Isotopic fractionation indicates anaerobic monochlorobenzene biodegradation

Abstract

The concentration and isotopic composition of monochlorobenzene (MCB) was monitored in the plume of an anaerobic, contaminated aquifer in Bitterfeld, Germany. An enrichment in the carbon isotopic composition of more than 4 delta units was found at the fringes of the plume relative to the center (-26.5 %), suggesting the occurrence of in situ biodegradation of MCB. A similar enrichment was measured in a detailed cross-section of the plume and in depth-specific samples obtained in a multilevel sampling well. The latter samples gave a good correlation of MCB concentrations and respective isotopic composition according to the Rayleigh equation. On the other hand, batch experiments using the aerobic MCB-degrading strains Ralstonia sp. DSM 8910, Acidovorax facilis UFZ B517, Rhodococcus erythropolis UFZ B528, and Pseudomonas veronii UFZ B547 showed that the known aerobic pathway initiated by dioxygenases does not result in a significant isotopic fractionation. Thus, a novel anaerobic pathway resulting in an isotopic fractionation appears to be the predominant process of MCB degradation in this aquifer. The study also clearly demonstrates the usefulness of isotopic fractionation analysis to prove biodegradation directly in the field, even when microcosm studies are not available and a metabolic pathway has not yet been elucidated.

Microbial degradation of chlorinated benzenes

“…However, CB was shown to be degraded in situ at the fringe of the contaminant plume in an anoxic aquifer (with a long history of CB contamination) based on evidence from isotopic fractionation data and incorporation of [ 13 C]-labeled CB into long chain fatty acids of bacteria (Kaschl et al 2005;Kastner et al 2006).…”

Compound-Specific Isotope Analysis (CSIA) Application for Source Apportionment and Natural Attenuation Assessment of Chlorinated Benzenes

“…Yet, if contaminants undergo a preferential transport along the plumes and/or when biodegradation processes are taking place, the δ 13 C sum interpretation is not an easy task [31]. However, assuming that the carbon pool is conservative during anaerobic biodegradation (supposing DCBs → MCB → benzene) [32,33] and that aerobic processes are not inducing significant fractionations, particularly for CBs [23][24][25], δ 13 C sum values should retain their signature along the flowpath, regardless of the occurrence of degradation. Eventually, with further degradation and mineralization of benzene to CO 2 , δ 13 C sum could show an enrichment along the plume [32,33].…”

“…In contrast to compounds such as chlorinated ethenes, for which there is a well-developed literature on CSIA applications, only few studies are available for CBs. Significant carbon enrichment factors were obtained in laboratory experiments under anaerobic conditions, ranging from −0.8‰ to −6.3‰ for several CBs [22][23][24][25][26], which allows for the use of CSIA for tracing in situ biodegradation of CBs in contaminated anoxic aquifers. Kaschl et al [24,27] demonstrated MCB anaerobic biodegradation at two contaminated sites, while [28] investigated anaerobic biodegradation of dichlorobenzenes (DCBs) and MCB by interpreting CSIA results on the basis of an isotope mass balance approach.…”

“…Significant carbon enrichment factors were obtained in laboratory experiments under anaerobic conditions, ranging from −0.8‰ to −6.3‰ for several CBs [22][23][24][25][26], which allows for the use of CSIA for tracing in situ biodegradation of CBs in contaminated anoxic aquifers. Kaschl et al [24,27] demonstrated MCB anaerobic biodegradation at two contaminated sites, while [28] investigated anaerobic biodegradation of dichlorobenzenes (DCBs) and MCB by interpreting CSIA results on the basis of an isotope mass balance approach. On the contrary, almost no isotope carbon fractionation for MCB and 1,2,4 TCB during aerobic biodegradation was demonstrated [23,24] -results that were confirmed by Liang et al [25].…”

“…Kaschl et al [24,27] demonstrated MCB anaerobic biodegradation at two contaminated sites, while [28] investigated anaerobic biodegradation of dichlorobenzenes (DCBs) and MCB by interpreting CSIA results on the basis of an isotope mass balance approach. On the contrary, almost no isotope carbon fractionation for MCB and 1,2,4 TCB during aerobic biodegradation was demonstrated [23,24] -results that were confirmed by Liang et al [25]. Thus, the lack of carbon isotope fractionation limits the use of carbon-CSIA for MNA applications in contaminated environments under oxic conditions.…”