Biodiversity of the marine ecosystem and carbon flux around Kerguelen (Southern Ocean) : role of small phytoplankton at the single cell level

• Main Author: Irion, Solène
• Other Authors: Laboratoire d’Océanologie et de Géosciences (LOG) - UMR 8187 (LOG), Institut national des sciences de l'Univers (INSU - CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD France-Nord ), Université du Littoral Côte d'Opale, Urania Christaki, Ludwig Jardillier
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2020
• Subjects: Small phytoplankton; Diatoms; Carbon fixation; Carbon export; Sequencing; NanoSIMS; Chemotaxonomy; Southern ocean; Petit phytoplancton; Diatomées; Fixation de carbone; Export de carbone; Séquençage; Chémotaxonomie; Océan Austral; [SDV.BID]Life Sciences [q-bio]/Biodiversity; [SDV.EE.ECO]Life Sciences [q-bio]/Ecology; environment/Ecosystems; antartic*; Southern Ocean
• Online Access: https://theses.hal.science/tel-03178263; https://theses.hal.science/tel-03178263/document; https://theses.hal.science/tel-03178263/file/These_Irion_Solene_version_definitive.pdf

This thesis focuses on small phytoplankton diversity (<20 µm) and its contribution to CO₂-fixation in contrasted marine ecosytems : the productive Kerguelen Plateau (KP) on the one hand and low productive surrounding waters on the other han. Iron-limited phytoplankton communities off-plateau are dominated by small cells all year long, whereas natural iron-fertilization over the KP promotes the seasonal development of chain-forming or large diatom blooms in spring and summer. Since the demonstration of natural iron fertilization on-plateau, the scientific community focused on large diatoms, assumed to promote carbon sequestration in the area. However, satellite data suggest that small phytoplankton dominate the phytoplankton communities outside of the bloom period on- and off-plateau. Consequently, this thesis had two main objectives. The first objective of this work was to obtain a precise image of the diversity of small and large phytoplankton after the diatom bloom (March 2018). A fragment of the 18S rRNA gene from small (0.2-20 µm) and large (20-100 µm) planktonic communities collected at discrete depths (down to 300 m), was sequenced (Illumina MiSeq) and used as an identity marker gene to determine the taxonomic diversity of small and large phytoplankton. At the surface, diatoms were dominant in the large-size fraction, while Phaeocystis antartica was particularly abundant in the small-size fraction, over the entire study area. As a whole, small phytoplankton communities were significantly different on- and off-plateau. High concentrations of silicic acid off-plateau favored the presence of a diverse assemblage of small diatoms, while high concentrations of ammonium on-plateau likely promoted the development of pico-sized Micromonas. Using chemotaxonomic pigments markers allowed the description of the temporal succession of phytoplankton communities on-plateau, dominated by diatoms from the onset to the decline of the bloom, while the contribution of small phytoplankton increased sharply after the bloom ...