Something has been right under scientists’ noses, and they hadn’t seen it — till now. Oh, they have known about bacteria for centuries, ever since Antony van Leeuwenhoek first glimpsed them in his homemade microscopes. Bacteria have been extensively classified and sequenced now. We know about their internal organelles, their genomes, and their interactions. But what came to light as recently as a decade ago is truly astonishing: some bacteria can join end to end to form cables that conduct electricity.

So-called “cable bacteria” were mentioned briefly on Evolution News back in February 2016 as potential agents of earth’s habitability. Cables of specialized microbes, extending several centimeters, appear to transfer electrons that operate the metabolism of other organisms living in deep sea sediments, and simultaneously prevent buildup of toxic wastes. Since then, living electrical wires are turning up everywhere. 

A Special Issue on Mud

Elizabeth Pennisi, writing in Science Magazine’s special issue on “mud” as “one of Earth’s most ubiquitous substances,” describes the disbelief among some scientists on hearing Lars Peter Nielsen announce in 2009 that he had found chains of bacteria conducting electricity in the “black, stinky mud” he had collected from a harbor in Denmark. Within days in his lab, the heavy doses of hydrogen sulfide in his mud samples disappeared, and so did the stink. Startled, he discovered that what he named “cable bacteria” were transferring electrons from the oxygen-deprived lower layers to the surface, allowing bacteria deeper in the mud to metabolize organic matter and get rid of hydrogen sulfide waste. In her article, “The Mud Is Electric,” Pennisi says,

When Nielsen first described the discovery in 2009, colleagues were skeptical. Filip Meysman, a chemical engineer at the University of Antwerp, recalls thinking, “This is complete nonsense.” Yes, researchers knew bacteria could conduct electricity, but not over the distances Nielsen was suggesting. It was “as if our own metabolic processes would have an effect 18 kilometers away,” says microbiologist Andreas Teske of the University of North Carolina, Chapel Hill. [Emphasis added.]

Now that they are believers, these and other scientists are finding that cable bacteria are almost as ubiquitous as mud itself. 

But the more researchers have looked for “electrified” mud, the more they have found it, in both saltwater and fresh. They have also identified a second kind of mud-loving electric microbe: nanowire bacteria, individual cells that grow protein structures capable of moving electrons over shorter distances… These nanowire microbes live seemingly everywhere — including in the human mouth.

The article’s lead photo shows a cross-section of mud with networks of strands, but these are not fungal hyphae one might find in garden soil. These are much thinner. “Threads of electron-conducting cable bacteria can stretch up to 5 centimeters from deeper mud,” the caption reads, “where oxygen is scarce and hydrogen sulfide is common, to surface layers richer in oxygen.” Basically, the deep organisms send electrons gained by “eating” organic matter up the cables to the top, and donate the electrons to oxygen and hydrogen, yielding water. This prevents buildup of toxic hydrogen sulfide. Similar oxidation-reduction (redox) reactions are the basis of all metabolism. 

Global Roles of Electric Mud

It might seem at first that these bacteria are acting selfishly, using a clever electrical trick to get food and eliminate waste. But when researchers started looking at the big picture, they saw a cooperative ecosystem coming into focus.

The discoveries are forcing researchers to rewrite textbooks; rethink the role that mud bacteria play in recycling key elements such as carbon, nitrogen, and phosphorus; and reconsider how they influence aquatic ecosystems and climate change.

Filip Meysman, the one whose first reaction was to call Nielsen’s theory “complete nonsense,” has come around. “We are seeing way more interactions within microbes and between microbes being done by electricity,” Meysman says. “I call it the electrical biosphere.”

Cable Structure

Working together, Nielsen and Meysman found out more details about these bacteria. They build a cylindrical sheath, possibly made of protein, within which the bacteria line up. These cylinders contain up from 17 to 60 protein “wires” where electrons are passed from cell to cell through the sheath. Many thousands of microbes can make up a single wire. The resulting cables conduct a current of electricity that, while not as efficient as copper wires, “are on par with conductors used in solar panels and cellphone screens, as well as the best organic semiconductors.” 

The other type of conductive microbe has been found almost everywhere microbiologists have looked. The infographic in Pennisi’s article shows that “nanowire bacteria” have a different structure but do the same job. Electrons gained from oxidation of organic compounds travel along “protein nanowires” to electron-accepting substances or cells. The nanowires are much shorter, on the order of 20 to 50 nanometers, but they can sprout from multiple parts of a bacterial membrane, probing the surrounding soil to connect the “terminals” of electrical currents that power their metabolism. But proteins were thought to be insulators; how can they conduct electricity? Nanowire conductance is not well understood, but it may have to do with sequences of amino acids bearing ring-shaped R-groups, called pilins.

Shared Benefits

These nanoscopic cables help the bacteria, but they also help other organisms. Without them, only the surface layers of soils and sediments would be viable, because toxic waste products would accumulate in the deeper, oxygen-deprived layers. These wires are “making mud more habitable for other life forms,” Pennisi says. It is also becoming apparent that they are natural clean-up agents in some ecosystems. 

Cable bacteria and protein nanowires are turning up everywhere, in both freshwater and saltwater. For example, they have been observed in the sides of worm tubes on the seafloor, probably helping make the tubes more habitable for the occupants. 

Nanowire bacteria are even more broadly distributed. Researchers have found them in soils, rice paddies, the deep subsurface, and even sewage treatment plants, as well as freshwater and marine sediments. They may exist wherever biofilms form, and the ubiquity of biofilms provides further evidence of the big role these bacteria may play in nature.

Pennisi catalogs some of the many roles that these electrically conductive bacterial cables play in nature. By creating gradients pH gradients, they undoubtedly play important roles in geochemical cycles involving elements and molecules as diverse as methane, arsenic, manganese, and iron. They may even be playing roles in the biofilms that form around our teeth! Whether that is good or bad remains to be seen, but Nielsen remarks, “It is dizzying to think about what we’re dealing with here.”

Global Ecosystem Engineers

As scientists learn more about electrically conducting microbes, we can expect more startling revelations about how central their roles are to global habitability. For example,

The microbes also alter the properties of mud, says Sairah Malkin, an ecologist at the University of Maryland Center for Environmental Science. “They are particularly efficient … ecosystem engineers.” Cable bacteria “grow like wildfire,” she says; on intertidal oyster reefs, she has found, a single cubic centimeter of mud can contain 2859 meters of cables, which cements particles in place, possibly making sediment more stable for marine organisms.

In separate but related findings, scientists are discovering more evidence that microbes really get around. At The Conversation, Predrag Slijepcevic writes that “Bacteria and viruses are travelling the world on highways in the sky” (see also, “Information Storage — In the Cloud(s)”). And some live on air. Researchers at the University of New South Wales report, “Microbes living on air [is] a global phenomenon,” even in polar climates where almost nothing grows. Most likely they influence carbon fixation and global climate. “There are whole ecosystems probably relying on this novel microbial carbon fixation process,” the senior author said, “where microbes use the energy obtained from breathing in atmospheric hydrogen gas to turn carbon dioxide from the atmosphere into carbon — in order to grow.”

Design Promotes Design

With all these benefits coming to light, it was inevitable that some would be thinking up biomimetic applications. A companion piece in the special issue of Science, also by Pennisi, has the provocative title, “Next up: a phone powered by microbial wires?” 

One potential use is to detect and control pollutants. Cable microbes seem to thrive in the presence of organic compounds, such as petroleum, and Nielsen and his team are testing the possibility that an abundance of cable bacteria signals the presence of undetected pollution in aquifers. The bacteria don’t degrade the oil directly, but they may oxidize sulfide produced by other oil-eating bacteria. They might also aid cleanup; sediments recover faster from crude oil contamination when they are colonized by cable bacteria, a different research team reported in January in Water Research. In Spain, a third team is exploring whether nanowire bacteria can speed the cleanup of polluted wetlands. And even before nanowire bacteria were shown to be electric, they showed promise for decontaminating nuclear waste sites and aquifers contaminated with aromatic hydrocarbons such as benzene or naphthalene.

There’s actually enough energy in moisture in the air, researchers have shown, to power a cellphone with genetically modified bacterial nanowire films. How’s that for “a revolutionary technology to get renewable, green, and cheap energy” in today’s energy-conscious society?

Down and Dirty

Bacteria, it is worth emphasizing, are living organisms with molecular machines built by and storing information in coded form. That means that even mud is loaded with complex specified information — what a thought! Pennisi comments, “Bacteria that conduct electricity are transforming how we see sediments.” It puts a new positive spin on “clear as mud.” 

There is no lack of clarity, however, in the conclusion that rapid, efficient, global ecosystem engineering through electrical cables sounds like a designing mind had the foresight to think of everything that a habitable planet would need for life to flourish.