From waste to electricity: Incredible applications of electric bacteria

An important breakthrough in bioengineering can turn what is now only a source of pollution and environmental hazard into a renewable source of energy by using bacteria to digest organic compounds through the cellular respiration progress to power a galvanic cell, thus generating a flow of electrons, and there we have electric energy.

There is quite a lot of terms to explain here: First, a galvanic cell is an electrochemical cell that turns the energy liberated by spontaneous reactions that happen inside the cell into electricity. A spontaneous reaction is a reaction that, as the term suggests, happens spontaneously and liberates energy during the process. What a galvanic cell does is to convert the chemical energy of the compounds prior to the reaction into electrical energy. A galvanic cell has an anode that acts as a receptor for electrons, from there starts a flow of current which travels all the way to the cathode, which on the other hand, attracts positive ions.

What bioengineers found is that there are some species of bacteria that instead of using ATP to power the various cellular processes, use electricity. This means that any flow of electrons flowing through the cell will power the organic processes happening inside the cell.

The cellular respiration process of those species of bacteria differs from the classical cellular respiration in some ways: Glucose (in this case the fuel of the reaction) reacts with oxygen turning into carbon dioxide as it normally happens, but hydrogen does not react with oxygen to form water right away. Instead, it separates into H+ (single proton nucleus) and free electrons, this creates the difference of potential that powers all the cellular functions.

What is interesting for our research though, is that those types of bacteria have specific proteins on their membrane, called cytochromes; those proteins are responsible for transporting electrons in and outside the cellular membrane, and this is how they generate electricity: They isolate electrons from their compounds and then pump them out in exchange for energy. What happens in nature is that the electrons are ceded to oxygen, which is very electronegative and thus attracts electrons, and this is the passage which gives power as output. The anode of a galvanic cell works just like oxygen, as it attracts electrons and makes them flow towards the cathode, where they meet oxygen and protons reacting and turning into water.

What researchers found was that by disposing a large number of bacteria on the anode of a galvanic cell and adding glucose around the anode (glucose is the primary source of energy for those bacteria), was that bacteria were expelling electrons from their cytochromes in order to maintain the difference of potential between the inside and the outside environment of the cell constant. The reaction taking place was the degradation of organic compounds into carbon dioxide, hydrogen nuclei, and free electrons. The free electrons were collected by the anode while free hydrogen nuclei (protons) were heading towards the cathode passing through a semipermeable membrane, this creates a difference of potential and thus electric energy.

The galvanic cell is structured in the following way:

There are two small pools of water, separated by a selective membrane which can only be trespassed by H+ ions (hydrogen nuclei), in the first pool the anode, which is made from conductive and corrosion-resistant material, is partially submerged in the water, and bacteria stick to it so that their cytochromes are directly connected to its surface.

The cathode, on the other hand, is placed inside the pool on the other side of the selective membrane, it attracts hydrogen nuclei, which creates a difference of potential between the two pools that sets the flow of electrons in motion.

This is a labeled diagram of how the system works:

If the circuit is closed, electrons would start to flow towards the cathode and combine with the hydrogen nuclei, reacting with oxygen, creating water as output. Thus the cell turns organic compounds and oxygen into water, carbon dioxide, and electricity using metabolic processes. Quite surprising as a result, but there’s more.

For the experiment, sugar was used to feed the bacteria and induce them into producing electricity, but as many experiments already proved, bacteria’s metabolism can be manipulated, and through genetic engineering, it is possible to make them digest any organic compound. If those bacteria were given the capability to digest the organic compounds present in sewage and wastewater, those could be used to be turned into electricity at the small cost of moderate carbon dioxide emissions.

This means that a product with no commercial value (actually something that people and industries pay to throw away) can be used to make a profit. With the current technology, the efficiency of this method allows producing 1kWh of energy from 1kg of sewage, which corresponds to approximately 0,16€ worth of energy. Not bad for some liquid waste.

Sources: MIT news, Ted Kinsman Science Source