Critics take aim at NASA 'arsenic life' study
Authors of study claiming bacteria used usually toxic arsenic to build DNA stand by results
Eight articles questioning a controversial study claiming that some bacteria can use the normally toxic substance arsenic to build DNA have been published in the journal Science.
The study, published last December in Science, was led by NASA scientist Felisa Wolfe-Simon and claimed that bacteria from a lake in California were able to substitute arsenic for phosphorus, normally an essential ingredient in DNA, fats and proteins.
University of British Columbia zoology professor Rosie Redfield was one of more than a dozen scientists from around the world who challenged those findings and whose comments were published Friday.
Science also published a response from Wolfe-Simon and her co-authors, who wrote that they welcomed the opportunity.
But they said they maintained that their interpretation "based on multiple congruent lines of evidence, is viable."
The researchers also said they will make the strain of bacteria they studied available for other scientists to test.
Wolfe-Simon and her colleagues described a new strain of Halomonadaceae bacteria found in Mono Lake, Calif., that seemed to be able to use arsenic — an element that is usually toxic to living things — instead of phosphorus.
"If true, this finding would raise important questions about life's basic requirements, since only six elements — carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus — make up the bulk of living matter," said Science in a statement published alongside the criticisms of the study.
"As would be expected after a study with such broad implications is published, scientists have asked comparably challenging questions."
The journal's editor-in-chief, Bruce Alberts, acknowledged in a statement that the article was "the subject of extensive discussion and criticism."
He added that the editors hope the series of new articles will "allow readers to better assess the research article's original claims and the criticisms of them."
Not 'final word': editors
Alberts and the other editors noted that they don't consider the paper to be "the final word" on the subject but something that provides "further opportunities for research and interpretation."
At that time, NASA said Wolfe-Simon would not be responding to those criticisms, as the U.S. space agency did not feel it was appropriate to debate the science using media and blogs. Instead, it said, the debate should occur in scientific publications.
On Friday, Redfield posted on her blog that Wolfe-Simon's and her colleagues' responses to her criticisms were "in some ways the most scientifically valid" because they provide more information about what precautions were taken to avoid contamination in the experiment.
However, she was largely critical of the researchers' statement.
"Most of their responses take the form of 'our interpretation could be correct on this point if...'," she wrote.
"In many cases, there is, indeed, a small possibility that it could, but there are so many of these points of interpretation, each with only a very small probability of being correct, that I don't think anyone will find the arguments convincing."
Summary of debate
Below is a summary of key criticisms made in the eight articles published Friday in Science, along with the response of the authors of the study from last December.
|The form of arsenic expected in DNA would be very unstable when exposed to water.||Arsenic compounds may be less flexible within biomolecules and therefore less vulnerable to water.|
|The bacteria may have managed to get trace amounts of arsenic from their environment.||Bacteria both exposed and not exposed to arsenic were grown in the same low phosphorus levels, and only survived if extra arsenic or phosphorus was added.|
|The bacteria may have used a different chemical pathway to survive in very low phosphorus.||If that were the case, arsenic would have been chemically modified, and there was no evidence of this.|
|The extremely low phosphorus levels found in the bacteria are within the range observed for other bacteria.||That holds only for individual cells, and not the average. The researchers measured the levels of all the cells in bulk.|
|The researchers averaged ratios of different elements incorrectly.||A revised calculation still supports the original conclusions.|
|The researchers didn't take enough measures to rule out contamination.||The researchers clarified what measures they did take, and said not taking the other measures shouldn’t have affected the measurements.|
|The arsenic couldn't be substituting for phosphorus because its ratio to carbon was too low.||The data for the two elements obtained using the technique in question cannot be compared.|
|The arsenic should have been converted to a different form, which was not observed.||No, that other form was not observed.|
|The researchers should have used more direct methods, such as tagging molecules with radioactive isotopes, to confirm their results.||The researchers hope other scientists will do that.|