Efficient mechanisms to overcome the deleterious effects of UVR are even more important during variable UVR conditions such as are experienced due to depletion of the ozone layer. There are many studies on physiological and biochemical responses by marine and freshwater organisms to UVR; however, there are many more questions begging to be answered. Although a lot is known about the distribution of UVR-absorbing compounds in aquatic organisms, there are many details left unstudied, such as how the damaging wavelengths of UVR are dissipated by the different types of UVR-absorbing compounds, where these compounds are synthesized and located and the mechanisms of bacterial or biochemical interconversion of these compounds. The study of DNA damage and repair or tolerance mechanisms is virtually an open field with only a few pioneering studies. Sensitivity to UVR is species dependent and is based on the differing abilities of organisms to attenuate UVR prior to absorption by UVR-sensitive cellular components and of the ability to repair or tolerate damage to proteins and DNA in particular. More emphasis needs to be placed in this particular area to modify techniques applied to bacterial systems for use in aquatic systems. Once established, these techniques can then be used to determine the importance of repair and tolerance mechanisms in aquatic organisms relative to the importance of screening and avoidance mechanisms.
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