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Chapter 24 : Microbial Fuel Cells as an Engineered Ecosystem
Category: Applied and Industrial Microbiology
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This chapter provides an overview of the microbial communities found in microbial fuel cells (MFCs), the interactions that drive the community structure, the processes performed by the communities, and how engineering affects the microbial resources within MFCs. Microbial electricity generation in MFCs relies on the drive of bacteria to acquire maximum energy. The electrode potential represents an important tool to control and increase the biocatalyst activity in relation to electricity generation. Moreover, the electrode potential will, as the key factor in the energy metabolism, determine the trade-off between fermenting and respiring organisms, thereby influencing the microbial composition. Several studies have described the microbial composition in MFCs. When comparing these data, several conclusions regarding the microbial community composition can be derived. First, various inocula can be used to successfully enrich electron-transferring organisms in an MFC. Second, several authors have concluded that MFCs strongly enrich organisms that utilize the electrode as final electron acceptor, both in a direct and in an indirect way. Third, although the MFC is an appropriate device to enrich electricity-producing communities, a typical electricity-generating microbial community has not been established yet. The majority of reported taxonomic classes are Proteobacteria (64%) followed by Firmicutes (13%) and nonclassified sequences (13%). The large variety of microorganisms found in MFCs suggests that many organisms can interact within the electricity-generating process. The influences of both the electron transfer interactions and the substrate transport within the biofilm on the development of the microbial community are discussed.
Key Concept Ranking
- Confocal Laser Scanning Microscopy
Overview of the possible microbial degradation pathways of acetate (A) and glucose (B) within the biofilm on the anode of an MFC. Fermenting cells are crosshatched. Cells using alternative electron acceptors are shaded. Microorganisms transferring electrons to the electrode by using mediators (M) or nanowires are dotted. Full lines indicate the use of acetate (A) or glucose (B) as the electron donor, while dotted lines indicate the use of intermediate electron donors. Striped lines represent possible losses of electron donors. Triangles represent gaseous components, and diamonds are aqueous components. Gibbs free values are expressed per mole of (intermediate) substrate as shown in reactions 1 to 12 and are calculated based on Thauer et al. ( 1977 ) and assuming substrate and intermediary product concentrations of 0.01 M, an anode potential of –150 mV versus standard hydrogen for both direct and mediated electron transfer, a pH of 7, and hydrogen partial pressures as indicated in panels A and B. Interspecies electron transfer is grouped by a frame; the division of the available Gibbs free energy is not taken into account. Reactions are as follows: (1) CH3COO– + 4H2O → 2HCO3 – + 9H+ + 8 e–; (2) CH3COO– + H2O → CH4 + HCO3 –; (3) CH3COO– + 4H2O → 4H2 + 2HCO3 – + H+; (4) H2 + 1/4HCO3 – + 1/4H+ → 1/4CH4 + 3/4H2O; (5) H2 → 2H+ + 2e–; (6) CH3COO– + SO4 2– → 2HCO3 + HS–; (7) HS– → S 0 + H+ + 2e–; (8) C6H12O6 + 12H2O → 6HCO3 – + 30H+ + 24e–; (9) C6H12O6 + 4H2O → 2CH3COO– + 2HCO3 – + 12H+ + 8e–; (10) C6H12O6 + 4H2O → 2CH3COO– + 2HCO3 – + 4H2 + 4H+; (11) C6H12O6 + 3H2O → 3CH4 + 3HCO3 – + 3H+; (12) C6H12O6 + 3SO4 2– → 6HCO3 – + 3HS– + 3H+.
Evolution of the polarization curves of an acetate-fed MFC during a 3-month period (adapted from Aelterman et al., 2006b ). While at Time 1 (152 days after start-up) a linear decrease of the voltage is noted at increasing currents, the polarization curve at Time 2 (175 days after start-up) is dominated by a sharp decrease of the voltage at maximum current production. At Time 3 (201 days after start-up) the overall performance of the MFC had increased and maximum current production had tripled. In addition, the sharp decrease of the voltage at high currents, attributed to mass transfer losses, was not noted any more.
Overview of the different taxonomic classes in microbial fuel cells, examined by clone libraries and sequencing in eight different setups ( Lee et al., 2003 ; Back et al., 2004 ; Kim et al., 2004 ; Phung et al., 2004 ; Rabaey et al., 2004 ; Logan et al., 2005 ; Aelterman et al., 2006b ; Kim et al., 2006 ). The different phyla are represented by different patterns. The relative amount of the different phyla is given between brackets: Proteobacteria, dotted pattern (64%); Firmicutes, black (13%); Bacteroides, vertical pattern (7%); other phyla, crosshatched (3%); nonclassified sequences, horizontal pattern (13%).
Overview of the potential working range of the different direct and indirect electron transfer mechanisms (direct cell contact, single nanowires, intertwined nanowires, and mediators), assuming a biofilm thickness of 100 μm.
Schematic overview of the cathode reactions catalyzed by microorganisms (represented by dotted ovals). (A) Direct transfer of electrons from the cathode to the microorganisms ( Gregory et al., 2004 ; Clauwaert et al., 2007a , 2007b ); (B) metal-mediated electron transfer ( Rhoads et al., 2005 ; Terheijne et al., 2006 ); and (C) bioelectrochemically driven dechlori-nation at the cathode ( Aulenta et al., 2007 ). The free Gibbs energy values are expressed per mole of electron acceptor and are calculated using the represented electrode potentials expressed versus standard hydrogen, a saturated oxygen concentration of 8 mg/liter and a NO3 concentration of 0.01 M, both at pH 7 unless noted otherwise.