spiritvast.blogg.se

First, Nils Risgaard-Petersen on Nielsen's team had to rule out a simpler possibility: that metallic particles in the sediment were shuttling electrons to the surface and causing the oxidation. One night, waking from his sleep, Nielsen came up with a bizarre explanation: What if bacteria buried in the mud were completing the redox reaction by somehow bypassing the oxygen-poor layers? What if, instead, they used the ample supplies of hydrogen sulfide as an electron donor, then shuttled the electrons upward to the oxygen-rich surface? There, the oxidation process would produce rust if iron was present.įinding what was carrying these electrons proved complicated. Moreover, a rusty hue appeared on the mud's surface, indicating that an iron oxide had formed. Yet, in Nielsen's laboratory beakers, the hydrogen sulfide was disappearing anyway. Bacteria produce the compound in mud by breaking down plant debris and other organic material in deeper sediments, hydrogen sulfide builds up because there is little oxygen to help other bacteria break it down. The vanishing hydrogen sulfide was key to proving it. 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. In eukaryotic cells, including our own, such "redox" reactions take place on the inner membrane of the mitochondria, and the distances involved are tiny-just micrometers. Energy harvested from these reactions drives the other processes of life. Most cells thrive by robbing electrons from one molecule, a process called oxidation, and donating them to another molecule, usually oxygen-so-called reduction. "We are seeing way more interactions within microbes and between microbes being done by electricity," Meysman says.

Scientists are also pursuing practical applications, exploring the potential of cable and nanowire bacteria to battle pollution and power electronic devices ( see sidebar below). 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. 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. These nanowire microbes live seemingly everywhere-including in the human mouth. 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. 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.īut the more researchers have looked for "electrified" mud, the more they have found it, in both saltwater and fresh. 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. When Nielsen first described the discovery in 2009, colleagues were skeptical. But the cables, by linking the microbes to sediments richer in oxygen, allow them to carry out the reaction long distance. Its absence would normally keep bacteria from metabolizing compounds, such as hydrogen sulfide, as food. 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. But the cause turned out to be far stranger: bacteria that join cells end to end to build electrical cables able to carry current up to 5 centimeters through mud. The first explanation, he says, was that the sensors were wrong. Given what scientists knew about the biogeochemistry of mud, recalls Nielsen, who works at Aarhus University, "This didn't make sense at all." Eventually, the microsensors indicated that all of the compound had disappeared. But 30 days later, one band of mud had become paler, suggesting some hydrogen sulphide had gone missing.

At the start of the experiment, the muck was saturated with hydrogen sulfide-the source of the sediment's stink and color. 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. For Lars Peter Nielsen, it all began with the mysterious disappearance of hydrogen sulfide.