The response of the scleractinian coral Turbinaria reniformis to thermal stress depends on the nitrogen status of the coral holobiont

The physiological response of the scleractinian coral Turbinaria reniformis to ammonium enrichment (3 mu mol l(-1)) was examined at 26 degrees C as well as during a 7 day increase in temperature to 31 degrees C (thermal stress). At 26 degrees C, ammonium supplementation had little effect on the coral physiology. It induced a decrease in symbiont density, compensated by an increase in chlorophyll content per symbiont cell. Organic carbon release was reduced, likely because of a better utilization of the photosynthesized carbon (i.e. incorporation into proteins, kept in the coral tissue). The delta N-15 signatures of the ammonium-enriched symbionts and host tissue were also significantly decreased, by 4 and 2 parts per thousand, respectively, compared with the non-enriched conditions, suggesting a significant uptake of inorganic nitrogen by the holobiont. Under thermal stress, coral colonies that were not nitrogen enriched experienced a drastic decrease in photosynthetic and photoprotective pigments (chlorophyll a, beta-carotene, diadinoxanthin, diatoxanthin and peridinin), followed by a decrease in the rates of photosynthesis and calcification. Organic carbon release was not affected by this thermal stress. Conversely, nitrogen-enriched corals showed an increase in their pigment concentrations, and maintained rates of photosynthesis and calcification at ca. 60% and 100% of those measured under control conditions, respectively. However, these corals lost more organic carbon into the environment. Overall, these results indicate that inorganic nitrogen availability can be important to determining the resilience of some scleractinian coral species to thermal stress, and can have a function equivalent to that of heterotrophic feeding concerning the maintenance of coral metabolism under stress conditions.