Interaction of antibacterial silver nanoparticles and microbiota-dependent holobiont revealed by metatranscriptomic analysis

Although antibacterial silver nanoparticles (AgNPs) are increasingly released into the environments and are very toxic to aquatic organisms, their effects on bacteria-based holobiont have been rarely studied. Here, we investigated the interactions between gut microbiota and zooplankton, Daphnia magna, under AgNPs exposure through bioimaging, metatranscriptomic and gut microbiota translational techniques. The results demonstrated that the toxic symptoms of zooplankton were mediated by gut microbiota at low concentrations of AgNPs, and the level of toxicity was influenced by the exposure time. At high AgNP concentrations, mortality of zooplankton was due to the combined effects of extinct gut microbiota and the accumulated toxicity. Exposure to AgNPs caused down-regulation of genes encoding proteasome, ATP synthesis, fatty acid biosynthesis, and β-oxidation in host, with a shorter body length, decreased digestion and reproduction ability. These perturbations were relieved possibly through microbial sulfidation of Ag+, and via the aggregation of nanoparticles and possibly the recovered synthesis of short-chain fatty acids (SFAs). The reduced fluorescence of AgNPs and the symptomatic palliation observed in the AgNP-tolerant microbiota inoculated D. magna further confirmed the detoxifying role of gut microbiota. Overall, our findings provide fundamental knowledge about the effects of AgNPs on bacteria-based holobiont.