Xylene aerobic degradation

Benzene is one of a group of related aromatic monocyclic hydrocarbons (BTEX—benzene, toluene, ethylbenzene, and xylene), and since these are water soluble, there has been concern for their dissipation and persistence in groundwater under both aerobic and anaerobic conditions. Although aerobic growth at the expense of benzene was established many years ago, the pathway for its degradation was established only much later. The aerobic degradation of benzene by bacteria is... 

Ryoo D, Shim H, Canada K, Barbieri P, Wood TK (2000) Aerobic degradation of tetra-chloroethylene by toluene-o-xylene monooxygenase of Pseudomonas stutzeri OX1. Nat Biotechnol 18 775-778... 

The pathways for the degradation of toluene and xylene under denitrifying and sulfate-reducing conditions have been studied most extensively, and they take place by reactions quite different from those used by aerobic bacteria. As an example, two anaerobes affiliated with known sulfate-reducing bacteria isolated from enrichments with crude oil were able to grow at the expense of a number of alkylated benzenes—strain oXySl with toluene, o-xylene, and o-ethyltoluene and strain mXySl with toluene, m-xylene, and m-ethyltoluene (Harms et al. 1999). 

All these substrates can be degraded nnder aerobic conditions and, althongh there appear to be important differences among the xylene isomers and mutant strains have been isolated that can degrade all three isomers (Di Lecce et al. 1997). Reviews have covered varions aspects of this problem ... 

In a single field study (Cozzarelli etal., 1990) where Fe(III) served as the terminal electron acceptor, oxidized products of m- and /(-xylene were observed in the groundwater at Bemidji, Minnesota. While it is not inconceivable that similar products can be formed by either aerobic or anaerobic metabolic routes, the study provides presumptive evidence for xylene degradation under iron-reduci ng condi tions. 

The vast amount of data from natural attenuation studies of petroleum hydrocarbon plumes generally supports anaerobic degradation, especially for benzene, toluene, ethylbenzene, xylenes (BTEX) under field conditions. The first-order degradation rates observed under unspecified anaerobic conditions (Suarez and Rifai, 1999) are typically one or two orders of magnitude lower than rates reported under aerobic conditions (Nielsen et al, 1996). 

Xylenes can also be degraded microbiologically to methylbenzoic acids and to xylenols [111, 112]. Since benzoic acids, phenols, xylenols etc. are hydrophilic decomposition products of volatile aromatics, it may be mentioned that these derivatives are easily biodegraded further by microbes [113-120]. Appart from these aerobic biodegredation reactions, also anaerobic biodegradation reactions are possible under special circumstances [113]. 

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