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发帖数: 571 | 1 Last week'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.
In the days since the story broke, various scientists including University
of British Columbia microbiologist Rosemary J. Redfield have posted
critiques of the work online. Among several concerns, Redfield tells C&EN,
is that the team didn't rigorously purify DNA samples "when it counted";
that is, when checking for arsenic content. She plans to submit a formal
letter to Science about the work.
"My research team and I are aware that our peer-reviewed Science article has
generated some technical questions and challenges from within the
scientific community," Wolfe-Simon tells C&EN in an e-mail. "Our manuscript
was thoroughly reviewed and accepted for publication by Science; we
presented our data and results and drew our conclusions based on what we
showed. But we welcome lively debate since we recognize that scholarly
discourse moves science forward," she adds. "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. NanoSIMS allowed the researchers to take
mass spectra at different points, in effect to take an MS picture of the
cell, Winograd says. "They've produced some solid analytical data," he says.
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 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 conclude that it's a
reasonable assumption that arsenic could be part of a DNA backbone,"
Hodgson says. However, "there's no direct proof in the X-ray absorption data
that the arsenic is a part of the DNA backbone."
The team "has not conclusively proven, in my view, that arsenic has been
incorporated into DNA," Hodgson says. It'll take studies on isolated
molecules with techniques such as X-ray crystallography or NMR to
unambiguously prove that, he says.
"The evidence in the paper does not establish arseno-DNA," says Steven A.
Benner, an expert in nucleotide chemistry at the Foundation for Applied
Molecular Evolution, in Florida. Esters made with arsenic, as would have to
appear in DNA, spontaneously fall apart in water with half-lives on the
order of minutes, a huge chemical challenge for any microbe to surmount, he
notes.
"On the other hand, there is not an easy explanation for the data that are
present," Benner says. The work shouldn't be dismissed outright, but more
biochemical tests should be done, such as checking for radioactivity in
purified nucleic acids from microbes fed radioactive arsenic, he says.
Most likely, the team has discovered a bacterium that aggressively
detoxifies arsenic species and scavenges phosphate from its surroundings to
survive, says Gerald F. Joyce of Scripps Research Institute, an expert in
RNA and origin of life research. "This is an amazing story about how life
can adapt to extreme conditions," he says.
"We've been concerned that some conclusions have been drawn based on claims
not made in our paper," Wolfe-Simon tells C&EN. In response, Science is
making the article freely available for a two-week period, says Ginger
Pinholster, director of the American Association for the Advancement of
Science's Office of Public Programs. And at a Dec. 7 lecture streamed on
NASA's website, coauthor Ronald S. Oremland of the U.S. Geological Survey
said the team will freely provide their bacterial strain to researchers
interested in testing it.
"It is a shame when a bright and enthusiastic researcher like Felisa isn't
better advised by her chemistry colleagues," says John D. Sutherland, an
organic chemist at the MRC Laboratory of Molecular Biology in Cambridge,
England, who has written a letter to the editor of C&EN critiquing the
findings. "Such a dramatic claim makes rock-solid characterization
absolutely mandatory."
Chemical & Engineering News
ISSN 0009-2347
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