We All Get Energy From Electrons

Animals need to eat and breathe oxygen to keep their metabolism going and stay alive. In your body, part of the food you eat turns into glucose, a special kind of sugar that provides energy to your cells. You basically have millions and millions of little power plants inside the cells of your body. In your cells, glucose molecules give away electrons, which are eventually given to oxygen molecules. This process gives you the energy you need to breathe, think, run, jump, and do all the other things you like to do.

Inside your cells, electrons are transferred from one place to another and, by doing so, electricity is generated. Electricity is simply the movement of electrons. In all living cells, electrons are transferred from molecules that give electrons away (like glucose) to molecules that take electrons (like oxygen), so you can say that cells generate electricity (Figure 1). In most living cells, electricity is generated on a very small scale. Electrons travel across very small distances of only a few nanometers (nm; one nanometer is one million times smaller than a millimeter).

For a long time, biologists thought that all living cells took care of their own energy demands. They also believed that electrons in cells could only travel across very short distances [1]. However, these ideas were challenged by the discovery of cable bacteria and their strange metabolism, back in 2012.

What are Cable Bacteria?

Bacteria live just about everywhere on Earth, including the air, soils, rivers, lakes, oceans, seafloor, as well as in and on the bodies of animals, including humans. Cable bacteria are a unique kind of bacteria that live in the sediment at the bottom of oceans and lakes. Cable bacteria are different from most bacteria in three ways:

Cable Bacteria are Multicellular and Very Long

While most bacteria have only one cell, cable bacteria have thousands of cells connected to each other in a long chain called a filament. Cable bacteria are therefore multicellular, meaning that they have multiple cells that work together. A cable bacteria filament can be up to a few centimeters long, which is enormous for bacteria. However, because the filaments are very thin, between 1 and 5 micrometer (μm; a micrometer is one thousand times smaller than a millimeter), we must use specialized microscopes to spot them (Figures 2A–C).

Cable Bacteria Transport Electrons Over Long Distances

When using a light microscope like those that you may have at school, cable bacteria look like a cable (Figure 2B). However, by using an electron microscope, which has a higher resolution, we can see parallel fibers running along each filament and connecting the cells to each other (Figure 2C) [1, 2]. These fibers serve as wires transporting electrons among the cells within the filament [3]. Because cable bacteria can be several centimeters long, they can transport electrons across distances that are more than 10 million times larger than in other bacteria [2]. This ability makes them the world champions of electron transport in the microbial world. How amazing is this?

We still do not know what the fibers transporting the electrons are made of. We do know, however, that they transport electrons very rapidly and efficiently, like a copper wire. These properties raised the attention of many scientists who are now investigating cable bacteria. So, maybe in the future, we can use cable bacteria to make environmentally friendly electronic devices [3].

Cable Bacteria Have Cells That Work Together

In most living cells, the transfer of electrons happens within a single cell and not across multiple cells. So normally, each cell is responsible for its own energy generation to survive and grow, even in multicellular organisms. It was a big surprise for biologists when they found that cells in cable bacteria must work together to generate energy! Like us, cable bacteria use oxygen, but they do not eat the same food we do. Instead of using glucose as their source of energy, cable bacteria use a molecule called hydrogen sulfide. In the sediment where cable bacteria live, hydrogen sulfide and oxygen are not present at the same location. The hydrogen sulfide is buried in the sediment, away from the oxygen that is present at the top of the sediment.

In cable bacteria, the electrons are taken from hydrogen sulfide. Then, the electrons are given to the electricity-conducting fibers, and they then move upward via these fibers to cells that are in contact with oxygen in the surface of the sediment. These surface cells take the electrons and give them to oxygen (Figure 3).

Unique Energy Generation

What is so unique about cable bacteria is that all cells within a filament must work together to generate energy [2]. This sort of “electrical teamwork” between cells has not been observed before, not even in other multicellular organisms. This teamwork gives cable bacteria an advantage, because it allows them to live in an environment where the resources are separated in space [1, 2]. Therefore, cable bacteria are found in many locations around the world and when they are present, there are a lot of them. For example, there can be up to 20,000 km of cable bacteria filaments per square meter of sediment [1].

Although cells within a cable bacteria filament must work together, the energy generated by this teamwork is not divided equally. Surprisingly, only the cells that take electrons from hydrogen sulfide can grow, whereas the cells that give the electrons to oxygen cannot grow. This happens because most of the energy is generated when the electrons are taken from hydrogen sulfide. When the electrons reach the oxygen, the electrons have no or only very little energy left (Figure 3). If a cell cannot generate enough energy, it will eventually die. However, if the cells at the top of the filament cannot give electrons to oxygen, the cells in the rest of the filament cannot take any electrons from hydrogen sulfide. This means that the entire filament cannot get any energy. Therefore, the reaction with oxygen is essential to keep the rest of the filament alive and able to grow. The activity of the oxygen-using cells can be thought of as a kind of “community service” to the filament.

Future Research

Currently, there is still a lot that we do not know. Do the oxygen-using cells “sacrifice” themselves for the greater good of the rest of the cable bacteria? If they keep giving electrons to oxygen without getting any energy, they will eventually die. It could also be that cells within a filament only stay in contact with oxygen for a while, and then another part of the filament takes over. If this is true, then the “community service” would only be temporary. Nevertheless, just like the “electrical teamwork”, the concept of “community service” within a multicellular organism is a new discovery in biology that requires more research to be better understood. Maybe someday you will be the one making exciting discoveries by studying the unique cable bacteria … or you might even figure out how to use cable bacteria to make environmentally friendly electronic devices!

Glossary

Metabolism: ↑ All chemical processes necessary for an organism to stay alive, including the conversion of food to energy required for running cellular processes.

Molecule: ↑ A molecule is the smallest unit of a substance that has all the properties of that substance. For instance, each glucose molecule has the same properties.

Electricity: ↑ Electricity is a form of energy where tiny charged particles called electrons move around. When electrons move, they carry energy from one place to the another.

Sediments: ↑ Naturally occurring solid material found at the bottom of lakes and oceans.

Filament: ↑ Long string of cells that are attached to one another, thus forming a multicellular bacterium.

Electron Microscope: ↑ A microscope that uses a beam of electrons to see tiny objects like bacteria.

Resolution: ↑ Within microscopy, the term resolution is used to describe the ability of a microscope to zoom in and see tiny details of the things you are looking at.

Hydrogen Sulfide: ↑ An energy-rich molecule that cable bacteria use as “food” to get energy for living and growing. Hydrogen sulfide can give electrons to the fibers in cable bacteria filaments.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

NG was supported by the Netherlands Organisation for Scientific Research (NWO PhD grant 023.005.049). JM was supported by the Ministry of Education via the Netherlands Earth System Science Centre. The NanoSIMS facility was partly supported by and NWO large infrastructure subsidy to JM (175.010.2009.011). FM was financially supported by the Netherlands Organization for Scientific Research (VICI grant 016.VICI.170.072), the Research Foundation Flanders (FWO grant G038819N) and University of Antwerp (TopBOF program).