Forget E.T. It’s time to meet the intraterrestrials.
They too are alien, appearing in bizarre forms and eluding scientists’ search efforts. But instead of residing out in space, these aliens inhabit a dark subterranean realm, munching and cycling energy deep inside the Earth.
Most intraterrestrials live beneath the bottom of the ocean, in an unseen biosphere that is a melting pot of odd organisms, a sort of Deep Space Nine for microbes. Many make their homes in the tens of meters of mud just beneath the seafloor. Others slither deeper, along fractures into solid rock hundreds of meters down.
Scientists are just beginning to probe this undersea world. In the middle of the South Pacific, oceanographers have discovered how bacteria survive in nutrient-poor, suffocating sediment. Off the coast of Washington state, other researchers have watched microbes creep into and colonize a borehole 280 meters below the seafloor, flushed by water circulating through the ocean crust. And near the underwater mountain ridge that marks the middle of the Atlantic Ocean, scientists have yanked up organisms that may be unlike any known sub-seafloor residents.
Such discoveries are helping biologists piece together a picture of a deep, seething ecosystem. Knowing how this world arose, researchers say, will help them understand more about the origin of life on Earth. One day intraterrestrials could even tell scientists more about extraterrestrials, by helping sketch out the extremes under which life can not only survive but even thrive.
Oceanic desert
Considering that oceans cover most of the planet, it’s a no-brainer to try to figure out what’s living in the mud and rock beneath them. “It’s really the most massive potential habitat on Earth,” says microbiologist Beth Orcutt of Aarhus University in Denmark.
By some estimates, as much as one-third of the planet’s biomass — the sheer weight of all its living organisms — is buried beneath the ocean floor. Many of these bacteria and other microbes survive on food that drifts down from above, such as the remains of plankton that once blossomed in the sunlight of the ocean’s upper reaches.
These hardy microbes manage to eke out an existence even where it shouldn’t be possible. In the middle of the South Pacific, for instance, lies an oceanic vortex where water circulates in a huge eddy, or gyre, twice the size of North America. Because the gyre is so far from any landmasses — from which nutrients wash off and help spur plankton growth and other ocean productivity — it is essentially a giant oceanic desert, says Steven D’Hondt of the University of Rhode Island’s oceanography school in Narragansett.
In some places in the gyre, seafloor mud builds up as slowly as eight centimeters per million years. That means if you wanted to plant a tulip bulb at the usual gardener’s depth of about 16 centimeters, D’Hondt says, you’d be digging into mud that is 2 million years old.
Such low-productivity regions in the centers of oceans are far more common than nutrient-rich coastal zones, but scientists don’t often visit the deserts because they are hard to get to. In the autumn of 2010, though, D’Hondt led a cruise to the South Pacific Gyre that drilled into the dull seafloor mud and pulled up cores. “We wanted to see what life was like in sediment in the deadest part of the ocean,” he says.
Among other things, the scientists discovered how microbes in the mud might cope. In other areas of the ocean, where more nutrients fall to the seafloor, oxygen is found only in the uppermost centimeter or two of mud; any deeper than that and it gets eaten up. But in the South Pacific Gyre, D’Hondt’s team found that oxygen penetrates all the way through the seafloor cores, up to 80 meters of sediment. To the scientists, this finding suggests that these mud microbes breathe very slowly and so don’t use up all the available oxygen. “That violates standard expectations,” says D’Hondt, “but until we went out there and drilled, nobody knew.”
Another possibility is that the microbes have a separate, unusual source of energy: natural radioactivity. Radioactive decay of elements in the underlying mud and rocks bombards the water with particles that can split H2O into hydrogen and oxygen, a process known as radiolysis. Microbes can then consume those elements, sustaining themselves over time with a near-endless supply of food. “That’s the most exotic interpretation,” D’Hondt says, “that we have an ecosystem living off of natural radioactivity that is splitting water molecules apart.”
Easy access
Thousands of miles north and east of drilling sites in the South Pacific Gyre, other scientists are exploring a very different alien realm in the Juan de Fuca Ridge, an underwater mountain range marking the convergence of several great plates of Earth’s crust. Juan de Fuca is one of those coastal areas getting plenty of nutrients from nearby British Columbia and Washington state, and scientists can get there relatively quickly.
As a result, the Juan de Fuca area may be the world’s best-instrumented seafloor. A network of observatories sprawls across the ocean bottom; in one spot, six borehole monitoring stations lie within about 2.5 kilometers of each other. One of the stations is hooked up to the shore via underwater cables, so that scientists sitting at their desks can track the data in real time. “We can do active experiments there that we can’t do anywhere else in the ocean,” says Andrew Fisher, a hydrogeologist at the University of California, Santa Cruz who helped set up much of the instrumentation.
Many of the stations are observatories known as CORKs, a tortured acronym for “circulation obviation retrofit kit,” which essentially means a deep hole in the seafloor plugged at the top to keep seawater out. Researchers lower a string of instruments into the hole, then come back several years later to retrieve them. Data from CORKs can reveal what organisms live at what depths within the borehole, as well as how microbial populations change over time.
CORKs are technically challenging to install, but sometimes glitches can yield unexpected discoveries. At one Juan de Fuca site, researchers tucked experiments down a hole in 2004. After retrieving rock chips that had dangled in the hole for four years, the team saw twisted stalks that looked like rust coating the surfaces. It turned out that the CORK hadn’t been properly sealed, and iron-oxidizing bacteria leaked in along with seawater.
Those bacteria initially colonized the borehole and built up the stalks, thriving on the cold and oxygen-rich conditions carried in by the seawater. But over the next few years the borehole began to warm up, thanks to volcanic heat percolating from below. Water from within the surrounding ocean crust began to rise and push out the seawater, reversing the flow within the hole. The iron-loving bacteria died and other types of organisms began to appear: bacteria known as firmicutes, which are found in similarly exotic environments such as the Arctic Ocean’s bottom. “For us that’s a really interesting finding and a kind of nice serendipitous experiment,” says Orcutt, who published the work with her colleagues last year in the International Society for Microbial Ecology Journal.
Research at Juan de Fuca also shows how water flushes through the ocean crust, offering clues to the best places to look for microbes. People tend to think of water sitting on top of the seafloor, says Fisher, but in fact water zips through undersea rocks — cycling the equivalent of the ocean’s entire volume through the crust every half-million years or so.
At Juan de Fuca, Fisher and colleagues have spotted two underwater volcanoes, about 50 kilometers apart, that help explain how such high rates of flow might happen. CORK observations reveal that water flows into one of the mountains and flushes out the other. “This is the first place anywhere on the seafloor where researchers have been able to put their finger on a map and say ‘the water goes in here and out here,’ ” Fisher says.
Those two volcanoes are arranged along a north-south line that tends to control much of the undersea activity at Juan de Fuca, he says. Most of the fractures in the ocean crust here run north to south, making that the probable direction in which microbes also move. The cracks serve as a sort of microbial superhighway, allowing the microbes to flow along easily, carried by water. Scientists looking for more sub-seafloor microbes might want to also focus on these areas, Fisher says: “You’ll see very different populations along the superhighways than along the back roads.”
Pond swimmers
Far from being monolithic, the seafloor is home to a surprising range of different environments. One new target, much different from Juan de Fuca or the South Pacific Gyre, is a spot in the mid-Atlantic known as North Pond. Geologists have studied this place, at 22 degrees north of the equator, since the 1970s for what it can reveal about the processes that form young crust at mid-ocean ridges. Now microbiologists are also targeting North Pond for what it can say about deep life.
The “pond” of North Pond is a pile of undersea mud, cradled against the side of tall jagged mountains. It lies about five kilometers from where seafloor crust is actively being born; all that violent geologic activity pushes water quickly through the mud and rocks and out into the ocean above. Compared with Juan de Fuca, the water at North Pond is much cooler — roughly 10Ë Celsius, as opposed to 60Ë C to 70Ë C — but flows much faster. “Nature finds a balance between temperature and flow,” says Fisher.
He and his colleagues, led by Katrina Edwards of the University of Southern California in Los Angeles and Wolfgang Bach of the University of Bremen in Germany, spent 10 weeks at North Pond last autumn. They installed two new CORKs, up to 330 meters deep, and pulled up samples of rock and water to test for any microbes that might be living there. The scientists also tucked long dangling strings of rock chips into the holes and plan to return in the years ahead to see what organisms might appear. “It was a great success,” says Edwards. “We set ourselves up for a good decade’s worth of work out at North Pond.”
For now, it’s up to microbiologists back on land to make sense of what’s there. Researchers are just starting to culture the slow-growing microbes pulled up at North Pond, but already they suspect they’ll find surprises.
Overall, studies at different locales reveal that deep-sea microbes are far more diverse than scientists had thought even a decade ago, says microbiologist Jennifer Biddle of the University of Delaware in Newark. Rather than just a couple of broad classes, researchers have found a rich diversity of bacteria along with archaea — other single-celled organisms with an older evolutionary history — plus fungi, viruses and more. “We were shocked it was so complicated,” says Biddle. “We thought there was maybe five Bunsens and 10 Beakers, and it turns out there’s the entire cast of the Muppets in there.”
By comparing microbes from different seafloor sites, Biddle has found surprisingly high amounts of archaea compared with bacteria in some places. She thinks that archaea may be thriving on organic matter in seafloor mud, so nutrient-rich coasts have more archaea than sediments in the middle of the ocean. “The jury’s still out on that one,” she says.
A new project known as the Census of Deep Life will help Biddle and others analyze and compare more of the sub-seafloor microbes. The census could take as long as a decade; the idea is to find overarching rules — if they exist — that describe where and how organisms thrive in the seafloor. “Right now you can get some idea of that by looking at the sorts of energy sources that are present in the subsurface,” says census leader Rick Colwell, a microbiologist at Oregon State University in Corvallis. “But do fractures in various subsurface environments, worldwide, contain certain types of microorganisms consistently?”
Plenty of data should be forthcoming. “We’re not suffering from a lack of things to do,” Orcutt says. Edwards and her team plan to return to North Pond in April to retrieve their first set of instruments. Fisher will go back to Juan de Fuca next summer, in what may be a final visit before turning his attention elsewhere. Next on his wish list: a site off Costa Rica where water flows through the crust some thousands of times faster than at Juan de Fuca.
One day, analyzing the deep biosphere may help NASA and other space agencies in their hunt for life elsewhere in the solar system. At North Pond, expedition scientists have tested out a new tool that, once lowered into a borehole, illuminates the hole’s walls using ultraviolet light. Because living cells turn fluorescent at specific wavelengths, the light can be used to spot films of organic matter coating the hole. This probe, or some elaboration on it, could end up flying on future space missions. And then the intraterrestrials could help scientists find extraterrestrials.
