This image shows a type of bacteria called GFAJ-1 that was said to incorporate arsenic in its cellular machinery.
Researchers say they ran a more rigorous version of the experiment that sparked a yearlong debate over the prospects for arsenic-based bacteria — but found no trace of arsenic within the organisms' DNA.
The findings, submitted to the journal Science this week and distributed openly via the ArxiV.org website, serve as the most definitive refutation to date of the "weird life" claims that caused such a stir in December 2010. "They match with what basically all the scientists had concluded a year ago," University of British Columbia microbiologist Rosie Redfield, the paper's senior author, told me.
Redfield had criticized the original study from the start, suggesting that the arsenic detected in a strain of bacteria known as GFAJ-1 was not actually incorporated into the machinery of life but was merely the result of insufficient purification. "We were much more meticulous about purifying the DNA before we analyzed it," she said today.
She and her colleagues worked with the same bacteria used for the original research, which had astrobiologist Felisa Wolfe-Simon as lead author and was published in Science. The bacteria were bred to live in a high-arsenic environment, with virtually no phosphorus present. The aim was to see whether arsenic compounds known as arsenates, which are typically poisonous to life as we know it, could be substituted for chemically similar phosphorus compounds known as phosphates. If that turned out to be the case, that would suggest that alien life forms could operate using biochemical processes radically different from Earth's.
In their paper, Wolfe-Simon and her colleagues said they saw evidence that the bacteria could be bred to live in the arsenic-rich environment, and that arsenates were detected in "macromolecules that normally contain phosphate, most notably nucleic acids and proteins."
Redfield and her colleagues were able to grow the bacteria amid high arsenic levels, under special conditions, but they found that the arsenic wasn't necessary for the bacteria's survival — and that the highly purified DNA from the bacteria did not contain detectable levels of arsenate.
Redfield noted that some arsenate stuck to the DNA even after what she thought would be sufficient purification, but was removed during a second round of washing. "That shows that arsenate does persist through steps in the DNA purification, but in a form that will wash away," she told me.
The researchers acknowledged that arsenate might occasionally get into the bacteria's biological machinery.
"Given the chemical similarity of arsenate to phosphate, it is likely that GFAJ-1 may sometimes assimilate arsenate into some small molecules in place of phosphate, such as sugar phosphates or nucleotides. Our results do not rule out the possibility that such assimilation could be beneficial," they wrote. "When it comes to DNA synthesis, however, GFAJ-1 does not appear to productively assimilate any arsenate."
Open review for results
The scientists behind the original study have said they would refrain from commenting on follow-up research until the peer-review and publication process is completed. Wolfe-Simon did not immediately respond to an emailed request for comment, but Science News' Rachel Ehrenberg quoted her as saying she and her colleagues never actually claimed that arsenate was being incorporated in GFAJ-1’s DNA.
"As far as we know, all the data in our paper still stand,” Science News quoted Wolfe-Simon as saying in an email. “Yet, it may take some time to accurately establish where the [arsenic] ends up."
That response left Redfield figuratively scratching her head — and literally wondering "WTF??" in a Twitter update. She pointed to several references in the original Science paper referring to DNA, including a sentence saying that the measurements "specifically demonstrated that purified DNA extracted from +As/-P [high-arsenic, low-phosphorus] cells contained As [arsenic]."
The paper written by Redfield and her colleagues is open for review and comment even in advance of its consideration for journal publication. That's consistent with Redfield's advocacy of an "open science" approach to research, as reflected in the regular updates posted to her RRResearch blog. Thanks to the blog, avid followers of the #ArsenicLife issue have known for weeks that the original results couldn't be replicated.
Redfield said she has received assurances that freely distributing the draft paper won't hurt the prospects for publication in Science — which goes against the traditional grain for peer-reviewed publication.
"What's happening, and I'm really pleased by this, is that interested people are reading the manuscript, and they're putting comments on it," she observed.
Redfield said she and her colleagues appreciated the feedback being posted to her blog by experts — as well as by non-experts. "Their comments are going to let us polish the manuscript to make it more accessible to non-experts," she told me.
More about the arsenic-life debate:
- A year later, the debate still percolates
- Nature: Study challenges existence of arsenic-based life
- Chemical & Engineering News: The arsenic aftermath
In addition to Redfield, the authors of "Absence of Arsenate in DNA From Arsenate-Grown GFAJ-1 Cells" include M.L. Reaves, S. Sinha, J.D. Rabinowitz and L. Kruglyak.
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.