δ¹³C and δ¹⁵N in tissues of reef building corals and the endolithic alga Ostreobium quekettii under their symbiotic and separate existence

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Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of organic matter of five species of reef building corals from Sesoko Island (Okinawa, Japan) and Mun Island (Nhatrang, Vietnam) and monodominant community of the endolithic alga Ostreobium quekettii inhabiting live and dead portions of these coral colonies were determined. There were no significant interand intraspecific differences in the isotope composition of soft tissues of tested corals as well as differences among corals containing and non-containing endolithic algae. Corals showed mean values of δ¹³C and δ¹⁵N −14.3±1.3‰ and 5.3±0.6‰, accordingly. Endolithic algae living in the neighborhood with coral polyps showed similar carbon and nitrogen isotope values (δ¹³C=−16.0±3.3‰, δ¹⁵N=5.1±0.6‰). At the same time, carbon of endolithic algae inhabiting dead portions of coral colonies was heavier approximately by 3.2‰ than carbon of endolithic algae inhabiting live corals while nitrogen was significantly lighter by 1.4‰. The similarity in carbon and nitrogen isotope values between coral polyps and endolithic algae may be attributed to a common source of these elements for zooxanthellae and endolithic algae, namely from respiration and metabolism of nitrogen by the coral host accordingly.