¹³C/¹²C isotope fractionation of aromatic hydrocarbons during microbial degradation

Abstract

The influence of microbial degradation on the 13C/12C isotope composition of aromatic hydrocarbons is presented using toluene as a model compound. Four different toluene-degrading bacterial strains grown in batch culture with oxygen, nitrate, ferric iron or sulphate as electron acceptors were studied as representatives of different environmental redox conditions potentially prevailing in contaminated aquifers. The biological degradation induced isotope shifts in the residual, non-degraded toluene fraction and the kinetic isotope fractionation factors alphaC for toluene degradation by Pseudomonas putida (1.0026 +/- 0.00017), Thauera aromatica (1.0017 +/- 0.00015), Geobacter metallireducens (1.0018 +/- 0.00029) and the sulphate-reducing strain TRM1 (1.0017 +/- 0.00016) were in the same range for all four species, although they use at least two different degradation pathways. A similar 13C/12C isotope fractionation factor (alphaC = 1.0015 +/- 0.00015) was observed in situ in a non-sterile soil column in which toluene was degraded under sulphate-reducing conditions. No carbon isotope shifts resulting from soil-hydrocarbon interactions were observed in a non-degrading soil column control with aquifer material under the same conditions. The results imply that microbial degradation of toluene can produce a 13C/12C isotope fractionation in the residual hydrocarbon fraction under different environmental conditions.

Hydrogen and Carbon Isotope Fractionation during Aerobic Biodegradation of Benzene

“…For example, Meckenstock et al found an enrichment factor of -2.6 ( 0.17‰ for aerobic degradation of toluene by Pseudomonas putida (17), which is in good agreement with our finding from aerobic biodegradation of benzene by Burkholderia sp. (-3.53 ( 0.26‰).…”

“…In contrast, physical processes such as dissolution, volatilization, or sorption are usually not accompanied by significant isotope fractionation (9)(10)(11)(12). Large isotope effects with respect to carbon have been observed during reductive dechlorination (8,13,14) and oxidation (15,16) of chlorinated solvents while much smaller isotope effects occurred during biodegradation of BTEX (13,17,18). As a consequence, carbon isotope ratios are not a very sensitive indicator for biodegradation of BTEX, and significant shifts of the carbon isotope ratio are only observed after a large fraction of the BTEX has been biodegraded.…”

Monitoring Biodegradation of Methyl tert-Butyl Ether (MTBE) Using Compound-Specific Carbon Isotope Analysis

“…A similar observation has previously been made for isotope fractionation associated with reductive dechlorination of chlorinated ethenes (24,26) and oxidation of dichloromethane (28), toluene (30), and 1,2-dichloroethane (29). The enrichment factors for MTBE degradation were similar for microcosm enrichments with MTBE as the only substrate and the cometabolic microcosm (Table 3), which may be due to a similarity in the enzymatic mechanism used by the bacteria for initial transformation of MTBE.…”

“…The enrichment factors for MTBE degradation were similar for microcosm enrichments with MTBE as the only substrate and the cometabolic microcosm (Table 3), which may be due to a similarity in the enzymatic mechanism used by the bacteria for initial transformation of MTBE. They were in the same range as enrichment factors obtained for aerobic and anaerobic oxidation of toluene (30) and smaller than the enrichment factors observed for biodegradation of chlorinated solvents (24,(26)(27)(28).…”

“…Large shifts in the carbon isotope composition of remaining contaminant have been observed during reductive dechlorination of chlorinated ethenes (24,26,27), aerobic oxidation of dichloromethane (28), and aerobic oxidation of 1,2-dichloroethane (29). In contrast, smaller shifts occurred during biodegradation of aromatic hydrocarbons (30). One of the reasons for this difference is the fact that the kinetic isotope effect mainly occurs with respect to the atoms of the bond that is formed or broken in the initial transformation step, while the measurement yields the average isotope ratio of all atoms of an element in the compound.…”