Stable isotopes (delta13C and delta15N) of organic matrix from coral skeleton.

The evolutionary success of reef-building corals in nutrient-poor tropical waters is attributed to endosymbiotic dinoflagellates. The algae release photosynthetic products to the coral animal cells, augment nutrient flux, and enhance the rate of coral calcification. Natural abundance of stable isotopes (delta13C and delta18O) provides answers to modern and paleobiological questions about the effect of photosymbiosis on sources of carbon and oxygen in coral skeletal calcium carbonate. Here we compare 17 species of symbiotic and nonsymbiotic corals to determine whether evidence for photosymbiosis appears in stable isotopes (delta13C and delta15N) of an organic skeletal compartment, the coral skeletal organic matrix (OM). Mean OM delta13C in symbiotic and nonsymbiotic corals was similar (-26.08 per thousand vs. -24.31 per thousand), but mean OM delta15N was significantly depleted in 15N in the former (4.09 per thousand) relative to the latter (12.28 per thousand), indicating an effect of the algae on OM synthesis and revealing OM delta15N as a proxy for photosymbiosis. To answer an important paleobiological question about the origin of photosymbiosis in reef-building corals, we applied this proxy test to a fossil coral (Pachythecalis major) from the Triassic (240 million years ago) in which OM is preserved. Mean OM delta15N was 4.66 per thousand, suggesting that P. major was photosymbiotic. The results show that symbiotic algae augment coral calcification by contributing to the synthesis of skeletal OM and that they may have done so as early as the Triassic.

ASSIMILATION OF PHOTOSYNTHETIC PRODUCTS OF ZOOXANTHELLAE BY A REEF CORAL.

1. The hermatypic coral, Pocillopora damicornis was incubated in the laboratory and in its reef habitat with Na214CO3 for 1-24 hours. Controls were incubated in darkness. 14C fixation in light exceeded that in darkness. 2. Fractionation of corals labeled on the reef for 24 hours revealed that 35-50% of the total 14C fixed appeared in the animal tissue lipid (as 14C-glycerol) and protein. From a comparison with dark controls it is concluded that photosynthetic products of zooxanthellae are translocated to host coral tissue. The skeletal organic matrix also acquires 14C. 3. Zooxanthehellae isolated from corals and incubated in a homogenate of host coral tissue selectively release glycerol and traces of other organic material including glucose, alanine, and glycolic acid confirming previous observations.