Polymer-dependent microbial responses to PBDEs-microplastic co-contamination in mangrove sediments

Mangroves are biogeochemical hotspots that mediate carbon, nitrogen, and sulfur cycling, yet they are increasingly threatened by the co-occurrence of persistent organic pollutants and microplastics. Although the individual impacts of polybrominated diphenyl ethers (PBDEs) and microplastics (MPs) have been documented, how different microplastic polymers modulate microbial responses under PBDE contamination remains poorly understood. In this study, a six-month microcosm experiment was conducted to investigate microbial community responses in mangrove sediment exposed to PBDEs alone and PBDEs combined with polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and polystyrene (PS). Under PBDE contamination, microplastic addition altered sediment physicochemical conditions, characterized by decreased Eh and polymer-specific changes in inorganic nitrogen concentrations. PBDEs significantly reshaped microbial community composition (PERMANOVA, R² = 0.685, p < 0.001), reducing the relative abundance of Proteobacteria while increasing Chloroflexi, without significantly affecting alpha diversity. Microplastic addition further induced polymer-dependent shifts. PE and PP increased Shannon diversity and enriched nitrogen-cycling taxa, including Rhodanobacter and Paenibacillus, accompanied by higher predicted abundances of nitrification and denitrification related functions. In contrast, PS and PET induced distinct beta-diversity shift and enriched taxa such as Mariprofundus, Acinetobacter, and Pirellula. Predicted functional profiles indicated higher abundances of aromatic hydrocarbon degradation, sulfate respiration, and C₁-compound metabolism related functions under PS/PET exposure, suggesting potential shift toward specialized carbon utilization and sulfur related pathways. Network analysis showed that microplastic exposure reduced network complexity, with edge numbers decreasing from 7736 to 2412-3168 and modularity increasing from 0.26 to 0.43-0.49, indicating more compartmentalized co-occurrence patterns. Overall, PBDEs acted as the primary driver of community shifts, whereas microplastics further modulated microbial composition, network structure, and predicted functional potential in a polymer-dependent manner.