STABLE ISOTOPE EVIDENCE FOR PHOTOSYMBIOSIS IN FOSSIL CORALS
Here we refine a proxy for ancient photosymbiosis using the stable nitrogen isotope ratio (δ15N) of the coral organic matrix (Muscatine et al., 2005), as well as stable carbon and oxygen isotope ratios (δ13C, δ18O) of fossil coral skeleton. To test the validity of this proxy, we analyzed samples of Modern, Pleistocene, Oligocene, and Triassic coral skeletons. Coral samples comprise both (interpreted) zooxanthellate and azooxanthellate fossil corals from the Oligocene and Triassic as well as zooxanthellate fossil corals from the Modern and Pleistocene to compare our findings to the results of Muscatine et al. (2005). Samples were screened for diagenesis by petrographic and scanning electron microscopy (SEM) to avoid sampling recrystallized material.
The nitrogen isotope ratio of coral organic matrix could provide a reliable proxy for photosymbiosis in fossil corals. A successful proxy for ancient photosymbiosis is an important step in defining the evolutionary relationship between symbiosis and coral reef-building corals.