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Microplastic fragments in sand alleviate the negative effects of heavy metals on plants
Summary
A mesocosm experiment found that microplastic fragments in substrate unexpectedly reduced cadmium uptake by plants despite increasing the metal's bioavailability in the soil — suggesting microplastics alter the chemistry of how plants access heavy metals through mechanisms not related to pH or soil moisture. The non-hyperaccumulator Arabidopsis thaliana showed increased biomass when exposed to both cadmium and microplastics together compared to either stressor alone. These counterintuitive interactions highlight how microplastics can complicate predicting heavy metal toxicity in contaminated soils.
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.