UV induzierte toxische Effekte von gelösten Huminstoffen auf Süßwasserorganismen

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In freshwater, the chromophoric UV/VIS-absorbing compounds of dissolved organic carbon (DOC) are mostly represented by humic substances (HS). By absorption of solar radiation, HS act like sun screens and protect aquatic organisms against harmful UV radiation. One consequence of this light absorption is the photolytical degradation of HS leading to the release of low molecular substances which may serve as potential nutrients for heterotrophic bacteria. At the same time, transient reactive oxygen species (ROS) are generated, which can be quenched by water or HS molecules itself. Some studies point to UV irradiation induced adverse effects on aquatic organisms in humic water, suggesting phototoxic effects caused by ROS. We want to investigate the phototoxic potential of HS in exposure experiments with heterotrophic bacteria, algae, and daphnids. The potential phototoxicity will be correlated with the photoinduced net-ROS, which will be quantified by electron paramagnetic resonance (EPR) using spin traps and laser spectroscopic methods. Furthermore, with daphnids the direct interaction between HS and light excitation shall be studied by EPR-tomography using spinlabels. The aim of this project is to figure out under which conditions and at which trophic level phototoxicity of HS may contribute to the structure of the ecosystem. EPR tomographic methods will be adapted to freshwater organisms for the first time.