EuropeanJournalofSoilScience,2013doi:10.1111/ejss.12051
Microbialseleniumtransformationsinseleniferoussoils
J.W.Fellowesa,R.A.D.Pattricka,C.Boothmana,W.M.M.AlLawatia,b,B.E.vanDongena,
J.M.Charnocka,J.R.Lloyda&C.I.Pearcec
aSchoolofEarth,AtmosphericandEnvironmentalSciencesandWilliamsonResearchCentreforMolecularEnvironmentalScience,
UniversityofManchester,Manchester,M139PL,UK,bHigherCollegeofTechnology,MinistryofManpower,Muscat113,Sultanateof
Oman,andcPacificNorthwestNationalLaboratory,Richland,Washington99352,USA
Summary
Selenium(Se)isanessentialtraceelementforanimalsanddisplaysanarrowrangebetweendietarydeficiency
andtoxicity.ThetoxicityofSedependsonitsbioavailability,whichisdirectlyrelatedtoitsoxidationstates,
ofwhichfouroccurintheenvironment(SeVI,SeIV,Se0andSeII−).Microbialcommunitiesdrivethecycling
ofSebetweentheseoxidationstates.InordertoinvestigatetheeffectofmicrobialactivityonSecyclingin
theenvironment,afieldsiteinCountyMeath,Ireland,wasidentifiedwithanomalouslylargeconcentrations
ofSeasaresultofweatheringofblackshaleswithintheLucanformation,leadingtocasesofSetoxicity
infarmanimals.SoilcoreswereextractedfromthesiteforSespeciationandmicrobialcommunityanalysis
priortomicrocosmexperimentstoassessSestabilityandmicrobialSetransformations.Seleniumwaspresent
asarecalcitrant,reducedorganicphasethatwasstronglycoordinatedwithcarbon,concordantwithsuggested
hypothesesofSephyto-concentrationwithinaclay-lined,postglacialmarshland.Seleniumwasnotmobilizedin
microcosmexperiments,andsupplementationwithSeVIresultedinrapidreductionandremovalfromsolution
asSe0.AdditionalelectrondonorsdidnotaffectSestabilityorremovalfromsolution,althoughnitratedid
hinderSeVIreduction.Terminalrestrictionfragmentlengthpolymorphismanalysisindicatedasignificantshift
inmicrobialcommunityafteramendmentwithSeVI.ThisworkextendsthecurrentknowledgeofSecycling
intheenvironment,andprovidesinformationonthebioavailabilityofSeinthesoil,whichdeterminesSe
contentoffoodstuffs.
Introduction
Selenium(Se)isanessentialtracenutrient,presentina
rangeofseleniferousproteinsandenzymesbutthesmall
rangebetweenhumandietarydefficiency(<40μgday−1)and
toxicity(>400μgday−1)requirescarefulcontrol,especiallywhen
monitoringhumandietsandsupplementingfeedstuffsforlivestock
(Fordyce,2007).Thebioavailabilitytoplantsand,thus,toxicityof
Seincropsislargelydependentonlocalgeochemicalconditions.
Inaqueoussolution,suchasgroundwaterandsoil-porewaters,
SeiscommonlyfoundastheSeIVandSeVIoxyanions,withthe
latterbeingpredominantandmobileundermoreoxic,alkaline
conditions(Oremlandetal.,2004;Fordyce,2007;Lenz&Lens,
2009).Underreducingconditions,insolubleSe0andSeII−mineral
phasesareexpected(Fordyce,2007;Lenz&Lens,2009).Volatile
organicspecies,suchasdimethylselenide,arealsofoundin
soilsandcontaminatedgroundandformedasaresultofbiotic
Correspondence:J.W.Fellowes.E-mail:Jonathan.Fellowes@Manchester.ac.ukReceived15August2012;revisedversionaccepted25February2013methylation(Lenz&Lens,2009).Biologicaltransformationsare
amajordrivingforceinthechangeofSefromonespeciesto
another,andavarietyofbiochemicalmechanismsareresponsible
forthelargerangeofseleniferouscompoundsfoundinthe
environment(Lietal.,2008;Lenz&Lens,2009).
GlobalsoilSeconcentrationsaresmall,typicallybetween0.1
and2.0μgg−1(Girling,1984;Berrow&Allan,1989),resultingin
theneedtoamendSe-deficientagriculturalsoilstoincreasedietary
uptake(Broadleyetal.,2006;Lietal.,2008).Theunderlying
lithologylargelydeterminestheSecontentofoverlyingsoils
(Fordyce,2007),andsoilsformedaboveSe-richrockssuch
asblackshalesandcoalmeasures,whencoupledwithevapo-
orphyto-concentrationmechanisms,cancontainelevatedSe
concentrationsinthemgg−1range(Fordyce,2007;Lenz&Lens,
2009).Accordingly,reportsofSetoxicityinhumansandlivestock
havebeenrecordedinsuchareas(Crinion,1980;Rogersetal.,
1990;Dhillon&Dhillon,1991;Fordyceetal.,2000).
CasesofSetoxicityinanimalsinIrelanddatebacktothelate
19thcentury(Parle&Fleming,1983),andRogersetal.(1990)list
sevencountiesaffectedbylargesoilSeconcentrations.Fleming
(1962)identifiesasitenearTrim,CountyMeath,withasoilSe
©2013TheAuthorsJournalcompilation©2013BritishSocietyofSoilScience12J.W.Fellowesetal.
concentrationof1.2mgg−1.TheoriginoftheSeispresumedtobe
theunderlyinginterbeddedmuddylimestone/shalesofthe‘Calp
Limestones’,partoftheDinantianLucanFormation(Parle&
Fleming,1983;McGrath&Fleming,2008).Weatheringofthese
selenium-containingrocksbyalkalinedrainagewatersfavours
theformationofmobileSeVIoxyanions,whicharetransported
intolocalizedlow-lying,clay-linedbasins,formedasaresultof
glaciationduringthelasticeage(Fleming,1962;Parle&Fleming,
1983;McGrath&Fleming,2008).Poordrainageresultsinthe
developmentoflow-lyingmarshland,allowingSetoaccumulate.
Inadditiontoitsroleasanutrient,seleniumisusedina
widerangeofchemicalandtechnologicalapplicationsbecause
ofitsuniquechemicalandphoto-opticalproperties,withaspecial
currentinterestinnano-particles(Oremlandetal.,2004;Lenz
&Lens,2009;Pearceetal.,2009;Fellowesetal.,2011).
Understandingthenaturalmicrobiologicalpathwaysdeveloped
toprocesslargeamountsofSeintheenvironmentprovides
theopportunitytocontrolitsbioavailabilityandtosynthesize
selenium-basedmaterialsbyanalternative,facile,‘green’route,
includingtheproductionofnovelbio-nano-materials(Oremland
etal.,2004;Pearceetal.,2008,2009)
Theaimofourworkwastocharacterizetheselenium
inhighlyseleniferoussoil(Fleming,1962)locatedinCounty
Meath,Ireland,examiningpossiblesourcesandenrichment
mechanismsandtherelationshipbetweensoilgeochemistryand
theincumbentmicrobialcommunity.Testingthehypothesisthat
soilmicroorganismsaredirectlyinvolvedintheaccumulation,
concentrationandremobilizationofseleniumwillprovidean
increasedunderstandingofgeo-microbiologicalinfluenceson
seleniumcyclinginseleniferousenvironments.
Materialsandmethods
Allchemicalsusedinthisworkwereofanalyticalgradeand
suppliedbySigmaAldrich,Dorset,UK,unlessotherwisestated.
Fieldsiteandsamplingtechniques
ThefieldsitewasidentifiedusingdatafromTeasgasc(Fayetal.,
2007)andpublishedliterature(Fleming,1962;Parle&Fleming,
1983;McGrath&Fleming,2008),whichhadrevealedanareanear
Trim,CountyMeath,ashavingrecordedsoilSeconcentrations
exceeding1.2mgg−1atadepthof15–30cm,andreportsof
seriousSetoxicityproblemsincattle(Fleming,1962;Crinion,
1980).Itisanareaofsubduedtopography(range30m),and
isaflatareaatthebottomofagentleslope.Followingthe
identificationofalocalizedseleniferoussoilbyFleming(1962),
theexactlocationofthepreviouslysampledseleniferoushorizon
wasfoundandthreesoilcoresweretakenincloseproximity
usinganEijkelkamp(Giesbeek,theNetherlands)auger.Thesoil
coreswereapproximately4cmindiameterand75cmlong,the
maximumattainabledepthatthissite.Forcomparison,afurther
threesoilcoresweretakenapproximately30maway,upslopeof
thefirstsamplingsite.Themaximumattainabledepthatthispointwasapproximately50cm.Ofthethreecorestakenateachsite,two
coreswerestoredaerobicallyforelementalandorganicconstituent
analysis,whilstthethirdwascollectedandstoredunderanoxic
conditionspriortomicrocosmandmolecularecologystudies(see
SupplementaryInformationfordetailedsamplingprocedure).
Soilcorecharacterization
Formineralogicalandchemicalanalyses,asoilcorewasdried
at70◦Candpowdered.Themineralcomponentofthesoilcore
wasthenexaminedbyX-raydiffraction(XRD)usingaBruker
(Coventry,UK)D8AdvancewithaCukαsource(1.54˚A).
Aliquotsofthesoilpowderwereseparatedforlossonignition
(LOI)analysistodeterminetotalorganiccarbon(C)contentof
thesoils.Aliquotswerealsousedtomakewax-mountedpelletsfor
X-rayfluorescence(XRF)analysisusingaPANalytical(Almelo,
TheNetherlands)AxiosPw4400.
Forlipidbiomarkeranalysesthesoilswerefreeze-dried,
groundandextractedwithadichloromethane:methanol(2:1v/v)
mixtureinasoxhlettoobtainthetotalextractablelipids.These
werefractionatedintoacid,apolarandpolarfractions,usinga
combinationofAgilent(Stockport,UK)Bond-Elut®andAl2O3columnchromatography,derivatizedandanalysedusinganAgi-
lent789AgaschromatographcoupledtoanAgilent5975CMSD
massspectrometer(seeSupplementaryInformationfordetails).
X-rayabsorptionspectroscopy(XAS)wasusedtodetermine
Sespeciationintheanoxicsoilcores,andundertakenon
thehighbrillianceX-rayspectroscopybeamlineID26ofthe
EuropeanSynchrotronRadiationFacility(ESRF),Grenoble,
France.Thestorageringwasoperatedatanominal6GeV
withacurrentof100–200mA.TheID26beamlineuses
Si<111>monochromatorcrystalstodeliveraspectralenergy
rangeof2.4–27keV,encompassingtheSeK-edgeataround
12.6keV.X-rayabsorptionnear-edgestructure(XANES)spectra
werecollectedinfluorescencemodeovertheenergyrange
12.63to12.70keV.Standardsweremeasuredintransmission
modeandcomprisedpowderedredamorphousandtrigonal
elementalSe(Se0),ironselenide(FeSe2,SeII−),selenomethionine
(C5H11NO2Se,SeII−),sodiumselenite(Na2SeO3,SeIV)and
sodiumselenate(Na2SeO4,SeVI)(Figure1),whichwereall
groundanddilutedwithboronnitridetooptimizetheedgejump.
Soilsandfrozenaliquotsfrommicrocosms(describedbelow)
weremountedanoxicallyontoamulti-samplestageforXAS
andmeasuredwithaliquidnitrogencryostat,allowingforXAS
determinationofSeinbothsolidandliquidphases.
Microcosmexperiments
Microcosmexperimentswereundertakentoassesstheroleof
microorganismsinthecyclingofSeatthefieldsite.Onegramof
anoxicsoilfromtheseleniferoushorizon(29–53cm)wasadded
to30-mlserumbottlesunderanN2atmosphere.Tenmillilitres
ofsyntheticgroundwaterwasaddedtoeachserumbottle.The
syntheticgroundwaterwasthesamecompositionasgroundwater
©2013TheAuthorsJournalcompilation©2013BritishSocietyofSoilScience,EuropeanJournalofSoilScience
