Physiology and ecology of deep-sea Bathymodiolus symbioses

The deep sea represents the largest ecosystem on Earth and sustains high biomass at hydrothermal vents and cold seeps. At these ‘oases of life’, Bathymodiolus mussels form large assemblages, thriving off biomass produced by their bacterial symbionts housed within specialised cells of the mussel gills. Through chemosynthesis, these symbionts use reduced chemical compounds from vent and seep fluids as energy sources to feed their mussel host. Life at vents and seeps is challenged by highly dynamic physico-chemical conditions, and it remained to be resolved how Bathymodiolus symbioses have adapted to flourish in these fluctuating environments. The research presented in this doctoral thesis unravels the physiological responses of the Bathymodiolus mussels and their symbionts to short- and long-term limitations of the energy sources that sustain the metabolism of the symbiotic partners. Through in-depth gene expression analyses of host and symbionts subjected to in situ and laboratory-based experiments, this thesis uncovers physiological adaptations that indicate how the symbiotic partners respond to their environment, and how this influences the cellular interactions between host and symbionts.