Microplastics as Selective Substrates for Marine Microorganisms: Beyond the Toxicological Paradigm

Microplastics (MPs) are widely studied as marine contaminants, yet their ecological roles in microbial systems remain incompletely understood. Conventional toxicological frameworks, largely developed for multicellular organisms, emphasize acute stress and cellular damage but capture only a limited subset of microbial responses. Here, we propose a stage-structured framework in which MPs function primarily as selective substrates rather than as conventional toxicants. Plastisphere assembly is governed by dynamically shifting selection across three stages: initial physicochemical filtering, substrate- and metabolism-driven selection, and biofilm-mediated spatial adaptation. Central to this process is the coupling between substrate transformation and reactive oxygen species (ROS) dynamics. Extracellular ROS arise from interfacial reactions of weathered plastic compounds, while intracellular ROS are generated during microbial carbon assimilation. These processes are mechanistically linked: plastic-derived compounds act as both substrates and stressors, and extracellular ROS further transform them into more bioavailable forms, reinforcing intracellular constraints. Biofilms introduce spatial organization that redistributes metabolic and oxidative burdens, enabling metabolic cooperation, stress partitioning, and HGT, allowing selection to operate at both individual traits and spatially organized community levels. Together, this framework shifts the perspective from toxicity-based interpretations toward a selection paradigm that integrates substrate availability, metabolic constraints, and spatial organization, providing a mechanistic basis for linking microscale microbial processes to broader ecological and biogeochemical dynamics.