Microplastic containing aggregates to not select for a distinct bacterial community Micro 2022 Sophia Reichelt1*(sophia.reichelt@aces.su.se ), Rehab El-Shehawy, rehab.el-shehawy@aces.su.se) and Elena Gorokhova1 (Elena.Gorokhova@aces.su.se ) 1Department of Environmental Science (ACES), Stockholm University, SE-106 91 Stockholm, Sweden Abstract: Natural particles, biopolymers together with micro-and nanoplastics (MNP) form aggregates in the aquatic environment termed marine snow. The degree of aggregation is dependent on environmental factors such as salinity, ion-content as well as the presence of microorganisms and related extra-cellular polymers and alter the bio-availability and toxicity of the particles. Filter feeders such as Daphnia magna interact with these particles by changing their size and composition. Furthermore, their microbiome gets transmitted to the aggregates during the passage through their digestive system and the degree of selection depending on the content of MNP of aggregates are unclear. Recently, it was debated whether those particles would subset the bacterial community and select for potential pathogen bacteria. In this study, we created environmentally realistic aggregates using different particle concentrations (TSS) (0.1-10 mg/l) with varying MNP ratios of (0-10%). We added Daphnia magna and allowed them to interact for 72h. After exposure, the aggregates were analyzed in terms of size as well as associated Daphnia microbiome. The aggregate associated microbiome was compared to the donor-daphniids as well as to the surrounding planktonic phase. Furthermore, we monitored Daphnia fitness parameters to assess toxicity related to particle size variability and MNP content. We observed no fitness decrease in Daphnia related to MNP content, TSS content or aggregate size. Microplastics seem not exert a selective pressure on the transmitted microbiome at environmentally realistic ratio to natural particles. However, particle density is a significant driver for a distinct microbial community. More data analyses are ongoing.