Selecting emergent wetland plants for constructed wetlands under microplastic stress: Physiological performance and transcriptomic mechanisms

Microplastics (MPs) have emerged as persistent contaminants in aquatic environments, posing potential risks to the stability of constructed wetlands (CWs). As wetland plants play a central role in pollutant removal and system resilience, understanding their adaptive responses to MP exposure is critical for the management of CWs. Five emergent wetland plant species were first screened under hydroponic conditions exposed to polyethylene MPs (0.1 mg ·L⁻¹). Among them, Iris pseudacorus exhibited the highest physiological stability, maintaining stable chlorophyll content (2.40 ± 0.09 mg·g⁻¹ FW), relatively stable net photosynthetic rates, elevated antioxidant enzyme activities, and low lipid peroxidation compared with other species. Subsequently, vertical-flow CW systems were operated for 120 days to compare planted (CW3) and unplanted (CW2) systems under MP exposure. CW3 achieved consistently high removal efficiencies of total nitrogen (91.5%), NH -N (up to 96.7%), and COD (80-90%), together with a peak total phosphorus removal of 88.0%. while NO₂⁻-N concentrations remained low (0.01-0.03 mg·L⁻¹), indicating complete nitrogen transformation. Although many representative genes involved in nitrogen assimilation, central carbon metabolism, antioxidant defence, and membrane lipid remodelling showed lower transcript abundance in CW3 than in CW1, these pathways remained significantly enriched, indicating selective pathway-level reorganization rather than broad transcriptional activation. In parallel, CW3 exhibited elevated dissolved organic carbon levels (62.65 mg C·L), suggesting enhanced rhizosphere carbon availability and compensatory metabolic adjustment under MP exposure. Together with the differential regulation of stress-responsive transcription factors (e.g., WRKY, NAC, and MYB), these responses indicate that functional stability in CW3 was maintained through integrated physiological buffering, rhizosphere carbon compensation, and selective transcriptional reorganization. Overall, this study demonstrates that appropriate emergent plant selection enables CWs to maintain treatment performance under MP stress through coordinated physiological resilience and pathway-level metabolic reorganization, providing practical guidance for the design of resilient constructed wetlands.