Bacteria as electric conductors

Just when I thought bacteria could not get anymore awesome, I come across studies that show that bacteria can conduct electricity. It seems that they can transport electrons from cell to cell over centimeter-long distances.

Mind-blowing.

In 2001, researchers first measured electric currencies in aquatic sediments. Soon afterwards, they found that this currency is due to the presence of specific bacteria that are able to exchange electrons with the surrounding.

These organisms were given the name cable bacteria and so far cable bacteria include the species “Candidatus Electrothrix” and “Candidatus Electronema”. These species belong to the Desulfobulbaceae family, which include bacteria that use sulfur or sulfur complexes for their metabolism to grow instead of oxygen.

Until now, Candidatus Electrothrix and Candidatus Electronema have not been characterised, as they usually live in in marine, freshwater, and salt-marsh sediments. Such conditions are pretty difficult to imitate, so that researchers can currently not grow them in a lab.

However, recent studies aimed to find out how these cable bacteria look like and how it is possible for them to conduct electricity over such long distances.

For this, researchers took water samples from different locations and visualised the harvested cable bacteria with different spectroscopic techniques. With these machines, researchers can magnify the tiniest things in the micrometer range and get high-resolution pictures of the bacterial cells.

Interestingly, these pictures revealed that one “cable bacterium” can rather be understood as a multi-cellular microorganism, because multiple bacterial cells reside within one such “cable”. As shown in the pictures below, the cells arrange according to their longitudinal axis so that they form a filament that can span up to 7 cm.

Cable bacteria imaged with different spectroscopic techniques. Cells are arranged along their longitudinal axis forming a long and insulated filament with a common outer membrane that functions as an insulator.
Cable bacteria imaged with different spectroscopic techniques. Cells are arranged along their longitudinal axis forming a long and insulated filament with a common outer membrane that functions as an insulator. Figures adapted from Cornelissen et al. and Trojan et al.

But how does the arrangement of cable bacteria allow electricity to flow? 

First of all, it is important to know that cable bacteria are Gram-negative bacteria, which have an inner and an outer membrane to protect the cell and all its content. Between the inner and outer membrane is a space that is called periplasm. This liquid space is important for the transport of compounds from the environment and for signalling. I describe one type of signalling in the periplasm here.

In cable bacteria, the surprising thing is, that the long cable filament is surrounded by one common outer membrane. This outer membrane does indeed work as a cable and insulates the inner cells from the surrounding. Therefore, within such cable each cell is surrounded by its inner membrane and all the cells within the cable share one periplasm. 

The cells are further held together by so-called fibers, which you can see as those long dark grey lines in the pictures above. These fibers go along the cells through the whole periplasm of the cable filament. As such, the fibers probably give the cable its structure, stabilise the cable and protect it from breaking. 

The researchers also suggest that the fibers are involved in the transport of the electrons, but this idea requires further evidence. 

But why would these bacteria transport electrons between cells and thus create an electric currency I hear you ask?

The short answer is to survive. 

As I said it before, all organisms just want to survive.

Cable bacteria reside in water sediments, which is rich in oxygen towards the water surface and rich in sulfur in the deeper layers. They sense the oxygen and by chemotaxis (I explain chemotaxis here) the filament aligns perpendicular to the water front with one end in the oxygen-rich area and the other end in the sulfur-rich area. Similar to the model below.

Now the filament works as a half cell.

The bacteria in the deeper layers perform sulfur oxidation that produces electrons (e) and protons (H+) as an anodic half-reaction. These electrons are transported along the cable filament to the upper part. Here, the bacteria consume the electrons to reduce oxygen in a catodic half-reaction.

A filament containing cable bacteria is aligned from the oxic zone to the sulfidic zone at the water surface. Near the water surface, bacteria reduce the available oxygen by consuming protons and electrons to molecular water. In the deeper water layers, bacteria oxidise sulfur thus producing protons and electrons. The electrons are then transported towards the bacteria residing in the oxic zone.
A filament containing cable bacteria is aligned from the oxic zone to the sulfidic zone at the water surface. Near the water surface, bacteria reduce the available oxygen by consuming protons and electrons to molecular water. In the deeper water layers, bacteria oxidise sulfur thus producing protons and electrons. The electrons are then transported towards the bacteria residing in the oxic zone. The figure was adapted from Geerlings et al.

Usually, these oxidation and reduction processes are part of each cell’s metabolism. This is what keeps every cell alive – including human cells, just that we do not use sulfur in the reduction reaction.

But cable bacteria found a way to uncouple the oxidation and reduction reactions and let them happen in different cells, which is the mind-blowing part.

These reactions also produce protons in the deeper layer and consume them in the surface layer. This leads to a drop in pH in the deeper layer and an increase in the pH near the surface, which can mineralise and demineralise metal complexes (similar to the mineralisation process I described here). 

Researchers suggest that these (de-)mineralisation processes also have an impact on the geochemistry within the local environment in the water. However, how exactly the metabolism of cable bacteria influences the environment and maybe other species is still not well understood.

Since cable bacteria were only recently discovered, it is understandable that there are still many questions unanswered. But I am convinced that we will hear many more interesting facts about them and who knows, maybe one day bacteria will fuel some kind of seawater-based batteries…

Take away from this week’s articles:

  • cable bacteria are filaments of a multitude of bacterial cells
  • they can produce and consume electrons and transport them from one end to the other
  • this electric currency shapes the local environment in water sediments

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