Photolysis of various chromophoric dissolved compounds results in the production of ROS, of which H2O2 is long-lived and may attack organisms. It easily penetrates biomembranes and, hence, contributes to the internal oxidative stress, which, in general, may be adverse to the exposed organisms. For instance, those leachates from aquatic macrophytes, which release the highest concentrations of H2O2, support microbial growth least. In addition to this information, the predominantly adverse effect of internal oxidative stresses, for instance from UV-irradiation or processing of xenobiotic chemicals, is well documented and comprises induction and modulation of stress response proteins and enzymes, reduction of photosynthetic activity, and increased membrane (lipid) peroxidation. However, the oxidative stress as a potential ecological driving force has not yet gained the attention it deserves.
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