphorus-poor lab conditions.
The team concluded that GFAJ-1 must be incorporating arsenic into its DNA in place of phosphorous, which is essential for the DNA of all other known organisms. (Get a genetics overview.)
The find was exciting to astrobiologists, who'd previously speculated that extraterrestrial life might survive in unexpected places if only such a swap were possible—arsenic and phosphorous being chemically similar. (Related: "Saturn's Largest Moon Has Ingredients for Life?")
Soon after the announcement, though, other researchers began saying they were having trouble replicating Wolfe-Simon's results. Those criticisms were finally given formal voice Sunday in the form of two different studies with very similar results.
The new studies, also published in Science, found that the bacterium did in fact grow in the conditions described in the 2010 study.
But when the amount of phosphorous was reduced even further than in Wolfe-Simon's experiments, GFAJ-1 stalled. Furthermore, biologist Rosemary Redfield writes in the new study, no signs of arsenic could be found in GFAJ-1's DNA.
The new conclusion: the arsenic-loving life-form does in fact need phosphorous to grow, but shockingly tiny amounts of it.
Not Backing Down
Wolfe-Simon, now of the Lawrence Berkeley National Laboratory, stands by her results. The new paper, she said, shows only that the arsenic doesn't show up in the DNA, not that the organism never uses it.
The fact that the organism has extreme resistance to arsenic and takes it up from the environment means that something unusual is happening with that arsenic, Wolfe-Simon said by email.
"We are working to define where the arsenate is [in the organism], rather than where it is not," she said.
"How does GFAJ-1 thrive in such high levels of arsenic? Where is the arsenic going? This is our continued focus."
Arsenic Tolerance No Sign of "Second Genesis"
For astrobiologists, the new finding is a disappointment but not a severe setback in the search for alien life.
The 2010 study sprang from a quest proposed by astrobiologist Paul Davies, director of the BEYOND Center for Fundamental Concepts in Science at Arizona State University, Tempe.
Davies encouraged scientists to look for organisms on Earth so exotic that they must have come not just from a different branch of our own tree of life but from an entirely separate founding ancestor.
If we could find such organisms, Davies suggested, they would indicate that life originated more than once here on Earth—a "second genesis." And if life began more than once here, it would seem more likely that life exists on other earthlike planets.
The new papers have no impact on this quest, he said, because genetic studies had already indicated that GFAJ-1 is related to other known bacteria.
"It was clear from early on," he said, "that GFAJ-1 did not constitute evidence for a second genesis."