Brood pouch-mediated polystyrene nanoparticle uptake during <i>Daphnia magna</i> embryogenesis

Nanoplastic debris is currently expected to be ubiquitously distributed in aquatic environments and an emerging environmental issue affecting organisms across trophic levels. While ingestion of particles receives most attention, other routes of uptake and cellular accumulation remain unexplored. Here, the planktonic filter feeder Daphnia magna was used to track routes of uptake and target tissues of polystyrene nanoparticles (PSNPs). A sublethal concentration of 5 mg L^-1 fluorescent PSNPs (25 nm) was used to monitor accumulation in adult animals as well as their embryos in the open brood pouch. A time series throughout embryonic development within the brood pouch revealed accumulation of PSNP in or on lipophilic cells in the early stages of embryonic development while the embryo is still surrounded by a chorion and before the beginning of organogenesis. In contrast, PSNP particles were neither detected in the gut epithelium nor in lipid droplets in adults. An ex vivo exposure of embryos to PSNP demonstrated a similar accumulation of PSNP in or on lipophilic cells, illustrating the likelihood of brood pouch-mediated PSNP uptake by embryos. By demonstrating embryo PSNP uptake via the brood pouch, data presented here give novel insights in bioaccumulation of nanoparticles and likely other lipophilic contaminants. Since this uptake route can occur within a diverse array of aquatic organisms, this study warrants consideration of brood pouch-mediated accumulation in efforts studying the hazards and risks of nanoparticle contamination.