At least two kinds of bacteria have evolved electric solutions to gaining energy. These microbes, first discovered in mud, separate the reduction and oxidation reactions that release the energy needed to fuel life. Learn more: ScienceMagArchives
For Lars Peter Nielsen, it all began with the mysterious disappearance of hydrogen sulfide. The microbiologist had collected black, stinky mud from the bottom of Aarhus Harbor in Denmark, dropped it into big glass beakers, and inserted custom microsensors that detected changes in the mud's chemistry. At the start of the experiment, the muck was saturated with hydrogen sulfide—the source of the sediment's stink and color.
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.
To see whether some kind of cable or wire was ferrying electrons, the researchers next used a tungsten wire to make a horizontal slice through a column of mud. The current flickered out, as if a wire had been snipped. Other work narrowed down the conductor's size, suggesting it had to be at least 1 micrometer in diameter."That's the conventional size for bacteria," Nielsen says.
As with cable bacteria, some puzzling sediment chemistry led to the discovery of nanowire microbes. In 1987, microbiologist Derek Lovley, now at the University of Massachusetts, Amherst, was trying to understand how phosphate from fertilizer runoff—a nutrient that promotes algal blooms—is released from sediments beneath the Potomac River in Washington, D.C. He suspected microbes were at work and began to sieve them from the mud.
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.
With vast swaths of the planet covered by mud, cable and nanowire bacteria are likely having an influence on global climate, researchers say. Nanowire bacteria, for example, can strip electrons from organic materials, such as dead diatoms, then shuttle them to other bacteria that produce methane—a potent greenhouse gas. Under different circumstances, cable bacteria can reduce methane production.
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