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Project 13

Symbiont transmission in bivalves from deep-sea chemosynthesis-based ecosystems

PhD position. Supervisors: Sébastien Duperron (UniParis) and Prof Andreas Schramm (UniAarhus) (with input from Oliver Gros, UniAntilles). Host: UniParis. Secondment internship: UniAarhus

Rationale

Mytilid, lucinid and thyasirid bivalves inhabiting deep-sea cold seeps, hydrothermal vents and organic falls rely on their sulphide- and/or methane-oxidizing symbionts for most of their nutrition. In mytilids, environmental acquisition of symbionts is suggested based on i) the lack of host / symbiont co-speciation; ii) the occurrence of non-parental symbiont phylotypes in at least two co-occurring species; and iii) the existence of extracellular bacteria closely related to endosymbionts in some small mussels. Lucinids, at least in shallow water species, apparently capture their bacteria, while thyasirids are not well-documented but also suspected to acquire their symbionts from the environment. However, the occurrence of free-living forms of bivalve symbionts is not yet proven, and early steps of host-symbiont interactions are poorly documented.

Objectives

Objectives are i) to investigate the existence of free-living forms of bivalve-associated symbionts at deep-sea vents, seeps and organic falls; ii) to assess the diversity, microheterogeneity, and phylogeny of bacteria associated with juvenile and adult bivalves using multiple marker genes; iii) to document early steps of host symbiont interactions.

Key methods

Colonization devices filled with carbonate, alfalfa and wood substrates were deployed at seeps (eastern Mediterranean, Haakon Mosby MV, Gulf of Cadiz) and vent sites (MAR) during EU programs DIWOOD and CHEMECO. Free-living bacteria as well as juvenile-to-adult stages of metazoans including bivalves were recovered. Symbiont-specific PCR primers and FISH probes will be tested on adult bivalves, and used to investigate the occurrence of symbionts within substrates. Juvenile bivalves will be identified using molecular methods and the phylogenetic relationships of associated bacteria will be investigated using multi-marker approaches to test hypotheses such as co-speciation or host shifts. The intraspecific microheterogeneity of symbionts will be analyzed in collaboration with Andreas Schramm (UniAarhus). Localization of symbionts will be investigated using electron microscopy and 3D-FISH, and their nutritional role in juvenile-to-adults using stable isotopes.