Freeze-thaw aging and microbial colonization converts microplastics into nitrogen cycling hotspots

As global warming intensifies, the frequency of freeze-thaw events increases, significantly impacting microbial metabolism and biogeochemical cycling. However, the synergistic effects of freeze-thaw cycles (FTCs) and pervasive microplastics (MPs) on microbial community assembly and nitrogen cycling remain poorly understood. Here, we conducted a microcosm experiment integrating metagenomic and random forest model to elucidate the co-regulatory mechanisms of FTCs and MPs on plastisphere microbial communities and nitrogen metabolism. Results revealed that FTCs accelerated the environmental aging of MPs, inducing surface cracking and oxidation, thereby creating microenvironments favorable for microbial colonization. In the experimental microcosms, the combined effects of FTCs and presence of MPs increased microbial richness and diversity, promoted community differentiation between sediment and plastisphere, and increased microbial niche specialization. Functional analyses showed that FTCs induced a functional reconfiguration of the plastisphere nitrogen metabolism, with a selective enrichment of key enzyme genes, such as nitrite reductase, which may enhance nitrite redox activity and N₂O emission capacity. In the plastisphere, the contribution of Acinetobacter to nitrogen cycling increased, whereas Nitrospira declined, possibly due to oxygen limitation. Overall, our findings suggested that FTCs may facilitate transformation of MPs from inert pollutants into potentially metabolically active microhabitats, providing critical insights for assessing emerging pollutants and climate change.