Symbiotic Scleractinian corals survive in oligotrophic reef waters owing to their efficient uptake of nutrients such as phosphorus and nitrogen. Although the use of nitrogen by corals has often been studied, this is not the case for phosphorus. The main aim of this work was to examine the use of dissolved inorganic and organic phosphorus (DIP, DOP) by corals under various environmental conditions, and to assess their metabolic demand for phosphorus. Results showed that DIP and DOP uptake, measured respectively with depletion experiments and with alkaline phosphatase activity assays, are dependent upon light, the presence of symbiotic Dinoflagellates within coral tissues, the availability of inorganic nitrogen, and the nutritional status of the host (repletion in particulate organic phosphorus, POP, ingested as plankton). During a temperature stress, alone or combined with a pH decrease, DIP uptake was affected. Upon DIP enrichment, coral calcification and photosynthesis increased, thus suggesting that, under oligotrophic conditions, phosphorus limits the symbiosis. Finally, nuclear magnetic resonance analyzes showed that phosphorus occurs as phosphate within the symbiosis, but also as phosphonates in the host, and as polyphosphates and phosphate esters in the symbionts. A first budget of the relative importance of DIP, DOP and POP was established at the end of this work, as well as a critical evaluation of the tools used to assess phosphorus limitation of the symbiosis.
