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 1,2dibromoethane (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 organohaliderespiring bacteria (i.e., either Dehalococcoides or Dehalogenimonas). Different epsilon bulk C values (-1.8 +/- 0.2 and -19.2 +/- 3.5 parts per thousand, respectively) were obtained, whereas their respective epsilon bulk Br values were lower and similar to each other (-1.22 +/- 0.08 and -1.2 +/- 0.5 parts per thousand), leading to distinctly different trends (?C-Br = Delta delta 13C/Delta delta 81Br approximate to epsilon bulk C /epsilon bulk 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 (SN2-reaction). The strongly pathway-dependent ?C-Br values illustrate the potential of this approach to elucidate the reaction mechanism of 1,2-DBA in the field and to select appropriate epsilon bulkC values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.