Extracellular electron transfer is a biological process allowing certain microorganisms to transfer electrons from their metabolism to substances outside their cellular boundaries. This capability is similar to a biological power cord extending from the cell into the external world. It allows them to “breathe” solid materials that they cannot physically engulf.
The Biological Need for External Respiration
All living organisms perform respiration to survive, a process that breaks down nutrients for energy and creates a flow of electrons that must be discarded. In many animals, these electrons are transferred to oxygen. However, environments like deep-sea sediments and waterlogged soils are devoid of oxygen.
Microbes in these anaerobic conditions require alternative molecules to accept electrons from their metabolism. Some use soluble acceptors like nitrate, but when these are scarce, certain microbes use solid materials like iron or manganese minerals. This adaptation is known as extracellular electron transfer (EET), a survival strategy that allows them to “breathe” minerals.
Pathways for Electron Export
Microorganisms use several strategies to move electrons to an external acceptor. One direct method involves proteins called outer-membrane cytochromes, which are embedded in the bacterium’s outer membrane. These specialized proteins can make physical contact with a mineral surface to facilitate a direct transfer of electrons, similar to plugging an appliance directly into a socket.
Another strategy uses electron shuttles, which are small organic molecules that act as intermediaries. Microbes secrete these molecules, which function like delivery trucks; they pick up electrons from the cell, travel to a distant acceptor, and deposit the electrons. This mediated transfer allows bacteria to interact with acceptors that are not in their immediate vicinity.
A third mechanism is the use of conductive protein filaments. Some bacteria produce networks of pili, which are protein-based appendages that function as biological “nanowires.” These filaments extend many times the cell’s length, conducting electrons over significant distances and creating a microscopic living electrical grid.
Electroactive Microorganisms
The bacterium Geobacter is an example of an electroactive microbe, often found in anaerobic soils and sediments. It is proficient at EET, using both direct contact via outer-membrane cytochromes and conductive nanowires to respire minerals. Its ability to form thick, electrically conductive biofilms makes it a powerhouse in environments where it is present.
Another electroactive microorganism is Shewanella, found in a wide range of aquatic habitats and sediments. Shewanella species are metabolically versatile and can perform direct electron transfer. They are especially noted for using the electron shuttle strategy, secreting small molecules to transfer electrons to minerals.
Environmental and Technological Implications
Electroactive microorganisms have profound environmental effects, driving global biogeochemical cycles for metals like iron and manganese. By reducing these metals—donating electrons to change their chemical state—these microbes alter the mineralogy of soils and sediments. This process influences nutrient availability and aids the breakdown of organic matter in oxygen-free zones.
Scientists are exploring ways to harness these microbes for technological purposes. One developed application is in microbial fuel cells (MFCs), where bacteria are grown on an electrode. As they break down organic waste from sources like wastewater, they transfer electrons to the electrode and generate a measurable electrical current. Other applications include bioremediation, where microbes are used to clean up toxic pollutants like uranium by changing their chemical state to a less soluble form.