Photolysis is an important process for the turnover of colored, photolabile compounds in ecosystems. In aquatic systems, this has a major impact on the penetration depth of damaging UV-A and UV-B irradiation and on the other hand, on the penetration of photosynthetic active radiation and control the vertical distribution of planktonic primary producers.

Due to photosensitization processes (indirect photolysis) also compounds which do not absorb solar energy themselves are subject to photolytic degradation. This is partly important for the degradation of xenobiotic compounds of both anthropogenic and natural origin such as oil spills and algal toxins like microcystins. Although photolysis does not lead to complete mineralization of most organic compounds, in many cases products are formed, which are substrates for heterotrophic bacteria. Photolysis, therefore, must be considered as the initial step of cleavage.

Furthermore, photolysis of dissolved organic matter plays an important role in the global cycle of carbon, and is involved in the binding and release of nutrients such as P and N. Photolysis of humic matter can alter the mode of organic complexion, for example, elemental toxicity of heavy metals can be modulated. Due to the low quantum yields and short lifetimes of ROS formed by humic matter in aquatic ecosystems, adverse effects on organisms are restricted to the upper photic layer and, probably more important, to the very proximity of the organism.

See also-. Acidification; Biodegradation.

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