Incorporation of Metabolic CO₂ into Coral Skeleton

Abstract

Two possible sources for the carbonate in the calcium carbonate coral skeleton are (a) soluble carbonates from seawater and (b) carbon dioxide produced in metabolism by the living coral tissue. Although radioisotopic tracer experiments have established that carbonate from sea-water is incorporated into the skeleton by many corals^1–3, evidence for incorporation of metabolic CO₂ into the skeleton is much less direct. Goreau¹ found that calcification rates measured by incorporation of radioactive carbonate from seawater were usually lower than those measured by using radioactive calcium; he proposed that the labelled carbonate might be diluted by unlabelled carbonate in coral tissue. In addition, findings that the ratios of ¹³C to ¹²C and of ¹⁸O to ¹⁶O in seawater differ from those in coral skeleton suggest that not all of the skeletal carbonate originates directly from carbonate in seawater^4–7. To examine further the possibility that some skeletal carbonate originates from metabolic CO₂, I fed ¹⁴C-labelled mouse tissue to small individuals of the coral Fungia scutaria, a solitary polyp containing symbiotic algae.