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REVIEW—ULTRAVIOLET RADIATION‐ABSORBING MYCOSPORINE‐LIKE AMINO ACIDS IN CORAL REEF ORGANISMS: A BIOCHEMICAL AND ENVIRONMENTAL PERSPECTIVE

The clear waters surrounding tropical coral reefs typically are oligotrophic, yet these reefs are highly productive and support dense populations of marine organisms. This paradox is resolved in part because many coral reef invertebrates accommodate unicellular autotrophs (‘‘zooxanthellae,’’ Symbiodinium spp.; Prochlorales, Prochloron sp.; cyanobacteria) within their tissues. These photoautotrophic symbioses entail an exchange of nutrients between the endosymbionts and the animal hosts. Organic compounds produced by the microalgal partners are released to the hosts for their nutrition while inorganic metabolic wastes are recycled to fertilize the algae (Muscatine 1990). The requirement for photosynthetically active radiation (PAR, 400–700 nm) also exposes such algalinvertebrate symbioses and other primary producers to high levels of environmental ultraviolet radiation (UVR 5 295–400 nm) in tropical waters (Smith and Baker 1979, Fleischmann 1989). These waters are often low in UV-absorbing particulate and dissolved organic matter, and sea level fluences of solar UV are high because of the short atmospheric path length and thinness of the stratospheric ozone layer above the tropics (Baker et al. 1980). Photoautotrophic symbiosis thus presents an evolutionary challenge by precluding the morphological development of an optically opaque barrier (such as hair, scales, and feathers in higher vertebrates) and potentially allows damaging UV radiation to reach vulnerable biomolecules in both partners. The problem is exacerbated because the tissues are hyperoxic (.250% air saturation: Kuhl et al.