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microbe swaps
arsenic for key
ingredient of life
Cells may use toxic element
to help construct their DNA
story one
By rachel ehrenberg
When cooking up the stuff of life, you can’t just sub- stitute margarine for butter. Or so scientists
had thought.
But now researchers report having
coaxed a microbe to build itself with
arsenic in the place of phosphorus, an
unprecedented substitution of one of
the essential ingredients of life. The
bacterium seems to have incorporated a
form of arsenic into its cellular machinery, and even its DNA, scientists report
online December 2 in Science.
Arsenic is toxic and is thought to be
too chemically unstable to do the work
of phosphorus, which takes on tasks such
as holding DNA in a tidy double helix,
activating proteins and forming bonds
that provide energy in cells. If the finding is validated, as many scientists are
arguing it needs to be, it would have huge
implications for basic biochemistry and
the origin and evolution of life, both on
Earth and elsewhere in the universe.
Courtesy of Science/AAAs
“This is an amazing result, a striking, very important and astonishing
result — if true,” says molecular chemist
Alan Schwartz of Radboud University
Nijmegen in the Netherlands. “I’m even
more skeptical than usual, because of
the implications. But it is fascinating
work. It is original and it is possibly very
important.”
The experiments began with sedi-
ment from eastern California’s Mono
Lake, which teems with shrimp, flies and
algae that can survive the lake’s strange
chemistry. Mono Lake formed in a closed
basin — any water that leaves does so by
evaporation — making the lake almost
three times as salty as the ocean. It is
highly alkaline and rich in carbonates,
phosphorus, arsenic and sulfur.
Researchers led by Felisa Wolfe-Simon
of NASA’s Astrobiology Institute and the
U.S. Geological Survey in Menlo Park,
Calif., cultured microbes from the sediment. Those microbes got a typical diet
of sugar, vitamins and some trace metals,
but no phosphate, biology’s favorite form
of phosphorus. Then the team started
force-feeding the critters arsenic, in the
form of arsenate, in greater and greater
quantities.
One microbe in particular, now identified as strain GFAJ- 1 of the salt-loving,
mostly marine family Halomonadaceae,
was plucked out and cultured in test
tubes. Some of the microbes were fed
loads of arsenate; others got phosphate.
While those subsisting on arsenate
didn’t grow as much as those getting
phosphate, they still grew steadily,
doubling their ranks every two days,
says Wolfe-Simon. And while the
team couldn’t eliminate every trace of
phosphate from the original culture,
detection and analytical techniques suggest that GFAJ- 1 started using arsenate
as a building block in phosphate’s place.
“These data show that we are get-
ting substitution across the board,” says
Wolfe-Simon. “This microbe, if we are
correct, has solved the challenge of being
alive in a different way.”
Arsenic sits right below phosphorus
in the periodic table and so isn’t that
different, chemically speaking, notes
5 μm
Scientists say they coerced a microbe in the mostly marine family Halomonadaceae
into incorporating arsenic into its cells by depriving the bacterium of phosphorus.
