Depth profiles of bacterioplankton assemblages and their activities in the Ross Sea

The identificationofbacterialcommunitystructurehasled,sincethebeginningofthe1990s,totheideathatbacterioplanktonpopulationsarestratifiedinthewatercolumnand thatdiverselineageswithmostlyunknownphenotypesdominatemarinemicrobialcommunities.Thediversityofdepth-relatedassemblagesisalsoreflectedintheirpatterns ofactivities,asbacteriaaffiliatedtodifferentgroupscanexpressdifferentactivities inagivenecosystem.Weanalysedbacterialassemblages(DGGEfingerprint-ing) andtheiractivities(prokaryoticcarbonproduction,protease,phosphatase,chitinase,beta-glucosidaseandlipaseactivities)intwoareasintheRossSea,differingmainlyintheirproductivityregime:twostationsarelocatedintheTerraNovaBaypolynyaarea(highlyproductiveduringsummer)andtwoclosetoCapeAdare(lowphytoplanktonbiomassandactivity).Ateverystationapronouncedstratificationofbacterial assemblageswasidentified,highlightingepipelagiccommunitiesdifferingsubstantiallyfromthemesopelagicandthebathypelagiccommunities.Multivariateanalysis suggestedthatpressureandindirectlylight-affectedvariables(i.e.oxygenandfluorescence)hadagreateffectonthebacterialcommunitiesoutcompetingthepossibleinfluencesoftemperatureanddissolvedorganiccarbonconcentration.Generallyactivities decreasedwithdeptheventhoughasignaloftheCircumpolarDeepWater(CDW)atoneofthenorthernstationscorrespondedtoanincreaseinsomeofthedegradativeactivities,generatingsome‘hotspots’intheprofile.Wealsofoundthatsimilarassemblagesexpresssimilarmetabolicrequirementsreflectedinanalogouspatterns ofactivity(similardegradativepotentialandleucineuptakerate).Furthermore,the presenceofeukaryoticchloroplasts’16SrDNAindeepsampleshighlightedhowinsomecasesthedensesurface-waterformation(inthiscaseHighSalinityShelfWater—HSSW) anddownwellingcanaffect,atleastforsomephylotypes,thebacterial(16SrDNAbased)communitystructureofthedarkocean.