World’s deepest zoo harbors clues to extraterrestrial life | Science News for Students

World’s deepest zoo harbors clues to extraterrestrial life

Scientists have discovered a variety of living things deep below Earth’s surface
Jun 15, 2017 — 7:10 am EST
worms stalactite

Life can be found even deep underground. For instance, these Monhystrella parvella worms (blue) were discovered living inside a stalactite almost a mile underground in South Africa’s Beatrix gold mine.

G. Borgonie

Maggie Lau works in a gold mine. But what she seeks here, more than a kilometer and a half (a mile) below the South African surface, may be more precious than gold. She’s looking for life.

It’s not easy work, lit only by headlamps. At times, it can feel as hot and humid as a sauna. Some spots smell like rotten eggs, due to sulfide gas emanating from holes dug in the rock.

Until about 20 years ago, scientists weren’t even sure if life existed deep below Earth’s surface. Then in 1992, Tullis Onstott and his colleagues discovered bacteria growing on the rocks retrieved from some 3 kilometers underground. Those rocks were more than 200 million years old, at least as old as the earliest dinosaurs. And the bacteria they analyzed may have survived from that time, Onstott now says.

He’s a geomicrobiologist — a scientist who studies how microbes interact with rocks and minerals. He heads the lab at Princeton University, in New Jersey, where Lau is now a graduate student.

Scientists like Lau and Onstott now travel the world over in search of deep life. They go deep underground in mines or caverns. They drill beneath the ocean floor and in oil fields. Some of these places are near-freezing; others are hotter than Death Valley.

“The challenge is in the hunt,” says Onstott. “It’s a fantastic journey.”

And these hunts are turning up a whole zoo of microscopic creatures. Some of the deeply buried critters feed on toxic chemicals such as arsenic and uranium. One day, other scientists might tap them to clean up toxic waste. Other microbes might produce useful substances, such as new types of germ-killing medicines known as antibiotics. And perhaps most intriguing, these organisms could also help biologists learn about life beyond Earth — true extraterrestrials.

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cave timelapse

Life in the depths

To reach her research site in South Africa, Lau has to drop down a mine shaft in a cage. It descends the nearly mile down in about a minute. That’s so quick that Lau’s ears pop, making it feel like she’s on an amusement park ride.

Next, Lau may walk or even crawl another 1.5 kilometers through narrow corridors. Some are flooded with water, which she’ll have to wade through. Through it all, Lau and her colleagues carry heavy backpacks and wear knee-high boots and overalls. “Some of my friends joke that we look like the Ghostbusters,” she says.

In her search for new life, Lau collects water from bore holes. These are narrow holes drilled deep into the ground, usually to explore for minerals, natural gas or oil. Sometimes these bore holes tap into underground water. That water can be as hot as 54 degrees Celsius (130 degrees Fahrenheit). At some sites, even the rocks can be warm to touch.

Maggie Lau
Maggie Lau collects borehole water in South Africa’s TauTona gold mine far below the Earth’s surface.
Image courtesy of Francois Vermeulen (Geosciences Manager, AngloGold Ashanti Limited)

As she sits on the hard, rocky ground and collects water in different-sized bottles, Lau can get splashed anywhere and everywhere with the hot water. “As you collect it,” she says, “you get wet, and you feel like you have been through a water fight.” After this tiring work, “You’re physically and mentally exhausted.” Although “intense,” Lau adds that it also is “very rewarding.”

This water once flowed on the planet’s surface. Although the ground looks solid, it’s more like Swiss cheese. Tiny holes all over allow the water to flow through and down, down, down. But this is a very slow process. Water from the surface may take thousands of years to trickle down to where Lau now works. Some of the water she studies is more than 50,000 years old. And on its travels, that water may have collected living creatures.

There’s very little life in this extreme environment. But Lau leaves equipment to filter thousands of liters (gallons) of water over weeks or even months. She needs to filter all that water to collect as many living cells as possible. Each teaspoon of water contains just a few thousand cells. That may be only a millionth as many cells as exist in every pinch of backyard soil.

Researchers have now identified many types of bacteria in underground water. Many of them stick to each other and to rocks in a slimy community called a biofilm. To study these microbes, scientists try to grow them in the lab. But so far, they have been successful with only about one out of every 100 bacterial species from down here.

Scientists also chemically identify the pattern of building blocks — known as sequences — that make up the bacteria’s genetic material. Those chemical sequences have let them identify most of the bacteria, based on their DNA and RNA. (DNA encodes the instructions for building a cell. RNA is a related messenger molecule. It’s needed to carry out those instructions.)

Lau isn’t just identifying the bacteria that live down here, though. She’s also trying to identify their diet. If scientists learn what these bacteria eat, they might be able to feed it to them in the lab, in what’s called a growth, or culture, medium. That would let researchers study these bacteria more closely, learn what makes them tick and see if they might be useful to us. But food is hard to come by deep underground. There is no sunlight and no plants. What’s more, the water that the bacteria live in contains very little oxygen.

At this depth, any microbes must get all of their energy — food — from the rocks and water around them. Reactions between rocks and water can produce different substances, including hydrogen, methane and sulfates. Scientists initially expected that these extremely deep-dwelling bacteria must feed mainly on these three materials.

To her surprise, Lau discovered, many don’t.

Indeed, fewer than half of the bacteria feed on them, she reports. This minority of bacteria produce other chemicals that the rest of the bacteria feed on. “That’s kind of new to us,” says Lau. “We expect to see more of this kind of cooperation, especially in these environments where it is difficult,” she says.

Knowing on what these bacteria dine might also inform the search for alien life. How? Scientists often look for the chemicals that living things feed on when they can’t find the living things themselves. From what they’ve learned about the new deep microbes, researchers know they might have to expand what kinds of menu items to scout for.

A whole lot of worms

Most of the first research on deep life focused on bacteria. But they’re far from the only species down here. Take nematodes.

These tiny worms usually live in the soil and feed on bacteria. When Gaetan Borgonie  heard there were bacteria deep underground, he wondered if there were also worms feeding on them. Borgonie is a zoologist at Extreme Life Isyensya in Gentbrugge, Belgium.

Kopanang roundworms
Roundworms on a biofilm almost a mile below the surface in South Africa’s Kopanang gold mine.
Gaetan Borgonie

“After 20 years working on worms, you realize these animals are incredibly resistant,” he says. “I wondered if they could actually go very deep down.” To find out, he went looking for worms in the same gold mines Onstott studied. “It’s always an experience,” he says. Down in these mines, “I feel like an astronaut who walks on another planet.”

On Earth’s surface, nematodes are everywhere. “If I take an ordinary coffee cup and I go out and take a soil sample,” Borgonie says, “you will have hundreds of thousands of nematodes.” But deep underground, these worms are quite rare. Borgonie had to look through about 60 bathtubs worth of water to find a single worm. But he succeeded. And by doing so, he became the first to identify any kind of multicellular life deep beneath Earth’s surface. And once he spied nematodes, Borgonie started wondering what else might live at such depths.

So he sampled the underground water for two years. Along the way, “We found this whole zoo of animals,” he says, “much more than I anticipated.” These animals, though, were all too small to be seen with the naked eye. Many looked like little worms or caterpillars. Others were related to crabs and lobsters and resembled tiny shrimp. Some were in 12,000-year-old water from bore holes. Others lived in the stalactites hanging from the mine ceiling.

Monhystrella parvella worm
A Monhystrella parvella worm found inside a stalactite almost a mile underground in South Africa’s Beatrix gold mine.
Gaetan Borgonie

The worms and other creatures deep underground were mostly the same species found atop Earth’s surface, Borgonie showed. Almost all were feeding on the deep-living bacteria. Indeed, he now suspects, living so deep underground might actually be a good thing for these critters. Unlike at the surface, there are no predators down there to gobble them up. “For them, it’s like a summer camp,” quips Borgonie.

He never suspected he’d find such a wide variety of deep life. “This is actually very good news for people searching for life on Mars or [Jupiter’s Moon] Europa,” he says.  Conditions deep beneath the surface of those space worlds could be similar to those deep inside the Earth. Maybe we’ll find a similar microscopic zoo beneath these other worlds.

Looking for alien life

Science may not have to wait long to apply the lessons learned from these underground creatures. Over the next several years, space missions will be exploring the solar system searching for signs of life.

Other planets and moons have much harsher conditions on their surface than those on Earth. “This means that if we were to find life elsewhere in the solar system, it’s going to be below the surface,” says Tori Hoehler. He works at the National Aeronautics and Space Administration’s Ames Research Center. As an atrobiologist there, he studies life on Earth and in space.

Europa spacecraft
This artist’s illustration shows what a mission to Europa might look like and the path that the spacecraft might take around Jupiter and its moons.

Life that would die on the surface of other planets or moons might be able to survive deep below. And reactions between underground rocks and water might provide the food needed to sustain them. Jupiter’s moon Europa or Saturn’s moon Enceladus may host rocks and water. But first scientists need a closer look. NASA will launch a mission to fly by Europa in the next five to 10 years. That could probe whether it might be able to support life.

Onstott also wonders if there might be creatures living deep below the Martian surface. Right now, it’s hard for life to survive on that planet’s surface. After all, it has almost no atmosphere. It’s also really cold and has no liquid water. But a long time ago, water may have flowed across the Martian landscape. And a thick atmosphere might have blanketed the planet. If there was Martian life on the surface then, maybe it trickled underground as critters have on Earth. “Down below they would have been nice and warm with plenty of water and plenty of energy,” says Onstott. That life could still be holed up there, captured in something like a time-capsule, waiting for us to dig it out.

Rovers have been exploring Mars for years. So far, none have ever done more than scrape the surface of the Red Planet. In 2018, though, the European Space Agency’s ExoMars Mission will get a deeper look. It won’t spy anywhere near as far below the surface as Lau and other scientists have searched back home. ExoMars plans to drill only about 2 meters (6.5 feet) down. That’s probably not deep enough to find life. But it might find signs that life once existed.

Onstott can’t wait. He imagines a time when humans might set up a base on Mars. Then we could drill deep below the surface, looking for water and life. Think about it, he says: “It would be an awesome undertaking.”

Power Words

(for more about Power Words, click here)

alien     A non-native organism. (in astronomy) Life on or from a distant world.

antibiotic     A germ-killing substance, usually prescribed as a medicine (or sometimes as a feed additive to promote the growth of livestock). It does not work against viruses.

arsenic     A highly poisonous metallic element. It occurs in three chemically different forms, which also vary by color (yellow, black and gray). The brittle, crystalline (gray) form is the most common. Some manufacturers tap its toxicity by adding it to insecticides.

astronaut     Someone trained to travel into space for research and exploration.

atmosphere     The envelope of gases surrounding Earth or another planet.

bacterial     Having to do with bacteria, single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

bacterium     (plural bacteria) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

biofilm     A gooey community of different types of microbes that essentially glues itself to some solid surface. Living in a biofilm is one way microbes protect themselves from stressful agents (such as poisons) in their environment.

bore hole     A narrow hole drilled deeply into ice or the surface of the Earth, often to sample or extract natural gas, crude oil or other mineral resources.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

culture     (in biology) The community of cells or tissue that is intentionally grown outside the body (or the wilds) for research purposes, usually in a laboratory. (in social science) The sum total of typical behaviors and social practices of a related group of people (such as a tribe or nation). Their culture includes their beliefs, values, and the symbols that they accept and or use. It’s passed on from generation to generation through learning. Once thought to be exclusive to humans, scientists have recognized signs of culture in several other animal species, such as dolphins and primates.

dinosaur     A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. They are distinguished by their hips. The lizard-hipped line became saurichians, such as two-footed theropods like T. rex and the lumbering four-footed Apatosaurus (once known as brontosaurus). A second line of so-called bird-hipped, or ornithischian dinosaurs, led to a widely differing group of animals that included the stegosaurs and duckbilled dinosaurs.

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

Enceladus     The sixth largest of Saturn’s more than 50 moons. Enceladus is bright white and covered with a thick shell of ice. Deep beneath that ice sits what appears to be a global ocean of salty liquid water. Enceladus is a round sphere, 500 kilometers (310 miles) across. It is a little less than one-third the width of Earth's moon.

encode     (adj. encoded) To use some code to mask a message.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

Europa     One of the moons of Jupiter and the sixth-closest satellite to the planet. Europa, 1,951 miles across, has a network of dark lines on a bright, icy surface.

extraterrestrial     Anything of or from regions beyond Earth.

filter     (in chemistry and environmental science) A device which allows some materials to pass through but not others, based on their size or some other feature. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

germ     Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some germs cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

Jupiter     (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

liquid     A material that flows freely but keeps a constant volume, like water or oil.

Mars     The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.

methane     A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view such tiny objects, such as bacteria or other one-celled organisms.

mineral     The crystal-forming substances, such as quartz, apatite, or various carbonates, that make up rock. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

moon     The natural satellite of any planet.

multicellular     Having or consisting of many cells. This includes all animals and plants, and many types of fungus.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.

natural gas     A mix of gases that developed underground over millions of years (often in association with crude oil). Most natural gas starts out as 50 to 90 percent methane, along with small amounts of heavier hydrocarbons, such as propane and butane.

nematode     A type of roundworm, usually found in soil, that can also live within other creatures as a parasite. It is very small, with no eyes, ears or nose.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen     A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

RNA     A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.

Saturn     The sixth planet out from the sun in our solar system. One of the four gas giants, this planet takes 10.7 hours to rotate (completing a day) and 29 Earth years to complete one orbit of the sun. It has at least 53 known moons and 9 more candidates awaiting confirmation. But what most distinguishes this planet is the broad and flat plane of seven rings that orbit it.

sequencing     Technologies that determine the order of nucleotides or letters in a DNA molecule that spell out an organism’s traits.

solar system     The eight major planets and their moons in orbit around the sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

solid     Firm and stable in shape; not liquid or gaseous.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

sulfate     A family of chemical compounds that are related to sulfuric acid (H2SO4). Sulfates occur naturally in drinking water.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

uranium     The heaviest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. Uranium atoms are radioactive, which means they decay into different atomic nuclei.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.


Journal: M. Lau et al. An oligotrophic deep-subsurface community dependent on syntrophy is dominated by sulfur-driven autotrophic denitrifiers. Proceedings of the National Academy of Sciences. Vol. 113, December 6, 2016, p. E7927. doi: 10.1073/pnas.1612244113.

Journal: G. Borgonie et al. Eukaryotic opportunists dominate the deep-subsurface biosphere in South Africa. Nature Communications. Published online November 24, 2015. doi: 10.1038/ncomms9952.

Journal: G. Borgonie et al. Deep subsurface mine stalactites trap endemic fissure fluid Archaea, Bacteria, and Nematoda possibly originating from ancient seas. Frontiers in Microbiology. Vol. 6, August 11, 2015. doi: 10.3389/fmicb.2015.00833.

Journal: G. Borgonie et al. Nematoda from the terrestrial deep subsurface of South Africa. Nature. Vol. 474, June 2, 2011, p. 79. doi: 10.1038/nature09974.