Carbon isotope composition of fatty acids and sterols in the scleractinian coral Turbinaria reniformis: Effect of light and feeding

The effect of auto‐ and heterotrophy on the carbon isotope composition (δ13C) of fatty acids (FA) and sterols was studied in the scleractinian coral Turbinaria reniformis. Colonies were maintained in aquaria for 6 weeks under two light levels (100 and 300 mmol photons m−2s−1) and/or two trophic conditions (starved and fed twice a week with zooplankton). In high‐light conditions, lipid biomarkers, both in the zooxanthellae and animal tissue, presented significantly higher δ13C values compared to those exposed to low light, supporting the widespread theory of enhanced 13C fixation with increasing irradiance and photosynthesis. This result and the similar lipid δ13C‐values of the host and the symbionts are a strong indication that most lipids are synthesized by the zooxanthellae. Feeding had a smaller effect than light on the lipid δ13C‐signatures and a more pronounced effect at low‐light level. Despite the depleted δ13C values of the zooplankton lipids, slight enrichment in 13C was observed in some FA from the host and zooxanthellae, whereas the main host sterol was depleted in 13C. These differences between FA and sterols give evidence for different biosynthesis pathways: FA might have been synthesized from the assimilation of carbohydrates or amino acids contained in the food, whereas sterols might derive from modification of dietary lipids.