Corals are what they eat: Insights on the robustness of the coral skeleton-bound organic matter δ15N proxy from a feeding and light experiment

Coral skeleton-bound organic matter δ¹⁵N (CS-δ¹⁵N) is increasingly being used to reconstruct past nutrient cycling, based on evidence that the CS-δ¹⁵N reflects environmental δ¹⁵N. However, the importance of photosynthetic endosymbionts in corals' internal N cycling raises the potential for secondary isotopic signals, in particular, in response to heterotrophy. We tested the response of Porites astreoides, a common coral on modern Caribbean reefs, to varying feeding and light conditions with a tightly controlled N source. A 4-fold increase in feeding frequency yielded a minor (0.6‰) decline in coral bulk δ¹⁵N, counter to previous suggestions that an increase in food availability can induce a substantial (≤3‰) increase in coral δ¹⁵N. Similarly, light levels across the experimental range (~50-250 µE) had no significant effect on coral bulk δ¹⁵N. Differences in δ¹⁵N of the bulk tissue, host tissue, and zooxanathellae symbionts provide insight into P. astreoides internal N cycling. Among the experimental corals, low-fed corals show a slight (0.6‰) elevation in symbiont δ¹⁵N relative to host tissue δ¹⁵N and a slight (0.7‰) elevation in bulk δ¹⁵N relative to the food source; these elevations are absent in the high-fed corals. Using a quantitative isotopic model of the coral host/symbiont system, we tentatively explain these slight δ¹⁵N differences in terms of an isotopic interaction between the leakage of metabolic ammonium to the environment and a concentration-dependent isotope effect for symbionts' ammonium assimilation, resulting in the leakage of modestly ¹⁵N-enriched ammonium in the high-fed case. If correct, this works against the typical tendency of organisms to increase in δ¹⁵N due to metabolic N leakage, weakening δ¹⁵N change due to nutritional conditions in this case. In sum, our data indicate that at least some coral species can reliably track the δ¹⁵N of their food even across a broad range of feeding rates and light conditions.