Toward a standardised protocol for the stable isotope analysis of scleractinian corals

The stable isotope analysis of carbon and nitrogen is a powerful tool in many ecological studies, but different sample treatments may affect stable isotope ratios and hamper comparisons among studies. The goal of this study was to determine whether treatments that are commonly used to prepare scleractinian coral samples for stable isotope analysis yield different δ¹⁵ N and δ¹³ C values, and to provide guidelines toward a standardised protocol.

The animal tissues and Symbiodiniaceae of two symbiotic scleractinian coral species (Stylophora pistillata and Porites lutea) were divided into subsamples to test the effects of the drying method, lipid extraction, acidification treatment and water washing. All the subsamples were analysed for their δ¹⁵ N and δ¹³ C values, using continuous flow elemental analyser/isotope ratio mass spectrometry.

The drying method and lipid extraction treatment had no substantial effects on the δ¹⁵ N and δ¹³ C values of Symbiodiniaceae and animal tissues. Acid treatment did cause significant differences in δ¹³ C values (mean differences ≤0.5‰, with individual samples becoming up to 2.0‰ more negative), whereas no ecologically significant differences were observed in δ¹⁵ N values. Animal tissue δ¹³ C values may vary depending on whether samples are washed or not.

To move towards a standardised protocol in coral research, we recommend using an available drying method (as they are equally acceptable) for the stable isotope analysis of scleractinian corals, examining the need for lipid extraction on a case-by-case basis, performing a direct acidification of Symbiodiniaceae and animal tissues, and avoiding washing animal tissue with distilled water.