Lead exposure modulates thermal vulnerability in Mytilus edulis under multi-stressor conditions.

Studying the cumulative impacts of interacting stressors is essential for predicting ecosystem-wide risks from anthropogenic stressors, yet multi-stressor experiments remain scarce. Here, we investigated how lead (Pb) contamination influences survival and cross-tolerance to salinity and thermal stress in Arctic blue mussels (Mytilus edulis). Mussels were exposed to environmentally relevant Pb concentrations, followed by sequential exposure to varying salinities (5-25) and aerial temperatures (5-36 °C). Survival patterns were assessed, and transcriptomic responses were analysed using RNA sequencing. Heat-induced mortality occurred across treatments, and Pb exposure elevated baseline mortality. However, elevated tissue Pb concentrations significantly attenuated temperature-driven mortality, indicating partial cross-tolerance between contamination and thermal stress. Reduced salinity increased vulnerability to heat stress and weakened the protective effect of Pb. Transcriptomic analyses revealed upregulation of cellular stress-response pathways, including detoxification-related genes, consistent with transcriptional front-loading mechanisms underpinning cross-tolerance. Gene expression responses were modest overall, reflecting the high physiological plasticity of Mytilus mussels. Our findings demonstrate that pollution can alter thermal sensitivity in non-additive ways, complicating predictions of organismal responses to environmental changes. Thus, accounting for multi-stressor interactions is critical to understand species resilience in a rapidly changing world.