Rhythmic variations in calcification and photosynthesis associated with the coral Acropora cervicornis (Lamarck)

Large populations of the algal endosymbiont, Gymnodinium microadriaticum, occur in the tissues of Acropora cervicornis, and most other species of hermatypic corals. Symbiont photosynthesis is directly proportional to light-enhanced calcification by the coral. Fresh isolates of G. microadriaticum exhibit daily rhythms in photosynthetic efficiency at saturating and subsaturating irradiation. With ambient solar irradia­tion, photosynthetic maxima occur at sunrise and sunset, and a minimum occurs at local solar noon. These rhythms provide an important homeostatic mechanism which enables A. cervicornis to maximize the portion of each day during which calcification can occur at a relatively high constant rate.