Microbial Fuel Cells: Plug-in andPower-on MicrobiologyThese devices already prove valuable for characterizing physiology,modeling electron flow, and framing and testing hypotheses

Kelly C. Wrighton and John D. Coates

If scientists have their way, “green” beerwon’t be limited to St. Patrick’s Daycelebrations anymore. Breweries are tak-ing their wastewater, which is rich inorganic material, and turning it into elec-

tricity with bacteria in microbial fuel cells(MFCs). MFCs can generate valuable commod-ities from a variety of organic wastes that areabundant and essentially free—bacteria havegenerated electricity from industrial wastewa-ters, sewage, and even sediment. In MFCs, bac-teria act as living catalysts to convert organicsubstrates into electricity. While this technology

may sound like an answer to our energy crisis,MFCs are not yet viable for most applications.Ongoing research is dedicated to optimizingtheir performance, with only recent attentionbeing given to the microbial details of waste-to-wattage conversion.

MFCs may well have a place in the futureenergy paradigm, as well as in bioremediationand industrial-chemical and hydrogen produc-tion. For MFCs to be considered more than a labnovelty, standardization of data expression isnecessary to allow reliable and accurate com-parison of results. Advances in the hardware,

operation, and microbial components arealso needed. However, MFCs are valuableresearch tools for characterizing the physi-ology and ecology of extracellular electrontransfer, modeling electron flow in complexmicrobial ecosystems, and framing and test-ing ecology theory.

Denizens of Power: the Role

of Bacteria in MFCs

Generating power in MFCs depends onoxidation-reduction (redox) chemistry.MFCs contain anodic and cathodic com-partments, each of which holds an electrodeseparated by a cation-permeable membrane(Fig. 1). In the anode chamber, microbialsubstrates such as acetate (an electron do-nor) are oxidized in the absence of oxygenby respiratory bacteria, producing protonsand electrons. The electrons are passedthrough an electron transport chain (ETC)and protons are translocated across the cellmembrane to generate adenosine triphos-phate (ATP).

Summary

• Microorganisms may be harnessed throughfuel cells to convert organic materials intoelectricity, hydrogen, or industrially usefulchemicals, or to remediate polluted environ-mental sites.

• In a typical microbial fuel cell (MFC), mi-crobes transfer electrons through a traditionalelectron transport chain onto an electrode sur-face generating electricity while producing aproton motive force for ATP generation.

• MFC-based research continues to expandknowledge about the diversity of extracellularelectron transfer processes, mechanisms usedin such processes, and biofilm ecology.

• Although gram-positive bacteria can generateelectrical energy in MFCs, how they transferelectrons without outer membranes is a mystery.

• Bioelectrical reactors do not produce electric-ity but furnish electrons to reduce remediationtargets such as uranium, perchlorate, chlori-nated solvents, and nitrate.

Kelly C. Wrighton isgraduate studentand John D. Coatesis a Professor ofMicrobiology in theDepartment ofPlant and MicrobialBiology, Universityof California, Berke-ley.

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Electrons and protons exiting the ETC typi-cally pass onto a terminal electron acceptor suchas oxygen, nitrate, or Fe(III). However, in the

absence of such acceptors in an MFC,some microorganisms pass the elec-trons onto the anode surface. Differ-ence in redox potentials (i.e., the abilityof a compound to donate or accept elec-trons, denoted Eo and measured involts) between the electron donor andthe electron acceptor is the determinantof the potential energy available to themicroorganism for anabolic processes.In an MFC the electrochemical redoxpotential difference of the anode andcathode determines how much energy isavailable.

Electrons produced in an MFC flowfrom the anode through an externalelectrical circuit to the cathode to gen-erate electrical current. While electronsmove externally, protons diffuse fromthe anode to the cathode via the cationmembrane to complete the internal cir-cuit (Fig. 1). At the cathode, the elec-trons and protons combine to reducethe terminal electron acceptor, which inmany applications is oxygen. Thereforebacteria in the anode are physically sep-arated from their terminal electron ac-ceptor in the cathode compartment.

The electrical power (measured inwatts) produced by an MFC is based onthe rate of electrons moving throughthe circuit (current, measured in amps)and electrochemical potential differ-ence (volts) across the electrodes. Manyfactors affect current production, in-cluding substrate concentration, bacte-rial substrate oxidation rate, presenceof alternative electron acceptors, andmicrobial growth. Electrochemical po-tential, on the other hand, depends onthe redox couple between the bacterialrespiratory enzyme or electron carrierand the potential at the anode, which isdetermined by the terminal electron ac-ceptor in the cathode and any systemlosses. (Fig. 1).

For bacteria to produce electricity inMFCs, the cells need to transfer elec-trons generated along their membranesto their surfaces. Yet, very little is

known about bacterial interactions with elec-trodes. While anodes and cathodes can functionin bacterial respiration, research has been fo-

F I G U R E 1

(A) Schematic of a microbial fuel cell illustrating oxidation of fuel by bacteria in the anodecompartment to produce electrons and protons. Electrons (red arrow) resulting frommicrobial oxidation flow from the anode through an external connection to the cathodeto generate electrical current, while protons (green arrow) travel thorough the cationmembrane. Together both function to reduce the terminal electron acceptor, which inthis case is oxygen, in the cathode. (B) Redox tower for components that are significantto current production in a MFC. For a redox couple, the electron donor must have agreater negative potential than the electron acceptor, this difference in potential isproportional to the amount of energy generated from the reaction. Current generation ina MFC is based on sequential redox reactions. First, bacteria oxidize fuel (potentialsnoted in blue) and transfer these electrons to an electron carrier at a more positivepotential (noted in red), thereby generating energy for the bacteria. The final powergenerated by an MFC is based on the current production and redox couple between thebacterial respiratory enzyme or electron shuttle and the potential at the anode, which isdetermined by the terminal electron acceptor in the cathode and any system losses.(References: redox potential of Shewanella produced flavin [Marsili, 2008] and aGeobacter (OmcB) outer membrane cytochrome [Mahadevan, 2006]).

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cused on understanding microbial anodic elec-tron transfer. Anode-respiring bacteria catalyzeelectron transfer in organic substrates onto theanode as a surrogate for natural extracellularelectron acceptors (e.g., ferric oxides or humicsubstances) by a variety of mechanisms (Fig. 2).

Bacteria transfer electrons to anodes eitherdirectly or via mediated mechanisms. In directelectron transfer, bacteria require physical con-tact with the electrode for current production.The contact point between the bacteria and theanode surface requires outer membrane-boundcytochromes or putatively conductive pili callednanowires. Although direct contact of an outer-membrane cytochrome to an anodic surfacewould require microorganisms to be situatedupon the electrode itself, direct electron transfermechanisms are not limited to short-range inter-actions, as nanowires produced by Geobactersulfurreducens have been implicated in electronconduction through anode biofilms more than50 �m thick. In mediated electron transfermechanisms, bacteria either produce or takeadvantage of indigenous soluble redox com-pounds such as quinones and flavins to shuttleelectrons between the terminal respiratory en-zyme and the anode surface.

Power Tools of Microbiology: MFC

Technology Advances Microbial Research

Besides generating energy, MFCs are powerfulresearch tools. With electrical current a proxyfor bacterial activity, MFCs are controlled sys-tems for addressing a range of questions aboutextracellular electron transfers. MFC-based re-search continues to expand knowledge aboutthe diversity of extracellular electron transfers,mechanisms used in such processes, and biofilmecology.

Microbial research from MFCs has revealedan expansive diversity of bacteria that transferelectrons onto external electron acceptors. Untilrecently, knowledge of electricity-generatingbacteria was limited to bacteria that transferelectrons to solid metals, thus phylogeneticallyconfining most MFC studies to members of theProteobacteria. However, culture-independentstudies of MFC anode biofilms indicate thatthe diversity of such microbial communities farexceeds that of the available electricity-produc-ing isolates, suggesting that many organisms

with this capability are yet to be discovered.This knowledge has spurred interest in using avariety of alternative inoculum sources, operat-ing conditions, and isolation methods to in-crease the known diversity of electrode-reducingorganisms.

For example, in our lab we are using MFCsoperated at 55°C, at which temperature the an-ode-reducing species Geobacter and Shewanellaspp. cannot survive. In this way we find anodecommunities dominated by gram-positive speciesand have isolated novel organisms from three ofthe five most dominant populations identified by16S rRNA gene clone libraries. Characterizationrevealed that the isolates use species-specific mech-anisms for electricity production, emphasizing notonly the phylogenetic diversity that exists in activeMFCs but also the phenotypic diversity within asingle community to perform the same function,i.e., transfer electrons onto an electrode surface.

One isolate, Thermincola strain JR, a memberof the Firmicutes, produces current comparableto that of the original MFC community andgreater than either Geobacter or Shewanellaspecies in similarly designed MFCs. Further-more, this is the first example of electrical powerproduction by a gram-positive bacterium with-out exogenous mediators. A second isolate,Geobacillus strain S2E, also a member of theFirmicutes, is the first member of this genusreported to respire using solid-phase iron ox-ides.

Our results indicate that microbial analysesfrom MFCs can result in the discovery of bacte-ria that are proficient at energy generation andhave unique metabolic functions. The signifi-cant enrichment of gram-positive bacteria in oursystems may signify a new ecological role forthese organisms in respiration of insoluble elec-tron acceptors. How these gram-positive bacte-ria, which lack outer membranes, transfer elec-trons extracellularly is a mystery.

To optimize MFC performance, we need tolearn more about these electrode-reducing mi-crobial communities. Molecular approachescharacterizing the microbiology communities inthese systems are needed to reveal the phyloge-netic diversity as well as the activity of electrode-respiring communities. Little is known aboutpopulation-level interactions within the anodecommunity, but it is foreseeable that these typesof interactions can be reinforced or controlled to

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increase substrate utilization or electron transferefficiency. Such research could also prove usefulfor determining the fate, transport, and biore-mediation of metals in the environment. Futurestudies could use MFCs set at different reduc-tion-oxidation potentials to better understandthe effects of redox potential and electron do-nors on microbial community structure and ac-tivity.

Elucidating Extracellular Electron

Transfer Mechanisms

MFCs as a research tool have expandedour knowledge of bacterial electrontransfer mechanisms. Unlike naturalexternal electron acceptors such asFe(III) or Mn(IV), anodes in MFCs donot participate in mineral dissolutionreactions, and electron transfer ratescan be quantified. Anodes also providea stable source of electron acceptor anddo not generate reduced products thatcan interfere with downstream genomicor proteomic applications. Addition-ally, colonized anodes can be adaptedto detect the presence, redox potential,and reversibility of electroactive com-ponents in biofilms.

The power of MFCs to elucidate mech-anisms of solid-phase electron transferwas convincingly demonstrated in 2008by researchers at the University of Min-nesota. Applying cyclic voltammetrytechniques to anode biofilms, theyshowed that Shewanella spp. excrete fla-vins which function in anode electrontransfer and metal chelation and mayaid in adhesion to anode surfaces. SinceShewanella use outer membrane cyto-chromes and putatively transfer electronsthrough conductive nanowires, this workshows that extracellular electron transfermechanisms are not mutually exclusivewithin a single species. This may accountfor observed discrepancies in researchfindings by different laboratories. Under-standing how bacteria attach to anodescould allow the design of more efficientelectron transfer systems. Genetic andmetabolic engineering of electrode activebacteria, including the overexpressionof essential cytochromes or shuttlingcompounds, could increase current pro-duction.

Modeling and Framing Biofilm Ecology

Consistent with their behavior in nature, bacte-ria in MFCs form biofilms on the anode surface.Because MFCs measure real-time bacterial ac-tivity and detect redox-active components, theyprovide a platform for addressing questionsabout biofilm ecology. For instance, on the basis

F I G U R E 2

Bacteria use direct and mediated mechanisms to transfer electrons from the cellmembrane to the anode surface. Electrons can be transferred from the cell or through aconductive biofilm using each method. (Figure adapted from Lovley, 2009.)

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of mathematical models to describe anode bio-films, Kato Marcus and collaborators at Ari-zona State University in Tempe determined thatthe entire anode biofilm is electrically conduc-tive and that biofilm density and detachment areimportant factors in electrochemical perfor-mance.

With these modeling results as a startingpoint, recent efforts are aimed at increasing theactive biomass capable of electron transfer tothe anode surface without altering mass transferevents or the physical environment within theanode biofilm. Researchers at the University ofMassachusetts found that biofilms formed un-der increased shear contain a higher density ofactive bacteria, increasing MFC performancethreefold. Future modeling studies could high-light discrepancies between predicted and ob-served power production in MFCs, suggestingabiotic and biotic areas of improvement.

Beyond modeling, we can explore the envi-ronmental and biological cues involved in bio-film formation and dissolution. The role andtemporal dynamics of quorum-sensing com-pounds on anode colonization and current pro-duction have not yet been evaluated in MFCs,which is unfortunate given that many signalingmolecules may also function as electron-shut-tling components in mediated electron transfer.MFCs provide an ideal platform to gain a betterunderstanding of the attachment, succession,dissolution, and interspecies interactions thatoccur within biofilms.

A Current Affair: Ongoing

Research and Challenges

MFC research endeavors are increasing eachyear. Much attention is dedicated to optimizingpower generation. Hardware and operationalconstraints, rather than microbial activity, pri-marily contribute to limitations in MFC powerdensities. Improvements in MFC design and ma-terials have significantly improved reactor per-formance by 10,000-fold since 1999. Despitethis advance, a further increase of 10- to 100-fold is required for MFCs to be considered forpractical applications.

A major focus is on reducing internal reactorresistance and increasing cathodic reaction effi-ciency to maximize power in lab-scale systems.However, in such systems the cost of componentmaterials and operation far exceeds the value of

energy generated. Economic feasibility studiesof large-scale implementation of MFC technol-ogy require pilot-scale applications to evaluatethe design, construction, operation, and micro-bial restrictions. We need to know which aspectsof MFCs will scale linearly and which will not,and cheaper and more durable electrodes andcation-permeable membranes are required.

Pilot-scale MFCs are being put to the test atAnheuser-Busch Inc. and Foster’s breweries, ofthe United States and Australia, respectively.Researchers at the two brewing companies sep-arately are evaluating whether MFC technologycan be used to treat organic-rich soluble waste-water while producing electricity. Thus, in Sep-tember 2007 researchers from the University ofQueensland and from Foster’s began a 1,000-LMFC experiment to address some of the ques-tions surrounding scale-up (Fig. 3a).

Full of Potential: Future Applications

of MFC Technology

Microbes participate in a vast array of biochem-ical reactions to satisfy their energy and resourcedemands. The microbial metabolism in MFCscan be harnessed for bioremediative, industrial,and hydrogen production applications to pro-duce valuable end products in an environmen-tally responsible way.

The conversion of biomass, especially organicwaste, to energy is considered an essential partof a sustainable global energy portfolio. A vari-ety of potentially valuable underutilized energysources exist in the United States. For example,assuming that its organic material is completelyoxidized to carbon dioxide, human waste couldproduce 34 billion kWh of energy annually.This represents an enormous untapped energysource. Moreover, MFCs can generate electric-ity from cellulose. Thus it is feasible for MFCs totreat wastewater from biofuel processing, re-moving waste material to recycle water whilegenerating electricity. Coupling these technolo-gies to minimize production costs and increaseenergy recovery could help make “green en-ergy” profitable and sustainable.

While the use of MFCs for wastewater treat-ment is in its infancy, MFCs as batteries forenvironmental sensors is nearing practical use.In contrast to traditional batteries, MFCs pow-ered by organic matter in sediments offer advan-tages as power sources because they can gener-

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ate energy without a need for recharging. Thesetypes of MFCs, called benthic unattended gen-erators (BUGs), have been used in inaccessibleareas such as river and ocean sediments. Oper-ation is technically simple; a graphite plate isdeployed into the anoxic sediment (anode) thatis electrically connected to another graphiteplate in the overlaying aerobic water (cathode)(Fig. 3b and 3c). Although power density isminimal, incorporating a capacitor in the elec-trical circuit stores the produced BUG energy foruse in short bursts. Using this approach, a BUGdeployed in creek sediment powered an environ-mental sensor and a wireless data transmitter to

monitor air and water temperature andtransmit this data to a shore-based re-ceiver.

MFCs have applications beyond elec-tricity production. MFCs are used topower cathodic reduction reactions forbioremedial or industrial processes.Since electricity is not being harvested—the biologically generated current isused to stimulate microbial metabolismon a cathode—these systems are notconsidered fuel cells, but are called bio-electrical reactors (BERs). An externalpower source usually provides the re-ducing equivalents in these systems, buta biological anode may be used. Cath-odes have served as electron donors forbacterial reduction of bioremediationtargets such as uranium, perchlorate,chlorinated solvents, and nitrate. Thistechnology could be applied to remedi-ate other contaminants including toxicmetals, dyes, pesticides, and herbicides.

BERs in which reducing equivalentsare produced at the anode may alsoyield industrially important chemicalssuch as hydrogen peroxide, sulfur, andbutanol. Using BERs to produce fuelssuch as propanol and butanol from or-ganic waste is very appealing. In thisprocess, organic waste with a sugarcontent too low to allow ethanol pro-duction would be microbially fer-mented in the absence of an electronacceptor into volatile fatty acids (VFA).These VFA can be fed to the cathodecompartment, where bacteria woulduse the electrons supplied from thecathode to reduce VFA into propanol

and butanol. This process, using hydrogenrather than MFC cathodes as a source of reduc-ing equivalents, is feasible, according to KirstenSteinbusch of the Wageningen Institute for En-vironment and Climate Research in the Nether-lands. Specific research hurdles include evaluat-ing the use of current rather than hydrogen forreducing equivalents, fine-tuning concentrationsof VFA and electrons for favorable thermody-namic conditions, and developing methods forseparating the desired end-products from thereactor liquor.

In addition to powering BERs, MFCs canalso be modified to produce hydrogen gas.

F I G U R E 3

Applications for MFCs. (A) Pilot-scale microbial fuel cell constructed by the advancedwater management centre at the University of Queensland and Foster’s brewery inYatala, both in Australia. Image is of graphite electrodes, which are contained in 12modules approximately 3 m in height. (Photo courtesy of Jeremy Patten, The Universityof Queensland.) (B) Sediment MFC prior to deployment in salt marsh sediments. (C)Active sediment microbial fuel cells. (Panels B and C published in Microbe in July 2006,p. 324, by Derek Lovley.)

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With transportation fuels accounting for up to25% of global fossil fuel consumption, alter-native, sustainable fuel sources are needed.Microbial electrolysis cell (MECs), which likeMFCs are based on bacterial oxidation oforganic substrates occurring at the anode andelectrons flowing to the cathode, can generaterenewable hydrogen from waste materials. InMECs an electrochemical potential achievedin the anode is supplemented with an addi-tional �250 mV from an exogenous source sothat electrolysis of water occurs at the cath-

ode, producing hydrogen. Over the past twoyears research in this area has advanced sig-nificantly, with the amount of hydrogen gen-erated per mol of oxidized glucose nearing theU.S. Department of Energy’s target for tech-nology viability. Hydrogen production in re-actors using existing technology is too low tomake large-scale MEC’s likely in the immedi-ate future, but a combination of improvedreactor design and treatment of organic-richwastewaters makes this an attractive proposi-tion for the future.

SUGGESTED READING

Logan, B. E., D. Call, S. Cheng, H. V. M. Hamelers, T. H. J. A. Sleutels, A. W. Jeremiasse, and R. A. Rozendal. 2008.Microbial electrolysis cells for high yield hydrogen gas production from organic matter. 42:8630–8640.Lovley, D. R. 2008. The microbe electric: conversion of organic matter to electricity. Curr. Opin. Biotechnol. 19:564–571.Marcus, A. K., C. I. Torres, and B. E. Rittmann. 2007. Conduction-based modeling of the biofilm anode of a microbial fuelcell. 98:1171–1182.Marsili, E., D. B. Baron, I. D. Shikhare, D. Coursolle, J. A. Gralnick, and D. R. Bond. 2008. Shewanella secretes flavins thatmediate extracellular electron transfer. Proc. Natl. Acad. Sci. USA 105:3968–3973.Pham, T. H., K. Rabaey, P. Alternman, P. Clauwert, L. De Schamphelaire, N. Boon, and W. Verstraete. 2006. Microbial fuelcells in relation to conventional anaerobic digestion technology. Eng. Life Sci. 6:285–292.Rabaey, K., J. Rodriguez, L. L. Blackall, J. Keller, P. Gross, D. Batstone, W. Verstraete, and K. H. Nealson. 2007. Microbialecology meets electrochemistry: electricity-driven and driving communities. ISME J. 1:9–18.Steinbusch, K., H. Hamelers, and C. Buisman. 2008. Alcohol production through volatile fatty acids reduction with hydrogenas electron donor by mixed cultures. Water Res. 42:4059–4066.Tender, L. M. 2002. Harnessing microbially generated power on the seafloor. Nature Biotechnol. 20:821–825.Thrash, J. C., and J. D. Coates. 2008. Review: direct and indirect electrical stimulation of microbial metabolism. Environ. Sci.Technol. 42:3921–3931.Wrighton, K. C., P. Agbo, F. Warnecke, K. A. Weber, E. L. Brodie, T. Z. DeSantis, P. Hugenholtz, G. L. Andersen, and J. D.Coates. 2008. A novel ecological role of the Firmicutes identified in thermophilic microbial fuel cells. ISME J. 2:1146–1156.

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