title: Subcellular investigation of photosynthesis-driven carbon assimilation in the symbiotic reef coral Pocillopora damicornis.
abstract: Reef-building corals form essential, mutualistic endosymbiotic associations with photosynthetic Symbiodinium dinoflagellates, providing their animal host partner with photosynthetically derived nutrients that allow the coral to thrive in oligotrophic waters. However, little is known about the dynamics of these nutritional interactions at the (sub)cellular level. Here, we visualize with submicrometer spatial resolution the carbon and nitrogen fluxes in the intact coral-dinoflagellate association from the reef coral Pocillopora damicornis by combining nanoscale secondary ion mass spectrometry (NanoSIMS) and transmission electron microscopy with pulse-chase isotopic labeling using [(13)C]bicarbonate and [(15)N]nitrate. This allows us to observe that (i) through light-driven photosynthesis, dinoflagellates rapidly assimilate inorganic bicarbonate and nitrate, temporarily storing carbon within lipid droplets and starch granules for remobilization in nighttime, along with carbon and nitrogen incorporation into other subcellular compartments for dinoflagellate growth and maintenance, (ii) carbon-containing photosynthates are translocated to all four coral tissue layers, where they accumulate after only 15 min in coral lipid droplets from the oral gastroderm and within 6 h in glycogen granules from the oral epiderm, and (iii) the translocation of nitrogen-containing photosynthates is delayed by 3 h.
  
  
  OBJECTIVE
  Our results provide detailed in situ subcellular visualization of the fate of photosynthesis-derived carbon and nitrogen in the coral-dinoflagellate endosymbiosis. We directly demonstrate that lipid droplets and glycogen granules in the coral tissue are sinks for translocated carbon photosynthates by dinoflagellates and confirm their key role in the trophic interactions within the coral-dinoflagellate association.
authors: Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland christophe.kopp@epfl.ch anders.meibom@epfl.ch.
authors: Christophe
authors: C
authors: Kopp
authors: MCAM UMR7245 CNRS-MNHN, Muséum National d'Histoire Naturelle, Paris, France.
authors: Isabelle
authors: I
authors: Domart-Coulon
authors: Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering (ENAC), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland.
authors: Stephane
authors: S
authors: Escrig
authors: Electron Microscopy Facility, University of Lausanne, Lausanne, Switzerland.
authors: Bruno M
authors: BM
authors: Humbel
authors: Aquarium Tropical, Établissement Public du Palais de la Porte Dorée, Paris, France.
authors: Michel
authors: M
authors: Hignette
authors: christophe.kopp@epfl.ch anders.meibom@epfl.ch.
authors: Anders
authors: A
authors: Meibom
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000469:parasitology* / Q000502:physiology; D002244:Carbon / Q000378:metabolism*; D004141:Dinoflagellida / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D010788:Photosynthesis; D018629:Spectrometry, Mass, Secondary Ion; D013559:Symbiosis
chemical_list: D002244:Carbon; D009584:Nitrogen