Anaerobic Decomposition of Halogenated Aromatic Compounds

Max M. Häggblom

Biotechnology Center for Agriculture and the Environment

Rutgers, The State University of New Jersey

59 Dudley Road, New Brunswick, NJ 08901-8520, USA

Halogenated compounds constitute one of the largest groups of environmental pollutants, partly as a result of their wide spread use as biocides, solvents and other industrial chemicals. The majority of these compounds are chlorinated, but brominated and fluorinated aromatic compounds are also in use. Our laboratory is examining the diversity of anaerobic processes in degradation of halogenated aromatic compounds and studied how alternate electron acceptors influence dehalogenation rates and degradation mechanisms. The objective of our work has been to determine the capacity of anaerobic microbial communities in anoxic sediments and soils for dehalogenation and degradation of organohalides. Anaerobic microbial processes that are of significance in soils and sediments include denitrification, iron/manganese reduction, sulfidogenesis and methanogenesis.

The biodegradability of halogenated compounds in the absence of oxygen is receiving increased attention. A critical step in degradation of organohalides is the cleavage of the carbon-halogen bond, however this step appears to be greatly influenced by alternate electron acceptors. Reductive dehalogenation is generally the initial step in metabolism under methanogenic conditions, which requires a source of reducing equivalents, with the halogenated compound serving as an electron acceptor. On the other hand, we have demonstrated that halogenated aromatic compounds can be degraded under a variety of different electron accepting conditions and that their complete oxidation to CO₂ can be coupled to processes such as denitrification, Fe(III)-reduction, sulfate reduction, and methanogenesis. For example, we have demonstrated that sulfidogenic and iron-reducing consortia can dehalogenate and mineralize chlorinated and brominated aromatic compounds. Dehalogenation rates were in general slower under sulfidogenic and iron-reducing conditions suggesting that dehalogenation was affected by the electron acceptor. Reductive dehalogenation is thus the initial step in degradation not only under methanogenic conditions, but can also occur under sulfate- and iron-reducing conditions.

The different substrate specificities and activities observed for the halogenated aromatic compounds suggests that distinct dehalogenating microbial populations are enriched under the different reducing conditions. We have observed very similar degradation activities at different sites indicating that the capacity for dehalogenation is widely distributed in anoxic environments. A detailed characterization of these anaerobic microbial communities is needed to fully understand the role that they play in the degradation of haloaromatics in anoxic environments. Identification of the phylogenetic and physiological groups is important for understanding the role of these communities in degradation of anthropogenic pollutants.