Electrical Bacteria as Ecosystem Engineers

Electric bacteria join cells end to end to build electrical cables able to carry current up to 5 centimetres through mud. The adaptation, never seen before in a microbe, allows these so-called cable bacteria to overcome a major challenge facing many organisms that live in mud: a lack of oxygen. Its absence would normally keep bacteria from metabolizing compounds, such as hydrogen sulfide, as food. But the cables, by linking the microbes to sediments richer in oxygen, allow them to carry out the reaction long distance…

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… Scientists are pursuing practical applications, exploring the potential of cable and nanowire bacteria to battle pollution and power electronic devices…

The Center for Electromicrobiology was established in 2017 by the Danish government. Among the challenges the center is tackling is mass producing the microbes in culture…Cultured bacteria would also make it easier to isolate the cable’s wires and test potential applications for bioremediation and biotechnology…

Electrical bacteria are everywhere. In 2014, for example, scientists found cable bacteria in three very different habitats in the North Sea: an intertidal salt marsh, a seafloor basin where oxygen levels drop to near zero at some times of the year, and a submerged mud plain just off the coast…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.

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.

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.

Excerpts from Elizabeth Pennisi, The Mud is Electric: Bacteria that Conduct Electricity are transforming the way we see sediments, Science, Aug. 21, 2020, at 902

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