Size-dependent and tissue specific accumulation of polystyrene microplastics and nanoplastics in zebrafish

As emerging pollutants, micro-nanoplastics (MNPs), have been widely detected in aquatic ecosystems. When engaging with fish species, MNPs can be initially accumulated in fish gills and intestinal tract, followed by subsequent transportation to other tissues and organs through the circulatory system. However, the tissue-specific accumulation of MNPs in relation to size differences remains uncertain. In this study, continuous exposure experiments with MNPs of different particle sizes were conducted to evaluate tissue-specific accumulation and to examine associated biochemical responses. Generally, fish intestine and gills accumulated MNPs of all size, whereas fish muscle and brain only accumulated nano-sized MNPs. Specifically, fish muscle and brain showed significantly higher MNPs enrichment of 25 nm (vs. 250 nm), since they can easily pass through the intercellular spaces or be endocytosed within cells. In terms of time-dependent accumulations, fish intestines and gills exhibited rapid initial uptake followed by fluctuating variations of MNPs as exposing lasted, since these organs possess both a high capacity for MNP accessibility and elimination. However, fish liver and brain showed a monotonic increase in MNPs accumulation with continuous exposure, which can be ascribed to the difficulty in draining the metabolic waste of these organs. When examining toxic effects, both AChE and CYP450 enzyme activities in all exposure groups were significantly higher than the control group, indicating that MNPs triggered neurotoxicity and metabolic detoxification. Our study highlights considering size and tissue-specific accumulations of MNPs when planning MNPs control for fish health and fishery products safety.