Effects of Co-exposure to Microplastics and Cadmium on the Binding of Protamine-like Proteins to DNA in Mytilus Galloprovincialis

Microplastics (MP) pollution is an increasingly serious environmental issue that affects both marine and terrestrial ecosystems.1 In our previous study, we demonstrated that MP induce alterations to the normal function of protamine-like (PL) proteins, which play a critical role in DNA packaging being the primary basic components of the sperm chromatin in Mytilus galloprovincialis.2 These alterations could have significant implications for the survival of the species, and highlight the wider environmental risks posed by microplastic pollution. Depending on the specific context and interactions, microplastics can increase or decrease the effects of heavy metals. They can act as vectors, transporting heavy metals and potentially increasing their bioavailability and toxicity. However, they can also absorb heavy metals, thereby reducing their mobility and availability in the environment. The present study examines the molecular effects of polystyrene microplastics (PS-MPs) on the binding of PL with DNA after exposing Mytilus galloprovincialis to PS-MPs at a dose of 1 µg/L for 24 hours, in combination with three doses of cadmium chloride (CdCl₂) (1, 10 and 100 nM), compared to exposure to cadmium chloride alone. The study indicated that, although acetic urea polyacrylamide electrophoresis (AU-PAGE) and SDS-PAGE did not reveal significant changes in the patterns of PL proteins in all exposure conditions, the PL proteins exhibited altered binding with DNA, as observed using Electrophoretic Mobility Shift Assays (EMSA), when mussels were exposed to cadmium chloride alone, particularly at the highest exposure dose, but not when exposed to cadmium chloride and PS-MPs simultaneously. In fact, the altered DNA binding ability observed in the PL proteins of mussels exposed to CdCl2 alone aligns with the altered release of PL proteins from sperm nuclei in response to salt treatment, particularly at the highest dose. This differs depending on whether the two stressors are treated separately or together. Interestingly, the presence of MP alongside this dose of cadmium chloride did not prevent the effects of cadmium. All the results suggest that, within certain limits, microplastics could mitigate the negative effects of cadmium in our experimental conditions. This could have significant implications for the reproductive health of Mytilus galloprovincialis and highlights the wider environmental risks posed by microplastic and cadmium pollution.