Arsenic and New Life—or Not
Claims of a new life form get whacked.
We have seen this movie before: scientists hold a press conference to announce much-hyped results (cold fusion in 1989, the Martian rock with signs of life in 1996). Then other researchers check out the claims, which proceed to vanish like a rabbit in a magic show. We have also seen the movie in which a leading science journal publishes a paper making extraordinary claims only to see them shown to be seriously flawed, as in recent claims about the genetic basis for longevity in Science, that other scientists raised serious doubts about.
So the concerns being raised about last week’s press-conference announcement, and Science paper, by researchers at NASA’s Astrobiology Institute, the U.S. Geological Survey, and elsewhere that bacteria scooped from Mono Lake are able (when grown in the lab) to incorporate arsenic rather than phosphorous in its nucleic acids and enzymes are not exactly surprising. The thrust of the claim is that this represents a new way to live—that although all previously known forms of life on earth use phosphorous in the backbone of their DNA and RNA, this bug can use arsenic, which is usually poisonous. That challenges the dogma that life requires the Big 6: oxygen, carbon, hydrogen, nitrogen, phosphorus, and sulfur. If there are other ways to be alive, the likelihood of life on other planets rises.
To their credit, the news pages of Science cited scientists who had serious doubts about the claim, with one researcher expressing “lingering concerns that the arsenic is simply concentrated in the bacterial cell’s extensive vacuoles and not incorporated into its biochemistry, and another saying that the claim of bacteria subbing arsenic for phosphorous “is, in my opinion, not established by this work.”
But those doubts have gotten a lot worse in the last few days. The beef is that the original researchers failed to show convincingly that the arsenic was really incorporated into the bacteria’s DNA and proteins, rather than just hanging around with the bugs in a way that fooled the researchers when they did their analyses. Geochemist Alexander Bradley of Harvard, who studies the evolution of life on earth, argues here that “the claim is almost certainly wrong.” The scientists overlooked evidence that “the DNA in question actually has a phosphate—not an arsenate—backbone,” he says: an arsenate DNA backbone hydrolyzes (falls apart) in water within minutes, yet during a multi-step chemical procedure the Mono Lake DNA, despite being in water for an hour or two, the DNA backbone held up fine. “Any arsenate-DNA would have been quickly hydrolyzed in the water, breaking down into fragments of small size, [but] phosphate-DNA would not hydrolyze quickly, and large-sized fragments might be recoverable. So what size are the fragments of DNA extracted from GFAJ-1? They are large ... If this DNA did not hydrolyze in water during the long extraction process, then it doesn’t have an arsenate backbone. It has a phosphate backbone. It is normal DNA.”
A more plausible explanation for the findings that there is arsenic in the Mono Lake bacteria, argues Bradley, is that “arsenate sticks to stuff. When you grow bacteria in media containing lots of arsenate, cellular material gets covered in arsenate. If you analyze this material chemically, you see a high arsenic background,” fooling you into thinking that the proteins and DNA of the cells actually incorporated arsenic.
Microbiologist Rosie Redfield of the University of British Columbia goes further, writing here that “NASA’s shameful analysis of the alleged bacteria in the Mars meteorite [that’s the mess I refer to above, from 1996] made me very suspicious of their microbiology, an attitude that’s only strengthened by my reading of this paper. Basically, it doesn’t present ANY convincing evidence that arsenic has been incorporated into DNA (or any other biological molecule).” She points out a number of methodological flaws, concluding this way: “Bottom line: Lots of flim-flam, but very little reliable information ... If this data was presented by a PhD student at their committee meeting, I’d send them back to the bench to do more cleanup and controls ... I don’t know whether the authors are just bad scientists or whether they’re unscrupulously pushing NASA’s ‘There’s life in outer space!’ agenda.”
If so many experts have raised so many red flags, how did the study get published without the authors being asked by reviewers to go back and do better-controlled experiments? (On this point, Redfield argues that “there’s a difference between controls done to genuinely test your hypothesis and those done when you just want to show that your hypothesis is true. The authors have done some of the latter, but not the former.”) Reviewers might have been in an impossible position, Redfield says: “I hesitate to blame the reviewers, as their objections are likely to have been overruled by Science’s editors in their eagerness to score such a high-impact publication.”
To make matters much, much worse, both NASA and the authors of the paper have refused to respond to these criticisms, basically saying they don’t want to debate science in the media—something they might have considered before they held the press conference. The phrase “live by the sword, die by the sword” comes to mind.
Scientists’ reluctance to enter the 21st century is more than a little distressing. The original arsenic paper “went through the peer-review mill, which means that at least three scientists [who] are credited as experts in the field have looked at it and given it a clean bill of health,” points out computational biologist Iddo Friedberg of Miami University in his blog, Byte Size Biology. “But once it got published, hundreds of microbiologists and biochemists had a look, and many were less than convinced of some of its claims.” Is it too much to ask that scores of “reviewers” after publication be accorded the standing of three anonymous referees before?