CompetitionbetweenFeIIIReducingandMethanogenic

CompetitionbetweenFe(III)-ReducingandMethanogenicBacteriaforAcetateinIron-RichFreshwaterSedimentsE.E.Roden,1R.G.Wetzel21DepartmentofBiologicalSciences,TheUniversityofAlabama,Tuscaloosa,AL35487-0206,USA2DepartmentofEnvironmentalSciencesandEngineering,TheUniversityofNorthCarolina,ChapelHill,NC27599-7431,USAReceived:18June2002;Accepted:18November2002;Onlinepublication:28March2003ABSTRACTThekineticsofacetateuptakeandthedepthdistributionof[2-14C]acetatemetabolismwereexaminediniron-richsedimentsfromabeaverimpoundmentinnorthcentralAlabama.Thehalf-saturationconstant(Km)determinedforacetateuptakeinslurriesofFe(III)-reducingsediment(0.8lM)wasmorethan10-foldlowerthanthatmeasuredinmethanogenicslurries(12lM)whichsupportedcomparableratesofbulkorganiccarbonmetabolismandVmaxvaluesforacetateuptake.Theendogenousacetateconcentration(Sn)wasalsosubstantiallylower(1.7lM)inFe(III)-reducingvsmethanogenic(9.0lM)slurries.Theproportionof[2-14C]acetatecon-vertedto14CH4increasedwithdepthfromca0.1intheupper0.5cmtoca0.8below2cmandwasinverselycorrelated(r2=0.99)toadeclineinamorphousFe(III)oxideconcentration.TheresultsoftheacetateuptakekineticsexperimentssuggestthatdifferencesintheaffinityofFe(III)-reducingbacteriavsmethanogensforacetatecanaccountforthepreferentialconversionof[2-14C]acetateto14CO2inFe(III)oxide-richsurfacesediments,andthatthedowncoreincreaseinconversionof[2-14C]acetateto14CH4canbeattributedtoprogressiveliberationofmetha-nogensfromcompetitionwithFe(III)reducersasFe(III)oxidesaredepletedwithdepth.IntroductionUnderstandingthebiogeochemicalcontrolsonmethaneproductioninhydromorphicsoilsandfreshwateraquaticsedimentsisimportantforpredictingspatialandtemporalpatternsoftheinputofthisimportantgreenhousegastotheatmosphere[5].NumerousstudieshaveindicatedthatmicrobialFe(III)oxidereductionplaysanimportantroleingoverningtheproductionandreleaseofmethanefromiron-richnaturalandagriculturalwetlandsoils[1,7,19,23,28–30].AvailableevidencesuggeststhatdissimilatoryFe(III)-reducingbacteria(FeRB)cansuccessfullyout-competemethanogenicbacteria(MGB)foracetateandH2(bothmajorintermediatesintheanaerobicdecompositionoforganiccarboninanaerobicenvironments[12]),therebysuppressingratesofmethaneproductioninsitu-ationswheremicrobiallyreducibleFe(III)oxidesareabun-dant[15].SuchcompetitiveinteractionsareanalogousCorrespondenceto:E.E.Roden;E-mail:[e-mail](2003)45:252–258DOI:10.1007/s00248-002-1037-9
2003Springer-VerlagNewYorkInc.towell-knowninteractionsbetweensulfate-reducingbac-teria(SRB)andMGBinanaerobicsediments[31].Al-thoughasubstantialbodyofliteratureexistsoninteractionsbetweenbacterialFe(III)reductionandmethanogenesisinanaerobicsoilsandsediments[2,7,14,15,23],anexplicitdemonstrationofthekineticnatureofcompetitiveinteractionsbetweenFeRBandMGBforace-tateisnotyetavailable.Herewedescribestudiesof[2-14C]acetatemetabolisminfreshwatersedimentswhichindicatethatdowncore(0–5cm)relationshipsbetweenFe(III)oxideabundanceandpathwaysof[2-14C]acetatemetabolismcanbeexplainedbydifferencesinthehalf-saturationconstantforacetateuptakebyFeRBvsMGBpresentinthewetlandsediment.MaterialsandMethodsStudySiteandSampleHandlingSedimentswereobtainedfromabeaverimpoundmentintheTalladegaNationalForestinnorthcentralAlabama,USA.TheTalladegaWetland(TW)sedimentsarerichinreactive(0.5MHClextractable)iron(ca5%drywt),andFe(III)oxidereductionmediatesasubstantialfractionofsedimentcarbonmetabolismandregulatesmethaneproductionintheupperfewcentimetersofunvegetatedsediment[23].Sulfateconcentrationsareinthelowrangeforfreshwatersystems(40lM),whichstronglylimitstheroleofsulfatereductioninsedimentcarbonmetabolismandFediagenesis[23].Undisturbedsedimentcoreswereobtainedwith10-cmIDPlexiglastubes.Coresweretransferredtoananaerobicchamber(CoyProducts)andeithersectionedat0.25to1.0-cmintervals,orusedtoprepareslurrieswithmaterialfromtheupper5–10cmofsediment.Theslurrieswereincubatedinthedarkatroomtemperatureforca2weekstodepleteFe(III)oxides,afterwhichportionswereoxidizedbystirringandspargingwithairfor24h.Thisprocedureconvertedvirtuallyall(90%)ofthesolidanddissolvedFe(II)inthesedimenttoamorphous,0.5MHCl–ex-tractableFe(III)oxide.Aninoculum(5%vol)ofreducedsedi-mentwasaddedtooxidizedslurriestoreestablishanaerobicbacterialpopulations.PortionsofhomogenizedsedimentorsedimentslurryweretransferredunderN2toserumbottles,andthebottlesweresealedwiththickrubberstoppers.Allsedimentslurrieswereincubatedinthedarkatroomtemperature.KineticsofAcetateUptakeThekineticsofacetateuptakeinFe(III)-reducingvsmethano-genicTWsedimentwasdeterminedbyquantifyingtheuptakeof[2-14C]acetateinslurriesamendedwithdifferentconcentrationsofunlabeledacetate[13,33].Triplicate3-mLportionsofslurrywereamendedwith0.3mLofsterile,anaerobicsodiumacetatesolutionsinordertoprovidearangeofaddedacetateconcen-trationsof0–16lM.Parallellargerbottlesofsedimentslurry(50mLinitialvolume)werepreparedintriplicatefordeterminationofFe(III)reductionandCH4productionrates.The[2-14C]acetateuptakeexperimentswereconductedaftera2-dpreincubationperiod,afterwhichtimerate