Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean

Type Article
Acceptance Date 2021 IN PRESS
Language English
Author(s) Irion SolèneORCID1, Christaki Urania1, Berthelot HugoORCID4, L’helguen Stéphane2, Jardillier LudwigORCID3
Affiliation(s) 1 : Université Littoral Côte d’Opale - ULCO, CNRS, Université Lille, UMR 8187 - LOG - Laboratoire d’Océanologie et de Géosciences, F-62930, Wimereux, France
2 : Laboratoire des Sciences de l’Environnement Marin (LEMAR), UMR 6539 UBO/CNRS/IRD/IFREMER, Institut Universitaire Européen de la Mer (IUEM), Brest, France
3 : Ecologie Systématique Evolution, Université Paris-Saclay, Centre National de la Recherche Scientifique - CNRS, AgroParisTech, Orsay, France
4 : Laboratoire des Sciences de l’Environnement Marin (LEMAR), UMR 6539 UBO/CNRS/IRD/IFREMER, Institut Universitaire Européen de la Mer (IUEM), Brest, France
Source The ISME Journal (1751-7362) (Springer Science and Business Media LLC) In Press
DOI 10.1038/s41396-021-00915-z
Keyword(s) Biogeochemistry, Microbial ecology, Stable isotope analysis
Abstract

Phytoplankton is composed of a broad-sized spectrum of phylogenetically diverse microorganisms. Assessing CO2-fixation intra- and inter-group variability is crucial in understanding how the carbon pump functions, as each group of phytoplankton may be characterized by diverse efficiencies in carbon fixation and export to the deep ocean. We measured the CO2-fixation of different groups of phytoplankton at the single-cell level around the naturally iron-fertilized Kerguelen plateau (Southern Ocean), known for intense diatoms blooms suspected to enhance CO2 sequestration. After the bloom, small cells (<20 µm) composed of phylogenetically distant taxa (prymnesiophytes, prasinophytes, and small diatoms) were growing faster (0.37 ± 0.13 and 0.22 ± 0.09 division d−1 on- and off-plateau, respectively) than larger diatoms (0.11 ± 0.14 and 0.09 ± 0.11 division d−1 on- and off-plateau, respectively), which showed heterogeneous growth and a large proportion of inactive cells (19 ± 13%). As a result, small phytoplankton contributed to a large proportion of the CO2 fixation (41–70%). The analysis of pigment vertical distribution indicated that grazing may be an important pathway of small phytoplankton export. Overall, this study highlights the need to further explore the role of small cells in CO2-fixation and export in the Southern Ocean.

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