Anaerobic Denitrifying Conditions

FIGURE 9.28 Transformation of 1-fluoronaphthalene by Cunninghamella elegans. (From Neilson, A.H. and Allard, A.-S., The Handbook of Environmental Chemistry, Vol. 3R, Springer Verlag, 2002, pp. 1-74. With permission.) 

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It has been shown that pure cultures of bacteria under anaerobic denitrifying conditions may produce benzylsuccinate as a metabolite of toluene (Evans et al. 1992 Migaud et al. 1996 Beller et al. 1996). Demonstration of this and the corresponding methylbenzyl succinates from xylenes has been used to demonstrate metabolism of toluene and xylene in an anaerobic aquifer (Beller et al. 1995, 2002). 

Barbaro JR, JF Barker, LA Lemon, Cl Mayfield (1992) Biotransformation of BTEX nnder anaerobic, denitrifying conditions field and laboratory observations. J Contam Hydrol 11 245-272. 

Phenol is an important intermediate in the anaerobic degradation of many complex and simple aromatic compounds. Tschech and Fuchs proposed that the carboxylation of phenol to 4-hydroxybenzoate is the first step in the degradation of phenol under denitrifying conditions. However, 4-hydroxybenzoate is not detected in the cultures or cell extracts of the denitrifying Pseudomonas species in the presence of CO2 and phenol, but it is detected if phenol is replaced by phenolphosphate. In contrast, 4-hydroxybenzoate is readily detected as an intermediate of phenol degradation in the iron-reducing bacterium GS-15, and 4-hydroxybenzoate may prove to be a common intermediate in the anaerobic transformation. Thus, in anaerobic degradation of phenolic compounds, it has been postulated that carboxylation reactions may play important roles. 

Thauera sp. strain DNT-1 is able to degrade toluene under aerobic conditions mediated by a dioxygenase, and under denitrifying conditions in the absence of oxygen by the anaerobic benzylsuccinate pathway (Shinoda et al. 2004). Whereas the tod genes were induced under aerobic conditions, the bss genes were induced under both aerobic and anaerobic conditions. 

It has become clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkylated arenes such as toluene and xylenes, and that carboxylation, hydroxylation, and reductive dehydroxylation are important reactions for phenols that are discussed in Part 4 of this chapter. The simplest examples include alkylated benzenes, products from the carboxylation of napthalene and phenanthrene (Zhang and Young 1997), the decarboxylation of o-, m-, and p-phthalate under denitrifying conditions (Nozawa and Maruyama 1988), and the metabolism of phenols and anilines by carboxylation. Further illustrative examples include the following ... 

The anaerobic metabolism of L-phenylalanine by Thauera aromatica under denitrifying conditions involves several steps that result in the formation of benzoyl-CoA (a) conversion to the CoA-ester by a ligase, (b) transamination to phenylacetyl-CoA, (c) a-oxidation to phenylglyoxalate, and (d) decarboxylation to benzoyl-CoA (Schneider et al. 1997). 

The metabolism of phenols under anaerobic conditions has been examined under denitrifying, sulfate-reducing, Fe (lll)-reducing, and anaerobic nonmethanogenic conditions. It is plausible to suggest a common pathway that has been elucidated for denitrifying bacteria. This comprises (a) activation of phenol by the formation of phenylphosphate, (b) carboxylation at a position para to... 

Nitrite oxizers are also reported to possess an anaerobic metabohsm and even to grow under denitrifying conditions (Freitag et al., 1987). Subsequent work showed that ATP was not produced during N02 reduction (Freitag and Bock, 1990). Very little work has been done in this area with marine N02 oxidizers, but N02 oxidation has been reported in environments where very htde oxygen is present (see later text). 

The degradation of o-, m-, and p-phthalates under denitrifying conditions has been examined (Nozawa and Maruyama 1988), and after an initial decarboxylation to benzoate followed the pathway for the anaerobic degradation of benzoate that has been noted above formation of the CoA ester, reduction to cyclohex-l-ene carboxylate, hydroxylation to 2-hydroxycyclohexane car-boxylate, and ring fission to pimelic acid that was further degraded by P-oxi-dation. 

Degradation of Toluene and Alkyl Benzenes—The anaerobic degradation of toluene under denitrifying conditions has been extensively investigated and different pathways have emerged. 

Hydrocarbons — Up until now, degradation of aromatic hydrocarbons has been limited to monocyclic representatives — particularly toluene, and there is only circumstantial evidence so far for the degradation of naphthalene and phenanthrene coupled to sulfate reduction under anaerobic conditions (Coates et al. 1996a) and the degradation of naphthalene, phenanthrene, and pyrene under denitrifying conditions (McNally et bal. 1998). This issue has been discussed more fully in Chapter 6, Section 6.7.3.2, and in the context of BTEX remediation in Section 8.2.6.I. 

This conclusion was supported by several reports showing a decrease in MTBE concentrations under denitrifying conditions in various locations [13]. Other experimental results [26] showed anaerobic degradation of MTBE in aquifer sediments in the presence of Fe(lll) as the electron acceptor and that unamended aquatic sediments produced " 002 and CH4 from [i C]-MTBE. These results were supported by a report of natural MTBE biodegradation under iron-reducing conditions [6]. 

Besides aerobic MTBE degradation, anoxic or even anaerobic degradation has also been described in the literature. Bradley et al. [35] described MTBE degradation under denitrifying conditions. There was also some evidence for MTBE degradation under denitrifying conditions at a test site in Leuna (Germany) [17]. 

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