Most aquatic organisms have evolved mechanisms to minimize damage by UVR. Many terrestrial species have additionally had to adapt to plant compounds (e.g., furanocoumarins) that are extremely toxic when activated by UVR. Over evolutionary time, it is unlikely that these compounds have been present in aquatic systems at concentrations sufficient to trigger adaptive responses. Within the last century, however, release of anthropogenic contaminants, particularly polycyclic aromatic hydrocarbons (PAHs), has greatly increased the potential for photoactivated toxicity in aquatic environments. Most phototoxic compounds exert toxicity via a photosensitizing process that produces (within tissues) ROS that ultimately damage biomacromolecules. Some phototoxic compounds, under certain conditions, may exert toxicity after they have been photochemically modified in the external environment. Both mechanisms require sufficient doses of chemical and UVR, particularly UV-A (315 to 400 nm). Assessment of the potential for phototoxic damage in aquatic systems requires thorough analysis of both of these elements, as well as species and lifestage-specific vulnerabilities. Because photoactivated contaminants are present in high concentrations at fairly isolated areas that may function as sinks, and are presently still being introduced into aquatic systems, consideration of phototoxicity will continue to be a significant ecological concern.

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