A massive "deep-living" study reveals billions of tons of microbes living far below the Earth's surface



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Billions of tons of tiny creatures are thriving far below the surface of the planet, according to an important "deep-living" study living in a habitat almost twice as large as the oceans.

Scientists have discovered that despite extreme temperatures, pressures and lack of nutrients, this hidden world is home to 70% of the Earth's bacteria and its cousins, the archaea.

Among the vast diversity of life were tiny worms and "zombie" microbes that the team described as "almost dead".

Comparing this habitat with the Amazon rainforest or the Galapagos Islands, they say that extreme conditions could help understand the origins of life on Earth and potentially on other planets.

The Deep Carbon Observatory's 10-year project involved deep drilling at the seabed and sampling of microbes from mines and holes more than 5 km deep.

Although its initiative only scratched the surface of the underground life spectrum, researchers estimated that up to 23 billion tons of microorganisms lived in this "deep biosphere" – responsible 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 hope to find life in," said team member Dr. Karen Lloyd of the University of Tennessee in Knoxville.

"Now, thanks to the ultra-deep sampling, we know that we can find them almost everywhere, although the sampling has obviously only reached a minimally infinite part of the deep biosphere."

The so-called "dark matter" of mysterious creatures consists mainly of bacteria and archaea, but within them there are millions of distinct types, many of which are probably still to be discovered.

Surprisingly, life on the deep Earth seems to have genetic diversity comparable to all life found above the surface.

Scientists originally found Candidatus Desulforudis audaxiser bacteria (purple) living in Mponeng Gold mine near Johannesburg. Scientists have found no other organisms in their samples, making this deep ecosystem the first to be found on Earth with only one species. Greg WangerGordon Southam)

While the record depth at which microbes were found underground is approximately three kilometers below the surface of the earth, the absolute limits of underground life have not yet been established.

A competitor for microbes that inhabit the most extreme conditions include one that lives at 121C in hydrothermal vents on the seabed.

Dr. Lloyd said that when the project began, very little was known about the creatures that inhabited these regions and how they survived.

"Today, we know that in many places, they invest most of their energy simply to maintain their existence and little for growth, which is a fascinating way of living," she said.

However, many mysteries still linger over these ecosystems, including how life spreads through the rocks that make up the planet.

They also want to understand if life began in the depths of the Earth – whether inside the crust or the hydrothermal vents on the ocean floor – or on the surface before migrating down.

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"Exploring the deep subsoil is similar to exploring the Amazon rainforest," said Dr. Mitch Sogin of the Marine Biological Laboratory Woods Hole, another member of the team.

"There is life everywhere, and everywhere there is an immense 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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