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Experiments were performed in controlled conditions in order to investigate the effect of light and/or feeding on the δ 15 N composition of zooxanthellae and animal tissue of a scleractinian coral, Stylophora pistillata. For this purpose, corals were cultured under 3 light levels (80, 200, and 300 µmol photons m -2 s -1 ) and for each light level, half of the corals were fed twice a week with nat- ural zooplankton while the other half, the control group, were starved. To understand how plankton ingestion changes the isotopic signal of the corals, zooplankton δ 15 N was also measured during the course of the experiment. It had a mean value of 6.75‰. No significant and consistent effect of light was observed on the δ 15 N of the tissue or zooxanthellae of S. pistillata. However, there was a signifi- cant depletion in tissue and zooxanthellae 15 N of fed corals compared to the controls. This depletion did not follow the general food-web rule in which predators are enriched in 15 N by 3.5‰ compared to prey, but can be explained by the fact that nitrogen is recycled within the symbiotic association. Indeed, in heterotrophic organisms, the enrichment is due to light nitrogen ( 14 N) being preferentially excreted in the form of waste products into the environment, which leads to an increase in the δ 15 N signature of the animal. In corals, light waste products are instead recycled by the zooxanthellae, therefore depleting the isotopic signature of the zooxanthellae, and also of the host, by transfer of photosynthates with light nitrogen. These observations support the nitrogen-recycling rather than the nitrogen-conservation theory.

Effect of light and feeding on the nitrogen isotopic composition of a zooxanthellate coral: role of nitrogen recycling