?Electric mud? teems with new, mysterious bacteria

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.Threads of electron-conducting cable bacteria can stretch up to 5 centimeters from deeper mud, where oxygen is scarce and hydrogen sulfide is common, to surface layers richer in oxygen.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. That is why so many researchers were skeptical of Nielsen’s claim that cable bacteria were moving electrons across a span of mud equivalent to the width of a golf ball.The vanishing hydrogen sulfide was key to proving it. “That’s the conventional size for bacteria,” Nielsen says.Lars Peter Nielsen discovered cable bacteria in mud from the local harbor.Ultimately, electron micrographs revealed a likely candidate: long, thin, bacterial filaments that appeared in the layer of glass beads inserted in the beakers filled with the Aarhus Harbor mud. Cultured bacteria would also make it easier to isolate the cable’s wires and test potential applications for bioremediation and biotechnology.Even as researchers puzzle over cable bacteria, others have been studying another big player in electric mud: nanowire bacteria, which instead of stacking cells into cables sprout protein wires spanning 20 to 50 nanometers from each cell.As with cable bacteria, some puzzling sediment chemistry led to the discovery of nanowire microbes. Whereas cable bacteria solve their redox requirements by long-distance transport to oxygenated mud, these microbes depend on each other’s metabolisms to satisfy their redox needs.Some researchers are still debating how the bacterial nanowires conduct electrons. (They didn’t find them in a sandy area populated by worms that stir up the sediments and disrupt the cables.) Elsewhere, researchers have found DNA evidence of cable bacteria in deep, oxygen-poor ocean basins, hydrothermal vent areas, and cold seeps, as well as mangrove and tidal flats in both temperate and subtropical regions.Derek Lovley detects nanowire bacteria by looking for electrical currents in mud samples.Cable bacteria have also shown up in freshwater environments. “The bacteria make [the burrow] more livable,” says Aller, who described these connections in a July 2019 paper in Science Advances.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.The bacteria also alter the mud’s chemistry, making layers closer to the surface more alkaline and deeper layers more acidic, Malkin has found. Such pH gradients can affect “numerous geochemical cycles,” she says, including those involving arsenic, manganese, and iron, creating opportunities for other microbes.With vast swaths of the planet covered by mud, cable and nanowire bacteria are likely having an influence on global climate, researchers say.