Microbial Competition in the Subpolar Southern Ocean: An Fe–C Co-limitation Experiment

• Published in: Frontiers in Marine Science
• Main Authors: Fourquez, Marion, Bressac, Matthieu, Deppeler, Stacy, L, Ellwood, Michael, Obernosterer, Ingrid, Trull, Thomas, W, Boyd, Philip, W
• Other Authors: University of Tasmania Hobart, Australia (UTAS), Université de Genève = University of Geneva (UNIGE), Laboratoire d'océanographie de Villefranche (LOV), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut de la Mer de Villefranche (IMEV), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), National Institute of Water and Atmospheric Research Wellington (NIWA), Australian National University (ANU), 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)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2020
• Subjects: iron; carbon; Southern Ocean; competition; heterotrophic bacteria; pico-nanoplankton; Fe uptake; bacterial production; [SDV.MP]Life Sciences [q-bio]/Microbiology and Parasitology; [SDE]Environmental Sciences; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography
• Online Access: https://cnrs.hal.science/hal-03831171; https://cnrs.hal.science/hal-03831171v2/document; https://cnrs.hal.science/hal-03831171v2/file/Fourquez%20et%20al.%202020.pdf; https://doi.org/10.3389/fmars.2019.00776

International audience Iron (Fe) is a paradox in the modern ocean-it is central to many life-critical enzymes but is scarce across most surface waters. The high cellular demand and low bioavailability of Fe likely puts selective pressure on marine microorganisms. Previous observations suggest that heterotrophic bacteria are outcompeted by small diatoms for Fe supply in the subantarctic zone of Southern Ocean, thereby challenging the idea of heterotrophic bacteria being more competitive than phytoplankton in the access to this trace metal. To test this hypothesis, incubation experiments were carried out at the Southern Ocean Time Series site (March-April 2016). We investigated (a) whether dissolved organic carbon (DOC), dissolved Fe, or both limit the growth of heterotrophic bacteria and, (b) if the presence of potential competitors has consequences on the bacterial Fe acquisition. We observed a pronounced increase in both bulk and cell-specific bacterial production in response to single (+C) and combined (+Fe+C) additions, but no changes in these rates when only Fe was added (+Fe). Moreover, we found that +Fe+C additions promoted increases in cell-specific bacterial Fe uptake rates, and these increases were particularly pronounced (by 13-fold) when phytoplankton were excluded from the incubations. These results suggest that auto-and heterotrophs could compete for Fe when DOC limitation of bacterial growth is alleviated. Such interactions between primary producers and nutrient-recyclers are unexpected drivers for the duration and magnitude of phytoplankton blooms in the Southern Ocean.