The thesis will be defended in French, and the jury is composed by:
. Mme Claire PRIGENT-COMBARET, UMR 5557 LEM Lyon, Rapportrice
. Mme Sophie ARNAUD-HAOND, IFREMER Sète, Rapportrice
. M. Christophe DESTOMBE, IRL 3614 Roscoff, Examinateur
. M. Alexis DUFRESNE, UMR ECOBIO Rennes, Examinateur
. M. Simon DITTAMI, UMR 8227 LBI2M Roscoff, Directeur de thèse
. Mme Catherine BOYEN, UMR 8227 LBI2M Roscoff, Directrice de Thèse
Abstract: Saccharina latissima is a kelp-forming species of brown algae and, as such, is considered a so-called ecosystem engineer. Several populations of this alga are exploited around the world. S. latissima is also currently undergoing increasing development in aquaculture. My PhD thesis aims to characterise the bacterial microbiota and study host-microbiota interactions within the S. latissima holobiont to understand how the microbiota composition influences algal growth. 16S metabarcoding analyses were used to study the microbiota of S. latissima natural populations from Roscoff (France), Helgoland (Germany) and Southern Norway (Skagerrak). They showed a difference in composition depending on the area of the blade, the geographical area and the algal physiological state. Bacterial isolates were used to gain access to the cultivable portion of the microbiota. Combining these results with the 16S metabarcoding data allowed us to select bacterial strains of interest. Algal-bacterial cocultures have shown their impact on algal growth, and unexpected results led us to consider other parameters: the general state of the starting seedling, the composition of its initial microbiota and the phenomenon of quorum sensing (QS). The results show a correlation between an increase of QS molecules and a decrease in algal growth. Together, these results on S. latissima microbiota’s composition help decipher its impact on the algal host and, more generally, contribute to a better understanding of the algal holobiont, supporting the idea of interdependence between the host and its microbiota.