Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current

Philipp Assmy*, Victor Smetacek, Marina Montresor, Christine Klaas, Joachim Henjes, Volker H. Strass, Jesus Arrieta, Ulrich Bathmann, Gry M. Berg, Eike Breitbarth, Boris Cisewski, Lars Friedrichs, Nike Fuchs, Gerhard J. Herndl, Sandra Jansen, Sören Krägefsky, Mikel Latasa, Ilka Peeken, Rüdiger Röttgers, Renate ScharekSusanne E. Schüller, Sebastian Steigenberger, Adrian Webb, Dieter Wolf-Gladrow

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

164 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)20633-20638
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number51
StatePublished - Dec 17 2013


  • Evolutionary arms race
  • Geo-engineering
  • Top-down control

ASJC Scopus subject areas

  • General

Cite this