news of the week ARSENIC BACTERIA BREED BACKLASH CONTROVERSY: Claim that microbe weaves arsenic into its DNA comes under fire
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HEN RY BO RT M AN
D EC E MBE R 1 3, 2010 EDITED BY WILLIAM G. SCHULZ & KENNETH J. MOORE
HIS MONTH’S high-profile announcement
about a bacterium that thrives on arsenic is drawing criticism from scientists in multiple fields, including chemists. The study claims that the microbe can swap phosphorus for arsenic in its biomolecules, including DNA (C&EN, Dec. 6, page 36; Science, DOI: 10.1126/science.1197258). But outside experts say the data presented don’t back this claim. NASA astrobiologist Felisa Wolfe-Simon and her colleagues have been the subject of worldwide media attention since they described a microbe, scooped out of California’s briny, arsenic-rich Mono Lake, that can grow under conditions nearly devoid of phosphorus, one of six elements common to all life on Earth. “My research team and I are aware that our peerreviewed Science article has generated some technical questions and challenges from within the scientific community,” Wolfe-Simon says in a statement. “We invite others to read the paper and submit any responses to Science for review so that we can officially respond.” Indeed, the team’s case for arsenic becoming part of the microbe’s DNA is not airtight because it depends on studies that, even when taken together, don’t provide all the evidence they need, experts say. Nanosecondary ion mass spectrometry (NanoSIMS) and inductively coupled plasma mass spectrometry, two of the techniques the team employed, are useful for measuring the relative concentrations of arsenic in microbial cells grown under various conditions, says Nicholas Winograd, a SIMS expert at Pennsylvania State University. But the way both types of MS experiments were run, “they break everything to pieces,” he says. The team can detect arsenic’s presence in cells, “but they don't know what the chemical environment of the arsenic is.” To learn more about arsenic’s chemical environment, the team performed X-ray studies. Extended X-ray absorption fine structure (EXAFS) spectroscopy, one technique the team used, can reveal arsenic’s oxidation state and average distances of any bonds it’s making, explains Keith O. Hodgson at Stanford University, an expert on X-ray techniques. In its report, the team “looks at the average distances, they make comparisons, and they MORE ONLINE
Wolfe-Simon at conclude that it’s a reasonable assumption that arsenic Mono Lake, pulling could be part of a DNA backbone,” Hodgson says. Howa tube of mud from ever, “there’s no direct proof in the X-ray absorption data which to start that the arsenic is a part of the DNA backbone.” It’ll take microbial cultures studies on isolated molecules with techniques such as in the lab. X-ray crystallography or NMR to unambiguously prove that, he says. AT ISSUE WolfeThe team also used a technique called X-ray Simon’s team claims fluorescence (XRF), which provides information its microbe replaces about which chemical elements are present in a phosphorus (red) in sample and in what concentration, explains Laszits DNA with arsenic; lo Vincze, an XRF expert at Ghent University, in other researchers Belgium. Combined with EXAFS, XRF can only say this claim demonstrate that arsenic is in an environment isn’t supported by consistent with the idea that arsenic is in DNA evidence. but cannot provide indisputable proof, he adds. Most likely, the team has discovered a bacte–O P O rium that scavenges phosphate from its surroundings to survive, says Gerald F. Joyce of Scripps ReO Base search Institute, an expert in RNA and origin of life O research. “This is an amazing story about how life can adapt to extreme conditions,” he says. O The team says it will freely provide their bacterial strain to researchers for testing. –O P O “It is a shame when a bright and enthusiastic O Base researcher like Felisa isn’t better advised by her chemistry colleagues,” says John D. Sutherland, O an organic chemist at the MRC Laboratory of Molecular Biology in Cambridge, England, who has O written a letter to the editor of C&EN critiquing the findings (see page 4). “Such a dramatic claim makes rock-solid characterization absolutely mandatory.”—CARMEN DRAHL
For more scientists’ comments, click on this story on C&EN Online, www.cen-online.org. WWW.CEN-ONLINE.ORG
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DECEMBER 13, 2010
