Effects of microplastics on denitrification and associated N₂O emission in estuarine and coastal sediments: insights from interactions between sulfate reducers and denitrifiers

Global estuarine and coastal zones are facing severe microplastics (MPs) pollution. Sulfate reducers (SRB) and denitrifiers (DNB) are two key functional microorganisms in these zones, exhibiting intricate interactions. However, whether and how MPs modulate the interactions between SRB and DNB, with implications for denitrification and associated N₂O emissions, remains poorly understood. Here, we simultaneously investigated the spatial response patterns of SRB-DNB interactions and denitrification and associated N₂O emissions to different MPs exposure along an estuarine gradient in the Yangtze Estuary. Spatial responses of denitrification to polyvinyl chloride (PVC) and polyadipate/butylene terephthalate (PBAT) MPs exposure were heterogeneous, while those of N₂O emissions were not. Gradient-boosted regression tree and multiple regression model analyses showed that sulfide, followed by nitrate (NO₃⁻), controlled the response patterns of denitrification to MPs exposure. Further mechanistic investigation revealed that exposure to MPs resulted in a competitive and toxic (sulfide accumulation) inhibition of SRB on DNB, ultimately inhibiting denitrification at upstream zones with high sulfide but low NO₃⁻ levels. Conversely, MPs exposure induced a competitive inhibition of DNB on SRB, generally promoting denitrification at downstream zones with low sulfide but high NO₃⁻ levels. These findings advance the current understanding of the impacts of MPs on nitrogen cycle in estuarine and coastal zones, and provide a novel insight for future studies exploring the response of biogeochemical cycles to MPs in various ecosystems.