Can bacteria help meet our energy needs by turning waste-water into electricity or hydrogen? The discovery that some bacterial species common in soil and water can generate electrical current in microbial fuel cells (MFCs) raises intriguing possibilities for a different kind of bioenergy – bioelectricity. In MFCs electrogenic bacteria generate electric power from metabolism of organic molecules in soil, sewage or waste-water. Anaerobically respiring bacteria transfer electrons from organic molecules via NAD+/NADH (redox reactions of glycolysis, pyruvate oxidation and the citric acid cycle) to an extracellular electron acceptor (the anode). The difference in redox potential between the anoxic environment of the anode and the oxygenated environment of the cathode generates a voltage difference, and electric current that depends on the overall rate of bacterial metabolism.
MFCs illustrate key concepts for introductory biology students:
- Respiration consists of electron-transfer reactions that take electrons from organic molecules and transfer them to an electron transport chain localized in the membrane, and ultimately to a terminal electron acceptor.
- In the absence of oxygen, microbial anaerobic respiration uses alternative terminal electron acceptors other than oxygen.
The Mudwatt by Keegotech is a simple microbial fuel cell. It features living soil, a container, and two electrodes. The key is that the two electrodes are placed in different redox environments. One electrode (the anode) is 5-6 cm deep in the soil, in an anoxic layer. The other electrode (the cathode) is at the top of the soil, exposed to air containing oxygen.
Microbes at the anode metabolize organic carbon through anaerobic respiratory pathways. As in aerobic respiration, anaerobic respiration oxidizes organic carbon to reduce NAD+ to NADH. The electrons flow from NADH through the electron transport chain localized in the bacterial plasma membrane to generate a proton gradient across the membrane. The resulting proton motive force powers oxidative phosphorylation of ATP from ADP and inorganic phosphate.
However, in the absence of oxygen, electrons flowing down the electron transport chain reduce (give electrons to) alternative electron acceptors such as nitrate or sulfate. The difference in redox potential between NADH (-320 mV) and the terminal electron acceptor determines how much ATP the bacterium can make per mole of organic carbon respired. Bacteria undergoing aerobic respiration make the most ATP because oxygen at +820 mV has the highest redox potential among electron acceptors, resulting in a net potential difference from NADH of 1.14 V. Bacteria in anoxic sediments that reduce sulfate (-220 mV) make much less ATP per mole of organic carbon respired, with only 100 mV of potential difference from NADH.
In microbial fuel cells, electrogenic bacteria that can reduce extracellular terminal electron acceptors form a biofilm coating the anode. Using either soluble electron carriers or electrically conductive pilli, they can reduce a solid metal or graphite anode. The microbial fuel cell provides an electrical circuit to conduct electrons from the anode to a cathode placed in an oxygen-rich air/water interface. At the cathode, the electrons combine with oxygen molecules and protons to form water molecules.
What are the source molecules for the electrons transmitted to the anode by the electrogenic bacteria?
Where is the electron transport chain located in bacteria?
How do these electrons get from the source molecules to the anode?
If oxygen were present at the anode, what would happen to the electricity output of the MFC?
What is the theoretical maximum voltage output for a single MFC?
If all the NADH produced by the anaerobically respiring bacteria are used to reduce the anode, can these bacteria make ATP? Explain.
Can bacteria running fermentation reactions generate electricity in a MFC?
Resources and references:
Gorby, YA, Yanina, S, McLean, JS, Rosso, KM, Moyles, D, Dohnalkova, A, Beveridge, TJ, Chang, IS, Kim, BH, Kim, KS, Culley, DE, Reed, SB, Romine, MF, Saffarini, DA, Hill, EA, Shi, L, Elias, DA, Kennedy, DW, Pinchuk, G, Watanabe, K, Ishii, S, Logan, B, Nealson, KH and Fredrickson, JK, 2006. Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms, Proc Natl Acad Sci USA 103:11358-11363. DOI: 10.1073/pnas.0604517103
B1510_module3_5_respiration_questions_2011Fall – slides with clicker questions