Role of Symbiotic Algae (Zooxanthellae) in Coral Calcification

Pearse, Vicki; Muscatine, Leonard

Members of many invertebrate groups live symbiotically with unicellular algae, but the symbiosis between corals and dinoflagellate algae (zooxanthellae) is especially interesting because it occurs in all species of tropical reef-building corals (see reviews by Droop, 1963; Yonge, 1963; McLaughlin and Zahl, 1966). Moreover, a significant effect of the algae on the physiology of corals has been clearly demonstrated and quantified : Corals with symbiotic algae calcify many times faster in light than in darkness, while corals which have lost their zooxanthellae calcify at rates which are slower and unaffected by light (Kawaguti and Sakumoto, 1948; Goreau, 1959; Goreau and Goreau, 1959). In the light, photosynthesis by zooxanthellae must somehow lead to higher rates of calcification by corals. Three mechanisms have been proposed to explain how zooxanthellae influence coral calcification : (1) removal of carbon dioxide in photosynthesis directly favors chemical equilibria leading to the precipitation of calcium carbonate (Goreau, 1959); (2) algal removal of phosphates, which may act as crystal poisons, enhances crystallization of calcium carbonate (Simkiss, 1964a, 1964b); and ( 3) organic products of photosynthesis, either specific materials required for skeletogenesis, or nutrients or general energy sources supplied to the coral, permit faster calcification (Goreau, 1959; Wainwright, 1963). So far, there has been no experimental evidence which conclusively supports or eliminates one hypothesis or another. One observation appears to be inconsistent with current ideas about the intimate relationship between algal photosynthesis and coral calcification. In the staghorn coral, Acropora cervicornis (Fig. 1), as in other branching forms, calcification rates are highest in the tips, decreasing progressively towards the base (Goreau and Goreau, 1959). However, very few symbiotic algae are found in the tips, their numbers increasing towards the base. Where abundant, they give the coral a deep brown color, contrasting sharply with the whiteness of the almost algae-free tips (Figs. 1 and 2). We undertook a study of the rapidly calcifying tips in order to clarify the problem of how the algae stimulate coral calcification rates.

Role of Symbiotic Algae (Zooxanthellae) in Coral Calcification

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