- Controversy has erupted over Lake Vostok, one of Earth's
last unexplored frontiers, which lies deep under the Antarctic ice.
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- Last week a team of Russian and French scientists claimed
the lake is sterile. But American scientists insist that it is a potential
source of undiscovered life forms, and are worried that Russian plans to
drill right through the ice will contaminate it.
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- At the heart of the dispute is a small but diverse group
of microbes found in the single core that has been drilled from the ice
above the lake. The Russians and French say these are contaminants from
the drilling and testing of samples in the labs. They also argue that the
lake itself is too toxic to sustain life because of its extremely high
levels of oxygen.
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- If they are right, it would be the first lifeless water
body found on Earth. As such, it could help us hone our techniques for
the search for life under the polar ice caps on Mars, and in the oceans
under the frozen surface of Jupiter's moon Europa.
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- Lake Vostok hit the headlines in 1996 when its extraordinary
size first came to light. Spanning some 14,000 square kilometres, and reaching
a depth of nearly 800 metres in parts, it has been shut away from sunlight
and the atmosphere for at least 15 million years. There has been much speculation
about the exotic ecosystems that might be found in this unique environment.
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- Accretion ice
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- French, Russian and American scientists looking for evidence
of climate change began drilling through the ice sheet above the lake in
1989. But as they were not looking for life forms, the equipment was not
sterilised and the samples were never properly stored. The drilling stopped
130 metres from the lake's surface to avoid contaminating it with antifreeze
and the drilling fluid, kerosene.
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- At this depth the ice is refrozen lake water, known as
accretion ice. Four groups of researchers have found organisms in samples
of this ice, suggesting that the lake supports a small but thriving microbial
ecosystem.
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- But molecular biologist Sergey Bulat of the Petersburg
Nuclear Physics Institute, Russia, has created a furore by questioning
the accuracy of this research. At last week's meeting of the Scientific
Committee on Antarctic Research in Bremen, Germany, Bulat claimed that
these microbes are contaminants.
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- His team compiled a list of 80 microbes found in their
lab and in the drill hole that could have contaminated the Vostok ice.
All but three of the organisms discovered in the ice are on the list. "We
find bacteria in the accretion ice, but can eliminate most of them using
our contamination database," says Bulat.
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- Other researchers say the decontamination techniques
used on the core sample, which involve washing it and removing the outer
layers, would be completely effective.
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- Glacial ice
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- Brent Christner of the Montana State University in Bozeman
has even tested for kerosene and other markers to distinguish the outside
from the inside of the core. He is confident the cells he has found are
authentic lake microbes.
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- "The numbers of microbes in the accretion ice are
two to seven times as high as numbers in the overlying glacial ice,"
says Christner. "This indicates that the lake is a source of life.
All the data points to the fact that there are microbes and they're alive."
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- But Bulat disagrees. "We don't have different results,
only a different interpretation," he says.
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- Others are impressed with Bulat's methods, but argue
he is being too stringent. "His methods are so severe they might cut
out things that could be in the lake," says Chris McKay, at NASA's
Ames Research Center in Moffett Field, California.
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- Yet others are reserving judgement on the issue. "Of
course the ice core is contaminated," says David Karl of the University
of Hawaii in Honolulu, whose 1999 analysis of Vostok accretion ice reported
up to 300 cells per millilitre of ice (Science, vol 286, p 2144).
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- "We dealt with that potential problem in our processing
of the core," he says. "I stand by the data that I published,
but I have an open mind and would certainly capitulate if and when I am
proven wrong."
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- 'Cold hell'
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- The disagreements do not end there. Bulat and colleagues
believe there are no bacteria in the ice because the lake is too toxic
to support life. If so, Lake Vostok will be the only known sterile body
of water on Earth.
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- Though the lake itself has not been sampled, theoretical
calculations predict that it has been pumped full of oxygen released from
air bubbles in the overlying ice. None can escape, so oxygen concentrations
are thought to have rocketed to around 50 times the norm for lakes.
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- "Oxygen is very toxic at these concentrations,"
says Bulat. Added to this, he says, the probable concentrations of other
by-products of this hyper-oxygenated environment, such as hydrogen peroxide
and highly reactive free radicals, would destroy living organisms.
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- Jean Robert Petit of the Laboratory of Glaciology and
Geophysics of the Environment at Grenoble, France, agrees. "It's a
cold hell," he says. "It was once an open pond which probably
contained life. Between 15 and 30 million years ago it started to freeze
over, then gases accumulated in the lake and finally it sterilised itself,"
he says.
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- This hypothesis has provoked scorn from John Priscu of
Montana State University in Bozeman, author of one of the original papers
on microbes in the Vostok accretion ice (Science, vol 286, p 2141). Priscu
questions the assumption that the lake is poisonous, pointing out that
Lake Vostok contains only around 10 times as much oxygen as other under-ice
lakes in a region of Antarctica called the Dry Valleys, which are full
of life.
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- Slow-growing microbes
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- "Free radicals are caused by radiation and ultraviolet
light, and neither exists down there," he says. Besides, microbes
can protect themselves against the hyper-oxygenation by producing antioxidants.
"We should look for bio-signatures of microbes that can produce these,"
says Priscu.
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- He believes the bottom of Lake Vostok, like that of many
lakes, could be anaerobic and so provide a refuge from high oxygen levels
in the upper parts. Other researchers also think that microbes will cope
with Vostok's challenging environment.
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- "Almost every place we look, we find new types of
organisms," says Scott Rogers from Bowling Green State University
in Ohio, who has also analysed Vostok accretion ice and found very low
numbers of what he thinks could be genuine bacteria. "I expect the
same in Vostok."
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- But what if the Russians are right? "It would be
bigger news if Lake Vostok were found to be sterile, than the more probable
result that it contains a small population of slow-growing microbes,"
says Karl. "We will not know the answer to this until we have a whole
lake sample in our hands for proper investigation."
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- Icy moons
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- This could happen soon. A Russian team aims to break
into the lake in the southern hemisphere summer of 2006-2007. Ignoring
a proposal from an international panel to drill a new, cleaner hole through
the ice and enter the lake using a self-sterilising "cryobot",
the Russians plan to enter the lake from the existing, highly contaminated
drill hole.
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- They calculate that high pressure in the lake will prevent
drilling fluid from polluting the water. Christner, among others, is concerned.
"The Vostok ice was not drilled under the cleanest of conditions,"
he says. "It is imperative that we employ methods to verify the authenticity
of samples we obtain."
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- But with a complete lack of consensus on how to distinguish
between authentic microbes and contaminant organisms, analysis of the lake
water could prove as controversial as that of the ice above it.
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- This has implications for the search for life outside
Earth, because the lake is a unique test area for exploration of icy moons
and planets. Considering that hundreds of microbes were found even in the
clean rooms of NASA's Jet Propulsion Laboratory, which built the Mars Rovers
that are now roaming the Red Planet, the arguments about contamination
raised by Lake Vostok look set to continue.
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