Self-sustaining microbial reductive debromination of brominated flame retardants driven by sewage sludge-derived endogenous organics amid competing electron acceptors.

Polybrominated diphenyl ethers (PBDEs) and tetrabromobisphenol A (TBBPA) are prevalent brominated flame retardants in wastewater and sewage sludge, yet the sustainability and robustness of their microbial reductive debromination remain poorly understood, particularly in the presence of co-existing electron acceptors (e.g., nitrate and sulfate). Here we revealed that sewage sludge-derived endogenous organic matter sustained efficient microbial reductive debromination of both TBBPA and PBDEs without external organics amendment. Over 99% TBBPA was transformed to bisphenol A within 30 days, whereas PBDE debromination occurred sequentially after TBBPA depletion, producing lower-brominated congeners (33.9%) and diphenyl ether (17.9%) after 120 days. Unexpectedly, amendment of external organic carbons (formate, acetate, pyruvate, and lactate) did not enhance debromination and instead stimulated methanogenesis, indicating diversion of electron flow towards competing reduction pathways. Reductive debromination remained highly active in the presence of 0.5-10 mM nitrate and sulfate, and similar debromination extent of both pollutants was achieved as the controls, although high nitrate and sulfate concentrations decreased TBBPA debromination rates by 42.8-67.4%. Notably, active debromination persisted even at sulfide concentrations exceeding 6 mM generated from sulfate reduction, revealing exceptional sulfide tolerance of sludge-associated organohalide-respiring bacteria (OHRB). Dehalococcoides and Dehalobacter were identified as obligate OHRB involved in debromination, with Dehalococcoides exhibiting high tolerance to nitrate-, sulfate-, and sulfide-associated stress. Moreover, the sludge microbiome was resilient and metabolically integrated despite redox perturbations. Collectively, these findings reveal sewage sludge as a self-sustaining and resilient platform for reductive debromination and provide a low-cost strategy for remediation of brominated pollutants in wastewater, sludge and other anaerobic environments.