Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification

• Main Authors: Pfister, A.C., Cardini, U., Mirasole, A., Montilla, L., Veseli, I., Gattuso, Jean-Pierre, Teixido, Nuria
• Other Authors: 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), Institut du Développement Durable et des Relations Internationales (IDDRI), Institut d'Études Politiques IEP - Paris
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2023
• Subjects: [SDE]Environmental Sciences; Ocean acidification
• Online Access: https://hal.science/hal-04281106; https://hal.science/hal-04281106/document; https://hal.science/hal-04281106/file/Pfister_etal_sbm_Scientific%20Reports.pdf

International audience Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we test whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we show that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural 2 carbon dioxide vents and show that microbial ammonification was reduced at lower pH. The presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6 to 17.4 with the greatest effect at CO2 vent sites. Seagrass and its microbial associates thus enhanced water column productivity and were a locus for nutrient cycling.