Effects of the Interaction of Salinity and Rare Earth Elements on the Health of Mytilus galloprovincialis: The Case of Praseodymium and Europium

The growing use of products containing rare earth elements (REEs) may lead to higher environmental emissions of these elements, which can potentially enter aquatic systems. Praseodymium (Pr) and europium (Eu) are widely used REEs with various applications. However, their ecotoxicological impacts remain largely unexplored, with poorly understood risks to wildlife. Moreover, organisms also face environmental stressors like salinity fluctuations, and the nature of the interaction between salinity variations and contaminants is not yet clear. Therefore, this study aimed to evaluate the influence of salinity shifts on the impacts of Pr and Eu on adult mussels and the sperm of the species Mytilus galloprovincialis after 28 days and 30 min of exposure, respectively. To do so, biochemical and histopathological alterations were evaluated in adults, while biochemical and physiological changes were analysed in sperm. Additionally, the Integrated Biological Index (IBR) was calculated to understand the overall impact of each treatment. The results showed that adult mussels were most affected when exposed to the combination of high salinity and each element, which altered the behaviour of defence mechanisms causing redox imbalance and cellular damage. On the other hand, sperm demonstrated sensitivity to specific REE-salinity combinations, particularly Pr at lower salinity and Eu at higher salinity. These specific treatments elicited changes in sperm motility and velocity: Pr 20 led to a higher production of O₂^- and a decrease in velocity, while Eu 40 resulted in reduced motility and an increase in irregular movement. At both lower and higher salinity levels, exposure to Eu caused similar sensitivities in adults and sperm, reflected by comparable IBR scores. In contrast, Pr exposure induced greater alterations in sperm than in adult mussels at lower salinity, whereas the reverse was observed at higher salinity. These findings suggest that reproductive success and population dynamics could be modulated by interactions between salinity levels and REE pollution, highlighting the need for further investigation into how REEs and environmental factors interact. This study offers valuable insights to inform policymakers about the potential risks of REE contamination, emphasising the importance of implementing environmental regulations and developing strategies to mitigate the impact of these pollutants.