I worked nearly every summer of my teenage years at a summer camp for Cub Scouts. On the first night of each new session the staff would officially welcome everyone by hosting an “Opening Campfire.” The campers and their families would gather on wooden benches set before a stage on the edge of the lake, and watch the staff perform skits, sing songs, and generally make fools of themselves.
Whenever an extra minute was needed between skits—to put on an awkward costume or assemble an unwieldy prop—a staff member would sprint through the audience, arms flailing, and shout, “It’s all around me!” as loud as they could. When they reached the stage, they would fall to the ground and, rolling around in the dust, continue to shout, “It’s all around me! IT’S ALL AROUND ME!” After a few moments, a second staffer would walk onto the stage and ask, “What’s all around you?” The first would sit up, look at the audience, and say, “My belt!”
Cue the groans of recognition, and the occasional “well duh,” and then it was on to the next skit. If ever I come to my senses and get out of this PhD program, perhaps I’ll end up back at a summer camp and I’ll petition them to replace the punchline, “my belt!” with, “microbes!”
Microbes really are all around us. It has become increasingly apparent that it is far easier to list all the places where microbes aren’t than where they are. Even environments previously assumed to be inhospitable to any form of life—a boiling Yellowstone hot spring, under a half-mile of Antarctic ice, inside salt crystals from Death Valley, CA—have been found to be teeming with microorganisms. To quote Dr. Ian Malcolm, “Life will not be contained […] Life finds a way.”
Scientists from the Deep Carbon Observatory (DCO) recently announced that another “impossible” environment has been crossed off the list: 5 km (3.1 mi) underground. And there is a lot of life down there.
The announcement, following nearly 10 years of research conducted by over 1,000 scientists from 52 countries, was made just before the annual meeting of the American Geophysical Union. These “deep life” studies of the “dark biosphere” estimate that subsurface microorganisms total 15 to 23 billion tonnes of carbon—245 to 385 times greater than the carbon mass of all humans.
DCO scientists also estimate that there are 200 to 600 octillion (2×1029 to 6×1029) individual microbes living within the continental subsurface. Let that number sink in. There are only 100 to 400 billion (1×1011 to 4×1011) stars in the Milky Way. There are just 7 quintillion five-hundred quadrillion (7.5 x 1018) grains of sand on Earth. The ground beneath you is full of life.
This subterranean biosphere has been called the “Galapagos of the deep.” Mitch Sogin of the Marine Biological Laboratory Woods Hole, USA has likened these studies to exploring the Amazon rainforest. “There is life everywhere, and there’s an awe-inspiring abundance of unexpected and unusual organisms.”
These subterranean organisms live very different lives from their surface-dwelling cousins and must survive the greatest extremes of pressure, temperature, and low nutrient and energy availability. For example, while the life cycles of many surface-dwelling organisms are tied to the solar or lunar cycles, the life cycles of deep microbes are tied to far slower geological cycles—earthquakes, volcanic eruptions, and the movements of tectonic plates. These barely living, “zombie” bacteria can exist in a dormant state for thousands of years, waiting for the rocks to shift and a new influx of energy and nutrients to spark their growth.
These studies were made possible by advancements in drilling and DNA sequencing technologies, which allow scientists to dig deeper and sequence more samples at a lower cost and an ever-increasing accuracy. Many samples from the continental subsurface were obtained from mines, such as the Beatrix Gold Mine in South Africa. Samples from subseafloor sediment were obtained by ocean-going ships with deep-sea drills that can reach depths of up to 2.5 km below the seafloor (Fig. 1). When taken from environments with extreme pressures—such as 10.5 km below the ocean surface—sampling devices maintain pressure to preserve the microbial life contained within.
With an estimated 70 to 80% of all Bacteria and Archaea living underground, these findings flip the entire field of microbiology on its head. They also raise deep questions (pun intended) about the origins of life on Earth: Did it start underground, far away from the sun, and then migrate up? Could we find uncharacterized branches of the tree of life living underground, branches which challenge Carl Woese’s three-domain concept? Could extraterrestrial life, extant or extinct, be found buried within the surfaces of Mars or Europa?
The subsurface—like the depths of the ocean—is a massive environment full of uncharacterized biological diversity. Further exploration into the depths of this “dark biosphere” is likely to uncover new details about the teeming microbial world lying beneath our feet, and the evolution of life on Earth from its very beginnings.