THE PHYSIOLOGY OF SKELETON FORMATION IN CORALS. IV. ON ISOTOPIC EQUILIBRIUM EXCHANGES OF CALCIUM BETWEEN CORALLUM AND ENVIRONMENT IN LIVING AND DEAD REEF-BUILDING CORALS

Abstract

Equilibrium exchange of Ca between the skeleton and the medium was measured in dead and living corals under various conditions, using Ca45 as isotopic tracer. The Ca which is deposited as aragonitic carbonate into the skeleton of living corals is not in contact with the. environment. Under the conditions of the experiments, no significant istopic Ca exchange occurred in Porites furcata and Millepora alcicornis over a period of 23 days and 156 days, respectively. In the dead parts of Porites furcata colonies where the corallum was exposed to direct contact with the medium, rapid isotopic exchange occurred so that the half-life of Ca45 activity on the skeletal surface was only about 5 days. In reef corals killed with formaldehyde, the amount of Ca45 activity picked up by the skeleton from the medium in 48 hours by equilibrium exchange was the same in colonies in which the coenosarc had been preserved as in those from which all tissues had been removed, so that the naked corallum was in direct contact with the radioactive sea water. This indicates, that, under the conditions of the experiments, the mechanical presence of the non-living coenosarc did not constitute a barrier to equilibrium exchange between the corallum and the environment. In living corals in which the calcification rate was depressed to extremely low levels by simultaneous removal of the zooxanthellae and inhibition of carbonic anhydrase with 10- 3 [image] acetazolamide, the amount of Ca45 activity incorporated was 50% to 70% less than that exchanged by the dead control colonies from which all tissues had been removed by maceration. This indicates that the living coenosarc is an effective barrier to equilibrium exchange of Ca between the skeleton and the environment, even when the Ca deposition rate is almost zero. The experimental evidence indicates that the living coenosarc is fairly Ca-proof and prevents the skeleton from exchanging with the environment. Unlike vertebrates, in which a large fraction of the bone Ca remains in dynamic equilibrium with the dissolved Ca of the body fluids, the skeleton Ca of corals, once it is deposited into the corallum, probably does not take part in steady-state or equilibrium exchanges with the overlying tissues.