Evaluating stable isotope proxies (δ¹³C, δ¹⁸O, and δ¹⁵N) for detecting photosymbiosis in fossil corals

Photosymbiosis is a mutually beneficial relationship that many scleractinian corals have developed with dinoflagellates called zooxanthellae. Photosymbiosis is often considered the evolutionary innovation that allowed corals to become major reef-builders; nevertheless it is difficult to determine whether ancient reef-building corals harbored symbionts because zooxanthellae are not preserved in the fossil record. Two stable isotope measurements were previously proposed as proxies for ancient photosymbiosis: δ18O/δ13C and δ15N. Modern zooxanthellate (Z) and azooxanthellate (AZ) corals can be differentiated by skeletal δ18O and δ13C due to fractionation caused by zooxanthellae photosynthesis (Stanley and Swart, 1995) and by the δ15N of their skeletal organic matrix because nitrogen is influenced by trophic lifestyle (Muscatine et al., 2005). In this study, Modern, Holocene, Oligocene, and Triassic coral skeletons with varying morphologies (Z-like and AZ-like morphologies) were analyzed for δ18O/δ13C and δ15N to test whether these geochemical techniques are applicable to the fossil record. To avoid altered signals due to recrystallization, samples were visually tested for diagenetic alteration through petrography and scanning electron microscopy (SEM).

The δ18O and δ13C data displayed enriched isotopic values for both C and O in fossil corals and higher variability in Triassic corals than previously reported suggesting that the proxy is not dependable when used on fossil corals. The δ15N analyses of fossil material failed to reproduce the significant Z vs. AZ offset originally identified by Muscatine et al. (2005). Although AZ-like specimens were specifically selected, all of the samples were collected from shallow-water platforms and it is possible that they were zooxanthellate despite their morphology. It is also possible that all corals shared the same nitrogen sources by living in the same environment thus leading to similar organic matrix δ15N values. Future research on this proxy should focus on understanding the influence of the nitrogen cycle on organic matrix δ15N and analyzing fossil deep-water corals to establish whether the δ15N of fossil coral organic matrix can serve as a reliable proxy for ancient photosymbiosis. The establishment of a successful proxy for photosymbiosis will allow scientists to define the evolutionary relationship between coral-zooxanthellae symbiosis and reef-building.