When Nasa announced a high profile press conference in December on extraterrestrial life the level of excitement was palpable. Rumours abounded on the internet that the space agency had finally discovered signs of ET. The actual announcement was a little less dramatic, with scientists reporting that they had found a terrestrial bacterium that incorporated arsenic into its biomolecules. This was still big news, however, because if organisms could use sizeable amounts of elements generally considered toxic then the available niches for life in the universe would be much bigger than previously thought. But then the backlash began.

Science bloggers and tweeters were quickly claiming that the results had been hyped. Microbiologists and chemists soon raised what they believed were serious flaws in the paper.

In response to this criticism Science, the journal which published the paper that caused the original hoo-hah, has now put together a collection of eight of the best technical criticisms of the paper and the response of lead author Felisa Wolfe-Simon and co-workers.

Perhaps some of the most stinging criticism came from Stephen Benner, from the Foundation for Applied Molecular Evolution in the US. He reminds the authors that, as Carl Sagan said, ‘extraordinary claims require extraordinary evidence’. If arsenate can replace phosphate in DNA, Benner says, then chemists would need to ‘set aside nearly a century of chemical data’ about these molecules. He adds that, if arsenate linkages existed in DNA, they would, according to predictions, have a half life of just a minute.

Stefan Oehler, at the Biomedical Sciences Research Center Alexander Fleming, Greece, agrees that more work was needed before such a far reaching claim could justifiably be made. Experiments that could have demonstrated that arsenic had replaced phosphorus in DNA, such as comparing the rate of hydrolysis of ordinary DNA with DNA thought to contain arsenic, were not performed. Other criticisms included poor presentation of data, a failure to rule out sample contamination and sufficient phosphate in the growth media for bacteria to replicate without needing to resort to using arsenate.

However, Wolfe-Simon and colleagues maintain that these bacteria did add arsenic to its biomolecules and set out some rebuttals to these points. They now plan further experiments, along the lines of those suggested in the technical comments. Also, samples of the bacteria are now available to labs, and researchers are certain to be keen to get their hands on them.

Patrick Walter

P L Foster, 2011, Science, DOI:10.1126/science.1201551
S Oehler, 2011, Science,DOI:10.1126/science.1201381
S A Benner, 2011, Science,DOI:10.1126/science.1201304
R Redfield, 2011, Science,DOI:10.1126/science.1201482
F Wolfe-Simon et al, 2011, Science, DOI:10.1126/science.120209
B Schoepp-Cothenet et al, 2011, Science, DOI:10.1126/science.1201438
D W Borhani, 2011, Science, DOI:10.1126/science.1201255
I Csabai and E Szathmáry, 2011, Science, DOI:10.1126/science.1201399
J B Cotner and E K Hall, 2011, Science, DOI:10.1126/science.1201943
E Pennisi, 2011, Science, DOI:10.1126/science.332.6034.1136

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