Microplastics aging potentially enlarge the ecological risk to wetland sediments as revealed by their interactive effects on γ-HCH dissipation and methane production

Microplastics (MPs) have garnered global concern, yet the environmental implications of their aging remain poorly understood. Especially, their interactions with co-occurring pollutants and impacts on biogeochemical processes in wetland sediments require further investigation. Through microcosm experiments, this study systematically elucidated the differential effects of pristine vs. aged MPs on γ-hexachlorocyclohexane (γ-HCH) behavior and methanogenesis. Aged MPs exhibited enhanced γ-HCH adsorption (666.7 vs. 500.0 mg kg for pristine MPs), yet paradoxically inhibited γ-HCH dissipation in wetland sediments. This demonstrates that previous studies might oversimplify the interaction between MPs and pollutants. Concurrently, Dhc functional gene abundance on aged MP-surface biofilms declined sharply from 3015.4 to 811.4 copies g dw over 60 days, suggesting impaired functional microorganisms. Notably, aged MPs amplified CH production (1.64 ± 0.10 vs. 1.25 ± 0.15 mg kg day pristine MPs) and HO generation (2.62 ± 0.12 vs. 2.06 ± 0.05 mmol L pristine MPs), with reduced microbial complexity and stability. Mechanistically, aging altered MP surface properties, selectively enriching mcrA functional genes and methanogenic archaea (Methanomassiliicoccus and Methanosarcina). These findings show aged MPs in suppressing pollutant dissipation (γ-HCH) while accelerating elemental cycling (CH), driven by reactive oxygen species and microbiome shifts. Given the widespread occurrence of aged MPs in wetlands, this study underscores their underappreciated yet critical influence on wetland sediment biogeochemistry, urging prioritized research to mitigate their cascading potential risks.