Rimicaris exoculata is a caridean shrimp that lives in the deep ocean around active high-temperature hydrothermal sites. This chemosynthetic vent species maintains dominant populations at the Mid-Atlantic Ridge and is highly adapted to life in this extreme environment. Feeding mechanism include grazing on sulfides from hydrothermal chimneys and living in symbiosis with bacteria that are cultivated inside their respiratory and digestive systems. The energy sources for these symbioses generally rely on the bacterial oxidation of hydrogen sulfide or methane emitted at vent sites. However, the recent discovery of iron oxide minerals within the gill chamber of shrimps collected from the ‘Rainbow’ vent site suggests that iron-oxidizing symbionts, as well as sulphide-oxidising symbionts, could be involved in the metabolism of vent communities. Our first objective will be to better understand the mechanism by which shrimps gain their energy. Cycling of redox-sensitive elements, such as Fe, by biological process has been the rule on Earth for at least several billion years. Over the last decade, the measurement of Fe isotopes has become an emerging technique for understanding biogeochemical cycles. Experiments investigating metabolic processing of Fe have shown that measurable Fe isotope fractionations are produced during bacterial Fe+3 reduction, as well as anaerobic photosynthetic Fe+2 oxidation. Iron isotopes represent therefore a potentially exciting tool to investigate the complex interplay between microbial activity, biogeochemical processes and Fe redox cycling at hydrothermal vent sites. Our second objective will be to provide a fuller understanding of the microbial pathways for iron in hydrothermal systems. This will be achieved by systematically measuring the Fe isotopic composition of the iron oxide mineralization found within R. Exoculata, as well as a few selected samples of hydrothermal fluids and black smoker sulfides.