Since 2011, blooms of holopelagic Sargassum have been observed in the Tropical North Atlantic Ocean which caused important beach stranding along the Caribbean and African shores. Main microbial contributors of the nitrogen cycle were studied in holopelagic or benthic Sargassum biofilm and in adjacent waters to understand their relative contribution to seaweed growth in the Great Atlantic Sargassum Belt (GASB) and in Caribbean coastal zones. In GASB Sargassum biofilm, the proportion of diazotrophs (composed of 76% of non-cyanobacterial-nifH sequences) predominated and declined in coastal samples. Denitrifying and nitrifying counterparts showed an opposite trend. Biofilm diazotrophs also exceeded those found in the nearby waters, whereas nitrifiers and denitrifiers presented the same trends in both sample types. Benthic Sargassum biofilm showed lower values for all investigated communities. Significant differences were observed between diazotroph diversity of pelagic and benthic biofilms or adjacent water microbiomes. The GASB seaweed mean δ¹⁵N value was very low (-2.04‰), and increased in coastal samples (-1.17‰), whereas lower values were recorded for benthic Sargassum (0.03‰) or POM (1.73-4.95‰). These results highlighted the atmospheric origin of nitrogen used during the pelagic Sargassum growth  in the GASB and extended the role of biofilm diazotrophs to non-Cyanobacteria.

Primary Presenter: Matéo Léger-Pigout, Aix-Marseille University (mateo.leger-pigout@mio.osupytheas.fr)

Authors:

Matéo Léger-Pigout, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (mateo.leger-pigout@mio.osupytheas.fr)

Elisabeth Navarro, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (elisabeth.navarro@univ-tln.fr)

Frederic Menard, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (frederic.menard@mio.osupytheas.fr)

Sandrine Ruitton, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (sandrine.ruitton@mio.osupytheas.fr)

François Le Loc’h, University of Brest, CNRS, IRD, Ifremer, LEMAR, F-29280, Plouzane, France  (francois.le.loch@ird.fr)

Sophie Guasco, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (sophie.guasco@mio.osupytheas.fr)

Jean-Marie MUNARON, University of Brest, CNRS, IRD, Ifremer, LEMAR, F-29280, Plouzane, France  (Jean.Marie.Munaron@ird.fr)

Delphine Thibault, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (delphine.thibault@mio.osupytheas.fr)

Thomas Changeux, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (thomas.changeux@mio.osupytheas.fr)

Solène Connan, University of Brest, CNRS, IRD, Ifremer, LEMAR, F-29280, Plouzane, France  (solene.connan@univ-brest.fr)

Valérie Stiger-Pouvreau, University of Brest, CNRS, IRD, Ifremer, LEMAR, F-29280, Plouzane, France  (valerie.stiger@univ-brest.fr)

Thierry Thibault, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (thierry.thibaut@univ-amu.fr)

Valérie Michotey, Aix Marseille University, University of Toulon, CNRS, IRD, MIO, 13288 Marseille, France  (valerie.michotey@mio.osupytheas.fr)