Dual (C, H) Isotope Fractionation in Anaerobic Low Molecular Weight (Poly)aromatic Hydrocarbon (PAH) Degradation: Potential for Field Studies and Mechanistic Implications

Anaerobic polycyclic aromatic hydrocarbon (PAH) degradation is a key process for natural attenuation of oil spills and contaminated aquifers. Assessments by stable isotope fractionation, however, have largely been limited to monoaromatic hydrocarbons. Here, we report on measured hydrogen isotope fractionation during strictly anaerobic degradation of the PAH naphthalene. Remarkable large hydrogen isotopic enrichment factors contrasted with much smaller values for carbon: epsilon(H) = -100 parts per thousand +/- 15 parts per thousand, epsilon(C) = -5.0 parts per thousand +/- 1.0 parts per thousand (enrichment culture N47); epsilon(H) = -73 parts per thousand +/- 11 parts per thousand, epsilon(C) = -0.7 parts per thousand +/- 0.3 parts per thousand (pure culture NaphS2). This reveals a considerable potential of hydrogen isotope analysis to assess anaerobic degradation of PAHs. Furthermore, we investigated the conclusiveness of dual isotope fractionation to characterize anaerobic aromatics degradation. C and H isotope fractionation during benzene degradation (epsilon(C) = -2.5 parts per thousand +/- 0.2 parts per thousand; epsilon(H) = -55 parts per thousand +/- 4 parts per thousand (sulfate-reducing strain BPL); epsilon(C) = -3.0 parts per thousand +/- 0.5 parts per thousand; epsilon(H) = -56 parts per thousand +/- 8 parts per thousand (iron-reducing strain BF)) resulted in dual isotope slopes (Lambda = 20 +/- 2; 17 +/- 1) similar to those reported for nitrate-reducers. This breaks apart the current picture that anaerobic benzene degradation by facultative anaerobes (denitrifiers) can be distinguished from that of strict anaerobes (sulfate-reducers, fermenters) based on the stable isotope enrichment factors.