In 2009, Victoria Orphan crammed into the submarine Alvin with a pilot and another marine scientist to see an ecosystem as strange and separate as another planet.
They dipped below the surface in the Caribbean Sea near Costa Rica, where the North American continent is pushing the oceanic plate into the Earth’s searing-hot mantle. There, the shifting geology is destabilizing an undersea mountain, causing it to spew methane gas and sulfides from deep-sea vents. Descending down the water column, they soon passed beyond the reach of sunlight. At first, when Alvin’s spotlights touched the seafloor, the scientists saw nothing but ocean mud through the windows, with maybe a sea cucumber here and there. As they got closer to the methane seeps, they saw what looked like pavement: a rocky build-up of carbon over the mud. Then, Orphan saw something remarkable.
“When we came up on the site—I’ll never forget that—there were teams of these crabs, and they were all waving their arms over the sulfide coming out of the vents,” Orphan says. “Crabs dancing on the seafloor was definitely a memorable sight.”
The crabs’ arms are laden with bacteria that are able to eat the sulfide chemicals from the vents. Every so often, the crabs eat the bacteria off their arms—a farm that fits on a claw.
Orphan, a marine geobiologist at the California Institute of Technology, says that this entire alien ecosystem depends on a partnership between two very different kinds of single-celled microbes: a species of archaea and a species of bacteria. They’re more different from each other than humans are from plants.
In the deep-sea vents, they can form a superorganism greater than the sum of its parts. Microbes that act as superorganisms have evolved to be deeply dependent on each other instead of competing. And this cooperation lets them survive in extreme places. Working together, the microbes are able to eat the methane from the seeps. In the process, they cause dissolved carbon molecules in the water to solidify on the seamount, accumulating into the rocky pavement. And that hard surface creates a base on which all the larger creatures, including the farmer-crabs, can survive.
Partnerships like this one aren’t just far away in the deep ocean—they’re also in our landfills and backyard ponds. And the bizarre way in which these microbes cooperate tells us something about what we might expect to find when we search for life on other planets.
An Extreme Partnership for an Extreme Place
The ways that simple, single-celled bacteria and archaea interact with each other are more complex than scientists ever imagined, and biologists only recently discovered a surprising partnership between two very different microbes that team up to survive the harshest of environments.
All life is powered by chemical reactions. On land and in the shallows of the ocean, organisms use light from the sun to power the photosynthetic reactions that form the base of the food web. But at the bottom of the ocean, there’s no light—only gases and hot fluids seeping from Earth’s crust. If life is to survive, it has no choice but to use them. There, an organism’s choices for chemicals to react are pretty limited. Two of the common chemicals leaking from the vents, methane and sulfate, can combine to produce carbonate, sulfide, and a little energy. Under normal undersea conditions, making this reaction happen is like climbing a mountain.
But microbes have found a way around the mountain through working together. Archaea cells cluster together in a sphere of about 100 cells. They take in methane gas and break it down, grabbing electrons from the molecules to power their life cycle. But if too much electric charge builds up within the cell, the process stops. It’s like a bucket line with nowhere to put the last bucketful: the line gets jammed. The archaea need to dump the electrons somewhere.
That’s where the bacteria partners come in. About 200 of those cells cluster around the inner sphere of archaeal cells. The archaea pass the extra electrons to the bacteria, which use the electrons to power their own reaction. They absorb sulfates from the deep-sea vent and add electrons to them, producing sulfide and a little bit of energy.
It’s a process called syntrophy, which literally means “feeding together.” Together, the sphere of cells makes a complete reaction and produces enough energy so both archaea and bacteria can survive.
Even the way in which they share electrons gave scientists a surprise. Last year, biologists at the Max-Planck Institute for Marine Microbiology found that the two species use tiny nanowires that carry electric current from the archaea to the bacteria.
The nanowires form “an electrical bridge between organisms,” says James Holden, a marine biologist at the University of Massachusetts-Amherst, who wasn’t involved in the study. This creates “a really tight, tight relationship.” We knew this could happen between two species of bacteria—but not two organisms as different as archaea and bacteria. It’s as though you and your petunias shared a physical connection.
“They are like a very old couple—one cannot [live] without the other,” says Gunter Wegener, a researcher at the Max Planck institute and the first author of the 2015 paper. “We can clearly speak of a superorganism, as the survival of the genomic information of one organism clearly depends on the other.”
Orphan says that while she’s been able to show that these two species can survive on their own, given the right conditions in the lab, this might be like growing human tissue on a culture plate that couldn’t really live on its own.
This isn’t the only way that microbes can link themselves, either. In the hydrothermal vents that Holden studies off the coast of Oregon, water heated by magma and carrying sulfides and dissolved metals gushes out of chimneys on the seafloor. The water can reach a sweltering 700° F. There, a different kind of cooperation has evolved.
Two archaea species at the hot vents actually do the reverse of what the microbial superorganisms do at the methane seeps. Instead of breaking down methane, they work together to build it. One archaeon eats chemicals coming out of the vent, producing hydrogen gas as waste. If too much of it built up, it up would stop the reaction.
That’s where the partner microbe comes in. This other archaeon clusters close to the one producing hydrogen to gobble up every bit, combining it with carbon atoms to make methane gas and a little bit of energy. Normally, this reaction wouldn’t be energetically favorable at the bottom of the ocean. But by working together, the two microbes make this unlikely process possible.
“The reaction just doesn’t want to go forward by itself,” Holden says. “So the only way you can cheat thermodynamics is if you tilt the scales.” Cooperating lets the microbes survive.
But these kinds of extreme partnerships aren’t limited to the bottom of the ocean. “It’s almost like everywhere you look, you start to see evidence of these interdependencies,” Orphan says. They extend beyond extreme environments to the seemingly benign backyard ecosystems.
Superorganisms in Your Backyard
In fact, while we’re just now figuring out how these superorganisms work, even the United States’ first president saw their effects on the environment. After winning the Revolutionary War, George Washington became free to pursue his scientific hobbies. Benjamin Franklin and another scientist had been arguing about a mysterious phenomenon, and Washington hoped to settle the debate. Franklin had found that if he lit a candle near the surface of a stream, it would catch fire and “burn for near half a minute.” The two couldn’t decide what could cause this, and Washington, along with writer Thomas Paine, boarded a boat on a nearby creek to figure it out.
Paine argued that flammable gases constantly bubbled up from streams, while Washington believed that flammable gases rose only when rowers disturbed the bottom of the stream. They sat at either end of a boat holding burning rolls of paper. Sure enough, when the soldiers rowing the boat stirred up the creek mud, fire descended from Washington’s paper to the surface then spread around the boat.
While they could not have known at the time, the gas they stirred up was methane produced by superorganisms—two kinds of archaea partnering to survive in an environment without oxygen.
It’s not just backyard ponds and creeks, either. Huge flames sometimes burst from landfills when lots of methane builds up and ignites. Elsewhere, methane-producing archaea live in wastewater treatment plants, rice fields, oil spills, and the bellies of cows. The same reaction-sharing strategy as in the deep-sea seeps has let them adapt to both human-built ecosystems and the insides of other creatures.
In these partnerships, one set of archaea cells takes in carbon-containing compounds and breaks off hydrogen groups, gaining electrons to eat and producing hydrogen gas as a waste. It passes the hydrogen to the partner cells, which then builds methane molecules from the hydrogen gas and carbon dioxide. By keeping the concentration of hydrogen low, the second partner keeps the whole system running.
“Hydrogen syntrophy is a very, very common process on Earth,” Holden says. “Most of the methane that ends up in our atmosphere every year comes from this process.” The same partnership that makes life possible in a deep-sea hot vent allows life to flourish in a steaming landfill.
How to Survive on Mars
It turns out that microbes that can handle the heat hydrothermal vent on Earth can also probably brave the cold of the Martian surface or the moons of Jupiter.
In 2006, scientists at the Maryland Astrobiology Consortium took a strain of archaea from a salty lake in Antarctica and tested it to see just how much cold and salt it could handle before its methane production shut down. They found that, at 28˚ F, around the freezing point of very salty water, the hardy microbe clumped together to share resources, kept eating their chemical food, and even grew their ranks.
To stick together, the archaea cells connected to each other with tiny fibers. The scientists shook up the test tube to see how strong the aggregation was. While the shaking broke it up, the cells reformed their network of fibers within a day. The vibrations, salt, and cold all failed to stop the archaea.
On Mars, those almost-freezing temperatures only occur on the warm days. But microbes in Antarctica have shown that cells have ways of surviving when the temperature dips below the range in which they can grow and feed. They’re able to basically freeze-dry themselves to survive the cold and high salt concentrations that result when most of the water freezes.
The superorganisms we find on Earth suggest that if we ever find microbial life on another planet, it might not be just simple single-celled organisms living only for themselves. Being a good competitor, it turns out, isn’t always the best strategy. Superorganisms show us that sometimes it pays to work together.
“Certainly in a place like Mars, it’s a tough environment for a lot of different reasons,” Holden says. “But if there were or is microbial life on Mars, then it wouldn’t surprise me if there’s some sort of cooperation occurring between different types of organisms.”
From the depths of the ocean to our own backyards, bizarre relationships between microbes are changing how we understand life. We’re discovering that what it means to be an individual isn’t the same throughout the living world, says Matt Haber, a philosopher of biology at Utah State.
“Individuals come in degrees,” Haber says. There are individuals like cats and dogs, but there are also social organisms that act as individuals, like ant colonies and beehives. The deep-sea clumps of microbes seem to be a borderline case of individuality, since it doesn’t seem to reproduce as a unit. “This could be an example of ways that bacteria and archaea can interact that may be approaching individuality but hasn’t reached that level yet,” he says.
Still, they’re very tightly connected. “The unit looks like it’s acting as an individual, at least in some sense, in the way that they’re interacting with the environment,” Haber says.
“We’re kind of in a golden age of microbes right now,” Haber says. “We’re really beginning to appreciate how much they challenge our views of biology.”