A flow-through mesocosm study: biochemical and physiological responses of a three- level marine food web to conventional polyethylene, compostable plastic, and tyre granules

Plastic pollution remains a threat to marine ecosystems, yet the ecotoxicological effects of alternatives such as biodegradable plastics (BDPs) remain poorly understood. This study assessed the biochemical and physiological responses of a three-level marine food web comprising Zostera marina, Mytilus galloprovincialis, and Marthasterias glacialis to BDP, tyre granules (TGs), and conventional polyethylene (PE) exposure under flow-through mesocosm conditions at ECIMAT (Ría de Vigo, Spain). Notably, this represents the first application of biochemical biomarker protocols in M. glacialis, a species recently proposed as a coastal bioindicator. Chemical characterisation revealed contrasting hazard profiles: TGs carried the highest composite chemical hazard index (CHI = 19.0), followed by BDPs (CHI = 9.5) and PE (CHI = 0). Despite this ranking, BDPs produced the most consistent biochemical response: significant inhibition of glutathione S-transferase (GST) activity was observed in both M. galloprovincialis (day 2) and M. glacialis (day 6). TGs induced GST exclusively in M. glacialis, consistent with trophic transfer of lipophilic compounds via dietary exposure. No treatment effects were detected in Z. marina. These findings demonstrate that multi-trophic approaches can reveal contaminant dynamics missed by single-species assessments and that chemical hazard indices alone are insufficient predictors of biological impact.