Deep Beneath Your Feet, They Live in the Octillions

By JoAnna Klein

The real journey to the center of the Earth has begun, and scientists are discovering subsurface microbial beings that shake up what we think we know about life.

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An unidentified nematode, found in the Kopanang gold mine in South Africa, lives nearly a mile below the surface. CreditCreditGaetan Borgonie, Extreme Life Isyensya, Belgium
JoAnna Klein

At the surface, boiling water kills off most life. But Geogemma barossii is a living thing from another world, deep within our very own. Boiling water — 212 degrees Fahrenheit — would be practically freezing for this creature, which thrives at temperatures around 250 degrees Fahrenheit.

No other organism on the planet is known to be able to live at such extreme heat.

But it’s just one of many mysterious microbes living in a massive subterranean habitat that until recently has been practically invisible. Over the past decade, scientists from around the world have banded together under the Deep Carbon Observatory to make sense of these hidden habitats. The observatory’s researchers presented some of their recent discoveries last week.

With high-tech drills, ROVs and submersibles, pressurized collection tubes, the latest DNA technology and computer modeling, the researchers have explored volcanoes, diamond mines, deep-sea hot springs, underwater mud volcanoes and other extreme sites beneath our oceans and continents. What they’ve found turns what we know about the world literally upside down.

So science fiction fans, rejoice. The real journey to the center of the Earth has begun.

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Altiarchaeales, originally found living in sulfidic springs in Germany.CreditChristine Moissl-Eichinger/Medical University of Graz, Austria

These Altiarchaeales belong to a domain of nucleus-lacking single-celled microbes called Archaea. Archaea and bacteria make up the majority of life in the deep subsurface, and it’s estimated that there are more of these kinds of microbes below ground than above.

Some 200 to 600 octillion microbes live beneath our continents, suggests an analysis of data from sites all over the world, and even more live beneath the seafloor. Together they weigh the equivalent of up to 200 million blue whales — and far more than all 7.5 billion humans. Subterranean diversity rivals that of the surface, with most underground organisms yet to be discovered or characterized.

That means most microbes on the planet may not resemble our mental picture of a microbe at all, said Cara Magnabosco, a computational biologist the Flatiron Institute in New York.

As scientists continue their studies, the organisms they find are challenging and expanding the tree of life.

Just as vegetation varies between deserts, rain forests and Arctic tundras, microbial communities vary between habitats — whether buried beneath sediments or sulfuric crust in the seafloor, or encased within granite, basalt, sandstone or clay beneath continents. There are even some fungi and multicellular organisms, like insects and worms, living deep below ground.

Candidatus Desulforudis audaxviator, the purplish-blue rod-shaped cells, survives on hydrogen.CreditGreg Wanger, California Institute of Technology; and Gordon Southam, University of Queensland

There are basically two kinds of feeders in the deep subsurface. Some scavengers survive on leftovers of photosynthesis from the surface that have been buried for up to hundreds of millions of years.

Chemolithoautotrophs, on the other hand, do a kind of sunless photosynthesis and breathe whatever’s around.

“We are familiar with oxygen breathing, but the microorganisms have multiple options,” said Isabelle Daniel, a geobiologist at Université Claude Bernard Lyon in France.

Candidatus Desulforudis audaxviator, the cobalt, rod-shaped bacteria in the picture above, breathes what’s released when certain rocks meet water: “You take a rock. Put it with water. Heat it up a bit, not even extreme heat, and it will produce everything that life needs to go,” said Karen Lloyd, a microbiologist at the University of Tennessee, Knoxville.

Other microbes even breathe uranium and expel the waste as teeny crystals.

Subsurface microbes might only reproduce every thirty years, or take even longer. If nutrients run low, the microbes enter a dormant stage and focus the little energy they have on maintenance.

They’ll reproduce when some other energy source comes along — and that takes time, perhaps geological time. It can take tens to thousands of years for a new population to replace an old one.

Scientists wonder if Earth's subsurface microbes may resemble life elsewhere in the solar system, like Jupiter's icy ocean moon Europa.CreditNASA/Reuters

We don’t know where, when or how they got here, but we do know that the chemistry in the deep subsurface supports life and that these deep-dwelling microbes seem to share a common ancestor with surface dwellers.

Earth’s early chemistry, before oxygen became present in abundance billions of years ago, may have been similar to the deep subsurface biosphere. That has led some scientists to ponder whether this could have been where life began.

By modeling the deep subsurface biosphere and how the right chemical reactions could give rise to organic matter, researchers are hoping to get closer to an answer about whether life emerged at the surface or in the deep.

Other scientists wonder if understanding life in the subsurface could point toward life elsewhere in the solar system, like Mars or Europa.

“Could there be a deep biosphere on these other worlds?” said Robert Hazen, a mineralogist at the Carnegie Institution’s Geophysical Laboratory and George Mason University who directs the observatory.

Until that gets worked out, this new-old world is “providing us with a vivid new way of thinking about ‘what is life?’” said Dr. Hazen.

A methane-producing microbe found beneath the Pacific Ocean seafloor, off the coast of Japan.CreditHiroyuki Imachi/Japan Agency for Marine-Earth Science and Technology

Scientists have discovered microbes thriving three miles below the continental surface, and methane-producing bacteria, like the one pictured here, six miles below the seafloor.

“If you snubbed out the sun tomorrow, these guys wouldn’t even care,” said Dr. Lloyd.

We still don’t know how much deeper life may go.

In the 1970s, Soviet scientists drilled 7.5 miles into Earth’s crust in what’s known as the Kola Superdeep Borehole. They didn’t find much living that deep, but they weren’t really looking for it either, Dr. Lloyd said.

Now, with a Japanese deep-drilling vessel called Chikyu, scientists like her are trying to make it all the way through to the mantle — an 1,800-miles-thick layer of rocks and minerals between the crust we live on and the core of the planet.

“They haven’t gone that far yet, but we’ve all got our fingers crossed,” said Dr. Lloyd. “We have yet to say that we’ve truly drilled past the point where life exists.”

It’s hard to know for certain if conditions will be too severe in the mantle to sustain life. These microbes, which can subsist on the tiniest drop of water (many don’t even need oxygen) are constantly testing life’s known limits.

“We find cells in places where we thought there wouldn’t be any cells,” said Dr. Daniel.

Heat rises nearer the core, and too much may destroy the proteins and fats that make up cells, as happens in the lab.

“How far does this extend?” Dr. Daniel asked.

We won’t know until we get there.