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Microplastics as pathway-selective modulators rewire nitrogen cycling in coastal wetlands: Polymer identity outweighs concentration
Summary
Coastal wetlands act like natural water filters, using bacteria to remove excess nitrogen pollution before it causes harmful algae blooms in our oceans. This study found that certain plastics—especially polystyrene (like foam cups) and polypropylene (common in packaging)—can disrupt this filtering process, causing wetlands to trap nitrogen as ammonium instead of safely releasing it as gas. This matters because it means plastic pollution isn't just about visible litter or ingestion risks—it could be quietly weakening one of nature's key defenses against water pollution that affects seafood safety and coastal ecos
Coastal wetlands are globally important nitrogen sinks that mitigate eutrophication via microbial denitrification and anammox. However, the impact of microplastic (MP) pollution on nitrogen transformation pathways remains poorly understood. Here, we conducted a field-based mesocosm experiment across high, middle, and low tidal marshes, exposing sediments to environmentally relevant concentrations (0.1-1.0% w/w) of four common polymers (PE, PP, PS, and PET). We revealed highly significant three-way interactions among tidal elevation, polymer type, and concentration, demonstrating strong context-dependency. Rather than acting as generic stressors, MPs functioned as pathway-selective modulators, fundamentally shifting nitrate fate from gaseous removal (N via denitrification/anammox) toward ammonium retention (NH via DNRA). Random Forest modeling indicated that polymer type exhibited higher predictive importance than concentration. Notably, in the low marsh, intermediate MP concentrations (0.5%) triggered a non-linear threshold response, elevating DNRA contributions from ∼10% to 65-70% of total nitrate reduction. Concurrently, nitrogen removal multifunctionality declined significantly under PS and PP treatments at concentrations of 0.5-1.0%, particularly in the middle marsh. These findings identify PS and PP as priority polymers for mitigation and underscore the necessity of incorporating polymer-specific characteristics into environmental policies to safeguard the nitrogen buffering capacity of coastal wetlands.