AThermophilicGramNegativeNitrateReducingBacterium

AThermophilicGram-NegativeNitrate-ReducingBacterium,Calditerrivibrionitroreducens,ExhibitingElectricityGenerationCapabilityQianFu,†HajimeKobayashi,*,†,‡HideoKawaguchi,†TatsukiWakayama,§HaruoMaeda,§andKozoSato*,†,‡†DepartmentofSystemsInnovation,GraduateSchoolofEngineering,TheUniversityofTokyo,Tokyo,Japan‡EngineeringforSustainableCarbonCycle(INPEXCorporation)SocialCooperationProgram,FrontierResearchCenterforEnergyandResource(FRCER),TheUniversityofTokyo,Tokyo,Japan§INPEXCorporation,9-23-30Kitakarasuyama,Setagaya-ku,Tokyo157-0061,Japan*SSupportingInformationABSTRACT:Toexploitthepotentialdiversityofthermo-philicexoelectrogens,two-chambermicrobialfuelcells(MFCs)wereinoculatedwiththermophilicanaerobicdigestersludgeandoperatedat55°Cwithoutsupplementingwithexogenousredoxmediator.TheMFCgeneratedamaximumpowerdensityof823mWm−2after200hofoperation.Molecularphylogeneticanalysessuggestedthatthemicrobialpopulationontheanodewasdominatedbyaspeciescloselyrelatedtoathermophilicnitrate-reducingbacteriumCaldi-terrivibrionitroreducens,forwhichastrain(Yu37-1)hasbeenisolatedinpureculture.Thus,apurecultureoftheC.nitroreducensstrainYu37-1wasinoculatedintoMFCtoexaminetheelectricitygenerationcapability.Withoutanexogenousmediator,MFCsstablyproducedelectricitywithamaximumpowerdensityof272mWm−2for400hofoperation.TheMFCcurrentrecoveredtotheoriginallevelwithinfewhoursaftermediumreplacement,suggestingthattheelectricitygenerationwascausedbytheanodicmicroorganisms.Cyclicvoltammetryindicatedthatredoxsystems(E3andEc)withsimilarpotentials(−0.14and−0.17V)madethemaincontributionstotheexoelectrogenicactivitiesofthesludge-derivedconsortiumandC.nitroreducensYu37-1,respectively.ThisstudyundertookthebioelectrochemicalcharacterizationofC.nitroreducensasthefirstexampleofathermophilicGram-negativeexoelectrogen.■INTRODUCTIONThemicrobialfuelcell(MFC)isapromisingenergygeneration/conversiontechnologythatcanbeusedinrenew-ableenergygeneration,wastewatertreatment,biosensing,andbioremediation.1−3AkeyfeatureofMFCsistheuseofmicroorganisms(collectivelycalled“exoelectrogens”)possess-ingthecapabilityofextracellularlytransferringelectronstoelectrodes,asbiocatalyststoharvestenergyfromorganicmaterials,includingvarioustypesofwastesandbiomass.InatypicalMFC,exoelectrogensanaerobicallyoxidizeorganicmatterandextracellularlyreleaseelectrons(astheterminalstepofanaerobicrespiration),whicharecollectedatananodeforcurrentgeneration.1−3Todate,exoelectrogenicactivityhasbeenreportedfor20microbialspecies,mostofwhicharemesophilicGram-negativebacteriaaffiliatedwiththephylumProteobacteria.1,4Amongthem,Geobactersulfurreducens5−10andShewanellaoneiden-sis11−16havebeenextensivelystudiedasmodelexoelectrogens.BiochemicalandgeneticstudieshaveindicatedthatG.sulfurreducenscandirectlytransferelectronstoananodeusingmembrane-boundc-typecytochromes8,10andconductivepili.6,7Membrane-boundc-typecytochromes14,15andcon-ductivepili12,16havesimilarlybeencharacterizedinS.oneidensis.However,theextracellularelectrontransferofS.oneidensisisalsomediatedbyredox-activeflavincom-pounds,11,13whicharesecretedbythebacteriaandfunctionaselectronshuttles.Incontrasttothemesophilicspecies,thermophilicmembersofexoelectrogensremainunderexploitedandhavesofarbeenlargelylimitedtotheGram-positivespeciesaffiliatedwiththegenusThermincolaofthephylumFirmicutes.Twothermophilicbacteriastrains,T.potensstrainJR17andT.ferriaceticastrainZ-0001,18havebeenshowntotransferelectrons,intheabsenceofexogenousmediators,directlytoanelectrode.Arecentstudyhassuggestedthatmultihemec-typecytochromeslocalizedtoReceived:June21,2013Revised:September16,2013Accepted:September20,2013Published:September20,2013Articlepubs.acs.org/est©2013AmericanChemicalSociety12583dx.doi.org/10.1021/es402749f|Environ.Sci.Technol.2013,47,12583−12590thecellenvelopeoftheT.potensstrainJR,whichincontrasttotheGram-negativeenvelopelacksanoutermembrane,butinsteadhasacellwall,areimplicatedintheelectrontransfer.19However,molecularphylogeneticanalyseshavesuggestedthatmorediversespeciesofthermophilicbacteriashareexoelectrogenicactivity.InthermophilicMFCsinoculatedwithmarinesediment-anddigestersludge-derivedconsortia,althoughFirmicutes(particularly,genusThermincola)-affiliatedspeciesdominatedtheanodicmicrobialcommunities,sequen-cesrelatedtootherphyla(includingDeferribacteres,Proteobacteria,Spirochaetae,Nitrospirae,Thermotogae,andCoprothermobacter)werealsodetected.17,20Moreover,inathermophilicMFCcontinuouslyfedwithdistillerywastewater,aBacteriodetesbacteriumwasthedominantspeciesintheanodeandFirmicuteswasthesecondmostpopulatedgroup.21Furthermore,inadigestereffluent-inoculatedthermophilicMFC,noFirmicute-relatedsequencewasdetectedattheanodeandthedominantspeciesbelongedtothephylaDeferribacteresandCoprothermobacter.22However,theexoelectrogenicactivityofsuchnon-Firmicutethermophileshasneverbeeninvestigatedinpureculture.IthasrecentlybeensuggestedthatthermophilicMFCsarepotentiallysuperiortomesophilicMFCsinperformance,withhigherreactionactivity,greaterdurability,andawidersubstraterange.17,20,23Withregardtopo