Billions of tonnes of tiny creatures are thriving far beneath the planet’s surface, according to a major study of “deep life” living in a habitat nearly twice the size of the oceans.
Despite extreme temperatures, pressures and a lack of nutrients, scientists found this hidden world is home to 70 per cent of Earth’s bacteria and their cousins the archaea.
Among the vast diversity of life were miniscule worms and “zombie” microbes the team described as “barely alive”.
Comparing this habitat to the Amazon rainforest or the Galapagos Islands, they say the extreme conditions could help researchers understand the origins of life both on Earth and potentially on other planets.
The 10-year Deep Carbon Observatory project involved drilling deep into the seafloor and sampling microbes from mines and boreholes up to three miles underground.
Though their initiative only scratched the surface of the spectrum of underground life, the scientists estimated up to 23 billion tonnes of micro-organisms lived in this “deep biosphere” – accounting for nearly 400 times the amount of carbon found in all humans.
“Ten years ago, we had sampled only a few sites – the kinds of places we’d expect to find life,” said team member Dr Karen Lloyd from the University of Tennessee at Knoxville.
“Now, thanks to ultra-deep sampling, we know we can find them pretty much everywhere, albeit the sampling has obviously reached only an infinitesimally tiny part of the deep biosphere.”
The so-called microbial “dark matter” of mysterious creatures consists mainly of bacteria and archaea, but within them are millions of distinct types, many of which are likely yet to be discovered.
Remarkably, deep Earth life appears to have comparable genetic diversity to all life found above the surface.
While the record depth at which microbes have been found is approximately three miles below the earth's surface, the absolute limits of life underground have yet to be established.
One of the microbes the team discovered can survive temperatures of 121C around thermal vents at the bottom of the sea.
Dr Lloyd said that when the project began, very little was known about the creatures inhabiting these regions and how they survived.
“Today, we know that, in many places, they invest most of their energy to simply maintaining their existence and little into growth, which is a fascinating way to live,” she said.
However, many mysteries still remain about these ecosystems, including how life spread through the rocks that make up the earth's crust.
They also want to understand whether life started deep in the Earth – either within the crust or at hydrothermal vents at the bottom of the ocean – or on the surface before migrating downwards.
“Exploring the deep subsurface is akin to exploring the Amazon rainforest,” said Dr Mitch Sogin of the Marine Biological Laboratory Woods Hole, another team member.
“There is life everywhere, and everywhere there’s an awe-inspiring abundance of unexpected and unusual organisms.”
The scientists presented their findings before the annual meeting of the American Geophysical Union in Washington DC.
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