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Microplastic-derived dissolved organic matter is associated with increased N2O emissions and altered denitrification-related microbial signatures in constructed wetlands
Summary
Microplastics breaking down in water treatment wetlands release chemicals that make helpful bacteria and fungi work differently, and this shift causes these wetlands to release more nitrous oxide—a greenhouse gas roughly 300 times more potent than CO2. The good news: these wetlands still did a solid job cleaning up pollutants like excess nitrogen; the trade-off is a bigger climate footprint, highlighting how microplastic pollution can have ripple effects beyond just contaminating water, potentially worsening climate change in ways we're only beginning to understand.
Microplastics (MPs) and aged MPs continuously release dissolved organic matter (DOM), yet the effects of microplastic-derived DOM (MPs-DOM) on carbon-nitrogen coupling and nitrous oxide (NO) emissions in constructed wetlands (CWs) remain unclear. Here, polypropylene- and polyethylene-derived MPs-DOM, in pristine (PP-DOM, PE-DOM) and UV-aged forms (LPP-DOM, LPE-DOM), was introduced into laboratory-scale CWs to evaluate pollutant removal, DOM transformation, NO fluxes, isotopocule-inferred source structure, and denitrification-related microbes. MPs-DOM maintained high NO₃⁻-N and COD removal and enhanced NH₄⁺-N and TN removal, especially in the LPE-DOM treatment, but also associated with increased NO emissions. MPs-DOM was removed; LPP/LPE-DOM responses were driven by composition and DOC. Peak NO fluxes in LPP/LPE-DOM treatments were 173%-194% higher than the control, and mean fluxes were approximately doubled. Effluent DOM became more oxidized and unsaturated, and was enriched in lignin- and tannin-like compounds, whereas protein- and lipid-like fractions declined. Isotopocule-based apportionment showed that bacterial-denitrification contribution declined to 38.5%-50.1% under MPs-DOM exposure, whereas isotopocule-attributed fungal-denitrification signatures rose to 46.0%-55.8%, exceeding bacterial denitrification in most MPs-DOM treatments, while chemodenitrification remained minor (2.8%-5.7%). Under MPs-DOM treatments, biofilm microbial abundance and denitrification-gene abundance were elevated, and denitrifier community structure, co-occurrence networks, and niche patterns were reshaped relative to the control. Overall, MPs-DOM enhanced pollutant removal but also increased greenhouse-gas risk in CWs. Under the present exposure design, the stronger responses in aged-MPs-DOM treatments should be interpreted as treatment-associated outcomes linked to DOM composition and higher influent DOC concentration, rather than aging-specific effects.