oceans
Huge hidden biomass lives deep beneath the oceans
* 22 May 2008
* NewScientist.com news service
* Catherine Brahic
It's the basement apartment like no other. Life has been found 1.6
kilometres beneath the sea floor, at temperatures reaching 100 °C.
The discovery marks the deepest living cells ever to be found
beneath the sea floor. Bacteria have been found deeper underneath
the continents, but there they are rare. In comparison, the rocks
beneath the sea appear to be teeming with life.
John Parkes, a geobiologist at the University of Cardiff, UK, hopes
his team's discovery might one day help find life on other planets.
He says it might even redefine what we understand as life, and,
bizarrely, what we understand by "age".
Parkes has been hunting for deep life for over 20 years. Recently,
he and his colleagues examined samples of a mud core extracted from
between 860 metres and 1626 metres beneath the sea floor off the
coast of Newfoundland.
They found simple organisms known as prokaryotes in every sample.
Prokaryotes are organisms that often have just one cell. Their
peculiarity is that, unlike any other form of life, their DNA is not
neatly packed into a nucleus.
Gradual descent
About 60% of the cells Parkes and his team found were alive. They
are related to organisms found in deep-sea hydrothermal vents.
Depending on the depth, between one in 20 and one in 10 of the cells
were dividing, which is the normal way prokaryotes reproduce.
Where cells living so far beneath the sea floor could have come from
remains a mystery. They may have been gradually buried in sediment
as millions of years passed by, and adapted to the increasing
temperatures and pressure, he says.
Another possibility is that they were sucked deep into the mud from
the sea water above. Hydrothermal vents pulse hot water out of the
seabed and into the ocean. This creates a vacuum in the sediment,
which draws fresh sea water into the marine aquifer.
It is important to understand the way the cells got down there,
because that has implications for their age. The cells are not very
active and according to Parkes they have very few predators. "We
find very few viruses, for example, down there," he says. "At the
surface, if you don't divide you get eaten. But if there are no
predators, the pressure to reproduce decreases and you can spend
more energy on repairing your damaged molecules."
Ancient life
This means it is conceivable – but unproven – that some of the cells
are as old as the sediment. At 1.6 km beneath the sea, that's 111
million years old. But in an underworld where cells divide
excruciatingly slowly, if at all, age tends to lose its relevance,
says Parkes.
Parkes' interest in prokaryotes goes far beyond those that are
buried deep in the Earth. He thinks the cells found there could lead
to life on other planets.
Previously, he has shown that the rocks beneath the oceans could be
home to the largest population of prokaryotes on Earth, and account
for one tenth of all living carbon. He estimates the combined
undersea biomass could be equivalent to that of all the plants on
Earth.
"We are all dominated by our surface existence where everything
relates to photosynthesis and oxygen," he told New Scientist.
The possibility that there could be more forms of life beneath the
surface than above it suggests that they have different and
effective ways of surviving – ways that could be independent of
light and oxygen. And if these "new" forms of life exist on Earth,
they could exist on other planets too.
Dense at depth
"That's what really excites me. This is not just about the deepest,
hottest, oldest – but also that we may have misunderstood life."
Life beneath the continents is very different. The temperature
increases more slowly with depth in the continental crust, which
allows life to go deeper.
"We have recovered living cells from depths of 3.2 km to about 5 km
in South Africa," says Tullis Onstott of Princeton University. "But
what I find most interesting in Parkes' samples is the high density
of microbial cells. They are about 100 to 1000-fold greater than in
our terrestrial environments at comparable depths or temperatures."
In 2002, Parkes had found prokaryotes at 842 metres beneath the
seabed, the previous record, and it seems likely he will be finding
life deeper yet in years to come. "The more you look the more you
find," comments Karsten Pedersen of the Deep Biosphere Laboratory at
Göteborg University, Sweden.
Journal reference: Science (DOI: 10.1126/science.1154545)
Mysteries of the Deep Sea -The deep sea is one of the harshest
habitats on Earth, but is home to many remarkable creatures. Learn
more in our comprehensive special report.
Evolution - Learn more about the struggle to survive in our
comprehensive special report.
Astrobiology - Learn more in our out-of-this-world special report.
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