Combined impacts of polybrominated diphenyl ethers and polystyrene microplastics on microbial community composition and network organization in mangrove sediments

Microbial communities play pivotal roles in mangrove sediment biogeochemistry, yet their responses to persistent organic pollutants and microplastics remain poorly understood. In this study, we examined how polybrominated diphenyl ethers (PBDEs) and polystyrene microplastics (PS-MPs), individually and in combination, affect sediment physicochemical properties and microbial assemblages in Kandelia obovata mangrove sediments over three months. PBDEs significantly altered Eh and nitrogen species, reduced α-diversity, and reshaped microbial community composition, whereas PS-MPs' effects depended on plant presence. Distance-based redundancy analysis (db-RDA) identified NH₄⁺ and PBDEs as key drivers of community variation. At the phylum level, PBDEs enriched Proteobacteria but suppressed Desulfobacterota and Chloroflexi, while PS-MPs promoted Chloroflexi and Planctomycetota. At the genus level, sulfur-oxidizing and sulfate-reducing taxa (e.g., Sulfurovum, Desulfobacca, Desulfosarcina) declined under pollutants, while Sulfurimonas and Desulfatiglans increased selectively. Correlation analysis further linked PBDEs and nitrogen species with sulfur- and carbon-cycling bacteria. Co-occurrence network analysis revealed that PBDEs markedly reduced network size and connectivity but increased modularity, PS-MPs maintained relatively higher connectivity, while combined exposure caused intermediate network patterns between the control and PBDEs treatments. The number and identity of connector taxa shifted under contamination: PBDEs favored anaerobic and sulfur-cycling connectors, PS-MPs enriched fermentative and sulfur-oxidizing taxa, and combined exposure generated metabolically versatile but fewer unique connectors. These results demonstrate PBDEs and PS-MPs jointly restructure microbial diversity, community composition, and interaction networks, highlighting the sensitivity of sulfur-cycling taxa and potential impacts on biogeochemical stability in mangrove sediments.