On the following subject: Study of the micro- and macro-organisms associated with holopelagic sargassum responsible for brown tides: impacts on algal blooms.
Member of the Jury
Ms Luisa MANGIALAJO (PR, UCA) - Examiner
Ms Valérie MICHOTEY (PR, MIO) - Thesis supervisor
Ms Élisabeth NAVARRO (CR, MIO) - Thesis co-supervisor
Mr Benjamin MISSON (PR, MIO) - Chairman of the jury
Mr Christophe LEBOULANGER (CR, MARBEC) - Rapporteur
Mr Jean-François BRIAND (MCF, MAPIEM) - Rapporteur
Guests Valérie Stiger (LEMAR) and Solenne Connan (LEMAR)
Summary
"Since 2011, a proliferation of holopelagic sargassum has been observed in the tropical North Atlantic, in a vast area known as the Great Atlantic Sargassum Belt (GASB). This phenomenon leads to massive groundings of sargassum on Caribbean coasts, with major environmental, health and economic repercussions for the regions affected. Several hypotheses have been put forward to explain this proliferation, but none has yet provided a complete and definitive explanation. Furthermore, as the phenomenon is transatlantic, sargassum can play a role in transporting organisms within a large geographical area. It is in this context that the aim of this thesis was to explore: (i) the diversity of organisms associated with Sargassum, and (ii) a new hypothesis according to which organisms in the Sargassum biofilm could influence its growth, and thus contribute to its proliferation. Using samples collected mainly from the GASB, the eukaryotic and nitrogen-fixing communities were characterised using 18S and nifH gene metabarcoding techniques. At the same time, quantification of the main players in the nitrogen cycle within the biofilm, as well as measurement of the isotopic ratio δ15N in sargassum tissues. The results obtained during this thesis made it possible to: (i) characterise the eukaryotic communities associated with holopelagic sargassum in the GASB, (ii) show that nitrogen fixed by diazotrophs is the main source of nitrogen in the open sea, in nutrient-poor areas, and that these microorganisms play a crucial role in the proliferation of sargassum. This thesis therefore highlights the complexity of the interactions between holopelagic sargassum and the organisms associated with it. These interactions appear to be potentially determining factors in the regulation of Sargassum growth and, consequently, in its large-scale proliferation. This work paves the way for numerous technical and experimental advances, enabling us to unveil the as yet unknown mechanisms underlying these complex interactions".
