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

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.