Seasonal microbial food web dynamics in contrasting Southern Ocean productivity regimes

• Published in: Limnology and Oceanography
• Main Authors: Chistaki, Urania, Gueneugues, Audrey, Liu, Yan, Blain, Stéphane, Catala, Philippe, Colombet, Jonathan, Debeljak, Pavla, Jardillier, Ludwig, Irion, Solène, Planchon, Frédéric, Sassenhagen, Ingrid, Sime-Ngando, Télesphore, Obernosterer, Ingrid
• 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 (ULCO), Laboratoire d'Océanographie Microbienne (LOMIC), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Observatoire océanologique de Banyuls (OOB), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire Microorganismes : Génome et Environnement (LMGE), Université Clermont Auvergne 2017-2020 (UCA 2017-2020 )-Centre National de la Recherche Scientifique (CNRS), Ecologie Systématique et Evolution (ESE), AgroParisTech-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS), Laboratoire des Sciences de l'Environnement Marin (LEMAR) (LEMAR), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Institut Universitaire Européen de la Mer (IUEM), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), We thank B. Queguiner, the PI of the MOBYDICK project, for providing us the opportunity to participate to this cruise, the captain and crew of the R/V Marion Dufresne for their enthusiasm and support aboard during the MOBYDICK-THEMISTO cruise (https://doi.org/10.17600/18000403) and the chief scientist I. Obernosterer. This work was supported by the French oceanographic fleet ("Flotte oceanographique francaise"), the French ANR ("Agence Nationale de la Recherche", AAPG 2017 program, MOBYDICK Project number: ANR-17-CE01-0013), and the French Research program of INSU-CNRS LEFE/CYBER ("Les enveloppes fluides et l'environnement"-"Cycles biogeochimiques, environnement et ressources"). This work was also supported by ULCO (Universite du Littoral) and CPER MARCO (https://marco.univ-littoral.fr/).We thank three anonymous reviewers and the editors for their comments which helped to improve this work., ANR-17-CE01-0013,MOBYDICK,Biodiversité des Ecosystèmes Marins et Dynamique du Carbone dans le secteur de Kerguelen : approche intégrée(2017)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2021
• Subjects: acl; productivity regimes; Southern Ocean; fluxes; Microbial food web; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; [SDE.BE]Environmental Sciences/Biodiversity and Ecology; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDV.MP]Life Sciences [q-bio]/Microbiology and Parasitology; Kerguelen; Indian; Kerguelen Island
• Online Access: https://univ-poitiers.hal.science/hal-03034631; https://univ-poitiers.hal.science/hal-03034631/document; https://univ-poitiers.hal.science/hal-03034631/file/Christaki_2020_HAL.pdf; https://doi.org/10.1002/lno.11591

International audience Spatial and seasonal dynamics of microbial loop fluxes were investigated in contrasting productivity regimes in the Indian sector of the Southern Ocean. Observations carried out in late summer (February-March 2018; project MOBYDICK) revealed higher microbial biomasses and fluxes in the naturally iron-fertilized surface waters of Kerguelen island in comparison to surrounding off-plateau waters. Differences were most pronounced for bacterial heterotrophic production (2.3-fold), the abundance of heterotrophic nanoflagellates (HNF; 2.7-fold). Independent of site, grazing by HNF was the main loss process of bacterial production (80-100%), while virus-induced mortality was low (< 9%). Combining these results with observations from previous investigations during early spring and summer allowed us to describe seasonal patterns in microbial food web fluxes and to compare these to carbon export in the iron-fertilized and highnutrient, low-chlorophyll (HNLC) Southern Ocean. Our data suggest an overall less efficient microbial food web during spring and summer, when respiration and viral lysis, respectively, represent important loss terms of bacterially-mediated carbon. In late summer, primary production is more efficiently transferred to bacterial biomass and HNF and thus available for higher trophic levels. These results provide a new insight into the seasonal variability and the quantitative importance of microbial food web processes for the fate of primary production in the Southern Ocean.