In New York State, when the snow melts, Oneida Lake starts collecting manganese. Combined with oxygen from the air, it makes manganese oxide which sinks into the lake bed. But, as Corey S. Powell reports for Popular Science, scientists didn’t find the compound at levels they’d expect, and the mystery of the missing manganese oxide set Kenneth Nealson, a microbiologist, searching for a microbe that seemed like it shouldn’t exist.
It took him a few years, but he found it—Shewanella oneidensis, a bacterium that lives off of a poisonous heavy metal, manganese.
Powell writes exactly why Shewanella is so odd:
For most living, air-breathing creatures, Nealson says, “The glucose that we eat supplies the electrons, the oxygen we breathe receives the electrons, and that electron flow is what runs our bodies.” That’s basic metabolism. The challenge for every organism is finding both sources of electrons and places to discard them in order to complete the circuit. Shewanella consumes electrons from carbohydrates, but it sheds them in an unusual way: “It swims up to the metal oxide and respires it.” Nealson says. “We call this ‘breathing rocks.’ ”
The bacterium grows special wires out of its membrane that transport electrons from inside the cell and deposit them on the heavy metal. Maganese oxide works, but so do other heavy metals like lead. Other discoveries revealed bacteria that are doing the reverse—they scavenge electrons from metal and minerals. The electron exchange completes that circuit. The result is life that eats and breathes electricity.
Moh El-Naggar, another researcher at USC, has produced videos that show these bacteria in action, growing those wire-like probes.
In 1988, when Nealson published his findings on Shewanella, it defied long-held assumptions about biology, to paraphrase Rebecca Fairley Raney’s profile of Nealson at AAAS.org. But now we know that Shewanella and other microbes are important drivers in the way Earth cycles metals.
Still, it gets weirder. One of Nealson’s graduate students, Annette Rowe, has found six new bacterial strains dredged from the ocean floor that don’t need a source of carbon at all, reports Powell. They can live off of electricity alone.
All studies of life at the extremes on Earth show scientists what life might look like on other planets. Powell writes:
Scrounging for electrons and sprouting nanowires are strategies for surviving when there is not enough food to do much growing and competing—just enough to help an organism hunker down and keep the flame of life lit. Such conditions are common in deep ocean sediments and far underground. If life exists on Mars and other worlds (Europa? Titan?), there’s a good chance that it, too, is huddled in resource-constrained settings far beneath the surface.
Future missions to detect traces of life on other planets may take the electron-gobbling bacteria into account. Nealson points out that on Earth, there is a gradient of electrical potential in the ground that decreases with depth. As you reach deeper, only electrons are available for food, so bacteria that live there adapt to eat electricity — thus setting up the gradient. To see that sign of life, all future missions would need to do is stick probes in the ground and measure it.