Stable Carbon Isotopes (δ 13C) in Coral Skeletons: Experimental Approach and Applications for Paleoceanography

Scleractinian corals obtain fixed carbon via photosynthesis by their endosymbiotic algae (zooxanthellae) and via hetertrophy (injestion of zooplankton, δ ¹³C ≈ -17 to -22‰ ). Carbon dioxide (CO₂) used for photosynthesis is obtained from seawater (δ ¹³C ≈ 0%) or from respired CO₂ within the coral host. The δ ¹³C of the carbon used in the formation of the underlying coral skeleton is fractionated as a result of both of these metabolic processes. Here I have pooled evidence from several field and tank experiments on the effect of photosynthesis and heterotrophy of coral skeletal δ ¹³C. In the experiments, decreases in light levels due to shading or depth resulted in a significant decrease in skeletal δ ¹³C in all species studied (Pavona gigantea, Pavona clavus, Porites compressa). Decreases in photosynthesis in bleached corals also resulted in a decrease in skeletal δ ¹³C compared to non-bleached corals growing under the same conditions and at the same location. Skeletal δ ¹³C also decreased at higher than normal light levels most likely due to photoinhibition. Thus, decreases in photosynthesis due to reduced light levels, due to bleaching-induced decreases in chlorophyll a concentrations, or due to photodamage-induced decreases in functional cholorphyll a, results in significant δ ¹³C decreases. Comprehensive interpretation of all of the data showed that changes in photosynthesis itself can drive the changes in δ ¹³C. In field experiments, the addition of natural concentrations of zooplankton to the diet resulted in decreases in skeletal δ ¹³C. Such a decrease was more pronounced with depth and in P. gigantea compared to P. clavus. In situ feeding experiments have since confirmed these findings. However under tank conditions with unaturally high feeding rates, enhanced nitrogen supply in the diet can disrupt the coral-algal symbiosis, stimlate zooxanthellae growth and photosynthesis, and cause an incrase in skeletal δ ¹³C. It is proposed that under natural field conditions corals feed on zooplankton below this `nutrient threshold' and that increases in heterotrophy should result in decreases skeletal δ ¹³C values. Overall, changes in photosynthesis and heterotrophy have significant effects on coral skeletal δ ¹³C. In shallower corals, photosynthesis drives the bulk of the variation in δ ¹³C. In addition, boron isotope data indicate that pH levels do not vary with changes in photosynthesis or heterotrophy suggesting that metabolically driven δ ¹³C fractionation during skeletogenesis is not pH driven. Thus the skeletal δ ¹³C records from shallow corals in non-upwelling regions where zooplankton concentrations are relatively constant should represent a reliable proxy of light variability. Due to the complexity associated with nutrients and heterotrophy, δ ¹³C records from upwelling regions or deep corals are still difficult to resolve.