Many questions remain as to how life spreads under the surface, which energy sources are the most important to sustain these organisms, and whether this is where life began on planet Earth.
On the eve of the American Geophysical Union's annual meeting, scientists with the Deep Carbon Observatory today reported several transformational discoveries, including how much and what kinds of life exist in the deep subsurface under the greatest extremes of pressure, temperature, and low nutrient availability.
Experts estimate the mass of "zombie bacteria" that live in the bowels of the Earth. This so-called microbial "dark matter" dramatically expands our perspective on the tree of life.
"Deep microbes are often very different from their surface cousins, with life cycles on near-geologic timescales, dining in some cases on nothing more than energy from rocks", the report says. A frontrunner for Earth's hottest organism in the natural world is Geogemma barossii, a single-celled organism thriving in hydrothermal vents on the seafloor.
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The group has approximated the 'size of the deep biosphere to be 2 to 2.3 billion cubic km (almost twice the volume of all oceans).
The record depth at which life has been found in the continental subsurface is approximately five km and the record in marine waters is 10.5 km from the ocean surface. "There is life everywhere, and everywhere there's an awe-inspiring abundance of unexpected and unusual organisms", says Mitch Sogin, a scientist at the Marine Biological Laboratory and co-chair of the Deep Carbon Observatory's Deep Life team, in a statement.
They found that the mass of life underground would fill up twice the volume of all the world's oceans and is so diverse that it has been dubbed a "subterranean Galapagos", with tiny creatures existing on odd diets of rock and methane which can live at temperatures up to 121°C.
"Now, thanks to ultra-deep sampling, we know we can find them pretty much everywhere", Lloyd went on to say.
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Expanding what we know about the limits of life on Earth could potentially give scientists new criteria for searching for life on other planets.
Gaining a better understanding of subsurface life on Earth can also help understand and better engineer climate-change fighting technologies that may one day sequester carbon from the atmosphere.
"Ten years ago, we knew far less about the physiologies of the bacteria and microbes that dominate the subsurface biosphere", said Lloyd.
"They are not Christmas ornaments, but the tiny balls and tinsel of deep life look like they could decorate a tree as well as Swarovski glass", said Dr Jesse Ausubel, of the Rockefeller University, a founder of the DCO, which is made up of dozens of worldwide researchers. Amazingly the variety of life discovered underground rivals or possibly even surpassed the diversity experienced on the surface.