Toxic effects of biodegradable polylactic acid nanoplastics on developing zebrafish (Danio rerio)

Plastic contamination represents a significant threat to the environment with potential health risks for all species. Recently, bio-based plastics have been introduced as a green alternative to fossil-based ones. However, bioplastic degradation products present potentially harmful effects comparable to the fossil-based ones. Polylactic acid (PLA) is one of the most widely used bioplastics in the world and it requires high temperatures found in industrial composting, to degrade fully. These conditions are not typical of natural environments, where PLA degradation leads to the accumulation of micro- and nanoplastics. According to this finding, the aim of the present study was to assess PLA nanoplastics (PLA-NPs) potential harmful biological effects. PLA-NPs exposure effects have been assessed through the in vivo study on early-stage zebrafish bioaccumulation, cellular stress induction, and morpho-physiology, and through the in vitro study on HDF, used to mimic one of the modes of contamination route in humans, to evaluate their cellular uptake potential. For this purpose, zebrafish embryos were exposed to fluorescent PLA-NPs at the concentrations of 0.1 and 1 mg/L, up to 5 days. In the study, the parallel experiments were conducted by exposing zebrafish embryos to polystyrene MPs (PS-MPs), used as a well-established harmful positive control. No alterations in the Zebrafish Embryo Acute Toxicity Test parameters were found; however, heartbeat rate alteration in the PLA-NPs-treated zebrafish at 96 and 120 hpf have been observed. No remarkable morphological alterations of brain and liver tissue have been detected. The bioaccumulation of PLA-NPs was detectable at 72 and 96 hpf, presumably in the gastrointestinal tract. The gene expression analysis of cellular stress markers (hmox1, nos2, sod1, sod2, il1β, tnfα, il4, il13, infγ, tbx21) showed inflammation and oxidative stress induction in zebrafish at 72 and 120 hpf. Finally, HDF demonstrated uptake potential, suggesting their ability to bypass the dermal human barrier. The obtained results, accompanied by those on the exposure of developing zebrafish, and HDF cells, to the same concentrations of PS-MPs, raise concerns about the biological impact of PLA-based bioplastics and their use as a safe alternative to petroleum-based plastics.