STABLE ISOTOPE EVIDENCE FOR PHOTOSYMBIOSIS IN FOSSIL CORALS

Photosymbiosis is a mutualistic relationship that many corals have developed with dinoflagellates called zooxanthellae. With this symbiotic relationship, corals gain increased metabolic rates, respiration, waste excretion and, most importantly, enhanced growth rates. Photosymbiosis is therefore considered the evolutionary innovation that allowed corals to become major reef-builders. Yet, studying this relationship in deep time is extremely difficult because zooxanthellae, which are housed in the coral tissue without their own hard parts, are not preserved in the fossil record. Without a direct record of zooxanthellae, it is difficult to determine whether reef-building corals had symbionts. Thus, a definitive proxy for coral-algal symbiosis is required.

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.