As living entities, corals are not only acted upon by the various elements of their environment, they also react or respond to them. And when changes in environmental factors pose a challenge to their continued existence, they sometimes take major defensive or adaptive actions to insure their survival.
We here consider coral responses to stress imposed by high solar irradiance. One simple example of such adaptation comes from studies of corals that exhibit a zonation of their symbiont taxa with depth, where symbiont algae that are less tolerant of intense solar radiation grow on corals at greater depths below the ocean surface (Rowan and Knowlton, 1995; Rowan et al., 1997).
A link between solar-induced tissue damage and the presence of photoprotective proteins has also been noted (Brown et al., 1999; Gates and Edmunds, 1999). For example, it has been demonstrated that zooxanthellae possess a number of light quenching mechanisms that can be employed to reduce the negative impacts of excess light (Hoegh-Guldberg and Jones, 1999; Ralph et al., 1999). Both the coral host and its symbionts also have the capacity to produce amino acids that act as natural "sunscreens" (Hoegh-Guldberg, 1999); and they can regulate their enzyme activities to enhance internal scavenging systems that remove noxious oxygen radicals produced in coral tissues as a result of high light intensities (Dykens and Shick, 1984; Lesser et al., 1990; Matta and Trench, 1991; Shick et al., 1996).
Although extremes of solar irradiance take their toll of corals now and then, coral reefs persist. Indeed, they have done so for millennia; and we are confident they will continue to do so for a very long time to come, as they employ the various strategies they have developed to respond to the high solar irradiance stresses to which they are periodically exposed.
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