We investigated the effects of microplastic (MP) and cadmium (Cd) on the performance of plant species with distinct Cd tolerance levels. We hypothesized that MP may affect Cd bioavailability via direct interactions or indirectly modifying pH, water content and bioavailability of other elements in the substrate.

Arabidopsis thaliana (non-hyperaccumulator) and Arabidopsis halleri (metal-hyperaccumulator) were exposed to different concentrations of Cd and MP. Plant biomass, Cd bioavailability in the substrate, Cd and other elements’ uptake by plants were assessed, and substrate’s properties monitored.

Biomass of A. halleri weakly responded to MP and Cd. A. thaliana biomass decreased when exposed to either Cd or MP and increased when exposed to both stressors. In both species, high MP concentrations in the substrate promoted higher Cd bioavailability, but reduced Cd uptake by plants. Cd bioavailability was not explained by substrate’s pH or water content. However, variation in other elements uptake in the two species was strongly correlated with Cd and weakly with MP concentrations.

The findings of this study suggest a direct interaction between MP and Cd mediating shifts in Cd bioavailability and Cd uptake in plants as well as indirect effects mediated by element uptake. This work provides insights on the complex interplay between MP and Cd.