Breaking new ground: Gadolinium and Microplastics co-exposure and biochemical alterations in marine clam Donax trunculus

Toxicity of single rare earth elements (REEs) or microplastics (MPs) on organisms has been reported widely, however, their combined toxicity on bivalves has limited investigation. In this study, the effects of gadolinium (Gd, 500 μg/L), microplastics (MPs, Ø ≤ 150 μm, with 0.1 μg/L and 100 μg/L) and their mixture (0.1 MPs μg/L + 500 Gd μg/L; 100 MPs μg/L + 500 Gd μg/L) were evaluated in the clam Donax trunculus under laboratory conditions. The impacts were assessed using a suite of biomarkers related to metabolism and energy reserves content, antioxidant and biotransformation enzymes, cellular damage and neurotoxicity. In the medium, at 500 μg/L Gd, the concentration measured after exposure was similar to the nominal concentration, whereas in the mix treatments it decreased, suggesting the adsorption of Gd onto the MPs. Regarding oxidative stress responses, MPs and Gd alone elicited distinct responses, with MPs showing mostly non-significant or inhibitory effects, while Gd increased oxidative stress. When both contaminants were combined, MPs acted as carriers for Gd, demonstrating their "Trojan horse" capability. As a consequence, the mix treatments induced the most severe oxidative stress. The toxicity order in clams was: 0.1 MPs μg/L ≈ 100 MPs μg/L < 500 Gd μg/L < 0.1 MPs μg/L + 500 Gd μg/L < 100 MPs μg/L + 500 Gd μg/L. This study advances understanding of the combined toxicity of MPs and Gd on marine bivalves, highlighting the need for further research on their interactions and broader toxic effects.