Lucinid bivalves, the most diverse bivalve group hosting chemosynthetic symbionts, are widespread in sulfidic coastal sediments worldwide and thought to be involved in major biological processes. The recent discovery that the chemosynthetic symbionts of lucinid bivalves can fix nitrogen (N₂) in addition to carbon opened up important questions about the relevance of N₂ fixation in organism and ecosystem functioning. Through the use of stable isotope labeling and nutrient budgeting both in the Laboratory and in-situ, the project aims to uncover the physiological and biogeochemical interactions between the lucinid symbiont, its host, and the ecosystem, with a focus on N cycling.