Combined Carbon and Hydrogen Isotope Fractionation Investigations for Elucidating Benzene Biodegradation Pathways

Abstract

Recently, combined carbon and hydrogen isotope fractionation investigations have emerged as a powerful tool for the characterization of reaction mechanisms relevant for the removal of organic pollutants. Here, we applied this approach in order to differentiate benzene biodegradation pathways under oxic and anoxic conditions in laboratory experiments. Carbon and hydrogen isotope fractionation of benzene was studied with four different aerobic strains using a monooxygenase or a dioxygenase for the initial benzene attack, a facultative anaerobic chlorate-reducing strain as well as a sulfate-reducing mixed culture. Carbon and hydrogen enrichment factors (epsilon(C), epsilon(H)) varied for the specific pathways and degradation conditions, respectively, so that from the individual enrichment factors only limited information could be obtained for the identification of benzene biodegradation pathways. However, using the slope derived from hydrogen vs carbon isotope discriminations or the ratio of hydrogen to carbon enrichment factors (lambda = deltaH/ deltaC approximately epsilon(H)/epsilon(C)), benzene degradation mechanisms could be distinguished. Although experimentally determined lambda values partially overlapped, ranges could be determined for different benzene biodegradation pathways. Specific lambda values were < 2 for dihydroxylation, between 7 and 9 for monohydroxylation, and > 17 for anaerobic degradation. Moreover, variations in lambda values suggest that more than one reaction mechanism exists for monohydroxylation as well as for anaerobic benzene degradation under nitrate-reducing, sulfate-reducing, or methanogenic conditions. Our results show that the combined carbon and hydrogen isotope fractionation approach has potential to elucidate biodegradation pathways of pollutants in field and laboratory microcosm studies.

Carbon and Chlorine Isotope Fractionation during Aerobic Oxidation and Reductive Dechlorination of Vinyl Chloride and cis-1,2-Dichloroethene

“…All isotope ratios are reported using the delta notation as δ ) (R/Rstd -1) × 1000‰ where R and Rstd are the isotope ratio of the sample and the standard, respectively. All δ 13 C values are reported relative to Vienna PeeDee Belemnite (δ 13 CVPDB) and δ 37 Cl values are reported relative to Standard Mean Ocean Chloride (δ 37 ClSMOC). Dilution of the samples to similar concentrations led to high precision measurements with a standard deviation of 0.3‰ for carbon isotopes and 0.1‰ for chlorine isotopes.…”

Anaerobic Catabolism of Aromatic Compounds: a Genetic and Genomic View

“…This methodology is based on the fact that the relative abundances of the stable (nonradioactive) isotopes are effectively constant for each element, but microbial processes make small, although significant, changes to isotopic compositions by preferentially utilizing the lighter isotopes. Isotopic fractionation studies to monitor in situ anaerobic bioremediation of BTEXcontaminated habitats have been successfully implemented (59,113,231,252,300). In some cases, the carbon isotopic ratio of the dissolved inorganic carbon in oil-contaminated seawater samples was used as a rapid method to directly assess the level of activity of microbes on the oil components (241).…”

A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

“…Data about fractionation effects for hydrogen isotopes are much scarcer; the enrichment factors for hydrogen were therefore set to values following the general trend of stronger enrichment in hydrogen compared to carbon isotopes . In a system with two transformation pathways, the ratio between the enrichment factors of two isotopic elements, for example of hydrogen and carbon (ε H /ε C ), can be indicative of a specific degradation mechanism (Meyer et al, 2009;Fischer et al, 2008). The enrichment factors for the two reaction pathways were therefore chosen such that they yield distinct ε H / ε C -ratios (Table 2).…”