Dual C-Br isotope fractionation indicates distinct reductive dehalogenation mechanisms of 1,2-dibromoethane in Dehalococcoides- and Dehalogenimonas-containing cultures

Abstract

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 organohaliderespiring 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 = Δδ13C/Δδ81Br ≈ εbulkC /εbulkBr ) 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 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 εbulkC values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.