Contributions of photoautotrophy and heterotrophy to the carbon and nitrogen nutrition of Anthopleura elegantissima in three symbiotic states

The Pacific coast sea anemone Anthopleura elegantissima is an excellent model organism for the study of temperate symbiosis due to its unique relationship with two microalgal symbionts, Elliptochloris marina and Symbiodinium spp. In addition to hosting one or both of these symbionts, A. elegantissima can live aposymbiotically, allowing for the comparison of nutritional compositions of hosts with different symbionts. This study used stable isotope analysis to quantify the photoautotrophic contributions by symbionts and heterotrophic contributions by hosts. Algal, anemone-only, and algae+anemone samples were isolated from A. elegantissima symbiotic with E. marina, Symbiodinium spp., or anemones lacking symbionts. Samples were analyzed for δ¹⁵N and δ¹³C in the OSU Stable Isotope Laboratory. Isotope analysis showed that symbiotic anemone-only and algae+anemone samples were very close to the high δ¹⁵N and δ¹³C values of aposymbiotic samples, while algal samples were much lower. These results indicate that symbiotic anemones are relying primarily on heterotrophic nutrition, and receiving little-to-no benefit from hosting either E. marina or Symbiodinium spp