Iron-driven synergism among microbial functional taxa facilitates recovery from denitrification toxicity

Denitrification serves as a key pathway in the nitrogen cycle and in mitigating nitrogen pollution, however it is highly susceptible to environmental conditions and exogenous pollutants. Herein, this study investigated the potentil of iron powder to restore the toxicity of sediment denitrification, inducing by Tetrabromobisphenol A (TBBPA, a typical brominated flame retardant), especially focused on the synergistic interactions between various microbial taxa (e.g., denitrifying bacteria and archaea), which have been largely overlooked in previous studies. The results demonstrated that iron powder addition significantly mitigated TBBPA-induced denitrification toxicity. Specifically, both NO₃^--N and total nitrogen removal efficiency increased approximately 3-fold following iron powder addition as compared to TBBPA-stressed group after 90-day's cultivation. Mechanistic studies revealed that iron powder restored organic carbon utilization by heterotrophic microbes, and maintained microbial activity (including urease activity, electron transport system activity, denitrifying enzyme activity) and microbial diversity. Further investigation shown that iron powder promoted the co-occurrence relationships between denitrifiers (e.g., Pseudomonas, Enhydrobacter, and Haloferax) and non-denitrifiers (e.g., Methanohalobium, Methanothrix, Geobacter, and Geothrix), providing potential electron donors for denitrifiers and facilitating electron transfer. Furthermore, iron powder also reinforced the interactions between iron-utilizing microbes and denitrifiers. These synergistic interactions between different microbial taxa, embedded within the functional relationships of different microbial communities, played a crucial role in facilitating the expression of denitrification genes (such as narG, nirS, and nirK), ultimately contributing to the mitigation of denitrification toxicity. These findings emphasize the critical importance of microbial synergism in the practical application of in-situ denitrification systems.