Ancient bacteria from nearly two miles below Earth’s surface: that’s what first drew Tullis Onstott to begin his search for life in the most unlikely of places.
The geomicrobiologist had just attended a 1992 U.S. Department of Energy meeting about rocks estimated to be more than 200 million years old—older than most dinosaurs. These prehistoric rocks had been unearthed from a gas exploration well, and they turned out to be teeming with bacteria.
“That was pretty amazing to me,” says Princeton University’s Onstott. “The idea that these bacteria had been living in these Triassic rocks since they were deposited at a time prior to the age of the dinosaurs, that idea caught my fancy,” he says.
These rocks were among the first substantial evidence that life existed miles underground, and they jumpstarted researchers’ efforts to study life in the so-called deep subsurface. Over the past 20 years, Onstott and others have found that there is a greater variety of life in a lot more inhospitable places than anyone had imagined.
Deep life has been found all over the world and under a variety of conditions—in oil fields and gold mines, beneath ice sheets in Greenland and Antarctica and in sediments and rocks below the ocean floor. These places can be extremely hostile environments, with pressures 10 to 100 times that at the surface. Temperatures can range from near freezing to more than 140 degrees Fahrenheit.
A mile or more below the surface there’s no sunlight and very little oxygen. In these austere environments, creatures have to scratch out a living on whatever energy they can muster from their surroundings. This means that the pace of life down there can sometimes be incredibly slow. These microbes can be a thousand- or million-fold less abundant than their brethren above ground. And some may have been around for hundreds, thousands or even millions of years—real microscopic Methuselahs.
These creatures of the deep are diverse, consisting of bacteria and other single-celled organisms called archaea. There are even multicellular animals miles below the surface, including tiny worms called nematodes.
“What has been surprising as we continue exploring this deep hidden universe, is that it’s more complex down there than we could have possibly imagined when we started looking at Triassic samples back in the ’90s,” says Onstott.
That complexity has opened up a world of possibilities for researchers, from cleaning up toxic waste to the search for extraterrestrial life. Some of these deep organisms feed directly on metals and minerals, and can affect groundwater by increasing or decreasing levels of arsenic, uranium and toxic metals. Scientists hope that these bacteria can soon be adapted to trap or remove such harmful substances from things like the wastewater leaking from a mine.
But perhaps most tantalizing is the idea that the conditions deep underground are so alien they may give researchers clues about where to find extraterrestrial life—and what that life might look like.
“It directly relates to whether life could be existing below the surface of Mars,” says Onstott. “That’s really what drew me into this field from the get-go, and still is a driver for me.”
Between the extreme environments and the relative scarcity of organisms, researchers go to great lengths—and depths—to study these microbes. They venture into mines and caverns or use drills to extract samples from below terrestrial sites or the ocean floor. In some areas it can take several days to get even a single sample. “Going to the ends of the earth and drilling, or going to the Arctic and going underground a mile to get a sample, it’s not easy,” says Onstott.
Probing the Hellish Depths
Almost a mile below Earth’s surface, deep within South Africa’s Beatrix Gold Mine, Maggie Lau looks for life. It’s hot and humid, and only headlamps breach the darkness as Lau, a geomicrobiologist in Onstott’s group at Princeton University, collects water from boreholes. These are holes drilled into the rock by geologists looking for gas and water pockets in advance of mining operations. Lau fills an assortment of vials with gas and water samples ranging in volume from less than a teaspoon’s worth to just over two pints.
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