Heavy Metal Loading Modifies Microplastic Mixture—Plant Interactions, Metal Availability, and Biochemical Responses in Aquatic Plants

Microplastics (MP), including tire wear particles, are increasingly detected in aquatic environments, where they may interact with co-occurring contaminants such as heavy metals (HM). However, the influence of metal loading on MP–plant interactions and subsequent biological responses remains poorly understood. This study investigated how heavy metal loading modifies the behavior of mixed MP and their effects on two aquatic macrophytes, Lemna minor L. and Spirodela polyrhiza (L.) Schleid. Mixtures of polyethylene, polypropylene, polystyrene, and tire rubber particles were tested in both virgin and HM-loaded forms, with the latter preincubated with Cu, Zn and Pb. Duckweed species were exposed to MP concentrations ranging from 20 to 10,000 particles/L for seven days. HM-loaded MP released Cu and Pb into the plant-free exposure medium, whereas Cu release was also detected from the nominally virgin MP mixture. Cu accumulation occurred in both duckweed species, indicating transfer of MP-associated metals to aquatic plants. MP adsorption increased with exposure concentration and was higher in L. minor than in S. polyrhiza, with polypropylene showing the strongest association with plant surfaces. Metal loading reduced MP adsorption at intermediate concentrations, indicating that adsorbed metals can influence MP–plant interactions in a polymer- and concentration-dependent manner. Although MP exposure did not significantly affect growth, chlorophyll a content and antioxidant/detoxification enzyme activities were altered, particularly in L. minor. Overall, the results demonstrate that heavy metal loading modifies microplastic behavior and influences metal availability and sublethal responses in aquatic plants.