Oligochaete-driven resilience: Mitigating microplastic toxicity in constructed wetlands through integrated plant-microbe responses

Aquatic oligochaete integration enhances constructed wetland (CW) performance, yet its role and mechanism in alleviating microplastic (MP)-related negative effects are poorly understood. Here, a laboratory-scale CW system was operated for 320 days to investigate the individual and combined effects of polystyrene MPs (PS-MPs, 100 μg/L, 100 μm) and the aquatic oligochaete Tubifex tubifex. PS-MPs showed clear time-dependent effects on CW performance, with short-term inhibition of nitrate-nitrogen removal and long-term suppression of ammonium-nitrogen and total phosphorus removal. The addition of Tubifex worms slightly improved NH-N removal (0.4%-1.7%) and more substantially enhanced NO-N and TP removal (9.0%-28.8% and 31.9%-41.8%, respectively). At the plant level, Tubifex worms significantly enhanced catalase activity, mitigating PS-MP-induced oxidative damage and improving the potential of plants for water purification. At the microbial level, Tubifex worms increased the relative abundances of taxa such as env.OPS_17, SBR1031, and Candidatus Alysiosphaera, which have been reported to be potentially involved in nitrogen and phosphorus transformations. It also reshaped the microbial interaction networks and enhanced the functional redundancy and stability of the system. Notably, although Tubifex worms inhibited nitrate reductase activity, they may help maintain overall denitrification performance by improving plant physiological status, which could potentially enhance nitrogen uptake, highlighting the importance of plant-microbe interactions. These findings not only deepen the understanding of the ecotoxicological effects of MPs but also provide a theoretical basis for utilizing ecosystem engineers such as Tubifex worms to enhance the resilience of CW to emerging pollutant stress.