Microbial endoliths in skeletons of live and dead corals: Porites lobata (Moorea, French Polynesia)

Carbonate skeletons of hermatypic corals harbor diverse populations of microboring organisms. The assemblages of euendolithic (boring) algae and cyanobacteria inhabiting the corallum of Live corals are different from those that colonize dead and denuded coral skeletons. The species composition of endoliths in live corals is a result of a selection in favor of oligophotic, positively phototropic, fast-growing taxa which can cope with the accretion rates of coral skeleton. Following the death and denudation of corals, the bioerosion of their skeletons changed profoundly with respect to (1) species composition of dominant endolithic microorganisms, (2) their boring patterns and (3) the direction of microboring activity. Skeletons of live colonies of Porites lobata were bored from the inside outward by the siphonalean chlorophyte Ostreobium quekettii. Dead and denuded parts of coral skeleton were colonized at the surface and bored inward by a succession of euendoliths, starting with colonization of the short-lived opportunistic pioneer endolith Phaeophila dendroides (Chlorophyta), followed by Mastigocoleus testarum and Plectonema terebrans (cyanobacteria), to establish within 2 yr a stable O. quekettii-dominated endolith community. The sequence of colonization and successional changes was determined using experimentally exposed carbonate substrates. During the Life of the coral, the endoliths are protected from grazers. Their bioerosive activity keeps pace with the rate of coral accretion but avoids the skeletal surface adjacent to the coral tissue. The green bands formed by endoliths within the skeleton of live corals bear relevance to sclerochronological interpretations. Their formation reflects either variation in coral growth rates or algal seasonality, but most Likely a combination of both. Endoliths in dead coral skeletons are exposed to grazing by mollusks, echinoderms and scarid fish, which significantly increases overall bioerosion rates. In the absence of intensive grazing, dead parts of coral skeleton are overgrown by epilithic algal turf. The series of successional biological changes triggered by the death and denudation of corals shifts the ecological balance between constructive and destructive forces on a coral reef in favor of the latter.