Metabolism of alkylbenzenes, alkanes, and other hydrocarbons in anaerobic bacteria

Aromatic and aliphatic hydrocarbons are the main constituents of petroleum and its refined products. Whereas degradation of hydrocarbons by oxygen-respiring microorganisms has been known for about a century, utilization of hydrocarbons under anoxic conditions has been investigated only during the past decade. Diverse strains of anaerobic bacteria have been isolated that degrade toluene anaerobically, using nitrate, iron(III), or sulfate as electron acceptors. Also, other alkylbenzenes such as m-xylene or ethylbenzene are utilized by a number of strains. The capacity for anaerobic utilization of alkylbenzenes has been observed in members of the alpha-, beta-, gamma- and delta -subclasses of the Proteobacteria. Furthermore, denitrifying bacteria and sulfate-reducing bacteria with the capacity for anaerobic alkane degradation have been isolated, which are members of the beta- and delta -subclass, respectively. The mechanism of the activation of hydrocarbons as apolar molecules in the absence of oxygen is of particular interest. The biochemistry of anaerobic toluene degradation has been studied in detail. Toluene is activated by addition to fumarate to yield benzylsuccinate, which is then further metabolized via benzoyl-CoA. The toluene-activating enzyme presents a novel type of glycine radical protein. Another principle of anaerobic alkylbenzene activation has been observed in the anaerobic degradation of ethylbenzene. Ethylbenzene in denitrifying bacteria is dehydrogenated to 1-phenylethanol and further to acetophenone; the latter is also metabolized to benzoyl-CoA. Naphthalene is presumably activated under anoxic conditions by a carboxylation reaction. Investigations into the pathway of anaerobic alkane degradation are only at the beginning. The saturated hydrocarbons are most likely activated by addition of a carbon compound rather than by desaturation and hydration, as speculated about in some early studies. An anaerobic oxidation of methane with sulfate as electron acceptor has been documented in aquatic sediments. The process is assumed to involve a reversal of methanogenesis catalyzed by Archaea, and scavenge of an electron-carrying metabolite by sulfate-reducing bacteria. Among unsaturated non-aromatic hydrocarbons, anaerobic bacterial degradation has been demonstrated and investigated with n-alkenes, alkenoic terpenes and the alkyne, acetylene.