Microbial Fuel Cells; An Alternative Energy Source for In-Water Equipment

Staying on the same theme as my previous post of alternative sources of energy, this post looks at microbial fuel cells (MFC) and the production of electricity. As the name suggests, MFCs rely on microbes to generate a movement of electrons (electricity) without the need of an initial electrical input. Organic compounds are catabolised by the microbes in an anaerobic chamber but instead of using an atom or compound as a terminal electron acceptor, the microbes transfer electrons to a conductive electrode which runs through a circuit, generating electricity.

A very current area of research for the applied use of MFCs is in marine sediments. For example, the US Navy is interested in setting up marine sediment MFCs to maintain the batteries of in-water sensors and navigation equipment positioned in oceans around the world. The main advantage of this is a decreased need for human maintenance (Arias-Thode et al 2011). How electricity can be harvested in this way involves two graphite plates. One is buried in the marine sediment (anode) and has to be completely anaerobic. Connected to the anode via an electrical circuit is the second plate which is held in place in the aerobic water column (cathode). Microbes use the anode as a terminal electron acceptor and at the same time releases protons (H+) into the sediment. The electrons are attracted through the circuit to the cathode by the presence of the highly oxidising molecular oxygen in the water column and combines with free protons to yield H2O. This movement of electrons can be used to trickle charge a battery.

Despite the clever way in which marine sediment MFCs work little is known on the microbes that grow on the anode plates. Holmes et al (2004) conducted the first detailed study looking into these communities. A number of MFCs were placed in different marine sediments around the USA. Sediment samples were collected adjacent to the anodes 3 months after initial placement. A control was also set up which consisted of just the anode plate with no cathode attached, which meant no electrical circuit. PCR amplification of 16rRNA was used for comparison to GenBank for species identification, it was also used to get an indication of cell numbers. Similar samples were taken from the cathode.

Results showed the anodes were dominated by bacteria of the Geobacteraceae family, within the δ-Proteobacteria class. When comparing to the control anode, Geobacteraceae were 100 times more abundant on the electrically active anodes. Geobacteraceae are normally iron reducers but in this case appear to be using the graphite anode as an electron acceptor instead. A possible explanation for this is that unlike other electron acceptors like oxygen or nitrate, iron is insoluble in water and a large insoluble graphite plate seems to be acting as a good alternative. The fact that cell numbers were far higher on the electrically active anodes indicates that this must attract and promote growth more than their normal iron reduction method. The cathode also showed unique bacterial colonisation. Nitrogen oxidising γ-Proteobacteria covered the surface of the cathode. Although not an uncommon group of bacteria they were significantly different from microbes normally found in the water column and nearby biofilms. Once again showing that active MFCs positively influence the bacterial populations.

Although this is not the most current paper on this topic, I reviewed it because it appeared to be the first study trying to specifically identify the microbes involved in marine sediment MFC electricity production. I had heard of the hydrogen fuel cell before which is a very similar concept but I had not heard of MFC before.

A Review of:
Holmes DE, Bond DR, O’Neil RA, Reimers CE, Tender LR and Lovley DR (2004) Microbial Communities Associated with Electrodes Harvesting Electricity from a Variety of Aquatic Sediments. Microbial Ecology. 48: 178-190.

Additional Reference:
Arias-Thode YM, Richter K, Wotawa-Bergen A, Chadwick DB, Kan J and Nealson K (2011) Development of Microbial Fuel Cell Prototypes for Examination of the Temporal and Spatial Response of Anodic Bacterial Communities in Marine Sediments.10.1109/Oceans-Spain.2011.6003535.