The discovery of deep-sea hydrothermal vents in 1977 was one of the most profound findings of the 20th century, revolutionizing our perception of energy sources fueling primary productivity on Earth. These ecosystems are based on chemosynthesis, that is the fixation of carbon dioxide into organic compounds as in photosynthesis, but using inorganic compounds such as sulfide, methane or hydrogen, as energy sources instead of sunlight. Hydrothermal vents support tremendous biomass and productivity of which the majority is generated through symbiotic microbe-animal associations. Bathymodiolus mussels are able to build extraordinarily large and productive communities at hydrothermal vents because they harbor symbiotic bacteria that use inorganic energy sources from the vent fluids to feed their hosts via carbon fixation. In addition to their beneficial symbionts, the mussels are infected by a novel bacterial parasite that exclusively invades and multiplies in their nuclei. In the work proposed here, I will use a wide array of tools that range from deep-sea in situ instruments to sophisticated molecular, 'omic' and imaging analyses to study the microbiome associated with Bathymodiolus mussels. The proposedresearch bridges biogeochemistry, ecological and evolutionary biology, and molecular microbiology to develop a systematic understanding of the symbiotic interactions between microbes, their hosts, and their environment in one of the most extreme and fascinating habitats on Earth, hydrothermal vents.