Biodegradation chlorinated aromatic compounds

The signiflcance of toxic metabolites is important in diverse metabolic situations (a) when a pathway results in the synthesis of a toxic or inhibitory metabolite, and (b) when pathways for the metabolism of two (or more) analogous substrates supplied simultaneously are incompatible due to the production of a toxic metabolite by one of the substrates. A number of examples are provided to illustrate these possibilities that have achieved considerable attention in the context of the biodegradation of chlorinated aromatic compounds (further discussion is given in Chapter 9, Part 1) ... 

Armenante, P. M., Kafkewitz, D., Lewandowski, G. Kung, C-M. (1992). Integrated anaerobic-aerobic process for the biodegradation of chlorinated aromatic compounds. Environmental Progress, 11, 113—22. 

Chatterjee, D. K., Kellogg, S. T., Watkins, D. R. Chakrabarty, A. M. (1981). Plasmids in the biodegradation of chlorinated aromatic compounds. In Molecular Biology, Pathogenicity, and Ecology of Bacterial Plasmids, ed. S. B. Levy, R. C. Clowes E. L. Koenig, pp. 519-28. New York Plenum Press. 

Chlorinated aromatic compounds are hazardous compounds that result from various industrial and agricultural activities. Water disinfection, waste incineration, and uncontrolled use of biocides are the major sources of chlorinated aromatics in the environment. Chlorinated compounds are also formed as subproducts of the biochemical reactions of herbicides containing chlorophe-noxy compounds. Treatment of chlorinated compounds has been studied using biological treatment, adsorption, air stripping, and incineration. Biodegradation of chlorinated compounds is a slow process that is ineffective for extremely low concentrations. Air stripping and adsorption simply trans-... 

Biodegradation of naphthalene by Cunninghamella elegans. (Adapted from Rochkind, M.L., J.W. Blackburn, and G.S. Sayler. 1986. Microbial Decomposition of Chlorinated Aromatic Compounds. EPA/600/2-86/090.)... 

Chlorinated aromatic compounds such as 2,4,6-trichlorophenol (278) are well known recalcitrant pollutants because of their slow biodegradation by microorganisms. Hydrogen peroxide oxidation of (U- C)-278 catalyzed by FePcS (iron tetrasulfophthalocyanine) was... 

Chlorinated benzoic acids have been shown to be intermediates in the biodegradation of several chlorinated aromatic compounds, for example, 4-chlorobenzoic acid is formed in the biodegradation of both polychlorinated biphenyls [103]. Mixed microbial cultures, which have been studied by a number of groups under aerobic conditions, can degrade a wide range of chlorinated benzoic acids, including 2-, 3- and 4-chlorobenzoic acid as well as 3,4-dichloro- and 2,4-dichlorobenzoic acid [104]. By contrast, under anaerobic conditions, only reductive dechlorination of meta-substituted benzoic acids has been observed [49,105,106]. Cometabolism of chlorobenzoic acids in the presence of unsubstituted benzoic adds leads to the formation of the corresponding... 

Korde VM, Phelps TJ, Bienkowski PR, et al. 1993. Biodegradation of chlorinated aliphatics and aromatic compounds in total-recycle expanded-bed biofilm reactors. Appl Biochem Biotechnol 39-40 631-641. 

Chlorinated aromatics are of special interest due to their extensive use as solvents and fumigants (e.g., dichlorobenzene), wood preservatives (e.g., pen-tachlorophenol), and as parent compounds for pesticides (e.g., 2,4-dichlorophenoxiacetic acid, also known as 2,4-D). An example of their biodegradation is that of 2,4-D, which takes place through a modified ort/io-cleavage pathway by enzymes that are not specific for their substrates. See Figure 9.11. The products, acetylSCoA and succinate, are part of... 

Altered Aromatic Metabolites. Loss of parent compound is insufficient evidence to support the occurrence of biodegradation. Frequently, the disappearance of the parent aromatic compound results in the formation of an altered aromatic metabolite. This product indicates that, although biotransformation has taken place, biodegradation has not. An example is the anaerobic O-demethylation of chlorinated guaiacols to chlorocatechols mediated by the acetogenic bacteria, Acetobacterium woodii and Eubacterium limosum (37). 

Chlorinated Aromatic Hydrocarbon Environmental Toxins. As a result of human endeavor, toxic compounds containing chlorinated benzene rings have been widely distributed in the environment. The pesticide DDT and the class of chemicals called dioxins provide examples of chlorinated aromatic hydrocarbons and structurally related compounds that are very hydrophobic and poorly biodegraded (Fig. 5.28). As a consequence of their persistence and lipophilicity, these chemicals are concentrated in the adipose tissue of fish, fish-eating birds, and carnivorous mammals, including humans. 

Another synthetic dye used to color natural and polymeric textiles are azo (azoic) dyes. Azo dyes are made from the reaction of both a coupling and diazo compound in water into a diazonium salt on liber substrates, typically at temperatures below ambient. These dyes are becoming less prevalent as their chlorinated aromatic amine byproducts are toxic and the dyes themselves are not biodegradable. 

Aerobic, Anaerobic, and Combined Systems. The vast majority of in situ bioremediations ate conducted under aerobic conditions because most organics can be degraded aerobically and more rapidly than under anaerobic conditions. Some synthetic chemicals are highly resistant to aerobic biodegradation, such as highly oxidized, chlorinated hydrocarbons and polynuclear aromatic hydrocarbons (PAHs). Examples of such compounds are tetrachloroethylene, TCE, benzo(a)pyrene [50-32-8] PCBs, and pesticides. 

Aust, S. D., Shah, M. M., Barr, D. P. Chung, N. (1994). Biodegradation of environmental pollutants by white-rot fungi. In Bioremediation of Chlorinated and Polycyclic Aromatic Hydrocarbon Compounds, ed. R. E. Hinchttetal., pp. 441-5. Boca Raton, FL CRC Press. 

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