Biodegradation of Polychlorinated Biphenyls

Polychlorinated biphenyls belong to persistent organic pollutants. Although their production was stopped long time ago, they still persist in the environment and represent a serious environmental problem. The aromatic ''doublering'' structure may be substituted with one to ten chlorine atoms. Although there are 209 individual compounds designated as congeners, but only 130 could be found in commercial PCB mixtures. The toxicity of such molecules is directly correlated to both number and position of the chlorine substituents. The coplanar PCBs (not substituted at the ring positions ortho-) tend to have dioxin-like properties, and generally are among the most toxic congeners. It has been demonstrated that PCBs are highly toxic to humans and animals, although their effects become apparent only after long-term exposure. The high resistance of these toxic compounds requires drastic conditions for decomposition, either high temperatures or chemical reagents, both very expensive processes. Hence, PCB contaminated material is disposed off at special sites to avoid exposure to human population. Alternatively to thermal and physico-chemical methods, bioremediation gains more public interest and acceptance as an effective and economic strategy for the removal of persistent toxic pollutants. PCBs can be degraded by aerobic bacteria via several pathways, however, the most important is co-metabolic process operated by biphenyl catabolic enzymes (Furukawa and Fujihara 2008). Unfortunately, this pathway leads to the formation of chlorobenzoic acids that tend to accumulate as dead-end products, since PCB degrading bacteria are not able to grow on these substrates (Kobayashi et al. 1996). The build up of these metabolites in the growth medium may result in a feed-back inhibition and impede or slow down PCB biotransformation (Adebusoye et al. 2008). Therefore, it is worthwhile to look for other microorganisms that are able to degrade PCBs.

Ligninolytic basidiomycetes have been also documented to be able to degrade PCBs. Besides P. chrysosporium, several other white-rot fungi: e.g., P. ostreatus (Beaudette et al. 1998; Kubatova et al. 2001), Coriolopsis polyzona (Vyas et al. 1994; Novotny et al. 1997), T. versicolor (Beaudette et al. 1998; Cloete and Celliers 1999). B. adusta (Beaudette et al. 1998), Lentinus (Lentinula) edodes (Sasek et al. 1993; Ruiz-Aguilar et al. 2002) are also known to metabolize PCBs. Mineralization of PCBs by P. chrysosporium was first reported by Eaton (1985) who found 8% mineralization of 0.3 ppm 14C-Aroclor 1254 to 14CO2 in a nitrogen-limited culture after 5 week incubation. Thomas et al. (1992) demonstrated mineralization of 2-chlorobiphenyl and 2,2',4,4'-tetrachlorobiphenyl to 14CO2. After 32-day incubation, only 1% of tetrachlorobiphenyl was mineralized, but 40% was found incorporated into fungal biomass. Dietrich et al. (1995) found that low chlorinated PCB congeners are degraded significantly, whilst higher chlorination becomes a limiting factor. After 28 days of incubation in liquid media, the degradation of 4,4'-dichlorobiphenyl was recorded 11% mineralization, in contrast, there was negligible mineralization of the 3,3',4,4'-tetrachorobiphenyl and 2,2',4,4',5,5'-hexachlorobiphenyl. Evidence for substantial degradation of Arochlor 1242 (60.9%), 1254 (30.5%), and 1260 (17.6%) after 30-day incubation was presented by Yadav et al. (1995). Contrary to an earlier observation in N-limited media, these authors found degradation also in high N-medium, and the most extensive in malt extract medium. Using congener-specific analysis, they documented that the degree of degradation is affected by chlorine number, but not by their position on the biphenyl ring. Based on the congener non-specificity and the fact that the degradation takes place also under non-ligninolytic conditions, these workers had a doubt in the role of ligninolytic enzymes, and supposed that the degradation might be due to free radical attack. These results were confirmed by Krcmar and Ulrich (1998) who described degradation of PCB mixture under non-ligninolytic conditions, but not under ligninolytic ones. Krcmar et al. (1999) performed an experiment where they observed in vitro degradation of PCB mixtures containing low and highly chlorinated biphenyls using intact mycelium, crude extracellular liquid and the enriched MnP and LiP. A decrease in PCB concentration during a 44 h treatment with mycelium (74%) or crude extracellular liquid (60%) was observed. In contrast, MnP and LiP isolated from the extracellular liquid did not catalyze any degradation. There are only a few publications describing possible pathways of PCB degradation and clarifying an involvement of fungal enzymes in the degradation. Kamei et al. (2006a) published a work about degradation of PCBs by the white-rot fungus Phlebia brevispora that was shown to degrade PCBs with the formation of methoxylated, p-dechlorinated and p-methoxylated metabolites. They performed another experiment where cultures of Phanerochaete sp. were contaminated with 4,4'-Dichlorobiphenyl (4,4'-DCB) and its metabolites and the metabolic pathway was partially elucidated by the identification of metabolites, namely 2-hydroxy-4,4'-DCB and 3-methoxy-4, 4'-DCB, 4-chlorobenzoic acid, 4-chlorobenzaldehyde, 4-chlorobenzyl alcohol, and 4-hydroxy-3,4'-DCB (Kamei et al. 2006b). Biodegradation of PCB 9 using the white rot fungus T. versicolor was studied by Koller et al. (2000). The cultivation took 4 weeks and reduction was recorded about 80% of the initial concentration and the formed metabolites were dichlorobenzenes, chlorophenols and alkylated benzenes. De et al. (2006) indicated that nitrate reductase gene of P. chrysosporium could be involved in dechlorination of hexachlorobiphenyl (PCB-153) under non-ligninolytic condition, when the nitrate reductase enzyme and its cofactor, molybdenum, were found to mediate reductive dechlorination of PCBs even in aerobic condition. Tungsten, a competitive inhibitor of this enzyme, was found to suppress this dechlorination and chlorine release assay provided further evidence for nitrate reductase mediated dechlorination of PCB. In conclusion, the ability of white rot fungi to degrade PCBs was proven, however, the enzymatic background and mechanism of the attack is still to be explored.

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