Effects of microplastics on bacterial communities in lake wetland sediments: a comparison between drought and flooded conditions

Under the backdrop of increasingly frequent wetland alternation between dry and wet conditions driven by global climate change, the differential impact mechanisms of microplastics (MPs) on microbial communities under drought and flooding conditions remain insufficiently characterized. This study established a sediment microcosm experimental system for Poyang Lake wetland to systematically analyze the regulatory effects of polyethylene (PE) and polypropylene (PP) microplastics on sediment bacterial community structure, functional genes, and ecological processes over 180 days under simulated drought and flooded conditions. The results demonstrated that microplastics significantly enhanced the richness (Chao1 index +95.9 %) and diversity (Shannon index +61.6 %) of sediment bacterial communities, with a more pronounced promoting effect under flooded conditions (η = 0.36). Furthermore, PE exhibited a more pronounced influence on community structure than PP, particularly under drought conditions. Under flooding microplastics upregulated nitrogen and sulfur cycling functional genes by enriching Nitrospira and Desulfatiglans, but concurrently increased the abundance of potential pathogenic bacteria (p < 0.001). Under drought conditions, Streptomyces emerged as the core genus, driving the expression of carbon metabolism genes (rho >0.56) and enhancing stress resistance. Null model analysis revealed that bacterial community succession was primarily driven by deterministic processes (contribution rate >51.3 %), while MPs restructured ecological niche by significantly reducing ammonium nitrogen (-14.35 %),and nitrate nitrogen (-13.64 %), while regulating pH and organic carbon levels. This study provides insights into the specific risks associated with "plasticosphere" formation under flooded and drought conditions: flooded conditions may exacerbate pathogen dissemination and metabolic dysregulation, whereas drought promotes carbon sequestration. Based on these findings, a hierarchical governance strategy is proposed, offering new insights for wetland ecological restoration.