Key mechanisms of micro- and nanoplastic (MNP) toxicity across taxonomic groups

Micro- and nanoplastics (MNPs) are ubiquitous in aquatic and terrestrial environments, and detrimental biological effects have been observed on a variety of organisms, from bacteria and alga to plants and animals. A fast-growing number of toxicological studies report diverse responses and wide species-dependent sensitivity upon MNP exposure. While studies are dominated by in vivo animal tests, our understanding of cellular toxicity and the corresponding toxicity mechanisms is still limited. This challenges the proper assessment of environmental hazards and health risks of MNPs. In this review, we gathered and analyzed the up-to-date studies on humans, animals, plants, alga, and bacteria, and identified the similarities and differences in key toxicity mechanisms of MNPs across different taxonomic groups. Particularly, human cell-based studies at the cellular level provide fundamental and valuable information on the key toxicity mechanisms, which are essential to answer the question of whether and how MNPs pose health threats. In general, toxicity mechanisms of MNPs depend on their size, surface characteristics, polymer type, as well as cell type. Plausible toxicity mechanisms mainly include membrane disruption, extracellular polymeric substance disruption, reactive oxygen species generation, DNA damage, cell pore blockage, lysosome destabilization, and mitochondrial depolarization. A deeper understanding of these key mechanisms in different taxonomic groups can also improve both in vivo and in vitro models useful for predictive impact assessments of plastic pollution on the environment and human health.