Talk:Shewanella oneidensis MR-1: Background and Applications
Application: Microbial Fuel Cells (MFCs)
The three methods of extracellular metal reduction have given Shewanella incredible metabolic versatility that scientists are eager to harness. The first application of Shewanella microbes has been the development of microbial fuel cells. Fuel cells (such as batteries) generate electricity by separating the electron donor (anode) from the electron acceptor (cathode) such that electrons must pass through some resistor (any a product that requires electricity: calculator, flashlight, car) in order to reach the electron acceptor. Traveling from an electron donor to an electron acceptor is a favorable and spontaneous process for electrons, and as an electron current travels through resistors, it powers appliances (Logan et al. 2006).
Microbial fuel cells (MFCs) harness the electrons generated by bacteria (for respiration) to power fuel cells. Specifically, the microbial biofilms are grown on the anode where they separate hydrogens from the substances provided to them as food (microbial food can include wastewater, acetate, formaldehyde, etc). In order to reach the cathode (spontaneous reaction), electrons must travel through the resistor, and the protons left beyond move to the cathode through a proton exchange membrane. At the cathode, oxygen can act as a terminal electron acceptor and the electrons and protons are combined with oxygen to make water (Fig 12; Slonczewski et al., 2001)
Transferring Electrons from Bacteria to the Anode
Microbial fuel cells have helped reveal the importance of electron shuttles, such as riboflavins, and nanowires in bacterial respiration. Particularly, confocal scanning laser microscopy of biofilms on the anodes has revealed that some of the viable, electron-transferring bacteria are actually at a distance from the anode. When bacteria on the anode were live-dead stained (red is dead and green is live), live cells were found in 3 dimensional structures tens of microns away from the anode surface. Further, dead bacteria were found directly on the anode surface perhaps because they had adhered there and then were unable to use the anode as an electron acceptor (Fig 13; Logan et al., 2006). Thus, the bacteria at the tops of the bacterial mounds must use either electron shuttles, such as riboflavins, or nanowires to reach the anode and maintain their metabolism.
Potential Uses of MFCs
Because MFCs generate electricity and can be fueled by any organic substance, they show great potential as alternative wastewater treatment reactors. Current wastewater treatment machines are high-cost and spend electricity. Thus, replacing them with MFCs would be an economically feasible way of producing energy as well as cleaning water (Logan et al., 2006). Further, recent studies indicate that MFCs produce enough energy to act as power sources for long-term, un-manned data collecting devices such as monitoring devices for environment and water treatment (Lowy et al,. 2006).
MFCs and Clean Energy
Aside from small monitoring devices and wastewater treatment, using MFCs to produce electricity on a large scale will be difficult because of their inefficiencies (see section 3.3). However, MFCs can be easily altered to produce hydrogen gas rather than electricity. Specifically, hydrogen gas is a high energy molecule that has been proposed as a cleaner alternative to petroleum because burning it produces only water rather than exhaust fumes, which cause tremendous pollution and have been implicated in global climate change. However, hydrogen is not produced naturally in large quantities and thus some type of fossil fuel combustion or other energy source is required to split water (water electrolysis) for hydorgen production (Hydrogen Fuel Cells, 2006). MFCs offer an interesting alternative to hydrogen production because removing oxygen from the cathode and applying a small voltage (~0.25 V) results in the production of hydrogen gas at the cathode. This is called bacterial electrolysis of organic matter because the electrons and hydrogens come from organic matter rather than water (Rozendal et al., 2007). While the splitting of water to make hydrogen gas is highly endothermic and thus requires an input of energy (~1.8 V), bacterial electrolysis is very slightly exothermic, enough so to provide energy to the bacteria. Adding a small amount of voltage (~0.25 V), gives bacterial electrolysis enough energy to produce hydrogen gas. Overall, hydrogen production by bacteria represents a net energy gain by a factor of 5.8 when compared to the net loss of energy required for water electrolysis (Logan et al., 2006). Thus, the MFCs hold great potential as efficient hydrogen producers.
Challenges of Microbial Fuel Cells
Although microbial fuel cells show great potential as sources of energy for wastewater treatment, monitoring devices, and hydrogen production, their efficiency is a continual challenge. Particularly, if the anode is not well sealed, oxygen can leak in providing an alternative electron acceptor to the bacteria such that they can respire without generating electricity (Liu et al., 2004). Further, wastewater contains a plethora of alternative electron acceptors, such as nitrate or sulfate, which can also be used as alternatives to the cathode so the bacteria will respire without producing electricity (He et al., 2005). Improving the efficiency of MFCs requires advanced infrastructure of the cell itself such as separate chambers for the anode and cathode to prevent leakage of electron acceptors. Further, running the cell at a higher power density also tends to increase efficiency perhaps because there is less time for electrons to be lost to alternative acceptors. These challenges are likely to be overcome through further study of the MFC infrastructure and the bacteria that colonize the anodes (Logan et al., 2006).
Conclusion
Overall paper length should be 3,000 words, with at least 3 figures.
References
Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.
