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<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported that bacterium GFAJ-1 can substitute arsenic for phosphorus in its macromolecules, including DNA and proteins. If such arseno-DNA exists, then much of the past century of work with arsenate and phosphate chemistry, as well as much of what we think we know about metabolism, will need rewriting. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported that a naturally occurring bacterium, strain GFAJ-1, can substitute arsenic for phosphorus in its biomolecules. However, straightforward experiments to support this claim, including density gradient centrifugation of DNA assumed to contain arsenic, were either not performed or not presented. As a result, the authors’ conclusions remain uncertain. </jats:p>

<jats:p>Concerns have been raised about our recent study suggesting that arsenic (As) substitutes for phosphorus in major biomolecules of a bacterium that tolerates extreme As concentrations. We welcome the opportunity to better explain our methods and results and to consider alternative interpretations. We maintain that our interpretation of As substitution, based on multiple congruent lines of evidence, is viable.</jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) argued that the bacterial strain GFAJ-1 can vary the elemental composition of its biomolecules by substituting arsenic for phosphorus. Although their data show that GFAJ-1 is an extraordinary extremophile, consideration of arsenate redox chemistry undermines the suggestion that arsenate can replace the physiologic functions of phosphate. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported that the bacterial strain GFAJ-1 can grow by using arsenic (As) instead of phosphorus (P), noting that the P content in bacteria grown in +As/–P culture medium was far below the quantity needed to support growth. However, low P content is a common phenotype across a broad range of environmental bacteria that experience P limitation. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported that bacterium GFAJ-1 can grow by using arsenic instead of phosphorus. However, the presence of contaminating phosphate in the growth medium, as well as the omission of important DNA purification steps, cast doubt on the authors’ conclusion that arsenic can substitute for phosphorus in the nucleic acids of this organism. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported an apparent stimulatory effect of arsenic on the growth of bacteria isolated from Mono Lake, California, which they interpreted as evidence that the cells can grow by using arsenic instead of phosphorus. Alternatively, arsenic may have stimulated the bacterium’s high-affinity phosphorus assimilation pathway, which is active when phosphate levels are low. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported that bacterial strain GFAJ-1 can substitute arsenic for phosphorus in its biomolecules, including nucleic acids and proteins. Unfortunately, their study lacks crucial experimental evidence to support this claim and suffers from inadequate data and poor presentation and analysis. </jats:p>

<jats:p> Wolfe-Simon <jats:italic>et al</jats:italic> . (Research Articles, 3 June 2011, p. 1163; published online 2 December 2010) reported the discovery of an unusual bacterium, strain GFAJ-1, that can grow in the presence of high concentrations of arsenate. The authors’ contention, however, that this microbe can appreciably vary the elemental composition of its fundamental biomolecules by substituting arsenic for phosphorus appears premature based on the data presented. </jats:p>