LUCY investigated the role of lucinid bivalves—the most diverse group of chemosymbiotic bivalves—in nitrogen (N₂) fixation and coastal ecosystem functioning. Widespread in sulfidic sediments worldwide, lucinid bivalves host chemosynthetic symbionts that contribute to both carbon and nitrogen cycling, yet their ecological significance was poorly understood. The project examined the physiological and biogeochemical interactions between lucinid hosts, their symbionts, and the surrounding ecosystem. Using stable isotope labeling and nutrient budgeting in both laboratory and field experiments, LUCY quantified the contribution of these associations to nitrogen cycling in coastal sediments.