Metabolic interactions with endosymbiotic photosynthetic dinoflagellate Symbiodinium spp. are fundamental to reef-building corals (Scleractinia) thriving in nutrient-poor tropical seas. Yet, detailed understanding at the single-cell level of nutrient assimilation, translocation, and utilization within this fundamental symbiosis is lacking. Using pulse-chase ¹⁵ N labeling and quantitative ion microprobe isotopic imaging (NanoSIMS; nanoscale secondary-ion mass spectrometry), we visualized these dynamic processes in tissues of the symbiotic coral Pocillopora damicornis at the subcellular level. Assimilation of ammonium, nitrate, and aspartic acid resulted in rapid incorporation of nitrogen into uric acid crystals (after ~45 min), forming temporary N storage sites within the dinoflagellate endosymbionts. Subsequent intracellular remobilization of this metabolite was accompanied by translocation of nitrogenous compounds to the coral host, starting at ~6 h. Within the coral tissue, nitrogen is utilized in specific cellular compartments in all four epithelia, including mucus chambers, Golgi bodies, and vesicles in calicoblastic cells. Our study shows how nitrogen-limited symbiotic corals take advantage of sudden changes in nitrogen availability; this opens new perspectives for functional studies of nutrient storage and remobilization in microbial symbioses in changing reef environments. IMPORTANCE The methodology applied, combining transmission electron microscopy with nanoscale secondary-ion mass spectrometry (NanoSIMS) imaging of coral tissue labeled with stable isotope tracers, allows quantification and submicrometric localization of metabolic fluxes in an intact symbiosis. This study opens the way for investigations of physiological adaptations of symbiotic systems to nutrient availability and for increasing knowledge of global nitrogen and carbon biogeochemical cycling.

International audienceWe used growth rates (estimated from otolith microstructure), morphometric (Fulton's K condition index) and lipid (triacylglycerols:sterols ratio, TAG:ST) condition indices measured on 0-group juveniles of common sole, to compare the habitat quality of 7 nursery grounds with differing levels of anthropogenic pressure. Along the French coast of the Southern Bight of the North Sea and the Eastern English Channel, we compared 2 sites located in intensively developed and industrialised harbour areas (Dunkerque and Calais), 1 site in a medium and less industrialised harbour (Boulogne), 1 site near a large and polluted estuary (Seine) and 3 other sites located near small estuaries less subject to human pressure (Canche, Authie and Somme). Indicators of human disturbance as well as hydrological and biotic data were used to establish between-site differences in environmental quality. Our results indicate consistent differences in the growth and condition indices of 0-group sole among the 7 sites. Sole had the slowest growth (mean G = 0.54 mm d–1) and lowest condition indices (mean K = 1.07; TAG:ST = 0.32) at Dunkerque and Calais and the fastest growth (mean G = 0.70 mm d–1) and highest condition indices (mean K = 1.19; TAG:ST = 2.18) at the Authie, Canche and Somme estuaries. The indices measured in this study correlated well with anthropogenic disturbance and may provide a useful tool to assess habitat quality. Sites with highest sediment chemical contaminants had the lowest habitat quality and, through growth and lipid-storage limitation, could dramatically lower over-winter survival of the juveniles living in these nursery ground