All life on Earth has to respire. For us this means inhaling oxygen to fuel our metabolism and exhaling waste gases like carbon dioxide. When we do this, we move the energy from the food we eat to the oxygen we breathe, creating a flow of energy that our cells use for growth. This is called respiration. However, not all life on Earth uses oxygen for this kind of respiration. These organisms have to find other ways to create electron flows to support their growth.
One shocking method used by some bacteria is to produce biological nanowires. These nanowires act like wiring in your house and allow for electrons to pass from cells like Geobacter bacteria directly onto metals. This process is similar to plugging your phone into the wall to charge. Electrons flow from the source to your device. When the biological nanowires touch a metal, a circuit is made, and Geobacter can then metabolize the foods they eat and grow even without oxygen. These bacteria can even use this amazing ability to get energy from uranium in nuclear wastewater, and clean the water as a result. By making an electrical circuit with uranium in contaminated water, Geobacter is able to mineralize the hazardous material and cleanse the water.
Scientists have been interested in further understanding biological nanowires and how they can be used to both clean metal contamination and recycle used metals through mineralization. This process of mineralization takes tiny atoms of metals and combines them together into a large clump. In this way, Geobacter can collect metals in the environment in large particles that scientists can then harvest for different uses such as producing batteries.
To understand the structure and flow of electrons in these nanowires, researchers from Michigan State University found a way to harvest the protein nanowires of Geobacter by genetically engineering them to hook up to an electrode. They then asked the question, “What metals can complete an electrical circuit by accepting electrons traveling through the nanowires from the electrode?” To answer this, the team measured the changes in electrical current as they slowly adjusted the voltage of the electrode when iron contaminated water was added to the system. By comparing the voltages used with the current produced they were able to identify a pattern of iron capture and release. This meant that metals were able to mineralize to the nanowires when electrons flowed out of the electrode and then float away when electrons were pulled back to the electrode.
Now that the scientists knew that their system worked like the living bacteria, they wanted to know if this could be used with other metals. The team chose cobalt for the second half of the study as it is a very expensive metal used in rechargeable batteries in addition to being toxic to the environment. Cobalt mining has been so feverish and so bad for the people who sell to large battery companies that it has been referred as the blood-diamond of batteries. A biological method to recycle cobalt would ease some of the pressure and eliminate some of the human costs associated with its mining.
With the expense of cobalt in mind, the Michigan State team made cobalt-contaminated water solutions and poured it onto their nanowire electrodes. What they saw wasn’t the clean flow of current like with the earlier iron experiment but something much more complicated. The electron flow seemed to increase and decrease in little peaks before finally peaking at a voltage almost too high for the electrode to handle. Using previous studies and insight into the physical properties of cobalt the team realized that these little peaks were a sign of growing nanoparticles. This meant that when one cobalt atom mineralized onto a nanowire another one would attach on top of it, and then another, creating a large metal particle on the nanowire.
These exciting results have set off the next stage of research to understand how this process can happen with live cells and how cobalt mineralization can be bumped up to the industrial scale. Perhaps one day soon, you may be reading this on a phone or laptop using cobalt batteries recycled by microbes for a cheaper price to you, to the miners, and to the environment.