Methanogenic transformation of aromatic hydrocarbons and phenols in groundwater aquifers

Abstract

Fermentative and methanogenic bacteria have been found repeatedly as important members of microbial flora in anoxic zones of the subsurface-in pristine as well as in contaminated groundwater aquifers. These bacteria, which together with obligate proton reducers form complex methanogenic communities, are significant as decomposers of organic matter under conditions of exogenous electron acceptor depletion. Their metabolic activity has been demonstrated in laboratory microcosms derived from aquifer material, and also in the subsurface in situ. Methanogenic communities have been shown to transform numerous organic pollutants, or even to completely degrade these compounds with the production of carbon dioxide and methane. Depending on the chemical structure of the pollutant, such a compound can be used as an electron donor and a carbon/energy source for fermentative microorganisms (which is typically the case with highly reduced compounds); alternatively, a highly oxidized pollutant can be used as a potential electron acceptor or electron sink. This review addresses fermentative/methanogenic degradation of chlorinated and nonchlorinated aromatic hydrocarbons and phenols by subsurface microorganisms; for comparison, it briefly relates also other types of anaerobic transformations (under sulfate-reducing, iron-reducing, and denitrifying conditions). Furthermore, it outlines transformation pathways, those that are proposed as well as those that are already partially proved, for aromatic hydrocarbons and phenols under fermentative/methanogenic conditions; finally, it discusses the relevance of these processes to bioremediation of contaminated groundwater aquifers.

Evidence for acetyl coenzyme A and cinnamoyl coenzyme A in the anaerobic toluene mineralization pathway in Azoarcus tolulyticus Tol-4

“…Most of the denitrifying toluene degraders can also catabolize toluene under aerobic conditions (17,20,35). Anaerobic degradation of toluene has also been observed under Fe(III)-reducing (30), methanogenic (13,23,24,40), and sulfidogenic (4,13) conditions, demonstrating a wide range of electron acceptors that might support the anaerobic biodegradation of BTEX compounds.…”

Metabolism of both 4-chlorobenzoate and toluene under denitrifying conditions by a constructed bacterial strain

“…The wide distribution of anoxic habitats in the environment and the recognition that different anaerobes have the ability to transform and degrade a broad variety of toxic chemicals (5,10,13,20,24) suggest that the diversity of anaerobic communities and their biodegradative capabilities has yet to be fully explored. In addition to classical procedures for isolation and taxonomic identification of novel anaerobic species, it may also be possible to expand the capabilities of selected anaerobes by molecular means as demonstrated here.…”

Intrinsic Aromatic Hydrocarbon Biodegradation for Groundwater Remediation

“…benzene and PAH, was considered unlikely due to the lack of destabilizing ring substituents. Only in recent years, oxygen independent hydroxylation and degradation of benzene was repeatedly observed, even though the degrading bacteria could not yet be isolated in most cases (Grbic-Galic, 1990;Weiner et al, 1998;Burland and Edwards, 1999;Schink, 2000). Degradation of PAH in the absence of oxygen has been documented only in a few reports.…”