We may earn revenue from the products available on this page and participate in affiliate programs. Learn more ›
This feature was originally published in the February 2015 issue of Popular Science.
Kenneth Nealson is looking awfully sane for a man who’s basically just told me that he has a colony of aliens incubating in his laboratory.
We’re huddled in his modest office at the University of Southern California (USC), on the fifth floor of Stauffer Hall. Nealson is wearing a rumpled short-sleeve shirt, a pair of old suede loafers, white socks—your standard relaxed academic attire—and leaning back comfortably in his chair. An encouraging collection of academic awards hangs on one wall. Propped behind him is a well-worn guitar, which he sometimes breaks out to accompany his wife’s singing. And across the hall is the explanation for his quiet confidence: beakers and bottles full of bacteria that are busily breaking the long-accepted rules of biology.
Life, Nealson is explaining, all comes down to energy. From the mightiest blue whale to the most humble microbe, every organism depends on moving and manipulating electrons; it’s the fuel that living matter uses to survive, grow, and reproduce. The bacteria at USC depend on energy, too, but they obtain it in a fundamentally different fashion. They don’t breathe in the sense that you and I do. In the most extreme cases, they don’t consume any conventional food, either. Instead, they power themselves in the most elemental way: by eating and breathing electricity. Nealson gestures at his lab. That’s what they are doing right there, right now.
“All the textbooks say it shouldn’t be possible,” he says, “but by golly, those things just keep growing on the electrode, and there’s no other source of energy there.” Growing on the electrode. It sounds incredible. Nealson pivots on his chair to face me and gives a mischievous grin. “It is kind of like science fiction,” he says. To a biologist, finding life that chugs along without a molecular energy source such as carbohydrates is about as unlikely as seeing passengers flying through the air without an airplane.
That discovery comes with some sizable implications. On a practical level, electric bacteria could be harnessed to create biological fuel cells or to clean up human waste. Nealson tells me that one of his former students just got a grant to build a bacteria-powered sewage system. But more to the point, such microbes appear to comprise a vast, largely unexplored realm of life on this planet. There’s a chance they are an important part of the biodiversity on planets beyond ours too.
Nealson never utters the word “aliens,” but it hangs heavily over the conversation. His bacteria are unlike anything we’ve ever encountered, and they are forcing us to rethink life as we know it.
The Discovery
Like any good alien story, this one begins with an abduction—though one of a decidedly scientific variety. The abductee in this case was not a person but a mineral. Nealson settles in to tell the tale.
In 1982, he was a professor at the Scripps Institution of Oceanography when he heard about strange goings-on in Oneida Lake in upstate New York. Each spring, snowmelt washes manganese out of the surrounding mountains and into the lake. Winds then whip up the waters, allowing the dissolved metal to combine efficiently with oxygen to form solid manganese oxide, which sinks to the lake bed. The trouble was, scientists didn’t find nearly as much as they anticipated. Something was making the manganese oxide vanish, at more than 1,000 times the geologically expected rate, and nobody could figure out what.
“If rates were really that fast, I knew it had to be due to biology,” Nealson says. He suspected bacteria in the lake were getting rid of the manganese oxide almost as quickly as it formed. That theory made perfect sense, but it ran counter to the textbook wisdom: that microbes cannot break down a raw piece of metal any more than you or I can. The mystery kept itching at him. In 1985, Nealson relocated to the University of Wisconsin–Milwaukee, and began research at Oneida Lake to prove his hunch correct.
After a two-year search, Nealson succeeded in identifying the manganese thief: Shewanella, a bacterium that functioned unlike any he had ever known. “As soon as I saw what Shewanella could do, I just went wacky,” Nealson says. “I called all my students into the lab and I said, ‘This is a very, very important organism to understand. Nobody’s going to believe it. It’s going to take us 10 or 15 years to convince the world it’s true.’?”
For most living, air-breathing creatures, Nealson says, “The glucose that we eat supplies the electrons, the oxygen we breathe receives the electrons, and that electron flow is what runs our bodies.” That’s basic metabolism. The challenge for every organism is finding both sources of electrons and places to discard them in order to complete the circuit. Shewanella consumes electrons from carbohydrates, but it sheds them in an unusual way: “It swims up to the metal oxide and respires it.” Nealson says. “We call this ‘breathing rocks.’?” Here is where the scientific heresies begin.
Shewanella‘s outer membrane is full of tiny chemical wires, enabled by specialized proteins, that let it move electricity out of the cell. The wires make direct contact with the manganese oxide, which is how it can deposit electrons and “breathe” a solid substance. Furthermore, Nealson realized that the bacterium doesn’t even care whether the substance on the outside of its membrane is manganese oxide or something else entirely, so long as it will complete the electric circuit.
While Nealson and his team were gathering proof that Shewanella is as extraordinary as it seemed, another microbiologist made a similar discovery. Derek Lovley, then a project chief at the U.S. Geological Survey, found an electron-moving bacterium, Geobacter, living on the bottom of the Potomac River. “Geobacter‘s proteins have a completely different evolutionary origin, but they solve the problem the same way,” Nealson says. Finding two unrelated microbes with an affinity for raw electricity provided reassuring evidence that Shewanella wasn’t some one-off weirdo.
At this point, Nealson realized the microbial landscape of the planet might be different than anyone had thought. He also realized he had probably only just begun to explore what electric bacteria are capable of.
“Nobody’s going to believe it. It’s going to take us 10 or 15 years to convince the world it’s true.”
Strange Metabolism
Annette Rowe, a postdoc researcher in Nealson’s group, is currently speeding through life’s outer limits in the lab across the hall from where I was talking with Nealson. There are fish tanks, test tubes, wires, incubators, and anaerobic chambers with push-through working gloves that look like old set pieces from CSI. I pass a large tank of slow-stirring liquid, with a family of Shewanella growing inside. (“Yeah, too bad you can’t see them,” Rowe says apologetically.) Motivational photos of Nealson gaze down from tall shelving racks. Sample captions: “I AM WATCHING YOU” and “GET YOUR ASS TO WORK.”
The place looks vaguely like an aquarium for microbes, and in fact that’s pretty much what it is. Just as Nealson found Shewanella in Oneida Lake, Rowe and her collaborators have been scouting local marine settings for other electric bacteria, the stranger the better, then cultivating them and trying to figure out what makes them tick.
“We’ve been working in Catalina Harbor. They have a really nice study system out there,” she says. Rowe has the slightly weary look of a graduate student who pulls a lot of late hours, but she lights up when she talks about getting into the field. “Basically, we pull up sediment and sieve it to get rid of invertebrates, and get a nice well-mixed system at the same time. We set up 10-gallon aquariums full of this sediment and bury electrodes in it. And then we look for signs of bacterial colonization.”
The electrode is the key to attracting the type of bacteria Rowe is looking for: not the kind that dumps electrons onto minerals, but the kind that scavenges electrons from them. Not breathers, but eaters. To those bacteria, a cathode looks like one enormous, electrically charged dinner table. Rowe adjusts the electric potential to mimic compounds the organisms might normally draw their energy from, and they swim right up.
As Rowe began sorting through her tanks of sedimentary muck, she was struck by the sheer diversity of bacteria she’d collected. “I’ve isolated a whole slew of electrode-oxidizing bugs,” she says—roughly a thousand strains in total. So far, she’s identified 30 of them, all previously unknown.
One important lesson that has emerged from Rowe’s work is that bacteria have a wide variety of mechanisms for moving electrons around. That finding suggests the ability evolved multiple times. Even more surprising, some of the bacteria, including Shewanella, can swing both ways. “A lot of organisms that can put electrons onto an electrode can also do the opposite and take electrons from one”—though not at the same time—Rowe says. That ability to reverse course surprises me, and Rowe, too. “I’d think it would be really hard on the organisms. You’re basically stealing energy from them. But they do okay.”
Another discovery is even more astonishing. Six of Rowe’s new bacterial strains can live on electrons alone. “It’s a crazy phenomenon,” she says, one that is well beyond anything Nealson had discovered up to now. “I’ve kept some of these bugs for over a month with no addition of carbon,” she says. They must be subsisting solely on electricity from the electrode, because there is nothing else.
These microbes are the ones that had Nealson so worked up in our earlier conversation. They are not just new to science; they require an entirely new method of collection and culture. The vast majority of Rowe’s strains must be grown on a cathode, not in a petri dish. And they indicate an immense and largely alien ecosystem here on Earth. The National Science Foundation calls it the “dark energy biosphere” and is funding Rowe to learn more about this parallel microbial universe.
These microbes indicate an immense and largely alien ecosystem here on Earth.
To Nealson, his protégé’s breakthrough both validates and stomps all over his own revelations about how life works: “I’ve been doing microbiology for 45 years,” he says. “It’s just wild to have your whole view change so drastically.”
Caught On Camera
As staggering as Rowe’s findings are, there is a certain level of intellectual remoteness to all that talk about electrons and energy levels. No matter how much I stare into the flask, I still keep wishing I could see what the bacteria are doing with my own eyes. That frustration dissipates when I stop in on Moh El-Naggar, who works a couple buildings over on the USC campus. He has actual videos of the microbes in action, unspooling wires and setting up microscopic electrical grids.
El-Naggar’s bacterial video project began as an effort to disprove a theory. Experiments Nealson had done with Shewanella showed that the bacteria can make contact with a metallic surface to deposit electrons. Other studies had revealed that bacteria sometimes produce hairlike appendages of unknown function. Some researchers dismissed those growths as unimportant, but a few wondered whether the hairs were actually “nanowires” created by the bacteria to move electrons.
Video: Electric nanowires stretch from the outer membranes of Shewanella oneidensis bacteria. Credit: El-Naggar et al./PNAS 2014, Courtesy USC
To El-Naggar, that reasoning seemed too tidy: “I kind of went into it thinking, it can’t really work that way, right? I’m going to do the measurements that show it doesn’t.” So El-Naggar did what any good home handyman would. He clipped a couple leads onto the wires to see if they conduct electricity. They do. Then he checked to see if the circuit is live, with current flowing across the wires. It is. Finally, he monitored the wires as they form, recording the cells lighting up with activity once they complete a circuit.
Afterward, he had a series of mind-boggling movies in which you can watch Shewanella reach out to an electrode in search of a place to deposit electrons. Sometimes the bacteria will link up with one another, possibly fobbing off electrons on cells that are able to accept them. El-Naggar describes the shock that runs through the room when he shows his videos at conferences: “You’re sitting there in the dark, you start the movie, and then you hear, ‘Ahh! Cool!’?”
Nanowires may be related to yet another widespread but newly discovered bacterial talent, the ability to connect into sausage-link cables thousands of cells long. As yet there is no indication whether Rowe’s electric bacteria form these kinds of cables (the research is far too new), but studies at Aarhus University in Denmark indicate that such cables do support a flow of electrons. El-Naggar speculates that the cables are like drinking straws, allowing bacteria buried deep in sediment to breathe from the top of the pile by pushing electrons up through the tube, from one cell to the next.
Just a few years ago, nobody imagined that any bacteria were capable of such behaviors. Now El-Naggar suspects that nanowires and cables are used widely by bacteria, and not just among the most extreme electron-eaters. He is collaborating with colleagues in the dental school at USC to examine evidence of nanowires in the bacterial films that form in people’s mouths; cell-to-cell electrical linkages might in fact be a general characteristic of biofilms, bacterial collectives, both benign and pathogenic, that take up residence on a surface.
Shelley Minteer, an electrochemist at the University of Utah, has probed even deeper into cell biology. She discovered that mitochondria—the power-generating units inside the cells of all complex cellular organisms, including humans—can interact electrically with surfaces outside themselves. That fits with a well-accepted theory that mitochondria evolved as free-living bacteria that later merged with other cells, forming a permanent partnership. Even after a billion years, mitochondria may retain some of the capabilities they had in their days of independence. It is possible, then, that we all have a smidgen of electric alien behavior locked away inside us.
It is possible we all have a smidgen of electric alien behavior locked away inside us.
Beyond Earth
My first trip from Nealson’s office took me across the hall. My last trip takes me to Mars. Not such a big leap, actually: Nealson has never made a clean philosophical distinction between the search for exotic life on Earth and the search for life on other planets. For several years he worked at NASA’s Jet Propulsion Lab (JPL), where he set up the astrobiology group. Now the ideas he developed there will get a formal test aboard the upcoming Mars2020 rover.
In some ways, getting to Mars is a cakewalk compared to the challenge of knowing what to search for once you arrive. The Viking missions in the 1970s landed just fine but got tripped up by things that smelled like life. The scientists studying the infamous Mars meteorite in the 1990s may have been led astray by things that looked like life. And the fancy new Curiosity rover has found intriguing whiffs of methane, but their connection to biology is utterly unknown. That’s what Nealson’s team grappled with at JPL. “Could you really figure out what the universal properties of any life must be? It’s very hard to solve this problem, because we can’t get away from our own biases,” he says.
SHERLOC is part of the answer. It is one of seven science instruments aboard Mars2020. One of Nealson’s former JPL employees, Rohit Bhartia, was a lead designer, and the instrument is heavily informed by the lessons of metal-breathing bacteria. Shewenella expanded scientists’ understanding of metabolism, and so SHERLOC will be looking for a wider spectrum of possible biosignatures. It will zap targets with ultraviolet rays and look for visual effects that indicate certain organic compounds and minerals.
Although SHERLOC will not be searching for life per se—only for the trail it leaves behind—electric bacteria suggest new ways to find active alien biology as well. All of the electric adaptations are responses to extreme environments. Scrounging for electrons and sprouting nanowires are strategies for surviving when there is not enough food to do much growing and competing—just enough to help an organism hunker down and keep the flame of life lit. Such conditions are common in deep ocean sediments and far underground. If life exists on Mars and other worlds (Europa? Titan?), there’s a good chance that it, too, is huddled in resource-constrained settings far beneath the surface.
While NASA gears up for Mars2020, Rowe and others in the USC group are bio-prospecting for more electric bacteria here on Earth, relocating their operation from the shallow waters around Catalina Island to deep boreholes in the Mojave desert and mines in South Dakota. These sites could not only expose more of Earth’s hidden biodiversity; they could also help guide thinking about possible alien biologies. “When we go to other planets, we look for life on the surface, but really there’s so much energy in the subsurface,” Nealson says. “I’ll be astounded if this extracellular electron transport isn’t the rule there.”
In the process of poking electrodes into different environments and rounding up electric microbes, Nealson’s team has noted a distinctive pattern: Stick a spike in the ground pretty much anywhere on Earth, and you can measure the electric potential steadily dropping off the deeper you go. That’s because microbes at each depth are chasing after whatever electrons are available. The most energetic organisms, using the most energetic reactions, live up top, where resources are the most abundant. The farther you go into the regions of scarcity, the more life has to grab at any energy it can get.
That electric gradient sure sounds like another good candidate for a universal signifier of life. “If there isn’t life, there shouldn’t be gradients,” Nealson says. So instead of running complicated chemical experiments that might miss some unfamiliar type of biological activity, he muses, why not stick a giant probe in Mars and replicate Rowe’s microbe hunting expeditions in Catalina? He envisions a whole flock of javelin-like probes that drop from an orbiter and penetrate the ground all around the planet. Each one would have a little transponder to send data up to the science satellites already circling the Red Planet. The probes would look for electrical gradients, flagging possible locations of biological activity for closer study. NASA and Russia have attempted simpler Mars penetrators, though both missions failed. Now the nonprofit Explore Mars is trying to raise funds for an “ExoLance” to seek subsurface life there.
“When we go to other planets, I’ll be astounded if this extracellular electron transport isn’t the rule there.”
Nealson is on a roll, so I goad him on: Could you do the same thing on Europa? He slows down for only a beat. “Europa is tough, because it’s all ice. . . . You would imagine that you would put something on the surface with a solar panel or a radioactive generator and just melt your way down in with the probe. You could radiation-harden just the little thing above the electronics.”
If they find no signs of electric biology, the probes could still measure geochemistry beneath the surface, which is valuable in and of itself. And if they do find it, popping champagne corks would be premature: You’d want to see if it is dynamic, changing with daylight or temperatures, for instance. That kind of additional signal would be strong circumstantial evidence of life. It still wouldn’t be the definitive discovery of ET, but it would tell you exactly where to go back—this time with a microscope.
The Shadow Biosphere
As we are talking, I find myself in the middle of a very different kind of conversation about the nature of life. At one point, Nealson pauses to inform other members of the lab that a close friend and colleague, Katrina Edwards, died over the weekend. Then he interrupts again, explaining that he has to drop off his retirement papers with the dean, easing himself into a four-year exit. When Nealson returns, he indulges in a little reflection. His only real regret, he tells me, is that he won’t have enough time to study Rowe’s all-electric bacteria himself: “It really pisses me off that I discovered this when I was 70 years old, because it’s important.”
Electrically active bacteria could have many practical benefits that researchers are now beginning to explore. They turn out to have an incredible talent for sewage treatment, for example. Stick an electrical anode in human waste and it attracts communities of bacteria that eat feces and breathe electrons. Hook them up to a fuel cell and you have a self-powered wastewater treatment system that produces significantly less sludge. One of Nealson’s former students, Orianna Bretschger, set up a test system at the J. Craig Venter Institute in San Diego, where it’s been running for five years with practically no maintenance. “My personal goal is developing these systems to a point where we could fly them into villages in the third world,” says Nealson, who still collaborates with Bretschger. “People would bring their sewage to the treatment plant and get clean water, and you wouldn’t need any outside power.”
Daniel Bond at the University of Minnesota is exploring the potential for electric bacteria to generate power and synthesize novel materials. The defense department is reportedly interested in underwater sensors driven by bacteria. El-Naggar suspects that electrical interactions between bacterial and human cells may have important, almost entirely unexplored health implications. After all, the sewage experiments indicate there are electrically active bacteria in the gut. He wonders aloud: Do they communicate with human cells as part of the body’s internal ecosystem?
All of these possible applications derive from the sheer unfamiliarity—the utter alien-ness—of Shewanella and its even stranger cousins. They are alien not just in what they do but in how they do it. Their Earth seems to be a world built on cooperation and sharing, a far cry from the more familiar world of cutthroat Darwinian competition. “Unless I miss my bet, that’s what we’re going to see when we get to the subsurface: little pockets of life with a socialist community, all working there together. But I won’t tell that to my Republican father because he won’t like it,” Nealson says.
I think of electric socialism as an exotic idea, but Nealson quickly convinces me otherwise. It may be the normal way things work in environments where resources are scarce and predatory competition is not an advantage. It may have been life’s reality through much of its history on this planet. (“I always thought that bacteria never learned to grow fast until predators evolved,” he says. “What’s the rush? You know, bacteria don’t eat other bacteria.”) In fact, it may suit more of today’s life than most scientists realize, since so much of Earth’s microbial ecosystem is still out of sight. By some estimates, 99.9 percent of all species cannot be cultured in a petri dish. Slow, collaborative living may be life’s way on many other worlds as well.
That’s a lot of maybes, so I put this to Nealson: Does he really believe in a shadow biosphere, built around electron sharing and microscopic collectivism? “Before I kick the bucket, I hope that is proven to be true,” he says. Then he corrects himself, like a proper open-minded scientist. “I mean, I don’t. It’s okay with me if it’s not true, but I’ll be really surprised. It makes so much sense, and life usually makes sense.”
How To Find Alien Life
1. Test For Metabolic Activity
The first serious attempt to find alien life took place in 1976, when the twin Viking probes sought out organisms by mixing Martian soil with nutrients and radioactive carbon. The results were negative (you probably knew that), but clouded by the complex soil chemistry.
2. Follow The Water
NASA’s current Mars research, led by the $2.5 billion Curiosity rover, focuses on learning whether the planet once had a warm, wet environment. Studies of Gale Crater look encouraging; unfortunately, these efforts show only that Mars could have sustained life, not that it actually did.
3. Scan For Organics
Taking lessons from Viking and Curiosity, NASA’s upcoming Mars2020 rover will include two instruments that scan the environment for signs of organic compounds. This technique can cover a lot of ground, and does not make specific assumptions about the metabolism of Mars life.
4. Look For Chemical Organization
Another approach would be to seek out chemical patterns suggestive of biological activity. For example, DNA is full of repeating molecular motifs. More subtly, there are almost no natural nitrogen-bearing minerals, so an array of nitrogen compounds would raise a red flag.
5. Measure Electric Potential
All life manipulates electrical energy. If the electric potential in the ground drops steadily with depth (as happens on Earth), that could indicate successive populations of microbes are pulling electrons from the environment. It would be a low-key First Contact, but revolutionary all the same.
Extreme Biology
Some of the best evidence for life in space lives right here on Earth: It’s weird, adaptable, and far hardier than we ever thought. —Alissa Zhu
