Tyre-derived ecotoxicity: Differentiating the effects from particles and chemical leachates on the blue mussel Mytilus edulis

Tyre particles contain complex chemical additives that can leach out into the aquatic environment, posing potential risks to marine organisms. Despite growing evidence of adverse effects, the relative importance of particle-driven versus chemically mediated toxicity remains poorly explored, especially under environmentally relevant exposure scenarios. This study used the blue mussel ( Mytilus edulis ) as a model to differentiate these effects by exposing individuals to cryomilled tyre particles (TP), their leachates (L) and pre-leached particles (TPL) over 36 days at the environmentally relevant concentration of 0.1 g/L. Chemical analysis confirmed uptake of key organic additives such as poly(1,2-dihydro-2,2,4-trimethylquinoline) (TMQ), N-(1,3-dimethylbutyl)- N ′-phenyl- p -phenylenediamine (6PPD), and 6PPD-quinone (6PPDQ), with certain compounds persisting after depuration. Particle-exposed mussels accumulated higher additive concentrations than those exposed to only leachates, indicating enhanced chemical release from particles. Biomarker responses revealed signs of oxidative stress and neurotoxicity in exposed mussels across all treatments, with earlier responses in leachate exposure and delayed responses during particle exposures. These results demonstrate that chemical additives are key toxicity drivers alongside physical particles, highlighting the importance of considering both pathways in environmental risk assessments. To our knowledge, this study is among the first to experimentally separate particle and leachate specific effects in mussels by using tyre particles before and after leaching to create contrasting chemical loads, thereby providing novel insights into their distinct and combined impacts on marine biota.