For Lars Peter Nielsen, it all started with the mysterious disappearance of hydrogen sulfide. The microbiologist collected the black, smelly mud from the bottom of Aarhus harbor in Denmark, threw it into large glass beakers and inserted special microsensors that detected changes in the chemical composition of the mud.
At the beginning of the experiment, the composition was saturated with hydrogen sulfide - the source of the odor and color of the sediment. But 30 days later, one strip of dirt turned pale, which indicates the loss of hydrogen sulfide. Eventually, the microsensors showed that the entire connection was gone. Given what scientists knew about the biogeochemistry of mud, recalls Nielsen of Aarhus University, "it didn't make sense at all."
The first explanation, he said, was that the sensors were wrong. But the reason turned out to be much more strange: the bacteria that connect the cells create electrical cables that can conduct current up to 5 centimeters through the dirt.
An adaptation never seen before in microbes allows these so-called cable bacteria to overcome a major problem faced by many organisms living in mud: lack of oxygen. Its absence usually keeps bacteria from metabolizing compounds such as hydrogen sulfide for food. But cables, by binding microbes to oxygen-rich deposits, allow them to react over long distances.
When Nielsen first described the discovery in 2009, his colleagues were skeptical. Philip Meisman, a chemical engineer at the University of Antwerp, recalls thinking, "This is complete nonsense." Yes, the researchers knew bacteria could conduct electricity, but not at the distances Nielsen suggested. “It was as if our own metabolic processes could affect a distance of 18 kilometers,” says microbiologist Andreas Teske of the University of North Carolina at Chapel Hill.
But the more researchers looked for "electrified" mud, the more they found it in both salt and fresh water. They also identified a second type of dirt-loving electrical microbe: nanowire bacteria, individual cells that grow protein structures that can move electrons over shorter distances.
These nanowire microbes are found everywhere, including in the human mouth
Discoveries force researchers to rewrite textbooks; rethink the role of mud bacteria in the processing of key elements such as carbon, nitrogen and phosphorus; and review how they affect aquatic ecosystems and climate change.
Scientists are also looking for practical applications, exploring the potential of bacteria containing cables and nanowires to combat pollution and power electronic devices. “We're seeing a lot more interactions within microbes and between microbes using electricity,” Meisman says. "I call it the electrical biosphere."
Most cells thrive by taking electrons from one molecule, a process called oxidation, and transferring them to another molecule, usually oxygen, called reduction. The energy gained from these reactions governs other life processes. In eukaryotic cells, including our own, such "redox" reactions occur on the inner membrane of mitochondria, and the distances between them are tiny - only micrometers. This is why so many researchers were skeptical about Nielsen's claim that cable bacteria move electrons through a layer of dirt the size of a golf ball.
Disappearing hydrogen sulfide was the key to proving this. The bacteria make a compound in the mud, breaking down plant debris and other organic materials; in deeper deposits, hydrogen sulfide accumulates due to lack of oxygen, which helps other bacteria to break it down. However, hydrogen sulfide still disappeared in Nielsen's beakers. Moreover, a rusty tint appeared on the surface of the dirt, which indicated the formation of iron oxide.
Waking up one night, Nielsen came up with a strange explanation: what if bacteria buried in the mud completed the redox reaction, somehow bypassing the oxygen-poor layers? What if, instead, they used the abundant supply of hydrogen sulfide as an electron donor and then funneled the electrons up towards the oxygen-rich surface? There, in the oxidation process, rust is formed if iron is present.
Finding what carries these electrons has proven difficult. First, Niels Riesgaard-Petersen of Nielsen's team had to rule out a simpler possibility: metal particles in the sediment carry electrons to the surface and cause oxidation. He accomplished this by inserting a layer of glass beads that do not conduct electricity into a pillar of dirt. Despite this obstacle, the researchers still found an electric current moving through the mud, suggesting that the metal particles were not conductive.
To see if a cable or wire was carrying electrons, the researchers then used tungsten wire to make a horizontal cut through the mud column. The current went out, as if a wire had been cut. Other work narrowed down the size of the conductor, suggesting that it should be at least 1 micrometer in diameter. “This is the normal size of bacteria,” says Nielsen.
Ultimately, electron micrographs revealed a likely candidate: long, thin bacterial fibers that appeared in a layer of glass beads inserted into beakers filled with mud from Aarhus Harbor. Each filament consisted of a stack of cells - up to 2,000 - enclosed in a ribbed outer membrane. In the space between this membrane and the cells stacked on top of each other, a plurality of parallel "wires" stretched the thread over its entire length. The cable-like appearance inspired the common name of the microbe.
Meisman, a former skeptic, was quickly converted. Shortly after Nielsen announced his discovery, Meismann decided to investigate one of his own samples of sea mud. “I noticed the same color changes in the sediment that he saw,” Meisman recalls. "It was Mother Nature's direction to take it more seriously."
His team began developing tools and methods for microbial research, sometimes working in conjunction with Nielsen's group. It was hard going. Bacterial filaments tend to deteriorate quickly after isolation, and standard electrodes for measuring currents in small conductors do not work. But once the researchers learned to pick out a single strand and quickly attach an individual electrode, “we saw really high conductivity,” Meisman says. Live cables cannot compete with copper wires, he said, but they match the conductors used in solar panels and mobile phone screens, as well as the best organic semiconductors.
The researchers also analyzed the anatomy of the cable bacteria. Using chemical baths, they isolated the cylindrical shell, finding that it contained 17 to 60 parallel fibers glued together inside. The shell is the source of conduction, Meisman and colleagues reported last year in Nature Communications. Its exact composition is still unknown, but it may be protein-based.
“It's a complex organism,” says Nielsen, who now heads the Center for Electro-Microbiology, created in 2017 by the Danish government. Among the problems that the center solves is the mass production of microbes in culture. “If we had a pure culture, it would be much easier” to test ideas about cell metabolism and the effect of the environment on conduction, says Andreas Schramm from the center. The cultured bacteria will also make it easier to insulate cable wires and test potential bioremediation and biotechnology applications.
While researchers are puzzling over the bacteria in the cable, others are looking at another major player in electrical mud: nanowire-based bacteria that, instead of folding cells into cables, grow protein wires 20 to 50 nm in length from each cell.
As with cable bacteria, the mysterious chemical composition of the deposits led to the discovery of nanowire microbes. In 1987, microbiologist Derek Lovely, now at the University of Massachusetts Amherst, tried to understand how phosphate from fertilizer wastewater - a nutrient that promotes algal blooms - is released from sediment under the Potomac River in Washington, DC. worked and began to weed them out of the dirt. After growing one, now called Geobacter Metallireducens, he noticed (under an electron microscope) that the bacteria had grown bonds with nearby iron minerals. He suspected that electrons were carried along these wires, and eventually figured out that Geobacter orchestrated chemical reactions in the mud, oxidizing organic compounds and transferring electrons to minerals. These reduced minerals then release phosphorus and other elements.
Like Nielsen, Lovely faced skepticism when he first described his electrical microbe. Today, however, he and others have registered nearly a dozen types of nanowire microbes, finding them in environments other than dirt. Many carry electrons to and from particles in the sediment. But some rely on other microbes to receive or store electrons. This biological partnership allows both microbes to "engage in new kinds of chemistry that no organism can do alone," says Victoria Orfan, a geobiologist at the California Institute of Technology. While cable bacteria solve their redox needs by being transported long distances into oxygenated mud, these microbes depend on each other's metabolism to meet their redox needs.
Some researchers still debate how bacterial nanowires conduct electrons. Lovley and his colleagues are convinced that the key are chains of proteins called pilins, which are made up of circular amino acids. When he and his colleagues reduced the amount of ringed amino acids in the pilin, the nanowires became less conductive. “It was really amazing,” says Lovely, because it is generally accepted that proteins are insulators. But others think that this question is far from being solved. Orphan, for example, says that although "there is overwhelming evidence … I still don't think [the conduction of the nanowire] is well understood."
What is clear is that electrical bacteria are everywhere. In 2014, for example, scientists discovered cable bacteria in three very different habitats in the North Sea: in a tidal salt swamp, in a seabed basin where oxygen levels drop to almost zero in some seasons, and in a flooded muddy plain near the sea. … shore. (They did not find them in a sandy area inhabited by worms that churn up sediments and disrupt cables.) Elsewhere, researchers have found DNA evidence of cable bacteria in deep, oxygen-poor ocean basins, hot spring areas, and cold conditions. spills, and mangroves and tidal banks in both temperate and subtropical regions.
Cable bacteria are also found in freshwater environments.After reading Nielsen's articles in 2010 and 2012, a team led by microbiologist Rainer Meckenstock re-examined sediment cores drilled during a groundwater contamination survey in Düsseldorf, Germany. “We found [the cable bacteria] exactly where we thought we would find them,” at depths where oxygen was depleted, recalls Mekenstock, who works at the University of Duisburg-Essen.
Nanowire bacteria are even more widespread. Researchers have found them in soils, rice fields, deep bowels and even sewage treatment plants, as well as in freshwater and marine sediments. They can exist wherever biofilms are formed, and the ubiquity of biofilms is further evidence of the great role these bacteria can play in nature.
The wide variety of electrical sludge bacteria also suggests that they play an important role in ecosystems. For example, by preventing the build-up of hydrogen sulfide, cable bacteria likely make dirt more habitable for other life forms. Meckenstock, Nielsen, and others have found them on or near the roots of seagrass and other aquatic plants that release oxygen, which bacteria probably use to break down hydrogen sulfide. This, in turn, protects the plants from the toxic gas. The partnership "seems very characteristic of aquatic plants," Meckenstock said.
Robert Aller, a marine biogeochemist at Stony Brook University, believes bacteria can also help many underwater invertebrates, including worms that build burrows that allow oxygenated water to enter the mud. He found cable bacteria sticking out from the sides of the worm tubes, presumably so they could use this oxygen to store electrons. In turn, these worms are protected from toxic hydrogen sulfide. “Bacteria make [the burrow] more livable,” says Aller, who described the links in a July 2019 article in Science Advances.
Microbes also alter the properties of dirt, says Saira Malkin, an ecologist at the University of Maryland's Center for Environmental Sciences. "They're especially effective … ecosystem engineers." Cable bacteria “grow like wildfire,” she says; On tidal oyster reefs, she found, One cubic centimeter of mud may contain 2,859 meters of cables that cement the particles in place, possibly making the sediment more resistant to marine organisms.
The bacteria also alters the chemical composition of the dirt, making layers closer to the surface more alkaline and deeper layers more acidic, Malkin found. Such pH gradients can affect “numerous geochemical cycles,” including those associated with arsenic, manganese and iron, she said, creating opportunities for other microbes.
Because vast swathes of the planet are covered in mud, the researchers say bacteria associated with cables and nanowires are likely to have an impact on the global climate. Nanowire bacteria, for example, can take electrons from organic materials like dead diatoms and then pass them on to other bacteria that produce methane, a powerful greenhouse gas. Under various circumstances, cable bacteria can reduce methane production.
In the coming years, “we will see widespread recognition of the importance of these microbes to the biosphere,” says Malkin. A little over ten years after Nielsen noticed the mysterious disappearance of hydrogen sulfide from the Aarhus mud, he says: "It is dizzying to think about what we are dealing with here."
Next up: a phone powered by microbial wires?
The pioneers of electrical microbes quickly thought about how to use these bacteria.“Now that we know that evolution has been able to create electrical wires, it would be a shame if we didn't use them,” says Lars Peter Nielsen, a microbiologist at the University of Aarhus.
One possible application is the detection and control of pollutants. Cable microbes seem to thrive in the presence of organic compounds like oil, and Nielsen and his team are testing the possibility that the abundance of cable bacteria signals the presence of undetected pollution in aquifers. The bacteria do not directly degrade the oil, but they can oxidize the sulfide produced by other oily bacteria. They can also help clean up; rainfall recovers faster from crude oil contamination when it is colonized by cable bacteria, another research group reported in January in the journal Water Research. In Spain, a third team is investigating whether nanowire bacteria can speed up the cleanup of polluted wetlands. And even before nanowire-based bacteria were electrical, they showed the promise of decontaminating nuclear waste and aquifers contaminated with aromatic hydrocarbons such as benzene or naphthalene.
Electrical bacteria can also give rise to new technologies. They can be genetically modified to alter their nanowires, which can then be cut off to form the backbone of sensitive wearable sensors, according to Derek Lovley, a microbiologist at the University of Massachusetts (UMass), Amherst. "We can design nanowires and adapt them to specifically bind compounds of interest." For example, in the May 11 Lovely issue of Nano Research, UMass engineer Jun Yao and their colleagues described a nanowire-based sensor that detects ammonia in concentrations needed for agricultural, industrial, environmental, and biomedical applications.
Created as a film, nanowires can generate electricity from moisture in the air. Researchers believe that the film generates energy when a moisture gradient occurs between the top and bottom edges of the film. (The top edge is more susceptible to moisture.) As the hydrogen and oxygen atoms of the water separate due to the gradient, charge is generated and electrons flow. Yao and his team reported in Nature on Feb.17 that such a film could create enough energy to light a light-emitting diode, and 17 such devices connected together could power a mobile phone. The approach is “a revolutionary technology for generating renewable, clean and cheap energy,” says Qu Lianti, a materials scientist at Tsinghua University. (Others are more cautious, noting that past attempts to squeeze energy out of moisture using graphene or polymers have been unsuccessful.)
Ultimately, the researchers hope to harness the electrical abilities of bacteria without having to deal with picky microbes. Catch, for example, persuaded the common laboratory and industrial bacterium Escherichia coli to make nanowires. This should make it easier for researchers to mass-produce the structures and study their practical applications.