In a study published online at Science Express on 2 December 2010, Felisa Wolfe-Simon and colleagues described a bacterium from Mono Lake, California, which they claimed substitutes arsenic for a small amount of the phosphorus in its DNA and other molecules. 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.
The study attracted widespread attention throughout the mainstream news media, the blogosphere, and elsewhere. After the research was published, Science received a wide range of correspondence from scientists, many of whom had concerns about the study’s methods and interpretations.
The journal has published eight Technical Comments that represent the main concerns, as well as a Technical Response by the Wolfe-Simon group, online at Science Express. This material, along with the final version of the Wolfe-Simon paper, will appear in the 3 June edition of Science.
In the initial study, Wolfe-Simon and colleagues grew samples obtained from arsenic-rich Mono Lake in laboratory cultures, to investigate whether the microbes might be capable of incorporating arsenic in place of phosphorus. In one set of cultures containing only very small amounts of phosphorus in the growth medium, the researchers gradually increased the amount of arsenic and observed the growth of a bacterial strain called GFAJ-1. This bacterium did not grow in a control culture with the same trace amount of phosphorus to which no arsenic was added. GFAJ-1 also grew readily in a third culture to which the authors added phosphorus but not arsenic.
Eastern California’s Mono Lake is an ancient alkaline lake known for unusual tufa formations (shown here), as well as for its hypersalinity and high concentrations of arsenic. | Photo © James J. Ludemann, licensed Creative Commons Attribution 3.0 Unported.
The researchers then used a variety of methods to investigate whether the microbes from the arsenic-rich conditions were incorporating the arsenic into their biomolecules.
The Technical Comments discuss many different aspects of the study, including whether the microbes might have been surviving off of the trace amounts of phosphorus in growth medium. One Comment points out that the form of arsenic that would be expected in DNA—compounds called arsenate esters—break apart much more quickly in water than phosphate esters do and would probably be very unstable in DNA. Other Comments critique the various lines of evidence that led the Wolfe-Simon team to conclude that the bacterial DNA contained arsenic.
Scanning electron micrograph of Halomonadaceae bacteria strain used in the Wolfe-Simon experiments.
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[Image courtesy of Science/AAAS]
In their Technical Response, Wolfe-Simon and colleagues clarify some points about the experimental setup and methods, and they respond to each of the Technical Comments. They conclude that their interpretation is still viable.
Science’s editors said the new material will provide further opportunities for research and education, help readers evaluate the research, and assist scientists attempting to replicate the findings. Wolfe-Simon and colleagues are providing their bacterial cultures for this purpose.
Read the Science Technical Comments and the Technical Reply from the Wolfe-Simon research group (free with registration).
Read the Science report, “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus,” by Felisa Wolfe-Simon and colleagues (free with registration).