Catabolic Pathways Involved in the Anaerobic Degradation of Saturated Hydrocarbons

Abstract

Structurally diverse saturated hydrocarbons (n-alkanes, branched alkanes, cycloalkanes) occur frequently and abundantly in microbial habitats. A diversity of enrichment and pure cultures of microorganisms which originate from such environments and degrade saturated hydrocarbons under strictly anoxic conditions have been characterized physiologically and phylogenetically. Typically, n-alkane-degrading anaerobic microorganisms exhibit more or less pronounced substrate specificities with respect to chain length range of utilizable n-alkanes; notably, very limited knowledge exists regarding anaerobic degradation of ethane. Currently, four different metabolic strategies are known to be employed by such organisms when growing anaerobically with n-alkanes. Best characterized is the pathway initiated by the addition of the hydrocarbon substrate to the co-substrate fumarate catalyzed by a glycyl radical enzyme. Other enzyme reactions apparently used for activation of the highly inert substrates include dehydrogenation/anaerobic hydroxylation, transformation to alkyl-coenzyme M, and “intra-aerobic” oxidation. Subsequent catabolic pathways necessarily differ depending on the chemical nature of the initial activation product. Branched alkanes and cycloalkanes appear to be metabolized through analogous activation reactions and catabolic pathways; however, their degradation by anaerobic microorganisms is less well understood.