Life is powered by the shuffling of electrons. When organisms break down a food source like a sugar, they're really extracting high-energy electrons, which they shuffle down through intermediate proteins before they end up in a final electron acceptor. For most of the life we're familiar with, that acceptor is oxygen. But for various microbes that thrive in the absence of oxygen, a variety of other chemicals are used.

A few bacteria don't even use a chemical receptor at all, instead transferring their excess electrons to metals in their environment (these can form the basis of microbial fuel cells). Now, researchers have witnessed the evolution of a bacteria that transfers its electrons to another bacteria, which goes on to put them to further use.

There are a number of symbiotic relationships like this among the microbes, some of which can metabolize an organic molecule, while others can transfer them to a low-energy chemical. Typically, the bugs exchange an organic chemical or hydrogen to a symbiotic bacteria that extracts further energy from it.

In the current paper, the authors forced two different species of bacteria to live in an anaerobic environment, and provided them with ethanol as food. Initially, they grew very poorly. After several transfers, however, the rate of growth improved, and small, colored nodules began to appear in the culture, which contained a mix of the two types of bacteria. The authors checked a number of the chemicals that are typically used to transfer electrons in these symbiotic cultures, but saw no evidence of their being used.

To figure out what was going on, they did whole-genome sequencing, and found only one change: a single base missing in the gene for a protein that regulates RNA production. Making a similar mutation in another strain also allowed those bacteria to form quick-growing nodules. The mutation appears to cause proteins involved in electron transfer to be expressed at increased levels. These proteins end up on pilli, arm-like structures that extend out from the bacteria.

As a result, the authors conclude that one of the two species of bacteria has evolved the ability to transfer electrons directly to their neighboring species, allowing both to get more energy out of the limited food available.

Science, 2010. DOI: 10.1126/science.1196526 (About DOIs).