Figure 3 Schematic presentation of photolytic production of reactive oxygen species (ROS) in an aquatic ecosystem. The major process is their release from illuminated dissolved chromophoric organic carbon. The ROS may interact with a great variety of water constituents, including organisms and dissolved organic compounds. The interaction of ROS with dissolved organic compounds leads to the well-known process of photobleaching.

The produced oxygen radical anion may react with water to generate the reactive "OH radical:

The production of OH-radicals transforms the habitats into (strongly) oxidizing media, and the organisms there must have developed an effective mechanism to protect themselves from this external oxidative stress. Furthermore, "OH may be also formed via direct photolysis of HS through an oxygen-independent mechanism which probably involves quinones as potential sources. OH-radicals are probably the most important driving force for the oxidation of organic compounds in sunlit ice, snow, and water.

Another important pathway of OH-radical formation is the classical Fenton reaction in which H2O2 is reduced and "OH produced:

When this reaction runs with light energy, where H2O2 comes from the photolysis of chromophoric organic substances, it is known as a photo-Fenton reaction. Simultaneous "OH formation along with the production of H2O2 and Fe(II) provide evidence of photo-Fenton-produced "OH. This is often the case in natural, particularly highly colored, iron-rich, acidic waters and may lead to carbon limitation (see below). Once formed, OH-radicals are unstable and react with many organic or inorganic species at rates near diffusion limit, either through an addition or H-abstraction pathway.

The impact of environmental factors on the ROSformation by humic matter will be demonstrated with O2. Like OH-radicals, *O2 is very reactive towards a wide range of electron-rich organic compounds such as alkenes, sulfides, phenols, or HS (photobleaching). With 1O2-formation, several interesting differences between terrestrial and aquatic chromophoric dissolved organic compounds emerge: Terrestrial isolates tend to possess higher quantum yields of 1O2-formation than those from freshwater, meaning that terrestrial material is more reactive. Furthermore, there is seasonality in the photoreac-tivity, since spring samples from freshwater habitats release more *O2 than autumn samples do (Figure 4). For the special case of *O2, we know that aquatic samples collected in spring apparently suffered less from the impact of solar irradiation than samples collected in the fall because they are exposed as long as they remain in the photic water column. In contrast, chromophoric organic carbon compounds in soils and peats, become photolyzed only at the surface.

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