Some microbes, simple as they may be, have an ability to gather energy from extreme sources like sulfur, formic acid, minerals, and... electricity? Yes, electricity. A team led by Peter Girguis from Harvard has discovered how a certain bacteria gets its energetic needs from electrons pulled from the environment. The results of this study were published in Nature Communications.

Rhodopseudomonas palustris are gram-negative bacteria that has remarkable dexterity in obtaining energy and is able to take cues from the environment to employ photoautotropic, photoheterotrophic, chemoautotrophic, or chemoheterotrophic metabolism. This flexibility has baffled microbiologists for some time. Girguis’s team focused on the phototrophic aspects of its metabolism in order to begin teasing out some answers.

Electrons are essentially the energy currency for most forms of life and they are exchanged through oxidation-reduction  reactions. R. palustris TIE-1 is somewhat different in that it is able to take electrons from materials in the solid phase, while most others require electron donors and acceptors to be in solution. One of these metabolic mechanisms allows the bacteria to obtain energy through extracellular electron transfer, though the cellular processes that accomplish this have been a mystery until now. 

These bacteria traditionally get electrons from iron, though Girguis was able to show that it wasn’t necessary—a critical breakthrough in understanding R. palustris’s metabolism. When the bacteria were exposed directly to an electrode, they were readily able to uptake the electrons and convert them into energy using carbon dioxide as an electron acceptor. Subsequent experiments showed that a certain gene is responsible for the majority of electron uptake. Without it, the microbe loses 66% of its ability to uptake free electrons. 

RuBisCo is the protein that is used to convert carbon dioxide into the energy-rich nutrients that the bacteria need. The gene that produces the protein is activated by sunlight, as is much of the ability to take in electrons from the environment. However, the ferrous materials used by the bacteria are below the ground where they would not have access to sunlight. The researchers found that while the bacteria stay on the surface, they are able to draw in electrons from the sediment beneath them and get the best of both worlds.

While it has been suggested that these bacteria could be used to create a functional battery, Girguis is not so sure it would be an efficient fuel source. He does note that there is a large opportunity to use them in the pharmaceutical industry where they could be altered to “produce something that is of interest” to researchers and merely need to be fed with electricity.