Freeze–thaw cycles restructure PET microplastics in sediments into new habitat of microbial carbon metabolism

Freeze-thaw cycling (FTC), a predominant climatic driver in alpine environments, remains an unaddressed knowledge gap regarding its impact on microplastic (MP) aging and associated microbial carbon metabolism. Here, we used controlled laboratory incubations coupled with multi-scale molecular and functional analyses to investigate MP-mediated carbon cycling under simulated FTC conditions. FTC exposure significantly modified the surface characteristics of MPs, particularly polyethylene terephthalate (PET), by inducing surface oxidation, increased roughness, and microcrack formation. These physicochemical transformations created reactive micro-environments that favored microbial colonization and metabolic potential. Under the combined stress of FTC and MPs, the abundance of sedimentary carbon-cycling genes declined by 30.39 to 31.27%. Conversely, the PET plastisphere exhibited a substantial enrichment of carbon-cycling genes (up to 3.93 × 10⁵), effectively establishing a localized carbon cycling habitat. A dominant functional module comprising fixation, composition, and oxidation pathways accounted for 88.9% of carbon-cycling genes within the PET plastisphere, with Brevundimonas and Comamonas emerging as the key functional taxa in here. These findings demonstrate that intensified FTC can transform inert PET MPs into metabolically active microscale cycling habitats with the potential to influence local carbon-cycling processes, and highlight the need for future in situ investigations to evaluate the environmental relevance of these effects.