In school we are taught that in order for life to exist, respiration must occur: that is, the taking of sugars like glucose and combining it with oxygen to give you carbon dioxide, water, and a small amount of energy. This energy is used to fuel metabolic processes in all life forms – boiling down to the ability to survive and reproduce. But there are exceptions, such as the many types of “electric bacteria” that use energy in the form of electricity.
I call it an exception but it’s not; the bacteria still respire but in a different way. The energy we accumulate from respiration is ultimately from the flow of electrons (and protons). Electrons are taken from the simple sugars we get from food and are passed onto Oxygen, with two Hydrogen ions, causing the formation of water. Oxygen in this type of respiration is called the final electron acceptor. It’s this flow of electrons to generate energy (in the form of the molecule ATP) that is the basis of all life. What’s different in these special bacteria is that instead of using Oxygen as the final electron acceptor, they can use metals. And instead of it all occurring inside the (micro)organism, it occurs outside.
They have the ability to obtain electrons extracellularly (outside the cell) from organic compounds, and these electrons are transported into the cell through bacterial nanowires – extensions of the cell membrane that contain a tonne of proteins that aid the movement of electrons. The electrons are then released from the bacteria to electron acceptors such as metal oxides. These bacteria are also more common than you might have thought, we already know about at least two genera: Geobacter and Shewanella. With the ability to induce a flow of electrons like this, it enables the bacteria to carry a current, opening the possibility for a wide range of uses.
Microbial Fuel Cells
A fuel cell converts chemical energy into electrical energy, the most common in recent years being the Hydrogen fuel cell. A microbial fuel cell works by manipulating bacteria such as Geobacter and Shewanella to drive an electric current. The fuel cell is divided into two chambers – where the two electrodes are separated. The bacterium is placed in the chamber with the anode along with fuel, i.e. food for the bacteria. The bacteria is in an anaerobic atmosphere so the only electron acceptor available is the anode itself. Electrons, products of respiration, travel from the anode to the cathode, in the next chamber; it is this movement that generates electricity. The next chamber is labelled the aerobic chamber as Oxygen is allowed to move in – so here Oxygen gains electrons from the cathode and Hydrogen ions (also products of respiration) to form water.
Although this hasn’t been put into major practice (mainly an engineering issue rather than an issue with the bacteria), many researchers in microbial fuel cells are optimistic about this new form of renewable energy. NASA researchers are even attempting to build a microbial fuel cell that can convert human faeces to electricity; the current procedure having the waste of astronauts shipped back to Earth.
Questioning what life really is
The discovery and further research into these electric bacteria has also made scientists question the fundamentals of life and what its bare minimum really is. This is a particularly interesting point when questioning whether life exists on other planets. Using electrodes to detect if these types of bacteria are present deep in extraterrestrial soil gives us, yet again, another method we could use for testing for life.
Bioremediation is the use of microbes to clean groundwater and soil. There are some Geobacter species that can remove uranium from contaminated groundwater and soil. Since Uranium is radioactive, large consumption of it must be avoided. The bacteria itself reduces (adds electrons to) Uranium(VI), which is soluble in water, to Uranium(IV), which is insoluble. This means Uranium can be separated from the water supply simply through filtration.
As well as all that, we’ve also found that they can use their bacterial nanowires to interlink and join onto neighbouring cells. Here, electrons can be moved several centimeters (which is a lot considering a single bacterium is about three to five micrometers long). This means that these so called electric bacteria could survive in incredibly harsh conditions: there is still a flow of electrons, there is still a constant supply of energy.
Electric bacteria simultaneously reminds us of the sheer complexity and beautiful simplicity of life. It is another example of a biological mechanism we can “hijack” to work for us, furthering biotechnology and research; as well as perhaps being a reminder of how important electricity (or rather, the transfer of electrons) is to the origin of life.
*Featured image: This image shows Shewanella oneidensis strain MR-1 growing on the surface of the iron oxide mineral hematite. This image was captured by PNNL’s Alice Dohnalkova. Courtesy of Pacific Northwest National Laboratory.