Elle Bethune discusses how bacteria can power batteries, and how these can be used for clean energy on Earth and finding life in space.
Clean energy has become a hot topic in science, and for good reason. While most of us have heard of wind, solar or even geothermal energy as alternatives to fossil fuels, a less well-known but equally as important resource are microbes.
Bacteria have a massively diverse range of applications in biotechnology, so perhaps it is unsurprising that they can also be used to generate electricity. Specifically, they can be used to construct microbial fuel cells, which are exactly what they sound like: batteries that contain bacteria instead of chemicals. Microbial fuel cells, or MFCs, are very similar to the typical galvanic electrochemical cells that are often used to make batteries, except instead of using a chemical reaction to power the electrical circuit, they harness the respiratory power of microbes.
So first of all, what do we mean by microbial respiration? Every form of life (that we know of) has to respire in order to convert environmental energy to biological energy that can be used for growth. Humans, and other aerobic organisms, do this by converting oxygen into carbon dioxide and water, a reaction that releases energy to be used by our cells. Many aerobic bacteria use a very similar reaction, however there are other types of bacterial respiration involving different molecules. The common factor is that all of these respiratory pathways involve the release of energy through the transfer of electrons from one compound to another. The compound that loses electrons becomes oxidised, and the one that gains them is said to become reduced. It is these reactions taking place in the bacteria that power MFCs.
In an MFC there are two compartments which contain electrodes. One of these electrodes is called the anode, and the other is the cathode. The anode compartment is where the microorganisms are. As these microbes respire, they deposit electrons onto the anode, which travel across a wire to the cathode. At the cathode, there must be a compound present which is capable of accepting these electrons to keep the circuit flowing. This compound is known as the oxidising agent because although it becomes reduced, it does this by oxidising something else (in this case, the cathode). In many MFCs this oxidising agent is molecular oxygen, which accepts both electrons and free protons to form water. It is the flow of electrons from the anode to the cathode which produces electricity, so as long as the bacteria are growing and there is an electron acceptor present at the cathode, the cell will produce power.
Bacteria are an incredibly diverse class of organisms, and as such there are many different types of respiratory pathways, so some types of bacteria work better in MFCs than others. For example, the species Escherichia coli is facultative anaerobic, which means it prefers to use oxygen as its electron acceptor for respiration, but in an anoxic environment it can use another compound as its terminal electron acceptor instead. This means it can continue to survive without oxygen but will not generate as much energy to grow. If something like E. coli is used in a MFC, a compound called an electron mediator needs to be added. This mediator acts as a terminal electron acceptor for the anaerobic respiration reaction, then diffuses across the cell membrane of the bacteria to deposit those electrons onto the anode. This extra step means that the power output of an E. coli powered MFC is much lower than a species that doesn’t require a mediator. An example of such a species is Shewanella oneidensis. This species has a different respiratory pathway which allows it to use an external electron acceptor instead of an internal one. This means that it can deposit its electrons directly onto the anode without an intermediate, so its power output is much higher. The upshot of this is that the choice of bacterial species to use in a MFC is an important consideration.
However, energy generation is not the only application of MFCs and therefore choosing the species with the highest energy output is not always the primary concern. For example, MFCs can also be used to produce useful compounds or eliminate unwanted waste products through the reduction reaction at the cathode. The choice of oxidising agent to supply at the cathode is therefore important for MFCs that are used in industrial processes. MFCs are already being implemented across different industries, such as in the processing and treatment of agricultural, industrial and municipal wastewater, providing a cleaner and more sustainable alternative to older techniques.
Another application for MFCs which has become of great interest is in the development of biosensors, which are designed to detect the presence of biological materials, such as contaminants in water or glucose in blood. MFCs are predicted by some to become the next generation of biosensing technology, as they have already been implemented in the detection of biochemical oxygen demand and toxicity of pollutants in the environment. There is even some research to suggest they could be used to detect extra-terrestrial life on distant rocky planets by showing an increase in power output and density in response to the presence of microbial respiration in a sample. Although this is not currently implemented on any planned space missions, it provides a good example of how diverse the functions of MFCs can be and how much opportunity there is for the future development of this fascinating technology.
Written by Elle Bethune and edited by Ailie McWhinnie.
Elle’s thoughts… I predict that we will hear a lot more about microbial fuel cells in the coming years as more industries and fields of research find ways of exploiting their huge potential, whether that be for clean energy generation or detecting life on distant planets.
Elle is an Astrobiology PhD student studying how anaerobic microorganisms grow on extra-terrestrial organic compounds from meteorites. Find her on LinkedIn @Elle Bethune.
