MeteorcruiseM137,Callao–Callao,06.–29.Mai,2ndWeeklyReport,14.May2017StefanSommerandtheM137Team

DespitetheshorttransitbetweenCallaoandthefirstworkingstationofonlyafewhours,thelaboratoriesandscientificgearswerereadyfordeploymentallowingtostartwiththestationwork.ThestudiesattheseafloorarefocusingontheshelfareaoffCallao,wherewesampleinwaterdepthsdownto350msedimentsandconductfluxmeasurementsaswellasexperiments.Wemakegoodprogresswithfluxmeasurementsusingthebenthicchamberlander(Fig.1).However,sometimesmothernaturewreckupourplans,especiallywhensmallcrustaceans(Pleuroncodesmonodon)onthesearchforoxygenhideinthevalvelidofthefluxchamberanddisturbourmeasurements(Fig.1).

Fig.1:Crustaceans(Pleuroncodesmonodon)hidinginthevalvelidofthebenthicchamberofthelander(Photos:C.Rohleder,S.Sommer).ExperimentsonboardoftheRVMETEORinvestigatingtheturnoverofforaminiferaandsulphurbacteriarepresentamajorfocusofthisresearchcruise.Thesinglecelledeukaryoticforaminifera(Fig.2A)andthefilamentoussulphurbacteria(Fig.2B)areimportantfortheturnoverofnitrogencompoundsandwithregardtosulphurbacteriaalsoforthephosphorouscycling.Therewith,bothgroupsoforganismsstronglycontributetotheoverallnitrogenandphosphorousbudgetofthePeruvianOxygenminimumzone(OMZ).Both

groupscolonizethesurfacesedimentswithintheOMZathighabundances.SulphurbacteriabelongingtothegeneraMarithioplocaandBeggiatoaoftenbecomesapparentasdistinctwhitepatchesatthesedimentsurface,whichcanbeeasilydetectedusingatowedcamerasystem.Theydominatetheappearanceofsurfacesedimentsfromtheshallowwaterdowntowaterdepthsof350m.Dependentonthebottomwateravailabilityofoxygen,nitrateandnitrite,membersofBeggiatoaarecapabletostorephosphorousaspolyphosphate.UnderanoxicconditionsthisstorageisdegradedtogainenergywhereatphosphateisreleasedresultingintransientlyhighP-fluxesfromtheseafloor.FirstmicroscopicanalysesonboardRVMeteorshowedthatBeggiatoasp.attheanoxicstationin244mwaterdepthhavelargepolyphosphateinclusions,whicharevisibleaswhitespots(Fig.2C)inthevacuolesofthesebacteria.Theactualorganismishardlytosee.

Abb.2:ALight-andfluorescencemicroscopicalimagesofforaminifera;B.InterwovenfilamentsofMarithioplocaandBeggiatoawithsedimentparticlesjustaftersampling;C.SingleBeggiatoafilamentwithpolyphosphateinclusions,whichareapparentaswhitespots.Thediameterofthefilamentsisabout14µm;(Photos:S.Roy,S.Sommer,S.Langer).Agroupoffivescientistsarededicatedfortheanalysisofforaminiferainsurfacesediments.Foraminiferathemselvesorinassociationwithbacterialendosymbiontsareabletousenitrateornitriteinsteadofoxygentogainenergyduringtheoxidationoforganicmatter.Theinvestigationofdenitrificationratesofsinglespecies,incubationoftheentiresedimentsystemaswellassamplingforlattergenomic,transcriptonomicandmorphologicalanalysesarethefocusofthesestudies.Hence,hundredsofthesesmallorganismsneedtobepickedevenundermovingshipconditionsusingahigh-resolutionbinocular.FirstratemeasurementsshowedthatthespeciesBolivinaseminudaundCassidulinasp.insedimentsobtainedfrom248mwaterdepthhadahighnitrateuptake.Remarkably,themuchbiggerspeciesCassidulinasp.hasasmalleruptakerateofabout56pmolperindividualanddaycomparedto88pmolperindividualanddayofthesmallerBolivina

seminuda.Bothorganismsfavouranoxicsedimentsastheirhabitat.Hence,weassumethatbothspeciesadopteddifferentlytoalifeunderanoxicconditions.Thediscoveryofanintra-cellularphosphatestorageofabout271pmolperindividualinCassidulinasp.isanotherremarkableresultoftheseinvestigations.Thishintstowardstheuseofanalternativeenergysourceofthisspecies.Theabundanceofbenthicforaminiferansinthiswaterdepthamountsupto600individualspercm2andtherewithmightbeveryimportantfortheoverallnutrientbudgetatthisstation.Despiteintensestationworks,themoodonboardisverygoodandtheatmosphereisveryfriendlytowhichthecaptainRainerHammacherandhisentireMETEORteamstronglycontributes–thankyou!Allonboardarefine,withbestregards,StefanSommerandtheentireM137-Team