Microplastics Amplify Greenhouse Gas Emissions from Freshwater Sediments through Synergistic Interactions

Microplastic pollution is pervasive in freshwater ecosystems, yet the ecological consequences of increasingly complex microplastic mixtures remain poorly understood, particularly under climate warming. Here, we establish 1264 aquatic microcosms containing microplastics from 1 to 12 types and expose them to 15 and 20 °C to assess their effects on greenhouse gas (GHG) emissions. We find that the increased microplastic chemodiversity, quantified by type number and chemical composition, significantly amplifies GHG emissions by up to 4.69-times especially under warming. This pattern is consistent across a 450-day organic carbon degradation cascade. Compared to single-type microplastics, mixtures of multiple microplastics emit greater amounts of GHG in ∼64% of cases, with the effects strengthening at higher chemodiversity. These effects are driven by synergistic interactions, which prevail when three or more microplastic types are combined, especially in the presence of conventional microplastics. The warming-induced increase in GHG emission rates is more pronounced with increasing microplastic chemodiversity, driven directly by microplastic diversity and indirectly by shifts in the compositions of microbes and dissolved organic matter. Our findings reveal the mixing strategies and interaction patterns among diverse microplastics in regulating greenhouse gas emissions, and advance understanding of how plastic pollution affects freshwater-carbon cycling.