From passive carriers to selective filters: how microplastic aging shapes antibiotic resistance gene abundance.

Microplastics (MPs) are widely recognized as vectors for antibiotic resistance genes (ARGs). However, pristine MPs are virtually nonexistent in nature. While environmental aging fundamentally alters MPs physicochemical properties, current studies report highly contradictory findings on how aging impacts ARG proliferation-ranging from synergistic enrichment to oxidative suppression-thereby precluding accurate ecological risk assessments. To resolve this conflict, we conducted a global meta-analysis of 701 independent observations. Aging significantly increased both absolute and relative ARG abundance (mean lnRR = 0.67 and 0.61, respectively), shifting MPs from passive carriers toward selective habitats. Critically, ARG abundance displayed non-linear temporal trajectories inferred from cross-study comparisons: absolute ARG abundance peaked at approximately 15 days of aging MPs, whereas relative ARG abundance showed a U-shaped pattern with a minimum at approximately 37.5 days. We propose a "boom-and-bust" conceptual model, in which an initial non-selective biomass surge is followed by late-stage, multi-stressor filtering; this model integrates our meta-analytic patterns with previously reported mechanisms and represents a hypothesis for direct experimental testing. Resistance mechanisms further diverged: efflux-pump genes peaked at mid-term aging on biodegradable polymers, while target-modification genes and mobile genetic elements remained enriched on high-surface-area recalcitrant plastics. Together, these findings suggest that aged MPs act as selective filters shaping the plastisphere resistome, and that aging state, temporal dynamics, and matrix-specific hotspots merit integration into antimicrobial resistance surveillance frameworks.