The elements carbon, hydrogen, oxygen, and nitrogen lie at the heart of the biological processes that make life possible, but they are by no means the only elements that facilitate those processes.
Two scientific papers, published in the journal Science, refuted the 2010 NASA finding that bacterium called GFAJ-1 not only tolerates arsenic, but actually incorporates the poison into its DNA, swapping out phosphorus. The finding, at the time, was hailed as a significant new discovery, which could increase the likelihood of discovering exotic life on other planets.
“Contrary to an original report, the new research clearly shows that the bacterium, GFAJ-1, cannot substitute arsenic for phosphorus to survive,” the journal said. “If true, such a finding would have important implications for our understanding of life’s basic requirements since all known forms of life on Earth use six elements: oxygen, carbon, hydrogen, nitrogen, phosphorus and sulfur.”
Phosphorous may not be one of the first elements that comes to mind when it comes to supporting life, but there are good reasons that phosphates are a critical component in plant fertilizers. Both DNA and RNA are dependent on the availablility of phosphorous, as are energy storage compounds such as ATP, and thus, the presence of biologically available (i.e., able to be metabolized by an organism) phosphorous is seen as an essential environmental requirement.
A 2010 NASA study garnered a lot of attention when its authors, observing bacteria in Lake Mono, California, reported that the bacteria were surviving on arsenic instead of phosphorous. Such a discovery would be of obvious interest to NASA, as it would expand the list of possible environments where life might be found beyond the confines of Earth. This bacteria, dubbed GFAJ-1, reportedly utilized arsenic in the construction of its cell wall as well as other systems.
“The newly discovered microbe, strain GFAJ-1, is a member of a common group of bacteria, the Gammaproteobacteria. In the laboratory, the researchers successfully grew microbes from the lake on a diet that was very lean on phosphorus, but included generous helpings of arsenic. When researchers removed the phosphorus and replaced it with arsenic the microbes continued to grow. Subsequent analyses indicated that the arsenic was being used to produce the building blocks of new GFAJ-1 cells,” the report noted at the time.
One independent team is disputing the finding, arguing that the bacteria are simply well-adapted to life in the extreme lake environment and able to efficiently use the scarce amount of phosphorous to meet its needs. The team notes that arsenic does not contribute to the growth of the microbe at all.
Meanwhile, another group led by Tobias Erb from the Institute of Microbiology at ETH Zurich targeted the claim that GFAJ-1 did not rely on the phosphorus present in the samples. While the original group insisted that the levels of phosphorus were simply too low to accommodate life, the new report found that the organism was incapable of surviving without some amount of the substance.
The lead researcher from the NASA study, Felisa Wolfe-Simon of NASA’s Astrobiology Institute, defended the study, saying that the data in the new findings do not, in fact, conflict with the study at all.
“We are working to define where the arsenate is [in the organism], rather than where it is not,” she said. “How does GFAJ-1 thrive in such high levels of arsenic? Where is the arsenic going? This is our continued focus.”
In addition, Michael H. New, astrobiology discipline scientist at NASA Headquarters’ Planetary Science Division, said in a statement that the U.S. space agency supports an open dialogue on the matter:
“NASA supports robust and continuous peer review of any scientific finding, especially discoveries with wide-ranging implications. It was expected that the 2010 Wolfe-Simon et al. Science paper would not be exempt from such standard scientific practices, and in fact, was anticipated to generate significant scientific attention given the surprising results in that paper. The two new papers published in Science on the microorganism GFAJ-1 exemplify this process and provide important new insights. Though these new papers challenge some of the conclusions of the original paper, neither paper invalidates the 2010 observations of a remarkable microorganism that can survive in a highly phosphate-poor and arsenic-rich environment toxic to many other microorganisms. What has emerged from these three papers is an as yet incomplete picture of GFAJ-1 that clearly calls for additional research.”
Skepticism over the validity of the findings is likely to leave alien hunters a bit disheartened. The 2010 study was the result of a test proposed by astrobiologist Paul Davies, director of the BEYOND Center for Fundamental Concepts in Science at Arizona State University, Tempe. Mr. Davies encouraged scientists to scour the Earth for organisms so exotic that they must have derived from other planets.
The findings were released Sunday in the journal Science.