Marine heatwaves modulate the toxicity of critical metals relevant to battery technologies: Understanding the results through integrated biological response index and predictive modelling

Marine coastal ecosystems face growing threats from chemical pollution and climate change, particularly through the intensification of marine heatwaves (MHWs). Critical metals, such as lithium (Li), cobalt (Co), and nickel (Ni), are increasingly detected in aquatic environments due to inadequate recycling and disposal practices. This study assessed the individual and combined effects of 100 µg/L of Li, Co, and Ni on the marine mussel Mytilus galloprovincialis under present-day temperature (17 °C) and simulated MHW conditions. Compared to 17 °C, under the MHW scenario, mussels exhibited enhanced mitochondrial activity and oxidative stress, particularly when exposed to the combination of the three elements. Temperature stress led to a decreased detoxification capacity, except in the case of Ni single exposure, which is consistent with the accumulation of elements in the mussels' soft tissues (with lower Ni accumulation under the MHW scenario but higher for Co and Li). The Integrated Biological Response (IBR) index increased from 8.3 to 13.6 under single-metal exposure to 18.1 and 13.9 in metal mixture treatments at 17 °C and MHW scenarios, respectively, highlighting the strong cumulative stress induced by combined metal exposure. For Co, higher IBR values were observed under MHW scenario, whereas for the remaining treatments, IBR values were similar (Li) or lower (Ni and Mix) under warming, indicating that higher temperatures imposed overwhelming conditions that limited the mussels' capacity to elicit an effective response. Together, these findings demonstrate that incorporating thermal anomalies and realistic contaminant mixtures into ecotoxicological risk assessments is therefore crucial to avoid underestimating the biological risks associated with metals from Li batteries. These insights highlight the need for urgent mitigation and circular economy strategies to reduce metal leakage into marine environments and protect ecosystem resilience in a rapidly warming ocean.