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604 ULTRAVIOLET STRESS J. MALCOLM SHICK University of Maine Solar ultraviolet (UV) radiation, because of its high energy, damages biological molecules such as DNA, proteins , and lipids. UV radiation penetrates coastal seawater, where it may kill organisms outright, adversely affect diverse physiological processes (embryological development , growth, photosynthesis, immune response), and evoke avoidance behaviors. Marine organisms have evolved biochemical defenses against the direct and indirect effects of UV radiation. The metabolic costs of maintaining these defenses represent a stress, particularly when the defenses must cope with enhanced UV irradiance. SOLAR UV RADIATION AT THE EARTH’S SURFACE AND UNDER WATER Because of atmospheric absorption and scattering, solar radiation reaching the earth is lowered in intensity and its spectral distribution is truncated (Fig. A). The shortest ultraviolet wavelengths (UVC, nm), which has a low energy content per photon (Fig. B) and is manifested as heat, constitutes just under half of the total solar energy incident on the earth (Fig. A). PAR (also involved in vision) accounts for roughly half. Combined UVA and UVB constitute only about %. Despite this low incidence , because of their high energy content per photon (Fig. B), UV wavelengths have large biological effects. The variable penetration of UV radiation into seawater is caused primarily by regional and temporal differences in its absorption by biologically derived dissolved organic matter and particulate organic matter (including detritus and living plankton). The attenuation of UV radiation is inversely related to wavelength, so that UVA penetrates deeper than does UVB. In clear seawater that is low in productivity, UV radiation penetrates to several tens of meters, whereas in productive coastal waters it reaches maximally to about – meters, with much variability among waters having different optical properties. In the coastal Gulf of Maine (United States), UVA is % of its surface value at about m; the % depth there for UVB is – m, but off Southern California (United States) UVB is detectable to m. Organisms living intertidally and in tidepools potentially are exposed to levels of UVR ranging from a few percent up to % of the local surface intensity. U BIOLOGICAL EFFECTS OF UV RADIATION Because their cyclic molecular structures contain conjugated double bonds (i.e., those that alternate with single bonds: C᎐C⫽C᎐C⫽C᎐C), in which the electrons are loosely bound, nucleotide bases (especially) and aromatic amino acids containing cyclic side groups (to a smaller extent) readily absorb UVB. The same is true of polymers containing these building blocks: the nucleic acids DNA and RNA, and proteins. These molecules are structurally altered when the loosely bound electrons are raised to higher energy levels on absorbing these energetic wavelengths. In DNA, the most common UVB photoproducts are pyrimidine dimers, in which two adjacent molecules of the pyrimidine nucleotide thymine are covalently linked in a cyclic structure that blocks transcription of the genetic information into RNA. UVB radiation causes mutations in DNA and can produce melanomas in mammals and fishes. Deleterious effects on proteins (e.g., loss of function, particularly in enzymes involved in photosynthesis) can be caused by both UVB and UVA, in the latter case primarily via the action of intracellular photosensitizing molecules that transfer the absorbed radiant energy to O, leading to the production of reactive oxygen species such as singlet oxygen, hydrogen peroxide (HO), and superoxide (O·- ) andhydroxyl(OH· )freeradicals.Freeradicalsarechemicals having at least one unpaired electron and in consequence are highly reactive. Oxygen free radicals and other reactive oxygen species in turn oxidatively degrade proteins, DNA, photosynthetic pigments, membrane lipids, and other cellular constituents, with widespread physiological effects. Reactive oxygen species normally are held in check by natural antioxidants such as carotenoids, ascorbic acid (vitamin C), and enzymes such as superoxide dismutase (SOD) and catalase. When there is an imbalance between the production of reactive oxygen species and the defenses against them, oxidative stress is the result. Reactive oxygen species are also produced in seawater through the interactions of dissolved organic matter with UV radiation and trace metals, with unknown effects on organisms. Any effects might be pronounced in tidepools, whose isolation at low tide and large biomass could lead to higher concentrations of precursors (dissolved organic matter) and products (reactive oxygen species). At the same time, high concentrations of dissolved organic matter (including algal exudates known to absorb UV radiation ) in tidepools might help to protect organisms there from the direct effects of UV radiation. In rare cases it has been possible to infer the proximal cause of the detrimental biological effects of UV radiation by comparing the...
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