Few scientific studies have drawn the level of criticism from the blogosphere and international scientific community lately as a study by scientists at the Stanford Synchrotron Radiation Lightsource (SSRL) — a division of the SLAC National Accelerator Laboratory (SLAC) — and astrobiologists at NASA last month.
The paper, titled “Bacterium that can grow using arsenic instead of phosphorus,” claims to have found a bacterium strain in California’s Mono Lake “which substitutes arsenic for phosphorus to sustain its growth.” To many readers, it implied that the bacterium strain, GFAJ-1 of the bacterium Halomonadacae, evolved in such a way that it was able to replace phosphorus in the bacteria’s cellular material.
News of the finding first splashed across headlines because the discovery of such an organism suggested to some that the possibility that other forms of life can exist in the extreme conditions of other planets, where the molecules we humans need to survive aren’t available. The newsprint hardly had time to dry, however, before heavy criticism from bloggers and renowned scientists, who said evidence for the paper’s conclusion is insufficient, started rolling in.
Several Stanford researchers expressed skepticism about whether these bacteria had used arsenic to build their DNA.
“There seems to be no doubt that these bugs grow on and incorporate a high level of arsenic,” chemistry professor Steven Boxer said. “The real question is whether all phosphorus is replaced by arsenic in these key molecules of life.”
Geology professor Norm Sleep, who has performed research in the field of astrobiology, was more critical of the study’s findings.
“The conclusion that their cell was making arsenic-based DNA is a highly outlandish, if not impossible one,” Sleep said. “It’s such a major conclusion…we need extremely good evidence, and the evidence of the paper falls short of this criteria.”
Sleep believes the study did not merit the type of press attention it received.
“The paper claimed that the microbe from Mono Lake could live by putting arsenic for phosphorus in its DNA…the press went way beyond this,” he said. “The microbe is closely related to…ancestor microbes that live in arsenic-poor environments; it is not a second independent form of life on Earth.”
According to Sleep, the research merely showed that the organism was able to tolerate extreme amounts of arsenic and could survive in a phosphorus-poor environment.
There are 10 complicated steps involved for the cell to make DNA, Sleep added.
“The chemistry has evolved to make ordinary phosphorus DNA, not arsenic DNA,” he said. “The [arsenic-DNA] made in the laboratory are unstable…so it is highly unlikely that arsenic-DNA could be made in any quantity,” he wrote in an e-mail to The Daily.
Sleep added that since the steps involved in the process of making DNA are so intricate, it would be “very improbable” to change one aspect of the cell without changing many others.
The study’s researchers have responded to criticism by putting together a collection of frequently asked questions in order to clarify the data and procedures that were involved.
“We are actively engaged in responding to formal questions submitted to Science magazine,” said Sam Webb, an SSRL staff scientist, who led the research at the Department of Energy’s SLAC National Accelerator Laboratory, in an e-mail to The Daily.
“Our manuscript was thoroughly reviewed and accepted for publication by Science,” he said. “We presented our data and drew our conclusions based on what we showed. But we welcome lively debate since we recognize that scholarly discourse moves science forward.”