title: C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ¹³C/δ³⁷Cl patterns for pathway identification and reveals surprising C-Cl bond involvement in microbial oxidation.
abstract: This study investigates dual element isotope fractionation during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via oxidative cleavage of a C-H bond (Pseudomonas sp. strain DCA1) versus C-Cl bond cleavage by S(N)2 reaction (Xanthobacter autotrophicus GJ10 and Ancylobacter aquaticus AD20). Compound-specific chlorine isotope analysis of 1,2-DCA was performed for the first time, and isotope fractionation (ε(bulk)(Cl)) was determined by measurements of the same samples in three different laboratories using two gas chromatography-isotope ratio mass spectrometry systems and one gas chromatography-quadrupole mass spectrometry system. Strongly pathway-dependent slopes (Δδ13C/Δδ37Cl), 0.78 ± 0.03 (oxidation) and 7.7 ± 0.2 (S(N)2), delineate the potential of the dual isotope approach to identify 1,2-DCA degradation pathways in the field. In contrast to different ε(bulk)(C) values [-3.5 ± 0.1‰ (oxidation) and -31.9 ± 0.7 and -32.0 ± 0.9‰ (S(N)2)], the obtained ε(bulk)(Cl) values were surprisingly similar for the two pathways: -3.8 ± 0.2‰ (oxidation) and -4.2 ± 0.1 and -4.4 ± 0.2‰ (S(N)2). Apparent kinetic isotope effects (AKIEs) of 1.0070 ± 0.0002 (13C-AKIE, oxidation), 1.068 ± 0.001 (13C-AKIE, S(N)2), and 1.0087 ± 0.0002 (37Cl-AKIE, S(N)2) fell within expected ranges. In contrast, an unexpectedly large secondary 37Cl-AKIE of 1.0038 ± 0.0002 reveals a hitherto unrecognized involvement of C-Cl bonds in microbial C-H bond oxidation. Our two-dimensional isotope fractionation patterns allow for the first time reliable 1,2-DCA degradation pathway identification in the field, which unlocks the full potential of isotope applications for this important groundwater contaminant.
authors.affiliation: Centre for Hydrogeology and Geothermics, University of Neuchâtel , 2000 Neuchâtel, Switzerland.
authors.forename: Jordi
authors.initials: J
authors.lastname: Palau
authors.forename: Stefan
authors.initials: S
authors.lastname: Cretnik
authors.forename: Orfan
authors.initials: O
authors.lastname: Shouakar-Stash
authors.forename: Martina
authors.initials: M
authors.lastname: Höche
authors.forename: Martin
authors.initials: M
authors.lastname: Elsner
authors.forename: Daniel
authors.initials: D
authors.lastname: Hunkeler
mesh_terms: D000332:Aerobiosis; D001673:Biodegradation, Environmental; D002247:Carbon Isotopes / Q000032:analysis*; D005591:Chemical Fractionation; D002713:Chlorine / Q000032:analysis; D005025:Ethylene Dichlorides / Q000032:analysis*; D060587:Groundwater / Q000737:chemistry*; D007554:Isotopes / Q000032:analysis; D007700:Kinetics; D010084:Oxidation-Reduction; D014874:Water Pollutants, Chemical / Q000032:analysis*; D020584:Xanthobacter / Q000254:growth & development*
chemical_list: D002247:Carbon Isotopes; D005025:Ethylene Dichlorides; D007554:Isotopes; D014874:Water Pollutants, Chemical; D002713:Chlorine; C024565:ethylene dichloride