Metabolic Pathways for Organic Pollutant Degradation

Different metabolic pathways are employed by microorganisms to degrade a variety of organic pollutants.

Aliphatic Hydrocarbons. Several bacteria (mainly belonging to the genera Nocardia, Pseudomonas, and Mycobacterium), and some yeast and fungi, which are capable of utilizing saturated aliphatic hydrocarbons for their growth, are the main candidates for new applications in bioscrubbers and biofilters for the removal of such air pollutants. The oxidation of the terminal carbon to an alcohol, catalyzed by a monoxygenase, is responsible for the introduction in the hydrocarbon structure of only one of the atoms of molecular oxygen, which acts as a cosubstrate, while the other is reduced to water:

monoxygenase

Further oxidations by NAD +-dependent alcohol and aldehyde dehydrogenases lead to a carboxylic acid that can be decomposed to acetyl-CoA by the /-oxidation route (46):

alcohol dehydrogenase

aldehyde dehydrogenase

/-oxidation

On the other hand, polychlorinated hydrocarbons can be cometabolized in the presence of isoprene-degrading bacteria (47).

Aromatic Compounds. The aerobic degradation of aro-matics is only possible if the aromatic ring undergoes an enzymatic cleavage. Benzene can be degraded by several microorganisms by two divergent pathways after preliminary dioxygenation to catechol: the so-called ortho-pathway, utilizing the intradiol cleavage catalyzed by the catechol-1,2-dioxygenase, and the so-called meta-pathway, following an extradiol cleavage. Mixtures of benzene, toluene, and xylene are preliminarily dioxygenated, decomposed mainly via the meta-pathway, and finally metabolized through the citric acid cycle (47).

The presence of an alkyl group in the benzene ring gives the microbe the opportunity to attack the compounds either on the side chain or the ring. In arenes with short side chains (up to seven carbons), both alternative routes are possible, mainly due to the widespread presence of transformable plasmids. In particular, the plasmid TOL (48), containing catabolic operons for both alkyl-group oxidation and ring cleavage via meta-pathway, gives several strains of Pseudomonas the ability to degrade toluene, m- and p-xylenes, and other monoalkylbenzenes (49,50). On the contrary, if the side chain is sufficiently long (more than seven carbons), its metabolization by initial side-chain attack via x- and /-oxidation (51), provides the cells of different species with enough energy for growth, and then the ring cleavage is not necessary.

Of the dialkylbenzenes, m- and p-xylenes can be biode-

graded by certain strains of Pseudomonas containing the TOL plasmid by preliminary oxidation of one of the methyl groups to a carboxylic group and methylcatechol (52), followed by meta-cleavage. Members of Nocardia are able to cometabolize m- and p-xylenes via the ortho-pathway, and o-xylene via the meta-pathway in the presence of alkanes as carbon sources (53). More recently, it has also been demonstrated that o-xylene can be metabolized as the sole carbon and energy source, via 3,4-dimethylcatechol and subsequent meta-cleavage, by some strains of Pseudomonas stutzeri (54) and Corynebacterium (55).

There are reports of different bacteria (Pseudomonas sp., Xanthobactersp., etc.) able to grow on styrene (58) and methylstyrenes (57) as sole carbon and energy sources, during which oxidation of the aromatic nucleus and subsequent meta-cleavage probably take place.

In several bacteria, after a preliminary monoxygenation to a catecholic structure, phenol is degraded (58) through the meta-pathway to directly give intermediates of the citric acid cycle (48). While for 4-chlorophenol degradation the chlorinated lactonic intermediate is directly dechlori-nated before its introduction into the cycle, for 4-methyl-phenol a specific enzyme catalyzes the isomerization of the related intermediate into a more easily metabolizable lac-tone (47).

Biphenyl and Fused-Ring Hydrocarbons. Although the biodegradation of biphenyl, naphthalene, and polycyclic aromatic hydrocarbons (PAH) is a subject of less relevance for biofiltration applications given their solid state at ambient temperature, their high toxicity has brought lively interest in the actual possibility of purifying low contaminated off-gases by utilizing biofilters inoculated with liquid suspensions of strains specifically adapted.

Apart some subtle differences, biphenyl is biodegraded through a pathway common to many different bacteria belonging to the genera Pseudomonas (59) and Nocardia (80), including a preliminary dioxygenation of the ring in the C2 and C3 positions to give a catechol-type intermediate, which is subsequently meta-cleaved and catabolyzed via 2-oxopenta-4-enoate and benzoate (80).

Naphthalene is degraded mainly by members of the genus Pseudomonas via dioxygenation to 1,2-dihydroxyna-phthalene, followed by extradiol cleavage of the ring between C1 and C9, whereas PAHS, which are potential carcinogens, mutagens, and tetragens, are dioxygenated and cleaved in two different ways, according to their phen-anthrenic or anthracenic structures (53). Less detail is available for higher molecular weight PAHs.

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