Warming alters temporal patterns of microbial-mediated nitrogen cycling under microplastics stress in intertidal sediment ecosystems

Intertidal sediments-hotspots of coastal nitrogen cycling-are preferential sinks for microplastics (MPs) influenced by terrestrial and marine inputs. How warming alters sedimentary microbial nitrogen-cycling functions under MPs stress remains unclear. We incubated sediment microcosms with polyethylene (PE) MPs (0, 0.3, 2.0 % w/w) at 25℃ and 30℃ for 31 days. Microbial community dynamics were tracked by 16S rRNA and metagenomics. While α-diversity was largely unaffected, PE-MPs (especially at 2.0 %) markedly altered microbial community composition from day 16 onward at both temperatures, especially at 2.0 %. At 25℃, the 2.0 % PE-MPs increased microbial interactions and network complexity, with interactions shifting from competition toward cooperation over time. Warming further intensified early competitive interactions in 2.0 % PE-MPs group, driving compositional shifts. Functionally, PE-MPs at 2.0 % modulated the expression of dissimilatory nitrate reduction (DNRA) reductases (nrfA and nrfH), attenuating the increase in sediment NH over time. Concurrently, upregulation of assimilatory nitrate pathway genes lowered NO. Expression of nitrification and DNRA genes was generally enhanced at 2.0 % MPs, accompanied by downregulation of glnA (NH assimilation) and nasB (assimilatory nitrate reduction). Thereby, warming at 30℃ reshaped MPs-driven community dynamics and nitrogen-cycling pathways, slowing the time-dependent declines of NH and NO relative to 25℃ and reducing the risk of nitrogen loss from intertidal sediments. These findings highlight the need to incorporate temperature and temporal dynamics into ecological risk assessments of MPs under global climate change.