Nitrogen Isotope Analysis of Nitrogen Cycling in Coral Host Tissue and Algal Symbionts: A Study of Acropora Spp. and Stylophora Spp. near the Fulong Reefs in Taiwan

Abstract

This study investigated the differences in δ15N values across Coral Host Tissue, Algal Symbionts, and Whole Tissue Sample in two coral genera, Acropora spp. and Stylophora spp., collected from a harbor structure at the National Taiwan Ocean University Aquatic Biological Research and Conservation Center near Fulong, Northern Taiwan in August 2024. Nine coral heads were sampled, subdivided into tissue fractions, and analyzed for stable nitrogen isotope composition using the denitrifier method, which reduces the sample’s nitrate (NO3 -) to nitrous oxide (N2O). Using nitrogen isotopes (δ15N) as a nutrient tracer, this study explored three hypotheses regarding nitrogen cycling within the coral holobiont: the Host Coral provides nitrogen to the Algal Symbionts directly through the system, the Host Coral provides nitrogen to the Algal Symbionts indirectly through the system, or the Algal Symbionts do not receive nitrogen from the Host Coral but instead fix dissolved inorganic nitrogen from the environment. Significant differences in δ15N were found between Coral Host Tissue and Algal Symbionts, suggesting that corals feed their symbionts indirectly. Genus-specific differences were also observed, with Acropora spp. exhibiting higher δ15N values across tissue types compared to Stylophora spp., likely due to variations in trophic strategies, metabolic demands, and morphological characteristics. Additionally, this study found that Algal Symbionts have a greater influence on Whole Tissue Samples δ15N than Coral Host Tissue, reflecting their varying diet and nutrient uptake patterns. This study contributes to the growing body of knowledge on coral nitrogen cycling, emphasizing how nitrogen isotopes can illuminate the complex interplay between coral hosts and their algal symbionts.