Dual C-Br Isotope Fractionation Indicates Distinct Reductive Dehalogenation Mechanisms of 1,2-Dibromoethane in Dehalococcoides - and Dehalogenimonas -Containing Cultures.

Brominated organic compounds such as 1,2-dibromoethane (1,2-DBA) are highly toxic groundwater contaminants. Multi-element compound-specific isotope analysis bears the potential to elucidate the biodegradation pathways of 1,2-DBA in the environment, which is crucial information to assess its fate in contaminated sites. This study investigates for the first time dual C-Br isotope fractionation during in vivo biodegradation of 1,2-DBA by two anaerobic enrichment cultures containing organohalide-respiring bacteria (i.e., either Dehalococcoides or Dehalogenimonas ). Different εbulk C values (-1.8 ± 0.2 and -19.2 ± 3.5‰, respectively) were obtained, whereas their respective εbulk Br values were lower and similar to each other (-1.22 ± 0.08 and -1.2 ± 0.5‰), leading to distinctly different trends (ΛC-Br = Δδ13 C/Δδ81 Br ≈ εbulk C /εbulk Br ) in a dual C-Br isotope plot (1.4 ± 0.2 and 12 ± 4, respectively). These results suggest the occurrence of different underlying reaction mechanisms during enzymatic 1,2-DBA transformation, that is, concerted dihaloelimination and nucleophilic substitution (SN 2-reaction). The strongly pathway-dependent ΛC-Br values illustrate the potential of this approach to elucidate the reaction mechanism of 1,2-DBA in the field and to select appropriate εbulk C values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.