A02: Effects of microplastic particles on model organisms of the soil-dwelling macrofauna and their associated microbiota In the present project A02 we want to characterize the effects of MP in two terrestrial model-organisms, the epigaeic soil-feeding earthworm Eisenia fetida and the soil-nesting omnivorous ant Camponotus floridanus. We aim to study the effects of MP on both model organisms, using model-MP-particles of the most abundant and thus most relevant types of plastic, with different particle morphologies, sizes and concentrations, to gain a better understanding of the mode of action of MP. For both, E. fetida and C. floridanus, we plan to study the effects of the different model-MP-particles (with their different physical and chemical properties) on the fitness of the model-organisms on the phenotypic level (mortality rate, number of offspring) as well as on the level of the transcriptome and proteome, in order to characterize sublethal stress- or immune-responses. In addition, potential effects of MP on the diversity and activity of the gut microbiome (such as altered levels provisioning the host with microbial fermentation products and other metabolites) will be characterized, as such alterations can indirectly affect host fitness. We expect that the mode of action and the biological effects induced by MP depend on their physical and chemical properties as well as their morphology and size.