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Michael: In some of the harshest environments on Earth, it seems like some form of life finds a way to get by, even inside solid rock. All kinds of tiny organisms, like bacteria, algae, fungi, or archaea, actually live in between the grains of rocks and minerals. We call them endoliths, from the Greek words meaning “inside” and “rock.”

Endoliths could survive in friendlier environments, but everything else vying for space and resources would probably win. So, they’re considered extremophiles – organisms that can live where nobody else can, and thrive when things get tough. One place where endoliths rule is in the cold desert of Antarctica: Temperatures can plummet to negative 60 degrees Celsius, winds can blow over 200 kilometers per hour, and the air is super dry.

For scientists researching there, it’s like a hostile alien world. But cryptoendoliths hidden in rocks on the Earth’s surface like it just fine. Crack open a porous rock, like sandstone, and just a millimeter below the surface you might find a grayish-greenish layer of microorganisms.

That could be lichen, which is a mixture of a fungus and photosynthetic algae or cyanobacteria cells that work together to survive. Or, it could just be bacteria, or it could just be algae. Pores in the sandstone let in a little sunlight and water from melted snow, which the endoliths use to photosynthesize.

That way, the rock keeps them protected from the worst of the Antarctic conditions. And even though a lot of the year is suffering in the cold, dry darkness, they can make just enough energy to live and reproduce. A couple thousand kilometers away in South Africa, endoliths have staked a claim in another extreme environment: the hot depths of the Earth’s crust.

Deep within gold mines, where the temperature of the rock reaches 60 degrees Celsius, the air pressure is double that of sea level, and workers are supplied with fresh air from the surface for oxygen... scientists found bacteria. One species of rod-shaped bacteria was hiding out nearly 3 kilometers down in a mine, and has possibly lived without contact with the surface for millions of years. These deep-biosphere endoliths are really far from sunlight, and get their energy through chemical reactions, or chemosynthesis.

The researchers who found this bacterium think that it survives by recycling dead cells, absorbing nutrients from rock, and getting carbon from gases, like carbon dioxide or carbon monoxide. Plus, radiation from deep in the rocks can break down water molecules that seep into the mine. This creates hydrogen atoms, which these bacteria can react with sulfate compounds to make energy.

Endoliths can even survive in the rock hundreds of meters beneath the ocean floor. Specialized research ships have deep-sea drilling projects, where they analyze cores of sediment layer by layer. One group of scientists found some bacteria that they think were tunneling through volcanic silica-containing rock, absorbing nutrients from the minerals and carbon compounds from seawater.

Others were found in fractured basalt rock, and might use chemicals like sulfides, nitrates, and carbon dioxide – kind of like the bacteria from the gold mine. Like the sediment they were found in, some of these microorganisms may be millions of years old, and reproduce once every couple thousand years. They’re basically living in slow motion.

Some scientists wonder whether these tiny organisms and their chemical interactions with rock and carbon in the deep ocean could affect global climate. But wherever we find them, endoliths are helping us discover the limits of how and where life can exist. They can give us lots of clues about how microorganisms make homes in extreme places here on Earth. And they could also hint at how life could survive on other planets.

Today, Mars has very little atmosphere and surface water, but scientists think that endolith-like organisms could’ve been some of the very last lifeforms – escaping the hostile surface by hiding in rocks. Endoliths’ amazing survival abilities have also fueled speculation about the Panspermia hypothesis, which is basically the idea that life could hitch a ride between worlds on asteroids or meteoroids.

If endoliths can survive in extreme conditions on Earth by hiding in rocks, maybe they could survive in rocks hurtling through space, too. And if they could do that, there’s a slight possibility that extremophiles are tucked all over our universe, waiting to be found.

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