Electricity is usually thought of as a very human thing. Animals and plants in nature may be capable of extraordinary feats of engineering, but there are still a few developments that humans claim as uniquely their own; fire, the wheel, and electricity.
For bacteria, on the other hand, the ability to push electrons down a small cable is just one more way to live, breath and communicate in a world full of niches to exploit.
Respiration is the process by which all living organisms gain energy. A food source such as sugar is broken down within the organism to produce electrons, which pass through a series of reactions before being handed over to a final electron acceptor, which in the case of almost all multicellular organisms is oxygen. Even plants use oxygen as a final electron acceptor to make energy, the carbon dioxide they take in is used in a separate process called photosynthesis which produces sugars.
Oxygen made its major debut into the atmosphere about 2.4 billion years ago, so before then living organisms had to rely on other electron acceptors to make energy, such as sulphur or nitrogen. Indeed many bacteria nowadays still use sulphur or nitrogen sources, instead of all this new-fangled oxygen stuff.
And some bacteria are even stranger. When Geobacter species that usually live in the soil were grown in the absence of oxygen, they started growing little hairs, or pili, extending from the cell surface. These little hairs would latch onto any available source of iron (FeIII for the chemically minded) and dump electrons onto it. In a certain sense, they were breathing into the rock. Reference 1, below, has some brilliant electron microscope pictures for those who can get through the pay-wall. For those that can’t here’s a summary:
What this means is that you have a significant flow of electrons through a small cable and there’s a word for that: electric current. The pili act as nanowires, transferring electrons into the surrounding iron. This fits in with historical detail as well, before oxygen joined the atmosphere there was a lot of soluble iron floating around that electrons could be donated too. Once oxygen erupted onto the world it started reacting with the iron and everything rusted, which lead to all the iron deposits being laid down that would later be dug up in the industrial revolution.
Talking with wires
It gets better. These little bacterial wire cables don’t just attach to the surrounding rock, they also attach to each other, producing a flowing electrical charge between bacteria. Work done on marine bacteria that live in the mud at the bottom of the sea (reference 2) showed that an electrical current was being propagated through the layers of mud. Bacteria at the bottom layers were creating energy, and then shuttling their electrons up to the top layer, where the electrons were released to react with oxygen (as there isn’t much oxygen in mud). Electrons were found to be travelling across a distance of 12 millimetres, which isn’t much to humans, but is 10 000 body lengths to bacteria. Although researchers haven't yet managed to grab a picture of these little cables, the presence and use of nanowires in other bacterial species makes this the most likely reason to explain the electron flow through the system.
The electrical circuits in the marine-mud bacteria are formed purely for the purposes of energy creation, but it’s interesting to speculate how much this system might play a part in communication. And in a way, even the energy transfer pili are an important part of communication. A bacteria passing electrons onto you ('you' being a fellow bacteria) means, “There’s no oxygen (or any electron acceptor) here, pass it on” whereas a bacteria that accepts electrons from your cautiously searching pili is saying “I can sense the oxygen” or “I’m next to a rock!” Communicating through an electrical current is relatively quick as well, and unlike releasing chemical signals it combines communication with the vital process of respiration.
And as for fire and the wheel? If bacteria don't already have them, I can't help thinking it's only a matter of time...
Reguera, G., McCarthy, K., Mehta, T., Nicoll, J., Tuominen, M., & Lovley, D. (2005). Extracellular electron transfer via microbial nanowires Nature, 435 (7045), 1098-1101 DOI: 10.1038/nature03661
Nielsen, L., Risgaard-Petersen, N., Fossing, H., Christensen, P., & Sayama, M. (2010). Electric currents couple spatially separated biogeochemical processes in marine sediment Nature, 463 (7284), 1071-1074 DOI: 10.1038/nature08790
Nealson, K. (2010). Geomicrobiology: Sediment reactions defy dogma Nature, 463 (7284), 1033-1034 DOI: 10.1038/4631033a