73SaltmarshplantsJuncusmaritimusandScirpusmaritimu

Saltmarshplants(JuncusmaritimusandScirpusmaritimus)assourcesofstrongcomplexingligandsAnaP.Muchaa,*,C.MarisaR.Almeidaa,AdrianoA.Bordaloa,b,M.TeresaS.D.Vasconcelosa,caCIMAR/CIIMAReCentroInterdisciplinardeInvestigac¸~aoMarinhaeAmbiental,UniversidadedoPorto,RuadosBragas,289,4050-123Porto,PortugalbLaborato´riodeHidrobiologia,InstitutodeCieˆnciasBiome´dicasdeAbelSalazar(ICBAS-UP),UniversidadedoPorto,LargoProfessorAbelSalazar,n2,4099-003Porto,PortugalcDepartamentodeQuı´mica,FaculdadedeCieˆncias,UniversidadedoPorto,RuadoCampoAlegre,687,4169-007Porto,PortugalReceived28December2005;accepted12September2007Availableonline11October2007AbstractThisworkaimedtoevaluate,invitro,thecapabilityofrootsofsaltmarshplantstoreleasestrongCu-complexingligandsandtoascertainwhetherCucontaminationwouldstimulateligands’exudationornot.ThesearushJuncusmaritimusandthesea-clubrushScirpusmaritimus,bothfromthelowerDouroriverestuary(NWPortugal),wereused.Plantswerecollectedseasonally,fourtimesayearin2004,duringlowtide.Aftersampling,plantrootswerewashedforremovalofadherentparticlesandimmersedfor2hinasolutionthatmatchedsalinity(3)andpH(7.5)oftheporewaterfromthesamelocationandspikedwithCu2þintherange0e1600nMtoobtainplantexudates.InthefinalsolutionsaswellasinsedimentporewatertotaldissolvedZnandCu,Cu-complexingligandconcentrationsandtherespectiveconditionalstabilityconstants(KCuL0)valuesweredeterminedbyvoltammetry.Thisstudydemonstratedthatplantsareabletorelease,inashortperiodoftime,relativelyhighamountsofstrongCu-complexingligands(56e265nmolgroot1),whichdifferedamongplantsandsamplingsitebutwereindependentofthesea-son.CucontaminationdidnotstimulateexudationofCu-complexingligands.Ontheotherhand,inmediacontaminatedwithCubothplantsaccumulatedrelativelyhighamounts(29e83%)oftheinitiallydissolvedCu,indicatingthattheyhavealternativeinternalmechanismsforCudetoxification.Cuexchangebetweenrootsandmedium(eitheraccumulationincontaminatedmediumorreleaseintheabsenceofCu)wasmoreintenseforS.maritimusthanforJ.maritimus.ItwasobservedthatexudatesolutionsobtainedintheabsenceofaddedCuandsedimentporewater(thedensitiesofrootsobservedinsidethesaltmarshwherecomparabletothoseusedintheinvitroexperiments),displayedsimilaritiesintermsoftotaldissolvedmetals,Cu-complexingligandsconcentrations,valuesofKCuL0(12logKCuL014),aswellaspatternsofvariationamongseasons(onlyobservedforZn).Theseresultsarenovelandpointoutthatsaltmarshplantsmaybethesourceatleastpartiallyofthestrongorganicligandsfoundinthesedimentporewaterinshallowmarginalareas.Thecapabilityofsaltmarshplantstoreleasestrongorganicligandsintotheenvironment,conjugatedwiththeirknowncapacitytooxidizeanaerobicsedimentaroundroots,indicatethattheseplantscanplayaroleincontrollingmetalspeciationinthewater/sedimentinterface.2007ElsevierLtd.Allrightsreserved.Keywords:Cu-complexingligands;copper;zinc;rootexudates;sedimentporewater;estuarineenvironment1.IntroductionSaltmarshsedimentsmayactbothassinksandsourcesofcontaminantsinurbanestuaries,astheyoftenreceivevariablepollutantloads,includingdischargesoftracemetalsfromin-dustrialandtransportationactivities.Thechemicalforms(orspeciation)inwhichthetracemetalsarepresentintheestuary(sediments,porewaterandwatercolumn)maycontroltheirbioavailabilityandtoxicity.Inthewatercolumn,speciationofmanybiologicallyactivetracemetalsiscontrolledbycom-plexationwithstrongorganicligands(Brulandetal.,1991).*Correspondingauthor.E-mailaddress:[e-mail](A.P.Mucha).0272-7714/$-seefrontmatter2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.ecss.2007.09.011Availableonlineat(2008)104e112(Cu)isprobablythemoststudiedmetalintermsoforganiccomplexationinseawaterandestuarineenvironments.ThisisbecauseCuisamicronutrient,butitisalsotoxic,forexampleformicroalgae,atrelativelylowconcentrationlevels.Inaddition,Cuformscomplexesofrelativelyhighstabilitywithvariousorganicligandsandthereareanalyticaltoolsavailableforitsanalysis.Autochthonousorganismsaremajorproducersoftheli-gandsthatdominatethecomplexingcapacityofopenoceanandcoastalproductivewaters(Crootetal.,2000;Gordonetal.,2000).Severalorganisms,suchascoccolithophores(Lealetal.,1999),cyanobacteriaanddinoflagellates(Crootetal.,2000)andheterotrophicbacteria(Gordonetal.,2000)havebeenobservedtoexcretestrongCu-complexingligands,mostlikelyasadefencemechanismagainstCutox-icity.Theriverinputofterrestrialhumicsubstanceshasalsobeenrecognizedasanimportantsourceofstrongligandstoestuarineenvironments(KogutandVoelker,2001;Shanketal.,2004b).RecentstudieshaveshownthatestuarinesedimentscanalsoactasasignificantsourceofCu-complexingligandstotheoverlyingwater,whichmaystronglyinfluencethebio-geochemistryandcyclingofdissolvedCubysediment/waterexchange(Skrabaletal.,1997,2000;Shanketal.,2004a).Amongthemainsourcesoftheorganicligandsfoundinsed-iments,Skrabaletal.(2000)haveconsideredseveralpossi-bilities,includingexudationbybacteria,macrofaunaorotherorganisms;sulphide,polysulphidesandothermicrobialmetabolites;anddiagenetic