Coral calcification:: autoradiography of a scleractinian coral Galaxea fascicularis after incubation in 45Ca and 14C

The uptake of Ca-45 and/or C-14 by the skeleton of coral colonies has been commonly used to investigate the processes of calcification. This study reports the differential uptake of these tracers within different regions of the skeleton and tissues of individual corallites and polyps of the hermatypic coral Galaxea fascicularis. Incubation in Ca-45 in the light resulted in 80 percent of the Ca-45 taken up being deposited in the skeleton. Autoradiography of transverse and longitudinal slices of freeze-substituted polyps and corallites showed that in the light Ca-45 was incorporated into the exsert septa, the outside of the thecal walls of the corallite and the inner edges of the septa. Incorporation did not occur in the costae. The radioactivity in the skeleton was considerably greater than in the tissues. In the dark, or in the presence of the photosynthetic inhibitor Diuron, Ca-45 was taken up by the exsert septa and was patchily distributed in the corallite walls which suggests that it was not a result of isotopic exchange. The differential incorporation of Ca-45 onto the exsert septa was confirmed by scintillation counting. Negligible radioactivity remained in the extrathecal coelenteron after a brief 5 min rinse in non-radioactive seawater. Only 0.1% of C-14 taken up in the light was incorporated into the skeleton and this was confirmed by autoradiography. In the presence of Diuron or in the dark, very little C-14 was incorporated into tissues or skeleton and in autoradiographs was either not evident in the skeleton or the distribution was similar to that seen in autoradiographs of Ca-45 uptake. These results show that the deposition of Ca-45, and therefore calcium carbonate, occurs at specific loci on the skeleton of a corallite. In the dark, deposition occurs specifically at the growing points of the corallite. Differential deposition of calcium carbonate within individual corallites has not been previously reported.