Organic Carbon and Nitrogen Isoscapes of Reef Corals and Algal Symbionts: Relative Influences of Environmental Gradients and Heterotrophy

The elemental (C/N) and stable isotopic (δ¹³C, δ¹⁵N) compositions and compound-specific δ¹⁵N values of amino acids (δ¹⁵N_AA) were evaluated for coral holobionts as diagnostic tools to detect spatiotemporal environmental heterogeneity and its effects on coral health. Hermatypic coral samples of eight species were collected at 12 reef sites with differing levels of pollution stress. The C/N ratios, δ¹³C values, and δ¹⁵N values of coral tissues and endosymbiotic algae were determined for 193 coral holobionts, and the amino acid composition and δ¹⁵N_AA values of selected samples were analyzed. δ¹⁵N values were influenced most by pollution stress, while C/N ratios and δ¹³C values depended most strongly on species. The results imply that δ¹³C and δ¹⁵N values are useful indicators for distinguishing the ecological niches of sympatric coral species based on microhabitat preference and resource selectivity. Using δ¹⁵N_AA values, the trophic level (TL) of the examined coral samples was estimated to be 0.71 to 1.53, i.e., purely autotrophic to partially heterotrophic. Significant portions of the variation in bulk δ¹⁵N and δ¹³C values could be explained by the influence of heterotrophy. The TL of symbionts covaried with that of their hosts, implying that amino acids acquired through host heterotrophy are translocated to symbionts. Dependence on heterotrophy was stronger at polluted sites, indicating that the ecological role of corals changes in response to eutrophication.