EssentialDataandTechniquesforConductingMicrobial

DOI:10.1002/cssc.201100604EssentialDataandTechniquesforConductingMicrobialFuelCellandotherTypesofBioelectrochemicalSystemExperimentsBruceE.Logan*[a]IntroductionStudiesofelectrochemicalactivitiesofmicroorganismshavein-creasedbothinnumberandcomplexityoverthepast100years.ThefirststudieswereconductedbyPotter[1,2]overonehundredyearsagoatauniversityintheUKthatiscurrent-lyknownasNewcastleUniversity.Potterusedaverysimplesystem(Figure1)toshowthatanelectrochemicalpotentialwasgeneratedbydifferentmicrobeswhentheyweregivencertainorganicsubstrates.HefoundpotentialsweregeneratedbytheyeastSaccharomycescerevisiaeandthebacteriumBacil-luscoli(laterrenamedEscherichiacoli[3]),bothofwhichfer-mentedsubstratesinthemedium.Hefailedtomeasureanyelectromotiveforcegeneratedwiththreeothermicroorgan-isms,buttheseothermicrobesdidnotgrowinthemedium.Fermentativegrowthofmicroorganismscansetupthermody-namicpotentialsintermsofsubstrates,pHandothergradi-ents,[4,5]whichcanoftenleadtothegenerationofanelectro-motiveforce,butnotappreciablecurrentgeneration.Onlymi-croorganismsthatgeneratesubstantialcurrentdensitieswith-outtheuseofexogenouselectronshuttlesormediators,calledexoelectrogens,areofprimaryinteresttoday.SincethisfirststudybyPotter,[1]over3000papershavebeenwritteninthegeneralareaofbioelectrochemicalsystems(basedonaThomsonReutersWebofSciencesearch),mostlyinthefieldofmicrobialfuelcells(MFCs),andtheequipmentneededtoconducttheseexperimentshasbeensubstantiallyadvanced.IncontrasttosimplecircuitsandmaterialsusedbyPotter,amodernbioelectrochemicallaboratorywillusemanydifferentinstrumentsforobtainingdata,suchasmultimetersforlowimpedancevoltagemeasurementsandpotentiostatsforelectrochemicalanalysis(Figure1).Aswelearnmoreaboutfactorsthataffectthegenerationofelectricalcurrentusingmi-croorganisms,ithasbecomeapparentthatbettercontrols,measurements,andcareareneededwhentakingmeasure-mentsandreportingresults.TheexperimentsbyPotterareagoodexampleinthisregard.BothS.cerevisiaeandE.coliarenotconsideredtobeexoelectrogenicmicroorganisms,asexogenousmediatorsneedtobeaddedwiththesestrainstoproduceappreciablecurrents.[6,7]E.colihavebeenusedinrecentstudiesasanegativecontrol,[8]althoughonegroupre-portedthatcurrentcouldbeproducedbyoneE.colistrainaftersuccessivetransfers.[9,10]However,purityoftheculturewasnotconfirmedafterthesetransfers,raisingthepossibilitythatcurrentgenerationwasduetocontamination.Inaddition,yeast-peptonemediumisknowntocontainmediators,[7]andmediatorssuchasflavins[11](forexample,Riboflavin,vitaminB2)areoftenaddedtoamediumdirectly[12,13]orindirectlyinyeastextract.[1]Thus,itisalwayspossiblethatcelldisruptionormediacarryovercanresultinsufficientconcentrationsofmedi-atorstoproducemeasurablebutlowcurrent.Alowlevelofcurrentproductionbymicrobesis,therefore,notsufficientproofthatthemicrobesareexoelectrogenic,thatis,thattheycangrowviaexogenouselectrontransfer.ExperimentsonMFCsandotherbioelectrochemicalsystems,likeallscientificexperiments,requirecarefulplanning,propercontrols,andathoroughanalysisofdata.ThemultidisciplinarynatureofMFCexperimentscansometimesmakeitdifficulttoestablishminimumstandardsforexperimentaldesign(suchasreplicates)orelectrochemicalmeasurements.Forexample,wastewatertreatmentstudiesareoftenconductedwithpilotorfullscalereactorsandthuslackseparatecontrolsduetothecostofthesystems.Electrochemistsevaluatechemicalreac-tionsandelectrodeperformanceusingsolutions(suchasMicrobialfuelcells(MFCs)andotherbioelectrochemicalsys-temsarenewtechnologiesthatrequireexpertiseinavarietyoftechnicalareas,rangingfromelectrochemistrytobiologicalwastewatertreatment.Therearecertaindataandcriticalinfor-mationthatshouldbeincludedineveryMFCstudy,suchasspecificsurfaceareaoftheelectrodes,solutionconductivity,andpowerdensitiesnormalizedtoelectrodesurfaceareaandvolumes.Electrochemicaltechniquessuchaslinearsweepvol-tammetrycanbeusedtounderstandtheperformanceoftheMFC,butextremelyslowscansarerequiredforthesebiologi-calsystemscomparedtomoretraditionalfuelcells.InthisMin-ireview,thecriticalinformationneededforMFCstudiesispro-videdwithexamplesofhowresultscanbebetterconveyedthroughafulldescriptionofmaterials,theuseofpropercon-trols,andinclusionofamorecompleteelectrochemicalanalysis.[a]B.E.LoganDepartmentofCivilandEnvironmentalEngineeringThePennsylvaniaStateUniversityUniversityPark,PA,16802(USA)Fax:[phone]E-mail:[e-mail]9882012Wiley-VCHVerlagGmbH&Co.KGaA,WeinheimChemSusChem2012,5,988–994perchloricacid)appropriatetounderstandmechanismsandchoosescanratesinsometeststhatareappropriateforfastchemicalreactions.However,theseconditionsmaynotprovideresultsusefulforunderstandingbiologicalsystemsthatareusuallyoperatedatneutralpHbyusingcarbonateandphos-phatebuffers.Thecommunityofresearchersstudyingelec-trodematerials,microorganisms,andotherfactorsthataffectcurrentgenerationinMFCsandotherbioelectrochemicalsys-tems(BESs)musttakecaretoobtaindataunderrelevantcon-ditions(suchasneutralpH)tobetterunderstandthefunction-ingofthesecomplexsystems.Asnotedintheexamplesbelow,fulloperationalcondition