Lucinid bivalves host sulfur-oxidizing and diazotrophic symbionts that enable them to thrive in
sulfidic, nutrient-poor environments such as tropical seagrass beds. While their role in nitrogen
fixation and sediment detoxification is recognized, their contribution to nitrogen cycling through
uptake and excretion of exogenous nitrogen remains poorly quantified. This study investigates the
capacity of the lucinid Codakia orbicularis to excrete and assimilate ammonium and nitrate under
chemosymbiotic conditions, and the fate of compound-specific nitrogen within the organism.
Using isotopically labeled nitrogen tracers (15NH3 and 15NO3-), along with effluent nutrient
analysis and respirometry, nitrogen fluxes and metabolic activity were measured over seven days.
C. orbicularis acted as a net sink for both nitrogen species, with ammonium assimilation exceeding
nitrate assimilation by two orders of magnitude. Isotopic enrichment confirmed the gills as the
primary site of nitrogen assimilation, with evidence of translocation to other tissues. Oxygen
consumption rates were consistent with those of other bivalves but exhibited major variability. The
novel two-phase flow-through incubation system developed to simulate natural gradients of
oxygen, sulfide, and nitrogen developed in this study provided a robust quantitative assessment of
exogenous nitrogen assimilation in C. orbicularis, positioning it as a potential nutrient sink in
tropical seagrass ecosystems.
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