It came from Mono Lake
But is NASAâs finding truly a previously undiscovered form of âweird lifeâ on Earth? Many scientists, including some noted experts at CU, have doubts
The New York Times, NASA and the prestigious journal Science announced startling news recently. âMicrobe Finds Arsenic Tasty; Redefines Life,â a page-one Times headline proclaimed.
The Washington Post and others went further, stating, âBacteria stir debate about âshadow biosphere,ââ a previously undetected environment that harbors âweird life.â
But the University of Colorado professors who coined the term âshadow biosphereâ contend that the studyâs findings do not buttress the headlines. Other CU microbiologists make similar arguments.
Felisa Wolfe-Simon, a NASA astrobiology research fellow in residence at the U.S. Geological Survey in Menlo Park, Calif. Photo by Henry Bortman.
Astrobiology is the study of the origin, evolution, distribution and future of life in the universe.
NASAâs press conference was scheduled to coincide with the publication in the journal Science of a study led by Felisa Wolfe-Simon, a NASA astrobiology research fellow in residence at the U.S. Geological Survey in Menlo Park, Calif.
NASAâs news release stated that its researcher had discovered âlife built with toxic chemical.â Wolfe-Simon stated, âWhat weâve found is a microbe doing something newâbuilding parts of itself out of arsenic.â
Wolfe-Simon and her colleagues had âtrained a bacterium to eat and grow on a diet of arsenic, in place of phosphorousâone of the six elements considered essential for life,â the Times reported.
The finding raised the possibility that other forms of life could exist on earth or elsewhere in the universe.
âThe definition of life has just expanded,â Ed Weiler, NASAâs associate administrator for the Science Mission Directorate at the agencyâs headquarters, stated.
Arsenic sits just below phosphorous on the periodic table (which means that the two elements have similar patterns of chemical reactivity), and the implication of the research was that the bacteriumâs DNA had substituted arsenic for phosphorous and kept growing.
Science itself published a news summary of the work, and while its headline stated that the bacterium âuses arsenic to build DNA and other molecules,â the story also quoted scientists who were skeptical. One scientist told Science that the proposition that the bacterium had replaced phosphorous with arsenic, âis, in my opinion, not established by this work.â
Within days, scores of scientists worldwide joined the debate, often expressing similar doubts.
Some of these experts are on the faculty at CU, are recognized experts in astrobiology and, in one case, have even collaborated with Wolfe-Simon and one of her colleagues, Paul. C.W. Davies.
A microscopic comparison shows shows bacteria grown in an arsenic-rich medium (left) vs. a phosphorous-rich medium (right). Photo by Jodi Switzer Blum
Not a âshadow biosphereâ
Carol Cleland, a philosophy professor and co-investigator at CUâs Center for Astrobiology. Photo by Noah Larsen.
Last year, Cleland and Wolfe-Simon were among several co-authors on an article titled âSignatures of a Shadow Biosphere,â which was published in the journal Astrobiology.
Additionally, Cleland has co-authored a paper on the definition of âlifeâ and is writing a book for Cambridge University Press titled âThe Quest for a Universal Theory of Life: Searching for Life as We Donât Know It.â
Cleland said Wolfe-Simonâs results are âintriguing,â especially if it were true that bacterial cells were using arsenic in place of phosphorous in their nucleic acids and proteins.
âUnfortunately, the evidence deployed in their paper does not support this claim,â Cleland said.
In the media and during the NASA press conference, experts also asserted that the results challenged our concept of what constitutes life and offered evidence of a possible shadow biosphereâpreviously undetected life on Earth.
âThis is not anything close to a shadow biosphere,â Cleland said. âThe most that their work shows, however, is that familiar life is chemically more flexible than previously thought.â
The bacterium from the bottom of arsenic-rich Mono Lake, in California, is from the same tree of life as all other known life on Earth, Cleland noted.
âWhen subjected to an arsenate-enriched, phosphate-depleted medium these bacteria grew, but it would be a mistake to characterize them as âthrivingâ (in the ordinary, everyday sense of the word),â Cleland wrote. âGrowth was faster and more extensive when phosphate was added to the culture medium.â
Further, Cleland notes, the bacteria in the arsenate-rich environments looked malformed compared to those in phosphate-rich media. âThis suggests that the bacteria were stressed and trying to cope with the high levels of arsenic in their environment.â
Instead of being âweird life,â the bacterium is an âextremophile,â an organism that has adapted to a harsh environment, Cleland said.
âThe metabolic pathways used by highly evolved, familiar, contemporary life cannot distinguish between phosphate and arsenate, which is why arsenic is poisonous to it.Ěý A truly novel form of life, descended from a separate origin, is unlikely to share the same high-level metabolic pathways as our form of life,â she said.
âAs a consequence, even if they established the substitution of arsenate for phosphate at significant levels in (the Mono Lake bacterium), their work does not challenge our current concept of life,â Cleland said. âAnd for the same reason, it does not provide support for the possibility of a shadow biosphere.â
âAt best, the work of Wolfe-Simon and colleagues adds another page to our growing knowledge of the astonishing environmental toughness and perhaps metabolic diversity of familiar Earth microbes.â
âPremature and misleadingâ
Shelley Copley, a CU professor of molecular, cellular and developmental biology, is co-investigator at CUâs Center for Astrobiology. Photo by Noah Larsen.
âThis finding gave rise to the idea that the cells might be using As (arsenic) in place of P (phosphorous). The authors focused on whether P in DNA was replaced by As, since that would indeed be a stunning finding,â Copley wrote.
âHowever, the data do not show that all of the P in DNA has been replaced by As, which seems to be the interpretation that is playing out in the media.
âTo be fair, nowhere in the paper do the authors make a quantitative statement. They say that the bacterium âcan vary the elemental composition of its basic biomolecules by substituting As for P,ââ Copley continued.
âIt is unclear whether they mean complete substitution or partial substitution, and whether substitution at a level of, say, 1 percent, would actually be very exciting.âĚý Based on the data in the paper, âit appears that there may be some low level incorporation of As into DNA, but the data are not completely convincing.â
Experiments that would have been conclusive were not performed, and some of the data are of dubious quality and were not interpreted critically, Copley said. The paper raises many questions that should have been addressed by the authors before this work was published, she added.
âUnfortunately, publication of work that makes premature and misleading claims in a high-profile venue like Science, followed by the excitement fomented by NASAâs promotion of the work, is likely to damage the credibility of origin-of-life-researchers and scientists in general in the eyes of the public.â
âProbably bogusâ
Norman Pace, a distinguished professor of molecular, cellular and developmental biology at CU and a co-investigator at CUâs Center for Astrobiology, said the conclusion of arsenate in the DNA is âprobably bogus.â
âNo evidence at all was presented that showed the specific incorporation of arsenic into any biomolecule,â Pace said. âAll of the data were inferential and easily explainable by contamination of analyzed samples with arsenate, on one hand, and phosphate on the other. The study was amateurish.â
Like Cleland, Pace said there is nothing particularly novel about the Mono Lake bacterium. âIt is a so-called gamma-group proteobacterium, a close relative of many well-known organisms, e.g.ĚýEscherichia coli. There are a lot of organisms known that metabolize arsenate and arsenite,â Pace said.
âBased on what I know, the story is largely hype; NASA needed some news, so they jumped on this.â
Michael Yarus, a professor of molecular, cellular and developmental biology, co-investigator at the Center for Astrobiology and author of the book âLife from an RNA World,â characterizes Wolfe-Simonâs article as making an âextraordinary claim,â which requires excellent evidence.
âStrikingly, strong evidence should be easily foundâif any mainline biochemical has a high frequency of arsenic instead of phosphorus (for example: DNA, RNA or AMP), that would be convincing, a âsmoking gun,ââ Yarus said.
âHowever, the published evidence does not approach this level. In fact, the attempt to demonstrate arsenic in DNA is very unconvincing and suggests instead that As may be present as a prevalent cellular contaminant, say as some small-molecule form made in high concentrations to detoxify an otherwise poisonous environmental hazard,â he added.
âI therefore remain to be convinced.â
As it is elsewhere, opinion about the NASA study is mixed at CU. Rob Knight, CU associate professor of chemistry and biochemistry, leads one of 13 interdisciplinary research projects on synthetic biology that were awarded by the National Academies Keck Futures Initiative. Knightâs research combines computational and experimental techniques to ask questions about the evolution of the composition of biomolecules, genomes and communities.
Knight said Wolfe-Simonâs findings âprovide a fascinating insight into the limits of metabolic adaptation: although more work remains to be done in order to confirm that As can substitute for P in a range of biological RNAs and DNAs, the results presented here are amazing in that they are the first direct evidence that life can substitute one of its key elements for another.â
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