The biodegradation pathway of HBCDs in the enriched microbial community

1,2,5,6,9,10-Hexabromocyclododecanes (HBCDs) have been widely used in industrial applications, leading to serious environmental contamination. Previously, we reported that the biotransformation of HBCDs in the enriched microbial communities might involve the interactions between degraders and other taxa. In the present study, we further investigate this phenomenon in these communities. We found that all microbial communities acquired HBCDs degradation capacity at the 24th transfer, with degradation rates ranging from 12.8% to 55.9%. The degradation of HBCDs was coupled with the growth of culturable bacteria in the communities. Alcanivorax (81.14%, relative abundance) maintained its dominance in the microbial community SZH2-24. Moreover, metatranscriptomic analysis revealed that dadAH and dadBH were the only dehalogenase genes induced by HBCDs. Functional genes of both Alcanivorax and other taxa were potentially involved in the downstream biodegradation pathway. Debrominated products were likely oxidized, hydrolyzed, and ring-cleaved by phenol 2-monooxygenase (Alcanivorax), toluene methyl-monooxygenase electron transfer component (Hyphomicrobium), quercetin 2,3-dioxygenase (Hyphomicrobium and Alcanivorax), and 4,5-DOPA dioxygenase extradiol (Alcanivorax). Subsequently, the ring-cleavage products were probably degraded via Fatty acid metabolism (Hyphomicrobiales, Alcanivorax, and Hyphomicrobium) rather than TCA cycle. Alcanivorax harbored a complete degradation pathway, which might have provided a competitive advantage over other taxa. In contrast, minor taxa might degrade downstream products.