Studies refute arsenic bug claim
The discovery of a bacterium that could substitute arsenic for phosphorus to survive is refuted by new research.
Six elements are considered essential for life - oxygen, carbon, hydrogen, nitrogen, phosphorus and sulphur - so the announcement in 2010 implied one of biology's golden rules had been broken.
The findings provoked an immediate backlash and now two new scientific papers suggest the bacterium needs phosphorus to grow after all.
The studies appear in Science journal.
One of the papers is authored by Tobias Erb from the Swiss Federal Institute of Technology in Zurich and collagues; the other comes from a team led by Marshall Reaves of Princeton University in New Jersey.
In December 2010, the same journal published a paper claiming to show that the GFAJ-1 micro-organism, found in the arsenic-rich sediments of California's Mono Lake, could incorporate arsenic into its DNA when phosphorus was unavailable.
The team, led by Felisa Wolfe-Simon - then based at the US Geological Survey (USGS) - had acknowledged very low levels of phosphate (a phosphorus-containing molecule) within their study samples.
But they concluded that this contamination was insufficient to permit GFAJ-1 to grow.
The new papers suggest instead that although the organism is able to survive in high arsenic and low phosphorus (phosphate) conditions, it still needs phosphorus to grow.
They conclude that Dr Wolfe-Simon's samples did in fact contain enough phosphate contamination to support GFAJ-1's growth.
The researchers argue that the bacterium - being highly adapted to the arsenic-rich environment of the lake - is thrifty, and has become adept at scavenging phosphorus under harsh conditions.
This helps explain why it can grow even when arsenic is present within the cells.
The latest studies also found no evidence that arsenic was incorporated into the microbe's DNA - as the authors of the original paper had suggested.
Despite the criticism directed at the December 2010 study, Science journal has never retracted the study by Dr Wolfe-Simon and her colleagues.
However, Science's editors have released a statement to accompany the new papers, in which they comment: "In conclusion, the new research shows that GFAJ-1 does not break the long-held rules of life, contrary to how Wolfe-Simon had interpreted her group's data.
"The scientific process is a naturally self-correcting one, as scientists attempt to replicate published results."
The original finding was the subject of a Nasa press conference, in which participants speculated about the finding's possible implications for life elsewhere in the Universe.
Dr Wolfe-Simon and her colleagues have already responded to a number of specific criticisms in the pages of Science journal, as well as in the press - where the subsequent debate was also played out.
The lead author subsequently left the USGS lab where she was based. It was originally reported that she was seeking a location with better molecular and genetic research facilities.
But in an interview with Popular Science magazine she said she was effectively "evicted" from the laboratory.