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Wolfe-Simon. And of the six essential
elements of life—carbon, hydrogen,
nitrogen, oxygen, phosphorus and sulfur (aka CHNOPS) — phosphorus can be
difficult to find in an available form. If a
microbe in a test tube can be coerced to
live on arsenic, perhaps life’s primordial
home was also arsenic-rich, and life that
used phosphorus came later. A “shadow
biosphere” of arsenic-based life may
even exist unseen on Earth, or on some
lonely rock in space.
“It isn’t about arsenic, and it isn’t
about Mono Lake,” says Wolfe-Simon.
“There’s something fundamental about
understanding the flexibility of life.”
Grind up any known organism and you
get CHNOPS, she says, but all those
organisms are also thought to come from
a single ancestor. “We have a single sam-
ple of life,” she says. “You can’t look for
what you don’t know.”
Similarities with phosphorus are also
what make arsenic poisonous to life as
we know it. Living things often can’t
distinguish between the two, so arsenic
can insinuate itself into cells. There, it
competes with phosphorus, grabs onto
sulfur groups, or otherwise gums up the
works, causing cell death. Some microbes
“breathe” by passing electrons to arsenic,
but even in those cases the toxic element
stays outside the cell.
Researchers are having a hard time
wrapping their minds around arsenate
doing the job of phosphate in cells. The
“P” in ATP, the energy currency for all
of life, stands for phosphate. And the
backbone of the DNA double helix, the
molecule containing the genetic instructions for life, is made of phosphate and
sugar. Basic biochemistry says that these
molecules would be so unstable that they
would fall apart if they were built with
arsenate instead of phosphate.
“Every organism that we know of uses
ATP and phosphorylated DNA,” says
biogeochemist Matthew Pasek of the
University of South Florida in Tampa.
He says the new research is both fascinating and fantastic. So fantastic that
NaSa scientist Felisa wolfe-Simon
samples sediment cores from Mono
lake in California. a microbe from the
samples was coaxed to munch arsenic.
a lot of work is needed to conclusively
show where in their cells microbes have
incorporated arsenate and how those
cells function.
Both phosphate and arsenate can
clump, and with their slightly negative
electric charge, slightly positive DNA
would be attracted to such clumps,
Pasek says. Perhaps the arsenic detected
in the DNA was actually a nearby clump
that the DNA wrapped itself around, he
speculates.
Back Story | OtherwOrldly lake
because water leaves only through evaporation, California’s mono Lake and the
surrounding basin have unusual chemistry and ecology.
Salt galore For every liter of water, mono Lake
has about 80 grams of salt. in the last century,
salinity has reached almost 100 grams per liter.
Compare that with the ocean, which has a
salinity of about 30 grams per liter.
limestone structures can form. these “tufa”
towers (shown above) often become visible
following a drop in the lake’s water level.
Bizarre microbes rich in chlorides, carbonates
and sulfates, the lake hosts microbes that can
survive extreme conditions. one such extremo-phile, Desulfonatronum thiodismutans, lives in
mono’s mud without sunlight, getting energy
from sulfates and other inorganic compounds.
tufa towers When underwater springs bring
calcium in contact with the lake’s carbonates,
Free from fish With a ph of 10, the lake is
about as alkaline as household glass cleaner.
this alkalinity is the reason fish don’t live in
the lake.
both images: © 2010 henry bortman
Bird haven brine shrimp and alkali flies thrive
in the lake’s waters. and with no dining
competition from fish, migratory birds have
made the lake a key stop. about 85 percent of
California’s breeding population of California
gulls (Larus californicus) nest there.
