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 δ15N and δ13C values, and to provide guidelines toward a standardised protocol.
Methods
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 δ15N and δ13C values, using continuous flow elemental analyser/isotope ratio mass spectrometry.
Results
The drying method and lipid extraction treatment had no substantial effects on the δ15N and δ13C values of Symbiodiniaceae and animal tissues. Acid treatment did cause significant differences in δ13C values (mean differences ≤0.5‰, with individual samples becoming up to 2.0‰ more negative), whereas no ecologically significant differences were observed in δ15N values. Animal tissue δ13C values may vary depending on whether samples are washed or not.
Conclusions
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.
