Carbonyl and Fouling Indices as Screening Descriptors for Ecotoxicological Responses to Aged Microplastics

Microplastics (MPs) inevitably undergo aging processes in the environment, which alter their physicochemical properties and may modify their ecological effects. To synthesize the current evidence on aged-MP ecotoxicity, we conducted a meta-analysis of 54 studies comprising 1400 observations across animals, microalgae, and plants. Compared with pristine MPs, aged MPs were generally associated with reduced locomotion and neurological performance in aquatic and terrestrial animals, suppressed microalgal growth, and apparent stimulation of some plant growth-related end points. These divergent responses were associated with aging-related descriptors, particularly the carbonyl index (CI) and fouling index (FI), although their relevance varied across taxa, end points, polymers, and exposure contexts. The CI was negatively associated with animal locomotor responses, while the FI was more closely related to physiology-related responses in the primary producers. Targeted validation experiments using polystyrene (PS) and polyvinyl chloride (PVC) further showed that an increased CI was accompanied by reduced zebrafish locomotion and elevated ROS levels, whereas FI-associated algal responses were accompanied by oxidative stress. Overall, the CI and FI can help compare aged-MP effects across studies and support more generalized and more realistic assessments of aged-MP ecotoxicity. However, these descriptors remain simplified indicators and cannot fully capture the complexity of actual environmental systems, including mixed aging processes, leachate chemistry, multistressor exposure, and matrix-specific interactions.