Anaerobic Hydrocarbon-Degrading Deltaproteobacteria

Despite being a remarkably biodiverse taxon, the class Deltaproteobacteria includes relatively few model hydrocarbonoclastic isolates. These organisms as well as notable enrichment cultures containing Deltaproteobacteria are able to anaerobically metabolize a wide variety of normal, iso-, and cyclic alkanes, as well as mono- and polycyclic aromatic hydrocarbons. The isolates are mostly recovered from marine environments and can readily couple the metabolism of their parent substrate with the consumption of sulfate as a terminal electron acceptor. As a consequence, the organisms reduce sulfate (and often other sulfur oxyanions) to sulfide. Fe(III) can also serve as electron acceptor for the biodegradation of aromatic compounds, and alkynes can be fermented by Pelobacter strains. In the absence of a respiratory electron acceptor, the hydrocarbonoclastic Deltaproteobacteria can participate as members of syntrophic consortia that transfer reducing equivalents ultimately to methanogenic Archaea. Under the latter circumstances, the major consequence is the transformation of parent hydrocarbons to the stoichiometrically expected quantity of methane. The mechanism of hydrocarbon activation by Deltaproteobacteria is fundamentally different from hydrocarbonoclastic aerobes in that molecular oxygen is not a coreactant. Rather, there are at least four biochemical strategies governing the primary attack on parent substrates. Identification of some functional genes has allowed for the detection and greater appreciation for the diversity of anaerobes capable of hydrocarbon metabolism. Despite the recognition that such organisms reside in varied environments, most cultivated representatives reflect rather limited environmental tolerance ranges. Given the importance of hydrocarbonoclastic Deltaproteobacteria in the biogeochemical cycling of carbon, the biocorrosion of steel, and prospects for methane recovery, studies on the full metabolic diversity inherent in this group of organisms seem particularly warranted.