It’s known as the deep biosphere, or the dark energy biosphere. What is it? Microbes - bacteria, archaea, even fungi - living not at the bottom of the ocean, but in the bottom of the ocean. We aren't just talking about the ocean version of soil bacteria, living in the seafloor mud (although that is part of the story). No, we're talking about microbes living in cracks in the rocks deep below that mud, in the most extreme case to date, a mile and a half below the seafloor.
“It’s still relatively shallow, if you think of the depth of our [planet], all the way from the core out,” says Julie Huber, Associate Director of the Bay Paul Center at the Marine Biological Laboratory. But she doesn’t discount the challenges for life in this realm - “really outstanding amounts of pressure, depth, and disconnection from the surface world.”
While some of the microbes that live beneath the seafloor are a lot like those that live above it, others are so odd that scientists think they may be varieties who trace their roots back billions of years and have spent the intervening millions of millennia evolving in isolation. They don't need the sun's energy, and many can't stand the presence of oxygen. Deep within sub-surface rocks, some grow so slowly they’re only reproducing once ever century or two.
Still, they’re there, they’re eking out a living, and they may be far more numerous than you’d think. Huber says early speculation that sub-surface life could account for 70-80 percent of the microbial mass on Earth was probably too high, but we don’t have a solid estimate yet.
“What we’ve been doing in the last fifteen years is try to put some numbers behind those estimates,” says Huber. “Making that global estimate is actually a bit harder than we thought.”
That’s because the abundance of microbes varies dramatically from place to place. At the places where two tectonic plates meet, magma and heated water fuel intense microbial growth near the seafloor. Along the edges of continents, organic material rains down, feeding microbial communities. But in other parts of the ocean, microbes can be quite rare. And global estimates haven’t really accounted for microbes in rocks.
All told, Huber says she wouldn’t be surprised if the final number came to more than fifty percent of total microbial mass.
“DNA sequencing tools have really opened our eyes,” says Huber, “not just to the sheer numbers but, even when you don’t have that many microbes, how many different types you might have, even in some of these extreme environments.”
That variety of life could teach us a lot, about the origins and evolution of life, and about how communities of microbes respond to environmental changes. They also hold the potential to provide new fuel sources and enzymes to drive any number of industrial processes. But Huber says, for her, it’s the pure excitement of exploring the inner workings of our planet that keeps her coming back for more.