Biofouled Micro- and Nanoplastics as Reactive Platforms for Potentially Toxic Element Transformation.

Micro- and nanoplastics (MNPs) are increasingly recognized not only as physical pollutants but also as dynamic chemical platforms. Upon environmental release, they are rapidly colonized by microbial biofilms (the plastisphere) biomolecules and natural organic matter (eco-corona) in the environment, transforming them from inert particles into mobile microreactors. These biofoulings provide redox-active constituents, photoreactive pigments, and ligand-rich polymers that can drive contaminant transformations (e.g., chromium redox cycling, arsenic oxidation, and mercury methylation). Such processes can alter contaminant speciation, mobility, and toxicity, yet they remain absent from most risk frameworks. For example, monitoring and risk assessments typically quantify MNPs by particle counts/mass and polymer identity (and occasionally total metal loads) but rarely measure speciation (e.g., MeHg vs Hg(II) or As(III)/As(V)) on biofouled MNPs. This perspective argues for recognizing MNPs as active pollutant platforms and outlines priorities to quantify biofouled MNP reactivity relative to natural particles, identify and test the toxicity of novel biofouled-metal-plastic complexes, and embed speciation-focused monitoring in policy. By integrating chemistry, ecology, health, climate, and environmental justice, this perspective discusses a forward-looking research and governance agenda to address this overlooked dimension of plastic pollution in a changing world.