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-res piring microorganisms has been known for about a century, utilization of hy drocarbons under anoxic conditions has been investigated only during the pa st decade. Diverse strains of anaerobic bacteria have been isolated that de grade toluene anaerobically, using nitrate, iron(III), or sulfate as electr on acceptors. Also, other alkylbenzenes such as m-xylene or ethylbenzene ar e utilized by a number of strains. The capacity for anaerobic utilization o f alkylbenzenes has been observed in members of the alpha-, beta-, gamma- a nd delta -subclasses of the Proteobacteria. Furthermore, denitrifying bacte ria and sulfate-reducing bacteria with the capacity for anaerobic alkane de gradation have been isolated, which are members of the beta- and delta -sub class, respectively. The mechanism of the activation of hydrocarbons as apo lar molecules in the absence of oxygen is of particular interest. The bioch emistry of anaerobic toluene degradation has been studied in detail. Toluen e is activated by addition to fumarate to yield benzylsuccinate, which is t hen further metabolized via benzoyl-CoA. The toluene-activating enzyme pres ents a novel type of glycine radical protein. Another principle of anaerobi c 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 ar e most likely activated by addition of a carbon compound rather than by des aturation and hydration, as speculated about in some early studies. An anae robic oxidation of methane with sulfate as electron acceptor has been docum ented 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 hyd rocarbons, anaerobic bacterial degradation has been demonstrated and invest igated with n-alkenes, alkenoic terpenes and the alkyne, acetylene.