Anaerobic Dechlorination

The observation that PCB contaminated sediments display a higher content of low chlorinated congeners than the mixture at the origin of the contamination was the first clue that anaerobic bacteria are able to degrade PCBs by a reductive dechlorination process. This process has been well documented for several aquatic sediments [30, 31, 32]. Anaerobic enrichment cultures able to dechlorinate PCBs were maintained in laboratory conditions, with and without sediment [33, 34, 35, 36, 37]. The dechlorination release preferentially meta and para chlorines [35, 38] but there are some reports of ortho chlorine release [39, 40, 41]. Brown et al described eight different microbiological processes responsible for the different dechlorination patterns and congener specificities [30, 42]. It is thought that these processes are performed by different strains and different enzymes. Until recently, all attempts to obtain a pure culture able to dechlorinate PCBs had failed. Now, one pure anaerobic strain and one consortium have been reported [43, 44]. However, isolation of more strains and studies of the enzymes responsible for the dechlorination are still to come. The effect of anaerobic dechlorination is double: first the removal of meta and para chlorines reduces the toxicity of the contaminated sediments by reducing the occurrence of co-planar congeners [45]. Second, the reduction of the number of chlorine atoms on the biphenyl ring potentially facilitates the aerobic degradation through the biphenyl pathway.

5. Degradation of PCBs in higher organisms

The first reports of PCBs degradation by white rot fungi date back to 1985 [46, 47]. The enzymes that are potential candidates for this activity are peroxidases and laccase. These enzymes are involved in lignin degradation and are also active against a wide range of aromatic compounds (PAH, substituted phenols, chlorinated pesticides [48, 49] etc.). The mechanism of degradation by lignin-peroxidase, manganese-peroxidases and laccases is described in degradation of many environmental pollutants. The ligninolytic enzyme system is non-specific, extracellular and free radical based that allows them to degrade structurally diverse range of xenobiotic compounds. Lignin peroxidase and manganese peroxidase carry out direct and indirect oxidation as well as reduction of xenobiotic compounds. Indirect reactions involved redox mediators such as veratrylalcohol and Mn2+. Reduction reactions are carried out by carboxyl, superoxide and semiquinone radicals, etc. Methylation is used as detoxification mechanism by WRF (white rot fungi). Highly oxidized chemicals are reduced by transmembrane redox potential. However, the mechanism of PCB degradation has not yet been fully understood. Several authors documented transformation of PCBs determined by fungi and some studies concluded that neither lignin peroxidases nor Mn-dependent peroxidases are involved in PCB degradation [50, 51]. A major problem is the adsorption of the different PCB congeners to the biomass which introduces a bias when the degradation is measured by substrate depletion. One alternative is to measure the complete mineralisation by using radiolabeled substrates but it is likely that many congeners cannot go through the entire process. The other alternative is the identification of the metabolites resulting from PCB degradation. Dietrich et al. [52] reported the presence of two metabolites produced from 4,4'-dichlorobiphenyl by P. chrysosporum confirming that this fungi can actually degrade this congener. Schultz et al. [53] described dehalogenation and metabolisation of chlorinated hydro-xybiphenyls by laccase from white rot fungi Pycnoporus cinnabarinus, Romero et al. [54] documented biotransformation of biphenyl by ascomyceteous fungi Talaromyces helicus giving hydroxylated biphenyls and 4-phenyl-2-pyrone-6-carboxylic acid. Several studies showed capacity of fungal cultures on di, tri, tetra-and penta-chlorinated phenols when first step is characterized by oxidative dehalogenation mediated by extracellular peroxidases to form benzoquinones. The level of degradation of halogenated aromatics, reported in scientific literature, is very different from one study to the other. More extensive investigation will be necessary to elucidate the PCB degradation pathway in these eukaryotic microorganisms.

Body Detox Made Easy

Body Detox Made Easy

What exactly is a detox routine? Basically a detox routine is an all-natural method of cleansing yourbr body by giving it the time and conditions it needs to rebuild and heal from the damages of daily life and the foods you eat and other substances you intake. There are many different types of known detox routines.

Get My Free Ebook


Post a comment