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Diverse microorganisms have been described to degrade petroleum hydrocarbons anaerobically. Strains able to utilizen-alkanes do not grow with aromatic hydrocarbons, whereas strains able to utilize aromatic hydrocarbons do not grow withn-alkanes. To investigate this specificity in more detail, three anaerobicn-alkane degraders (two denitrifying, one sulfate-reducing) and eight anaerobic alkylbenzene degraders (five denitrifying, three sulfate-reducing) were incubated with mixtures ofn-alkanes and toluene. Whereas the toluene degradationers formed only the characteristic toluene-derived benzylsuccinate and benzoate, but non-alkane-derived metabolites, then-alkane degraders formed toluene-derived benzylsuccinate, 4-phenylbutanoate, phenylacetate and benzoate besides the regularn-alkane-derived (1-methylalkyl)succinates and methyl-branched alkanoates. The co-metabolic conversion of toluene by anaerobicn-alkane degraders to the level of benzoate obviously follows the anaerobicn-alkane degradation pathway with C-skeleton rearrangement and decarboxylation rather than the β-oxidation pathway of anaerobic toluene metabolism. Hence, petroleum-derived aromatic metabolites detectable in anoxic environments may not be exclusively formed by genuine alkylbenzene degraders. In addition, the hitherto largely unexplored fate of fumarate hydrogen during the activation reactions was examined with (2,3-2H2)fumarate as co-substrate. Deuterium was completely exchanged with hydrogen at the substituted carbon atom (C-2) of the succinate adducts ofn-alkanes, whereas it is retained in toluene-derived benzylsuccinate, regardless of the type of enzyme catalysing the fumarate addition reaction.