Corals and light: From energy source to deadly threat

The bathymetric distribution of reef-building, zooxanthellate corals is constrained by the attenuation of underwater light. The products of algal symbiont photosynthesis provide a major share of the energy, supporting the metabolic needs of the animal host. The two orders of magnitude in the underwater light field spanned by corals are overcome by photoacclimative responses of both the algae and the animal host. The corals decrease the photosynthetic fraction of their energy budget as light dims with depth, increasing their heterotrophic dependence on predation. In shallow water, corals are exposed to photodynamic dangers to which both host and symbiont are susceptible. These effects are mitigated by an array of antioxidative mechanisms that wax and wane in a diel periodicity to meet the concomitant fluctuation in oxygen evolution. The nocturnal tentacular extension seen in many corals is terminated at dawn by light, probably mediated by the photosynthesis of the zooxanthellae. The Goreau paradigm of “light-enhanced calcification” is summarized while recently documented exceptions are discussed. The role of light as an information source, besides its acknowledged role as an energy source, is evident in the poorly understood role of lunar periodicity in triggering the spectacular mass-spawning episodes of pacific corals. The characteristics of the underwater light field, its intensity and directionality, control the architecture of zooxanthellate coral colonies, favoring the optimization of the light exposure of the zooxanthellae. This rejview summarizes the main gaps in our understanding of the interaction of corals and light, thereby suggesting promising future research directions.