Abiotic and biotic influences on silver nanoparticle fate and effects in aquatic model ecosystems

Silver nanoparticles (AgNP) are used dominantly for disinfection purposes due to their high toxicity against bacteria. Even though this property is desired during use, it may become unwanted, if AgNP are released into the environment. As the toxicity of AgNP is not limited to bacteria, also other taxonomic groups may suffer from release of AgNP into the environment. Release of silver ions from the AgNP represents an important source for toxicity, however, also AgNP themselves can act toxic. In aquatic systems, toxicity of AgNP is highly affected by their colloidal stability (dissolution and agglomeration) in the medium, which is in turn closely related to the coating of the used AgNP. As also the test conditions, such as pH, medium composition, light intensity, or test duration interact with the coating, generalizations on the mode of action or toxicity of AgNP to aquatic organisms can only be made for a single coating or a certain test design. Still, the high number of studies using AgNP provide a good reference for investigating the relation between colloidal stability in the test medium and toxicity in more detail. For this dissertation, two rarely considered aspects of this relation were chosen 1) the influence of surface area and surface properties on colloidal stability and actual concentrations of AgNP in the test medium, and 2) the effect of resource reduction on AgNP toxicity. By using these two topics, biological as well as chemical influences on AgNP toxicity could be investigated. For this purpose, an aquatic model system was used including two differently coated AgNP, two green algae, Raphidocelis subcapitata and Desmodesmus subspicatus, and the big water flea, Daphnia magna, in varying compositions. The investigation of surface-related effects revealed that differences in surface area were of minor importance for AgNP colloidal stability and fate compared to surface properties. Higher hydrophobicity increased the attachment of detergent stabilized AgNP to the test vessela s surface, thus reducing the actual exposure concentrations and causing lower levels of toxicity in this test vessel. In case of variation of the AgNP surface itself, the coating, higher degrees of attachment of the AgNP to each other, so a higher degree of agglomeration, increased uptake of the corresponding AgNP and caused higher levels of toxicity. These results support the importance of surface properties for the fate of AgNP in a given test system and identified hydrophobicity as well as surface charge as most important properties for the attachment of AgNP to surfaces. In addition, the interactions between biological surfaces and citrate coated AgNP were identified as most probable link between colloidal stability and observed toxicity, suggesting further investigations on this topic. The reduction of resources had a close connection to the other topic of research, as changes in media composition, as required for changes in nutrient supply for algae, highly affected colloidal stability of the AgNP. By the use of intensive analytics, however, effects resulting from changes in colloidal stability could be separated from changes caused by differences nutrient reduction. Resource reduction caused an increase in AgNP toxicity in both trophic levels with the response also differing between the two algae species. Consequently, AgNP toxicity can be expected to be higher for various taxa when resource provision is low, but the intensity of this change is likely to vary between species.