Infiltrating Biodegradable Microplastics in Zebrafish Blood: From Size Barrier Penetration to Risk

The widespread use of biodegradable plastics (BPs) has increased aquatic organisms' exposure to BP-derived microplastics (BMPs). Blood serves as both the primary site and critical transport system for MPs after crossing biological barriers, yet BMPs' barrier-crossing ability and risk under environmentally related concentration conditions remain unclear. Using an integrated bioimaging platform (electron microscopy, confocal imaging, and an activatable NIR-I/II probe with near-infrared (NIR) imaging), this study investigated polylactic acid (PLA) and its photoaged form (UV-PLA) in zebrafish blood at 50-500 μg/L. We first demonstrated that PLA entered zebrafish circulation with a ∼3 μm size threshold. In circulation, PLA interacted with blood cells, leading to oxidative stress in blood cells and redox imbalance plus tissue damage in the liver. The benchmark dose (BMD) model was developed to assess the dose-response relationships of PLA and UV-PLA for different end points. The mean BMD value of UV-PLA (32.9 ± 12.4 μg/L) was lower than that of PLA (51.6 ± 47.0 μg/L), and notably, the derived BMDs were lower than the reported PLA level in highly polluted areas (e.g., 150 μg/L), suggesting potential risks. This study provides in vivo evidence for the blood translocation and sublethal toxicity of PLA, emphasizing the need to develop sensitive end points and environmentally relevant testing.