title: Trophic dynamics of scleractinian corals: stable isotope evidence.
abstract: Reef-building corals form symbioses with dinoflagellates from the diverse genus Symbiodinium. This symbiotic association has developed adaptations to acquire and share nutrients, which are essential for its survival and growth in nutrient-poor tropical waters. The host is thus able to prey on a wide range of organic food sources (heterotrophic nutrition) whereas the symbionts acquire most of the inorganic nutrients (autotrophic nutrition). However, nutrient fluxes between the two partners remain unclear, especially concerning heterotrophically acquired carbon and nitrogen. We combined physiological measurements and pulse-chase isotopic labeling of heterotrophic carbon and nitrogen, as well as autotrophic carbon to track nutrient fluxes in two coral species, Stylophora pistillata and Turbinaria reniformis, in symbiosis with Symbiodinium clades A, and C,D respectively. We showed a rapid acquisition, exchange and a long-term retention of heterotrophic nutrients within the symbiosis, whereas autotrophic nutrients were rapidly used to meet immediate metabolic needs. In addition, there was a higher retention of heterotrophic nitrogen compared with carbon, in agreement with the idea that tropical corals are nitrogen-limited. Finally, a coupling between auto- and heterotrophy was observed in the species S. pistillata, with a higher acquisition and retention of heterotrophic nutrients under low irradiance to compensate for a 50% reduction in autotrophic nutrient acquisition and translocation. Conversely, T. reniformis conserved an equivalent heterotrophic nutrient acquisition at both light levels because this coral species did not significantly reduce its rates of gross photosynthesis and autotrophic carbon acquisition between the two irradiances. These experiments advance the current understanding of the nutrient exchanges between the two partners of a symbiotic association, providing evidence of the complexity of the host-symbiont relationship.
authors.affiliation: Centre Scientifique de Monaco, 8 quai Antoine 1er, MC-98000, Monaco pascale_tremblay@globetrotter.net.
authors.forename: Pascale
authors.initials: P
authors.lastname: Tremblay
authors.affiliation: LEMAR - UMR 6539 UBO/CNRS/IRD, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, Plouzané 29280, France.
authors.forename: Jean François
authors.initials: JF
authors.lastname: Maguer
authors.affiliation: Centre Scientifique de Monaco, 8 quai Antoine 1er, MC-98000, Monaco.
authors.forename: Renaud
authors.initials: R
authors.lastname: Grover
authors.affiliation: Centre Scientifique de Monaco, 8 quai Antoine 1er, MC-98000, Monaco.
authors.forename: Christine
authors.initials: C
authors.lastname: Ferrier-Pagès
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000378:metabolism*; D052818:Autotrophic Processes; D002244:Carbon / Q000378:metabolism*; D002247:Carbon Isotopes; D004141:Dinoflagellida / Q000378:metabolism*; D052836:Heterotrophic Processes; D008027:Light; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes; D010788:Photosynthesis; D013045:Species Specificity; D013559:Symbiosis
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D002244:Carbon; D009584:Nitrogen
keywords: Autotrophy; Carbon; Heterotrophy; Irradiance; Isotopic labeling; Nitrogen; Stylophora pistillata; Symbiodinium; Symbiosis; Turbinaria reniformis