Phylogeny and diversity of symbionts from whale fall invertebrates

Whale falls form oases at the bottom of the ocean that provide an energy source for a diverse and abundant fauna, including a new symbiosis type between Osedax worms and heterotrophic bacteria. Osedax infiltrate whale bones with their root tissues. These roots are filled with bacteria hypothesized to provide their hosts with nutrition by extracting organic compounds from the whale bones. This thesis is made of three thematic parts. The first part is a review on the ecology and evolution of Siboglinidae. The second part focuses on the diversity of the symbionts associated with Osedax mucofloris, where a higher diversity of Oceanospirillales bacteria was identified with eight monophyletic clusters. The symbiont clusters were not uniformly distributed, but one cluster dominated the population and each individual. In addition, when several clusters co-occurred in one individual they were not mixed but spatially separated. Statistical analyses showed that each O. mucofloris individual has a significant effect on symbionts diversity and distribution. Thus, each O. mucofloris individual has its own specific endosymbiont community. Our results suggest a horizontal transmission of the symbionts. Several scenarios explaining the observed symbionts distribution are considered including selection by the host, variability of the available symbionts, and competition between the symbionts. The third part of this thesis focuses on the symbionts of a polycheate worm, a Ctenodrilidae, Raricirrus beryli. Among the epibacteria, bacteria forming a monophyletic cluster with thiotrophic symbionts of bathymodiolin mussels were found. This is the first report of a polychaete host for these bacteria.