Chemical and toxicological fingerprints of polyethylene primary microplastics deployed along nine European rivers

Plastics can adsorb both organic and inorganic contaminants from surrounding aquatic environments, with potential toxic effects on a wide range of species. In this study, polyethylene pellets were immersed along nine European river-to-sea continuums for one month. Adsorbed contaminants were characterized, and their toxicity assessed using DMSO extracts on the marine bacteria Aliivibrio fischeri and Pacific oyster (Magallana gigas) embryos. A diverse array of organic pollutants was annotated, including plastic additives, pesticides, and per- and polyfluoroalkyl substances (PFAS). Spatial trends were observed for trace elements, with higher zinc adsorption downstream, while iron was more concentrated upstream. Standardized bacterial toxicity tests revealed significant effects at 29 % of the sites, with estuarine and intermediate-salinity locations exhibiting higher toxicity than upstream sites. Redundancy analysis identified manganese, copper, zinc and iron as the primary contributors of the DMSO extracts toxicity, although iron was negatively correlated with toxic effects. Individual trace element concentrations in DMSO extracts remained below EC₅₀ values reported in the literature. Overall, this study demonstrates that caging polyethylene pellets could serve as effective passive sensors, enabling the monitoring of a wide range of environmental contaminants along river-to-sea gradients and highlighting spatial variations in both contaminant accumulation and toxicity.