Pop culture has introduced us to many superhumans with the ability to harness electricity. For instance, the nordic god, Thor, channels electricity in the form of lightning using his mythical hammer Mjölnir. Like Thor, we have also learned to harness the electrical energy in our own way via the use of cables, batteries, and the likes. Recently, scientists have found that there are others on Earth who share this ability; the microorganisms. Microorganisms have, in fact, developed natural and organic apparatuses to channel electron flow across conductive surfaces outside of their cellular boundaries.

Microbes with the ability to carry out extracellular electron transfer are regarded as being “electroactive” and the branch of science studying them is called electromicrobiology.

An organic circuit board  

Electroactive microbes can transfer electrons to or from naturally conductive minerals in the environment and in the lab and can be studied using special bioelectrochemical reactors. In all cases, electrons can only flow via electrically conductive materials or electron carrying compounds. For instance, we use conductive copper wires to channel the flow of electrons to turn on a light bulb in the house. For electroactive microbes, they make their own conductive proteins for the electrons to hop on and travel across the cell membrane. 

Microbial extracellular electron transfer can be broadly grouped into two modes. In the first mode, the conductive proteins can act as an electron shuttle which carries the electrons, travels outside the cell, discharges the electrons to a conductive surface, and travels back into the cell to “shuttle” more electrons. In the second mode, the conductive proteins can associate with cellular structures like pilli or membrane extensions that discharge electrons to the extracellular conductive material via direct surface contact; like an organic electrical cable1! While these are the currently known modes,the discovery of more and more electroactive microbes means that we might soon see new and unknown microbial electroactive pathways.    

The perks of being an electroactive microbe 

Like humans, some microbes need to eat food and respire (or breathe) oxygen to live to grow. Instead of oxygen and unlike humans, some microbes can also respire other chemical compounds like nitrate and sulfate to carry out their metabolic processes. In essence, respiration is a series of chemical reactions involving electron flow from one compound to another resulting in energy generation. Electrons cannot exist alone in solution and respiration cannot proceed if the cell cannot find a compounds to pass the electrons onto. Most compounds used for microbial respiration are water-soluble and are able to diffuse and cross the cell membranes and the respiratory processes are contained inside the cell.

But, under certain conditions, like where there are no such soluble compounds available, electroactive microbes are able to use insoluble and solid conductive substances instead to transfer the electrons onto. This way, they can continue to respire, gain energy and grow! 

This ability to link intracellular processes to the extracellular environment provides electroactive microbes with an advantage allowing them to adapt better to the environment. A notable example is the filamentous cable bacteria which is able to link its respiratory processes centimeters across the sediment owing to its ability to conduct electrons across the length of its cells2. For us, it would be like trying to breath with an underwater snorkel that is thousands of metres long! In some cases, one electroactive microbe may also send electrons to another electroactive microbe to carry out coordinated metabolic processes3. Thus, while the superhero Thor may use his “electroactivity” to fight off bad guys, the microbes may use theirs to inhabit environments and conditions that are off-limits to non-electroactive microbes. 

The implications of microbial electroactivity

Despite their small size, the ubiquity and abundance of microbes makes them integral to the functioning of the Earth. Equally abundant are naturally occurring conductive minerals, from iron oxides in soils and sediments to hydrothermal vents in the deep oceans. As such, it is very likely that these microbe-mineral interactions (involving extracellular electron transfer) could be occurring in the soils right below our feet and in our oceans at this very moment affecting the earth’s biogeochemical cycles.

In addition, electroactivity has been demonstrated in many different kinds of microbes from both the bacterial and archaeal branches of life. Scientists have even found some of our gut microbes to be electroactive – imagine a microbial electrical network happening right in your belly!  

Electroactive microbes are also biotechnologically important. Scientists are now using the conductive proteins of these electroactive microbes in bioelectronics devices. Even in well established industries, such as wastewater treatment plants and anaerobic digesters, system efficiencies can be improved by stimulating electroactivity. For instance, adding conductive iron oxide particles increases organic matter breakdown and promotes methane production by electroactive methanogenic archaea4. 

Although the advantages are plenty, microbial electroactivity does have a few negative impacts. Electroactivity contributes to corrosion of manmade iron infrastructure, which results in huge economic  losses every year. In addition, the methane gas produced by methanogenic archaea is  a greenhouse gas that contributes to climate change and is known to be stimulated by electroactivity in environments such as rice paddy fields4. Understanding how microbial electroactivity impacts these processes is necessary to develop better mitigation practices. 

A electrical superhero

Thirty years ago, it would have just been science fiction to consider extracellular electron-transferring microbes. Today, scientists have developed devices that convert water vapour to electricity based on microbial conductive proteins5. Although Electromicrobiology is a relatively new field of science, it is growing fast due tothe interdisciplinary nature of the field, environmental impact, and vast applications in technology. Indeed, the next time we’re in need of an electrical superhero, we need not look to comics but just remember that they are already everywhere around us, you just need to look through a microscope!

Glossary

Pili - A short hair-like appendage found on the surface of many bacteria and archaea.

Metabolic - Relating to metabolism, which is the set of life-sustaining chemical reactions in organisms

Filamentous - Long and thin in diameter; resembling a thread

Biogeochemical cycles - Any of the natural pathways by which essential elements of living matter (e.g. carbon, nitrogen, etc) are circulated. The term biogeochemical is a contraction that refers to the consideration of the biological, geological, and chemical aspects of each cycle. (ref: Britannica.com)

Archaeal - Of Archaea; one of the three branches of life. Microorganisms which are similar to bacteria in size and simplicity of structure but radically different in molecular organization.

Discussion questions

1. What are the two broad modes of microbial extracellular electron transfer?

2. What advantages does being electroactive give to a microbe? 

3. Can you think of other living organisms that are able to manipulate electrical charge?

4. What kind of applications can you think of that could make use of biological conductive proteins?

5. What are some of the environments that electroactive microbes can be found?

6. What do you think are some advantages of electroactive microbes’ ability to inhabit conditions that are off-limits to others?

Reference

1. Shi, Liang, et al. “Extracellular electron transfer mechanisms between microorganisms and minerals.” Nature Reviews Microbiology 14.10 (2016): 651-662.

2. Teske, Andreas. “Cable bacteria, living electrical conduits in the microbial world.” Proceedings of the National Academy of Sciences 116.38 (2019): 18759-18761.

3. Rotaru, Amelia-Elena, et al. “A new model for electron flow during anaerobic digestion: direct interspecies electron transfer to Methanosaeta for the reduction of carbon dioxide to methane.” Energy & Environmental Science 7.1 (2014): 408-415.

4. Rotaru, Amelia-Elena, Mon Oo Yee, and Florin Musat. “Microbes trading electricity in consortia of environmental and biotechnological significance.” Current Opinion in Biotechnology 67 (2021): 119-129.

5. Liu, Xiaomeng, et al. “Power generation from ambient humidity using protein nanowires.” Nature 578.7796 (2020): 550-554.