(1984). Polychlorinated Biphenyls (PCBs) and Polybrominated Biphenyls (PBBs): Biochemistry, Toxicology, and Mechanism of Action. CRC Critical Reviews in Toxicology: Vol. 13, No. 4, pp. 319-395.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated biphenyls and their biodegradation

This review summarizes recent research results on the biodegradation of polychlorinated biphenyls (PCBs). These compounds, commonly believed to be indestructible, have repeatedly been shown to biodegrade under a variety of conditions. Two distinct classes of bacteria have now been identified that biodegrade PCBs by different mechanisms. The focus of this manuscript is current research involving the aerobic biodegradation of PCBs (natural strains, recombinant organisms, and soil applications) and the dramatic new results demonstrating microbial reductive dechlorination of even highly chlorinated PCBs under anaerobic conditions.These two PCB-degradative systems include aerobic bacteria which live in oxygenated environments and anaerobic bacteria which live in oxygen free environments such as aquatic sediments. The aerobes attack PCBs oxidatively, breaking open the carbon ring and destroying the compounds. Anaerobes, on the other hand, leave the biphenyl rings intact while removing the chlorines. Thi...

Aerobic and Anaerobic Biodegradation of PCBs: A Review

We designed a rapid assay that assesses the polychlorinated biphenyl (PCB)-degradative competence and congener specificity of aerobic microorganisms, identifies strains capable of degrading highly chlorinated biphenyls, and distinguishes among those that degrade PCBs by alternative pathways. Prior attempts to assay PCB-degradative competence by measuring disappearance of Aroclors (commercial PCB mixtures) have frequently produced false-positive findings because of volatilization, adsorption, or absorption losses. Furthermore, these assays have generally left the chemical nature of the competence obscure because of incomplete gas chromatographic resolution and uncertain identification of Aroclor peaks. We avoided these problems by using defined mixtures of PCB congeners and by adopting incubation and extraction methods that prevent physical loss of PCBs. Our assay mixtures include PCB congeners ranging from dichloro- to hexachlorobiphenyls and representing various structural classes, e.g., congeners chlorinated on a single ring (2,3-dichlorobiphenyl), blocked at 2,3 sites (2,5,2'5'-tetrachlorobiphenyl), blocked at 3,4 sites (4,4'-dichlorobiphenyl), and lacking adjacent unchlorinated sites (2,4,5,2',4',5'-hexachlorobiphenyl). The PCB-degrative ability of microorganisms is assessed by packed-column gas chromatographic analysis of these defined congener mixtures following 24-h incubation with resting cells. When tested with 25 environmental isolates, this assay revealed a broad range of PCB-degradative competence, highlighted differences in congener specificity and in the extent of degradation of individual congeners, predicted degradative competence on commercial PCBs, and (iv) identified strains with superior PCB-degradative ability.

Rapid assay for screening and characterizing microorganisms for the ability to degrade polychlorinated biphenyls.

Toxic aromatic pollutants, concentrated in industrial wastes and contaminated sites, can potentially be eliminated by low cost bioremediation systems. Most commonly, the goal of these treatment systems is directed at providing optimum environmental conditions for the mineralization of the pollutants by naturally occurring microflora. Electrophilic aromatic pollutants with multiple chloro, nitro and azo groups have proven to be persistent to biodegradation by aerobic bacteria. These compounds are readily reduced by anaerobic consortia to lower chlorinated aromatics or aromatic amines but are not mineralized further. The reduction increases the susceptibility of the aromatic molecule for oxygenolytic attack. Sequencing anaerobic and aerobic biotreatment steps provide enhanced mineralization of many electrophilic aromatic pollutants. The combined activity of anaerobic and aerobic bacteria can also be obtained in a single treatment step if the bacteria are immobilized in particulate matrices (e.g. biofilm, soil aggregate, etc.). Due to the rapid uptake of oxygen by aerobes and facultative bacteria compared to the slow diffusion of oxygen, oxygen penetration into active biofilms seldom exceeds several hundred micrometers. The anaerobic microniches established inside the biofilms can be applied to the reduction of electron withdrawing functional groups in order to prepare recalcitrant aromatic compounds for further mineralization in the aerobic outer layer of the biofilm. Aside from mineralization, polyhydroxylated and chlorinated phenols as well as nitroaromatics and aromatic amines are susceptible to polymerization in aerobic environments. Consequently, an alternative approach for bioremediation systems can be directed towards incorporating these aromatic pollutants into detoxified humic-like substances. The activation of aromatic pollutants for polymerization can potentially be encouraged by an anaerobic pretreatment step prior to oxidation. Anaerobic bacteria can modify aromatic pollutants by demethylating methoxy groups and reducing nitro groups. The resulting phenols and aromatic amines are readily polymerized in a subsequent aerobic step.

Enhanced biodegradation of aromatic pollutants in cocultures of anaerobic and aerobic bacterial consortia

Of 36 pure isomers (chlorine numbers 1 to 5) of polychlorinated biphenyls examined, 23 compounds were metabolized by Alcaligenes sp. strain Y42, and 33 compounds were metabolized by Acinetobacter sp. strain P6. The major pathway of many polychlorinated biphenyl isomers examined was considered to proceed through 2',3'-dihydro-2',3'-diol compounds, concomitant dehydrogenated 2',3'-dihydroxy compounds, subsequently the 1',2'-meta-cleavage compounds (chlorinated derivatives of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acids), and then chlorobenzoic acids. The meta-cleavage products were usually converted to chlorobenzoic acids upon further incubation in many polychlorinated biphenyls, but they accumulated specifically in the metabolism of 2,4'-, 2,4,4'-, and 2,5,4'-chlorobiphenyls, which are all chlorinated at the 2,4'-position in the molecules in common. Dihydroxy compounds accumulated mainly in the metabolism of 2,6-, 2,3,6-, 2,4,2',5'-, 2,5,2',5'-, and 2,4,5,2',5'-chlorobiphenyls by Acinetobacter sp. P6. The 2,3,2',3'-, 2,3,2',5'-, and 2,4,5,2',3'-chlorobiphenyls, which are chlorinated at the 2,3-position of one of the rings, were metabolized in a different fashion. Two major metabolites of a chlorobenzoic acid and an unknown compound accumulated always in the metabolism of this group of polychlorinated biphenyls. 2,4,6-Trichlorobiphenyl was metabolized quite differently between the two organisms. Alcaligenes sp. Y42 metabolized this compound very slowly to trichlorobenzoic acid by the major oxidative route. In contrast, Acinetobacter sp. P6 metabolized it to a trihydroxy compound via a dihydroxy compound.

Effect of chlorine substitution on the bacterial metabolism of various polychlorinated biphenyls.

Biodegradability of commercial polychlorobiphenyl mixtures (Kaneclors, KC 200 to KC 500) and their metabolic products by Acinetobacter sp. strain P6 were studied by gas chromatography-mass spectrometry analysis. KC 200 (primarily dichlorobiphenyls) rapidly degraded after 4 h of incubation with the P6 resting cells, showing predominant accumulation of monochlorobenzoic acids. KC 300 (primarily trichlorobiphenyls) were also degraded after 4 h of incubation, producing various metabolic intermediates such as mono- and dichlorobenzoic acids, dihydroxy biphenyl compounds with two and three chlorines, and the ring meta-cleavage compounds with two and three chlorines. KC 400 (primarily tetrachlorobiphenyls) were also susceptible to biodegradation by the same organism. Chlorobenzoic acids (chlorine number 1 to 3), dihydroxy compounds (chlorine number 2 to 4), and the ring meta-cleavage compounds (chlorine number 2 to 3) were observed as the products from KC 400. In addition to such products, a large amount of unknown compounds with two chlorines in the molecule, which can be derived from 2,3,2',3' - or 2,3,2',5'-tetrachlorobiphenyls or both, accumulated. In contrast to KC 200, KC 300, and KC 400, KC 500 (primarily pentachlorobiphenyls) were resistant to degradation and hardly metabolized. Only dihydroxy compounds of certain pentachlorobiphenyls were detected.

A Kamibayashi

Papers

Effect of chlorine substitution on the bacterial metabolism of various polychlorinated biphenyls.

Metabolic breakdown of Kaneclors (polychlorobiphenyls) and their products by Acinetobacter sp.