A
reservoir of briny liquid buried deep beneath an Antarctic glacier
supports hardy microbes that have lived in isolation for millions of
years, researchers report this week in the journal Science.
The
discovery of life in a place where cold, darkness and lack of oxygen
would previously have led scientists to believe nothing could survive
comes from a team led by researchers at Dartmouth and Harvard. Their
work was funded by the National Science Foundation, NASA, and Harvard's Microbial Sciences Initiative.
Despite their profound isolation, the microbes are remarkably
similar to species found in modern marine environments, suggesting that
the organisms now under the glacier are the remnants of a larger
population that once occupied an open fjord or sea.
"It's
a bit like finding a forest that nobody has seen for 1.5 million
years," says Ann Pearson, Thomas D. Cabot Associate Professor of Earth and Planetary Sciences
in Harvard's Faculty of Arts and Sciences. "Intriguingly, the species
living there are similar to contemporary organisms, and yet quite
different -- a result, no doubt, of having lived in such an
inhospitable environment for so long.
Jill Mikucki, PhD
"This briny
pond is a unique sort of time capsule from a period in Earth's
history," says lead author Jill Mikucki, now a research associate in
the Department of Earth Sciences at Dartmouth and visiting fellow at Dartmouth's Dickey Center for International Understanding and its Institute of Arctic Studies. "I don't know of any other environment quite like this on Earth."
Chemical
analysis of effluent from the inaccessible subglacial pool suggests
that its inhabitants have eked out a living by breathing iron leached
from bedrock with the help of a sulfur catalyst. Lacking any light to
support photosynthesis, the microbes have presumably survived by
feeding on the organic matter trapped with them when the massive Taylor
Glacier sealed off their habitat an estimated 1.5 to 2 million years
ago.
Mikucki, Pearson, and colleagues based their
analysis on samples taken at Antarctica's Blood Falls, a frozen
waterfall-like feature at the edge of the Taylor Glacier whose striking
red appearance first drew early explorers' attention in 1911. Those
"Heroic Age" adventurers speculated that red algae might have been
responsible for the bright color, but scientists later confirmed that
the coloration was due to rust, which the new research shows was likely
liberated from subglacial bedrock by microorganisms.
Because water flows unpredictably from below the glacier at Blood
Falls, it took Mikucki a number of years to obtain the samples needed
to conduct an analysis. Finally, in the right place at the right time,
she was able to capture some of the subglacial brine as it flowed out
of a crack in the glacial wall, obtaining a sample of an extremely
salty, cold, and clear liquid for analysis.
"When I
started running the chemical analysis on it, there was no oxygen," she
says. "That was when this got really interesting. It was a real
'Eureka!' moment."
The fluid is rich in sulfur, a
geochemical signature of marine environments, reinforcing suspicions
that the ancestors of the microbes now beneath the Taylor Glacier
probably lived in an ocean long ago. When sea level fell more than 1.5
million years ago, they hypothesize, a pool of seawater was likely
trapped and eventually capped by the advancing glacier.
The
exact size of the subglacial pool remains a mystery, but it is thought
to rest under 400 meters of ice some four kilometers from its tiny
outlet at Blood Falls.
Mikucki's analysis showed
that the sulfur below the glacier had been uniquely reworked by
microbes and provides insight into how these organisms have been able
to survive in isolation for so long.
The research
answers some questions while raising others about the persistence of
life in such extreme environments. Life below the Taylor Glacier may
help address questions about "Snowball Earth," the period of geological
time when large ice sheets covered Earth's surface. But it could also
be a rich laboratory for studying life in other hostile environments,
and perhaps even on Mars and its ice-covered moon, Europa.
Mikucki
and Pearson's co-authors are David T. Johnston and Daniel P. Schrag at
Harvard, Alexandra V. Turchyn at the University of Cambridge, James
Farquhar at the University of Maryland, Ariel D. Anbar at Arizona State
University, John C. Priscu at Montana State University, and Peter A.
Lee at the College of Charleston.
Written by Steve Bradt, Harvard University
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