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לבקשת כמה קוראים, הנה ההודעה המקורית של נאס"א בעניין החיידק מאגם מונו.
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TEMPE, Ariz. – Evidence that the toxic element arsenic can
replace the essential nutrient phosphorus in biomolecules of
a naturally occurring bacterium expands the scope of the search
for life beyond Earth, according to Arizona State University
scientists who are part of a NASA-funded research team
reporting findings in the Dec. 2 online Science Express.
It is well established that all known life requires phosphorus,
usually in the form of inorganic phosphate. In recent years,
however, astrobiologists, including Arizona State University
professors Ariel Anbar and Paul Davies, have stepped up
conversations about alternative forms of life. Anbar and
Davies are coauthors of the new paper, along with ASU
associate research scientist Gwyneth Gordon. The lead
author is Felisa Wolfe-Simon, a former postdoctoral scientist
in Anbar's research group at ASU's School of Earth and
Space Exploration and Department of Chemistry and
Biochemistry in the College of Liberal Arts and Sciences.
"Life as we know it requires particular chemical elements
and excludes others," says Anbar, a biogeochemist and
astrobiologist who directs the astrobiology program at ASU.
"But are those the only options? How different could life be?"
Anbar and Wolfe-Simon are among a group of researchers
who are testing the limits of life's chemical requirements.
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"One of the guiding principles in the search for life on other planets,
and of our astrobiology program, is that we should 'follow the
elements,'" says Anbar. "Felisa's study teaches us that we ought
to think harder about which elements to follow."
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Wolfe-Simon adds: "We took what we do know about the 'constants'
in biology, specifically that life requires the six elements CHNOPS
(carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) in
three components, namely DNA, proteins and fats, and used
that as a basis to ask experimentally testable hypotheses even
here on Earth."
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From this viewpoint, rather than highlighting the conventional
view of the "diversity" of life, all life on Earth is essentially
identical, she says. However, the microbe the researchers have
discovered can act differently.
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Davies has previously speculated that forms of life different from
our own, dubbed "weird life," might even exist side-by-side with
known life on Earth, in a sort of "shadow biosphere." The particular
idea that arsenic, which lies directly below phosphorous on the
periodic table, might substitute for phosphorus in life on Earth,
was proposed by Wolfe-Simon and developed into a collaboration
with Davies and Anbar. Their hypothesis was published in
January 2009, in a paper titled "Did nature also choose arsenic?"
in the International Journal of Astrobiology.
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"We not only hypothesized that biochemical systems analogous to
those known today could utilize arsenate in the equivalent biological
role as phosphate," notes Wolfe-Simon "but also that such
organisms could have evolved on the ancient Earth and might
persist in unusual environments today."
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Wolfe-Simon, now a NASA astrobiology research fellow in
residence at the U.S. Geological Survey, was one of the participants,
along with Anbar, at a workshop titled "Tree or Forest? Searching
for Alternative Forms of Life on Earth," that was organized in
December 2006 by the BEYOND Center, a "cosmic think tank"
at ASU.
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"That's where it all began," says Davies, a cosmologist, astrobiologist,
theoretical physicist and director of the BEYOND Center.
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"Felisa's talk was memorable for being a concrete proposal," Davies says.
"Many of the talks at the workshop discussed searching for radically
alternative forms of life with suggestions of the form 'maybe something
roughly like this,' or 'maybe a bit like that.' But Felisa said, quite
explicitly, 'this is what we go look for.' And, she did."
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"The idea was provocative, but it made good sense," notes Anbar.
"Arsenic is toxic mainly because its chemical behavior is so similar
to that of phosphorus. As a result, organisms have a hard time
telling these elements apart. But arsenic is different enough that
it doesn't work as well as phosphorus, so it gets in there and sort
of gums up the works of our biochemical machinery."
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After leaving ASU, Wolfe-Simon began a collaboration with Ronald
Oremland of the U.S. Geological Survey to chase down the
hypothesis. Oremland was a natural choice to bring into the project
because he is a world expert in arsenic microbiology. What
Wolfe-Simon discovered is presented in the Science Express
paper titled "A bacterium that can grow by using arsenic
instead of phosphorus."
The latest discovery is all about a bacterium – strain GFAJ-1
of the Halomonadaceae family of Gammaproteobacteria – scooped
from sediments of eastern California's Mono Lake, which is
extremely salty with naturally high levels of arsenic.
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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.
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Key issues that the researchers needed to address were the levels
of arsenic and phosphorus in the experiments and whether arsenic
actually became incorporated into the organisms' vital biochemical
machinery, such as DNA, proteins and the cell membranes. A
variety of sophisticated laboratory techniques was used to nail
down where the arsenic went, including mass spectrometry
measurements by Gordon at the W.M. Keck Foundation
Laboratory for Environmental Biogeochemistry at ASU.
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Commenting on the significance of the discovery, Davies says:
"This organism has dual capability. It can grow with either
phosphorous or arsenic. That makes it very peculiar, though
it falls short of being some form of truly 'alien' life belonging
to a different tree of life with a separate origin. However, GFAJ-1
may be a pointer to even weirder organisms. The holy grail
would be a microbe that contained no phosphorus at all."
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Davies predicts that the new organism "is surely the tip of a big
iceberg, and so has the potential to open up a whole new
domain of microbiology."
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It is not only scientists, however, who will be interested in this
discovery. "Our findings are a reminder that life-as-we-know-it
could be much more flexible than we generally assume or can
imagine," says Wolfe-Simon, noting that because microbes are
major drivers of biogeochemical cycles and disease this study
may open up a whole new chapter in biology textbooks.
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"Yet, this story isn't about arsenic or Mono Lake," Wolfe-Simon
says. "If something here on Earth can do something so unexpected,
what else can life do that we haven't seen yet? Now is the time
to find out."
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