Variation in the elemental stoichiometry of the coral–zooxanthellae symbiosis

Organismal elemental stoichiometry is important at all scales of ecological interactions, particularly in symbiosis. Symbiotic partnerships are found extensively in corals, where coral hosts and their photosynthetic dinoflagellate partners trade essential nutrients. Using an ecological stoichiometry framework, we assessed variations in carbon (C), nitrogen (N), and phosphorus (P) concentrations in coral hosts and symbiotic zooxanthellae. Our aim was to assess whether corals and zooxanthellae differ in the stoichiometry of C, N and P, which may indicate that corals regulate their symbionts by restricting their access to nutrients. We also investigated the influence of biological and environmental factors on the elemental stoichiometry of coral–zooxanthellae symbiosis. Analysing field data from four locations on the Great Barrier Reef using boosted regression tree models, we observed that C:P and N:P ratios are significantly higher in zooxanthellae than in their coral hosts, suggesting P limitation for zooxanthellae, although the degree of P or N limitation in zooxanthellae varied among coral taxa. Coral taxonomy was also a significant driver of coral–zooxanthellae stoichiometry variations at the species level. Geographical location had a small influence on the variations in stoichiometry. Our analyses suggest that coral species may differentially control the access of zooxanthellae to nutrients which may lead to variation in reef coral responses to nutrient enrichment.