Anaerobic decomposition of halogenated aromatic compounds

Halogenated compounds constitute one of the largest groups of environmental pollutants, partly as a result of their widespread use as biocides, solven ts and other industrial chemicals. A critical step in degradation of organo halides is the cleavage of the carbon-halogen bond. Reductive dehalogenatio n is generally the initial step in metabolism under methanogenic conditions , which requires a source of reducing equivalents, with the halogenated com pound serving as an electron acceptor. Dehalogenation is greatly influenced by alternate electron accepters; e.g. sulfate frequently inhibits reductiv e dehalogenation. On the other hand, a number of halogenated aromatic compo unds can be degraded under different electron-accepting conditions and thei r complete oxidation to CO2 can be coupled to processes such as denitrifica tion, iron(III)-reduction, sulfate reduction and methanogenesis. Reductive dehalogenation was the initial step in degradation not only under methanoge nic, but also under sulfate-and iron(III)-reducing conditions. Dehalogenati on rates were in general slower under sulfidogenic and iron-reducing condit ions, suggesting that dehalogenation was affected by the electron acceptor. The capacity for dehalogenation appears to be widely distributed in anoxic environments; however, the different substrate specificities and activitie s observed for the halogenated aromatic compounds suggest that distinct deh alogenating microbial populations are enriched under the different reducing conditions. Characterization of the microbial community structure using a combination of biomolecular techniques, such as cellular fatty acid profili ng, and 16 S rRNA fingerprinting/sequence analysis, was used to discern the distinct populations enriched with each substrate and under each electron- accepting condition. These combined techniques will aid in identifying the organisms responsible for dehalogenation and degradation of halogenated aro matic compounds. (C) 2000 Elsevier Science Ltd. All rights reserved.