title: Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current.
abstract: Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.
authors.affiliation: Norwegian Polar Institute, 9296 Tromsø, Norway.
authors.forename: Philipp
authors.initials: P
authors.lastname: Assmy
authors.forename: Victor
authors.initials: V
authors.lastname: Smetacek
authors.forename: Marina
authors.initials: M
authors.lastname: Montresor
authors.forename: Christine
authors.initials: C
authors.lastname: Klaas
authors.forename: Joachim
authors.initials: J
authors.lastname: Henjes
authors.forename: Volker H
authors.initials: VH
authors.lastname: Strass
authors.forename: Jesús M
authors.initials: JM
authors.lastname: Arrieta
authors.forename: Ulrich
authors.initials: U
authors.lastname: Bathmann
authors.forename: Gry M
authors.initials: GM
authors.lastname: Berg
authors.forename: Eike
authors.initials: E
authors.lastname: Breitbarth
authors.forename: Boris
authors.initials: B
authors.lastname: Cisewski
authors.forename: Lars
authors.initials: L
authors.lastname: Friedrichs
authors.forename: Nike
authors.initials: N
authors.lastname: Fuchs
authors.forename: Gerhard J
authors.initials: GJ
authors.lastname: Herndl
authors.forename: Sandra
authors.initials: S
authors.lastname: Jansen
authors.forename: Sören
authors.initials: S
authors.lastname: Krägefsky
authors.forename: Mikel
authors.initials: M
authors.lastname: Latasa
authors.forename: Ilka
authors.initials: I
authors.lastname: Peeken
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Röttgers
authors.forename: Renate
authors.initials: R
authors.lastname: Scharek
authors.forename: Susanne E
authors.initials: SE
authors.lastname: Schüller
authors.forename: Sebastian
authors.initials: S
authors.lastname: Steigenberger
authors.forename: Adrian
authors.initials: A
authors.lastname: Webb
authors.forename: Dieter
authors.initials: D
authors.lastname: Wolf-Gladrow
mesh_terms: D000864:Antarctic Regions; D005075:Biological Evolution; D002244:Carbon / Q000378:metabolism*; D017377:Diatoms / Q000502:physiology*; D017753:Ecosystem; D007501:Iron / Q000378:metabolism*; D009792:Oceans and Seas; D010839:Phytoplankton / Q000502:physiology*; D012825:Silicon / Q000378:metabolism*
chemical_list: D002244:Carbon; D007501:Iron; D012825:Silicon
keywords: evolutionary arms race; geo-engineering; top-down control