Competitive Inhibition and Pathway Truncation During Biotransformation of Traditional and Novel Brominated Flame...

The ubiquitous co-contamination of traditional and novel brominated flame retardants (TBFRs and NBFRs) in anaerobic environments necessitates a comprehensive understanding of their combined environmental fate. This study investigated the anaerobic biotransformation of BDE 99 (a legacy aromatic TBFR) and β-TBCO (a cycloaliphatic NBFR) using a Dehalogenimonas-containing mixed culture (QY2-S1) under single- and co-exposure conditions. In a single-exposure system, QY2-S1 achieved efficient transformation of both substrates, with the observed first-order kinetic constants (k_obs) of 1.31 ± 0.09 d^-1 and 2.35 ± 0.13 d^-1 for BDE 99 and β-TBCO, respectively. However, the corresponding k_obs values decreased by approximately 3- to 5-fold in a co-exposure system, demonstrating pronounced reciprocal kinetic inhibition. Notably, while culture QY2-S1 maintained a unique ortho-regioselectivity for BDE 99, its stepwise debromination was truncated at the tetra-BDE stage during co-exposure. Absolute quantitative 16S rRNA sequencing identified Dehalogenimonas as the sole organohalide-respiring bacterium, with its abundance initially increasing in tandem with substrate transformation. However, the dehalogenation-coupled cell growth of Dehalogenimonas was significantly modulated by substrate type and combination, with its absolute biomass following the order of: single β-TBCO > co-exposure > single BDE 99. Collectively, these results suggest that the reciprocal kinetic inhibition and the truncation of the BDE 99 debromination pathway under co-exposure are collectively driven by the suppressed growth and substrate preference of Dehalogenimonas. This study provides critical insights for predicting ecological risks and developing bioremediation strategies for co-occurring BFRs in real-world scenarios.