Corals access inorganic seawater nutrients through their autotrophic endosymbiotic dinoflagellates, but also capture planktonic prey through heterotrophic feeding. Correlating NanoSIMS and TEM imaging, we visualized and quantified the subcellular fate of autotrophic and heterotrophic C and N in the coral Stylophora pistillata using stable isotopes. Six scenarios were compared after 6 h: autotrophic pulse (13C-bicarbonate, 15N-nitrate) in either unfed or regularly fed corals, and heterotrophic pulse (13C-, 15N-labelled brine shrimps) in regularly fed corals; each at ambient and elevated temperature. Host assimilation of photosynthates was similar under fed and unfed conditions, but symbionts assimilated 10% more C in fed corals. Photoautotrophic C was primarily channelled into host lipid bodies, whereas heterotrophic C and N were generally co-allocated to the tissue. Food-derived label was detected in some subcellular structures associated with the remobilisation of host lipid stores. While heterotrophic input generally exceeded autotrophic input, it was more negatively affected by elevated temperature. The reduced input from both modes of nutrition at elevated temperature was accompanied by a shift in the partitioning of C and N, benefiting epidermis and symbionts. This study provides a unique view into the nutrient partitioning in corals and highlights the tight connection of nutrient fluxes in symbiotic partners.
Bibliographical noteFunding Information:
This work was supported by a Swiss National Science Foundation grant no. CR2312-141048 to AM. The NanoSIMS instrument was acquired with funding from ERC Advanced grant no. 246749 (BIOCARB) and from EPFL to AM. The Red Sea Simulator was funded by an Israel Science Foundation grant to MF. Samuel Donck is acknowledged for help with maintaining the system during the experiments. We thank Dr Emma Gibbin for help with the pyruvate samples. The Electron Microscopy Facility (EMF at University of Lausanne) is acknowledged for access and help with sample preparation; in particular, we thank Dr Jean Daraspe. The manuscript benefited from advice on the histological terminology from Prof Esther Peters.
© 2018, The Author(s).