Combined Exposure of Microplastics and Climate Warming Affects the Bacteria-Driven Macrophyte Litter Decomposition in an Urban Lake

Lake ecosystems are increasingly threatened by multiple stressors, notably climate warming and microplastic (MP) pollution, which have emerged as a major environmental concern. Although there is evidence of harmful effects on aquatic organisms, their interactive effects on macrophyte litter decomposition in lake ecosystems remain poorly understood. Here, we conduct a 30-day microcosm experiment to examine the individual and combined impacts of warming and polystyrene MPs (PS MPs) on Vallisneria natans litter decomposition dynamics and linked microbial structure and function in the lake ecosystem. The results demonstrated that combined warming and PS-MPs treatments did not significantly affect litter decomposition at low PS-MPs concentrations but promoted it at high concentrations, indicating a concentration-dependent effect. The combined warming and PS-MPs significantly increased bacterial biomass and some extracellular enzymatic (β-1,4-xylosidase, acid phosphatase, and leucine-aminopeptidase) activities; this enhancement is likely attributable to the abundance and diversity of bacteria at higher PS concentrations. Notably, the combined warming and PS-MPs significantly increased the bacterial diversity and the relative abundance of unclassified Paludibacteraceae and Treponema. The presence of pathogens such as unclassified Paludibacteraceae and Treponema on MPs highlights significant potential risks to public health and aquatic food webs. These shifts suggest that warming and PS-MPs selectively enrich taxa with functional traits adapted to disturbed environments, thereby driving enhanced decomposition. Our findings highlight the need to consider stressor interactions when assessing the ecological and health risks posed by global change to lake ecosystems.