The response of the scleractinian coral Turbinaria reniformis to thermal stress depends on the nitrogen status of the coral holobiont.
Résumé
The physiological response of the scleractinian coral Turbinaria reniformis to ammonium enrichment (3 µM) was examined at 26°C as well as during a 7d increase in temperature to 31°C (thermal stress). At 26°C, ammonium supplementation slightly changed 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 due to a better utilization of the photosynthesized carbon (i.e. incorporation into proteins, kept into the coral tissue). The δ15N signature of the ammonium-enriched symbionts and host tissue was also significantly decreased by 4 and 2‰ respectively, compared to the non enriched conditions, suggesting an important 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, β carotene, diadinoxanthin, diatoxanthin, peridinin), followed by a similar 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, which maintained rates of photosynthesis and calcification to ca. 60% and 100% of those obtained under control conditions respectively. These corals however lost more organic carbon into the environment. Overall, these results indicate that inorganic nitrogen availability can be important to determine the resilience of some scleractinian coral species to thermal stress, and can have an equivalent function as heterotrophic feeding concerning the maintenance of coral metabolism under stress conditions