delta C-13 and delta N-15 values in reef corals Porites lutea and P-cylindrica and in their epilithic and endolithic algae

In summer 1998, shallow water corals at Sesoko Island, Japan (26 degrees 38'N, 127 degrees 52'E) were damaged by bleaching. In August 2003, partially damaged colonies of the massive Porites lutea and the branching P. cylindrica were collected at depths of 1.0-2.5 m. The species composition of epilithic algal communities on dead skeletal surfaces of the colonies ('red turfs', 'green turfs', 'red crusts') and the endolithic algae (living in coral skeletons) growing close to and away from living coral polyps was determined. Carbon and nitrogen stable isotope values of organic matter (delta C-13 and delta N-15) from all six of these biological entities were determined. There were no significant differences in the isotope composition of coral tissues of the two corals, with P. lutea having delta C-13 of -15.3 to -9.6 parts per thousand and delta N-15 of 4.7-6.1 parts per thousand and P. cylindrica having similar values. Polyps in both species living close to an interface with epilithic algae had similar isotope values to polyps distant from such an interface. Despite differences in the relative abundance of the algal species in red turfs and crusts, their delta C-13 and delta N-15 values were not significantly different from each other (-18.2 to -13.9, -20.6 to -16.2, 1.1-4.3, and 3.3 to 4.9 parts per thousand, respectively). The green algal turf had significantly higher delta C-13 values (-14.9 to -9.3 parts per thousand) than that of red turfs and crusts but similar delta N-15 (1.2-4.1 parts per thousand) to the red algae. The data do not suggest that adjoining associations of epilithic algae and coral polyps exchange carbon- and nitrogen-containing metabolites to a significant extent. The endolithic algae in the coral skeletons had delta C-13 values of -14.8 to -12.3 parts per thousand and delta N-15 of 4.0-5.4 parts per thousand. Thus they did not differ significantly from the coral polyps in their carbon and nitrogen isotope values. The similarity in carbon isotope values between the coral polyps and endolithic algae may be attributed to a common source of CO2 for zooxanthellae and endolithic algae, namely, from respiration by the coral host. While it is difficult to fully interpret similarity in the nitrogen isotope composition of coral tissue and of green endolithic algae and the difference in delta N-15 between green epilithic and endolithic algae, the data are consistent with nitrogen-containing metabolites from the scleractinian coral serving as a significant source of nitrogen for the endolithic algae.