Symbiodiniaceae Are the First Site of Heterotrophic Nitrogen Assimilation in Reef-Building Corals

Coral reefs depend on the highly optimized mutualistic relationship between corals and Symbiodiniaceae dinoflagellates. Both partners exchange nutrients obtained through heterotrophy of the host and autotrophy of the symbionts. While heterotrophy helps corals withstand the harmful effects of seawater warming, the exchange of heterotrophic nutrients between the two partners is poorly understood. Here, we used compound-specific δ¹⁵N and δ¹³C of amino acids (δ¹⁵N_AA and δ¹³C_AA) and a ¹⁵N pulse-chase experiment with Artemia salina nauplii in two coral-dinoflagellate associations to trace the assimilation and allocation of heterotrophic nutrients within the partners. We observed that changes in the trophic position (TP_Glx-Phe), δ¹⁵N_AA, and δ¹³C_AA with heterotrophy were holobiont-dependent. Furthermore, while TP_Glx-Phe and δ¹⁵N of all AAs significantly increased with heterotrophy in the symbionts and host of Stylophora pistillata, only the δ¹⁵N_AA of the symbionts changed in Turbinaria reniformis. Together with the pulse-chase experiment, the results suggested a direct transfer of heterotrophically acquired AAs to the symbionts of S. pistillata and a transfer of ammonium to the symbionts of T. reniformis. Overall, we demonstrated that heterotrophy underpinned the nutrition of Symbiodinaceae and possibly influenced their stress tolerance under changing environmental conditions. IMPORTANCE Coral reefs rely upon the highly optimized nutritional symbiosis between corals and Symbiodiniaceae dinoflagellates. Heterotrophic feeding on plankton is key to the resistance of corals to environmental stress. Yet, a detailed understanding of heterotrophic nutrient assimilation and utilization within the symbiosis is lacking. Here, we used the advanced tools of compound-specific isotope analysis of amino acids and ¹⁵N-labeling of plankton to show that heterotrophy underpinned the nutrition of Symbiodinaceae. Symbionts received either heterotrophically acquired amino acids or recycled ammonium due to their association with the coral host. This study brought new insight into the nutrient exchanges in coral-Symbiodiniaceae associations and allowed a better understanding of the mechanisms involved in coral resistance to environmental stress.