In the darkness found a mile underground, biophysicist Moh El-Naggar and his graduate student Yamini Jangir traversed a mine’s network of passages in search of a rusty metal pipe. They siphoned some of the pipe’s ancient water, directed it into a vessel, and inserted a variety of electrodes. They hoped the current would lure their prey, a little-studied microbe that can live off pure electricity.
Carbon-fiber electrodes (gray) to lure electricity-eating microbes (orange). These microbes grow incredibly slowly, so attracting them can take time. The researchers left this electrode underground for five months.
As the tally of electron eaters grows, scientists are beginning to figure out just how they work. How does a microbe consume electrons out of a piece of metal, or deposit them back into the environment when it is finished with them? A study published last year revealed the way that one of these microbes catches and consumes its electrical prey. And a not-yet-published work suggests that some metal eaters transport electrons directly across their membranes—a feat once thought impossible.
Though eating electricity seems bizarre, the flow of current is central to life. All organisms require a source of electrons to make and store energy. They must also be able to shed electrons once their job is done. In describing this bare-bones view of life, Nobel Prize-winning physiologist Albert Szent-Györgyi once said, “Life is nothing but an electron looking for a place to rest.” Humans and many other organisms get electrons from food and expel them with our breath. The microbes that El-Naggar and others are trying to grow belong to a group called lithoautotrophs, or rock eaters, which harvest energy from inorganic substances such as iron, sulfur, or manganese. Under the right conditions, they can survive solely on electricity.