Tolerance, the ability to endure an external condition such as ultraviolet radiation (U VR), is dependent on the balance between the processes of damage and repair. Aquatic organisms have a variety of physiological and biochemical mechanisms available for reducing the damage incurred by exposure to UVR, including screening, quenching and repair. Screening mechanisms include both physical and chemical barriers and involve many types of UVR-absorbing compounds such as mycosporine-like amino acids (MAAs) and scytonemin, among others. When screening does not eliminate penetration of UVR into the cell, there is a variety of quenching and repair processes available to overcome damage to sensitive cellular components. Quenching is provided by various antioxidants and by carotenoids. Damaged proteins are usually replaced by de novo synthesis (turnover) whereas damaged DNA is generally repaired. Several different mechanisms exist for repairing DNA including photoreactivation, nucleotide excision repair, dimer bypass and recombinational repair. Most research on physiological and biochemical mechanisms of photoprotection has focused on screening mechanisms and in particular on the many kinds of UVR-absorbing compounds present in marine and freshwater species. There has been very little research on DNA and protein repair in aquatic organisms with most of our current knowledge of these processes being derived from bacterial systems.

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