Electricigens, when bacteria produce electricity
Posted on March 1, 2012
Electricity is a part of life. Electrical impulses coordinates the neural system in animals and even some species are able to use electrical discharges as a defense mechanism. And of course, electrical energy is involved in many cellular metabolic processes. In 1911, M.C. Potter affirmed that “The disintegration of organic compounds by microorganisms is accompanied by the liberation of electrical energy”.
But, what is a electricigen? Basically they are microorganisms that are able to completely oxidize organic matter to CO2 at the same time that electrons are transfered to electrodes. In other words, electricigens are able to convert renewable biomass and organic detritus into electricity without consume the fuel wasting energy in heat form. This process is very efficcient and, therefore, the importance of the discovery of this kind of organism for technologies such as the so called Microbial Fuel Cells (MFC).
The typical setup of a MFC consists on two chambers separate by a semipermeable membrane. In one chamber, under anoxic conditions it is placed an anode. On the other chamber, under oxic conditions, a cathode is placed and connected to the anode, creating a electrical current between both electrodes (figure 1).
The most important practical applications of MFC are two: to supply energy to small efficient electrical devices in remote locations (eg scientific monitoring instruments in deep sea basins) and the electrical cogeneration during the wastewater treatment, although advances on bioremediation (removing pollution from the environment using living organisms) have demonstrated that some electricigens are very useful on groundwater remediation of some heavy metals, as Uranium.
How electricigens produce electricity?
The explanation of this processes is based on the redox reactions, where a chemical compound is oxidized (loses an electron) at the same time that other is reduced (gain an electron). This transfer of electrons are usually given, by some dissimilatory metal reducing bateria (DMRB), to poorly soluble (hydr)oxides of Fe and Mn. Surprisingly, electricigens can send this electrons to electrodes, using them as sole electron acceptors, which allow the functioning of MFCs (the different mechanism for this process, as a more in depth view, will be discusses on future articles of the blog).
Some of the most well known electricigens belongs to the family of the Geobacteraceae, specially G. sulfurreducens, although more recently Shewanella oneidensis (figure 2) has attracted a lot of attention due to the discovery of its ability to produce electrical nanowires under conditions of electron-acceptor limitation (that will be treated in a future article on this blog).
With this the small introduction to electricigens is finished. I will write about this wonderful microorganism in later articles. If this post has awaken your interest, I recommend the next papers. from where I learnt most of the info about electricigens and MFCs:
1. Lovley, D. R. Bug juice: harvesting electricity with microorganisms. Nature Reviews Microbiology 4, 497–508 (2006).
2. Lovley, D. R. & Nevin, K. P. Electricity Production with Electricigens. Bioenergy 295–306 (2008).
3. Logan, B. E. Exoelectrogenic bacteria that power microbial fuel cells. Nature Reviews Microbiology 7, 375–381 (2009).
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