Stable isotopes (δ13C and δ15N) of organic matrix from coral skeleton

International audienceThe evolutionary success of reef-building corals in nutrient-poortropical waters is attributed to endosymbiotic dinoflagellates. Thealgae release photosynthetic products to the coral animal cells,augment nutrient flux, and enhance the rate of coral calcification.Natural abundance of stable isotopes (13C and18O) providesanswers to modern and paleobiological questions about the effectof photosymbiosis on sources of carbon and oxygen in coralskeletal calcium carbonate. Here we compare 17 species of sym-biotic and nonsymbiotic corals to determine whether evidence forphotosymbiosis appears in stable isotopes (13C and15N) of anorganic skeletal compartment, the coral skeletal organic matrix(OM). Mean OM13C in symbiotic and nonsymbiotic corals wassimilar (26.08‰ vs.24.31‰), but mean OM15N was signifi-cantly depleted in15N in the former (4.09‰) relative to the latter(12.28‰), indicating an effect of the algae on OM synthesis andrevealing OM15N as a proxy for photosymbiosis. To answer animportant paleobiological question about the origin of photosym-biosis in reef-building corals, we applied this proxy test to a fossilcoral (Pachythecalis major) from the Triassic (240 million years ago)in which OM is preserved. Mean OM15N was 4.66‰, suggestingthatP.majorwas photosymbiotic. The results show that symbioticalgae augment coral calcification by contributing to the synthesisof skeletal OM and that they may have done so as early as theTriassi