Although I studied physics and chemistry in college, I have always held an inherent appreciation for how life works. And now, researching microbes in action, I am continually amazed by their diverse habitats and metabolisms. It is remarkable to think how despite the diminutive size of microbes, they are dominant members of our biosphere and play a key role in every known biogeochemical cycle. This list includes carbon, sulfur, nitrogen, and oxygen as well as trace metals such as iron, cobalt and zinc. I have started thinking that if there is an available compound on Earth, then microbes have found a way to use it.
What does it mean that microbes are involved in these biogeochemical cycles? It usually implies that a microbial species has found a way to use a form of that specific element while the microbe undergoes cellular respiration. Although respiration is complex, I tend to think of it in a simplified manner as a transport of electrons from one compound to another by a living organism. The organism utilizes this electron transport to produce energy in order for it to stay alive and reproduce. As humans, we take electrons from the carbon compounds we consume as food and transport those electrons to the oxygen we breathe. Since we use oxygen, this is aerobic respiration. One can think of the food we eat as electron donors and oxygen as the electron acceptor. Now, a fascinating aspect about microbes is that different species have evolved to use a variety of electron donors and acceptors in respiration process. Not only is this how microbes are involved in Earth’s biogeochemical cycles, but this is why certain microbes can be found in such extreme conditions, such as hydrothermal vents which have been discussed earlier.
Since microbes can use a variety of compounds as electron acceptors, what if there was a way to harness these electrons to produce electricity? In 1911, M.C. Potter was reported to be the first person to observe electricity production from microbes but it has only been within the past 30 years that this research has gained momentum. In these systems, which are called microbial fuel cells, the material the microbes are using as an electron acceptor is called an electrode, more specifically an anode. By connecting the anode with a metal wire to a region with even higher electron affinity, termed a cathode, we can generate a circuit. Oxygen is one of the best electron acceptors and typically these systems have oxygen present near the cathode and the microbes grow in an chamber without oxygen in order for electrons to flow from the anode to the cathode.
Personally, I find the mechanism for microbes to transport electrons to a solid surface fascinating as a basic science question and think that understanding this process better would shed some light on how microbes do this in nature. Right now, there are a handful of microbes known to utilize a solid surface as an electron acceptor and I am curious about what others are out there that have not yet been identified. What do these different species have in common? What influences growth and thus electron generation? How is the structure of the microbes growing on the anode as a biofilm influenced by the anode properties, such as material composition or surface patterning? How does the internal properties within the biofilm correspond to electron generation, such as pH? To address these questions, I perceive microbial fuel cells as a useful system to explore how microbes form biofilms on surfaces that act as an electron acceptor.
Further, as a new area of research with potentially huge contributions to be made more knowledge will improve how this mechanism is utilized in technology not only for energy production but also for organic waste removal. An attractive aspect of this technology is that electricity generation by the microbes goes hand-in-hand with organic waste removal because the microbes use the organic compounds as their electron donors, or food. In wastewater treatment, this would allow for the water to become purified at the same time electricity is generated. Electricity production is still quite low with one report stating that their microbial fuel cell design could power 16 60-watt light bulbs. There is still room for improvement in power generation and I believe that addressing the mechanistic questions about microbial electron transport to an outside electron acceptor will aid in both improving our basis for building these technologies and our understanding of how our microbes are major players in our world’s biogeochemical cycles.