Effects of plastispheres and pristine microplastics on sediment microbial communities and nitrogen cycling under...

Microplastics suspended in freshwater ecosystems can develop plastispheres that transport exogenous microbiota into sediments. However, their effects on sediment microbial communities and functions remain poorly understood, especially under global warming. Here, through in situ incubation and laboratory microcosm experiments, we combined high-throughput sequencing with ¹⁵N isotope pairing techniques to investigate the effects of plastispheres and pristine microplastics on sediment microbial communities and nitrogen cycling at different temperatures (25°C, 29°C, and 35°C). Results showed that both plastispheres and pristine microplastics increased microbial diversity, altered community composition, and enhanced network complexity but reduced network stability. These effects were stronger for plastispheres than for pristine microplastics, and polylactic acid exerted greater influences on microbial communities than polyethylene. Community assembly analysis indicated that microbial changes were primarily driven by deterministic processes, while the contribution of stochastic processes increased with temperature. Importantly, plastispheres and pristine microplastics exhibited opposite effects on sediment nitrogen cycling. Plastispheres significantly decreased denitrification rates (21.75%) and N₂O emissions (39.45%), whereas pristine microplastics increased denitrification rates (31.42%) and N₂O emissions (23.81%). These contrasting effects were mainly caused by changes in denitrifier communities. Pristine microplastics increased the abundance of nirS-type in denitrifier and decreased nirK-type denitrifiers, while plastispheres induced the reverse pattern. Sediment microbial communities and nitrogen cycling functions converged across plastisphere and pristine microplastic treatments under high temperature, indicating that warming reduced the differential effects between the two treatments. This study offers new insights into the influences of plastispheres and pristine microplastics on sediment microbial dynamics in the context of global climate change.