Aerobic Degradation
Aromatic Hydrocarbons like Benzene, are acted upon by dioxygenases to labile cis cis-dihydrodiol that spontaneously convert to catechols. The dihydroxylated aromatic ring is opened by oxidative ortho cleavage resulting in “cis cis-muconic acid”. This is further converted to β-ketoadipic acid, which is oxidatively cleaved to the common tricarboxylic acid cycle intermediates, succinic acid and acetyl Co A. The catechols may also be opened by meta cleavage adjacent to hydroxyl groups yielding 2-hydroxy cis, cis muconic semialdehyde, which is further metabolized to formic acid, pyruvic acid and acetaldehyde.
Bacteria oxidise aromatic hydrocarbons to cis diols whereas fungi and algae produce trans diols, which are carcinogenic
Condensed
aromatic structures like naphthalene, if degradable,
are attacked by dihydroxylation and opening of one of the rings. Opened ring is
degraded to pyruvic acid and carbon dioxide. The second ring is attacked in the
same manner.
Many condensed aromatic
compounds are degraded with difficulty
or not degraded. Cometabolism can help in degrading them. In presence of lower molecular weight aromatic compounds, in the environment, enzymes are induced which hydrolyse them and also polynuclear aromatic hydrocarbons, by cometabolism.
3 or 5 or more ring containing polyaromatic hydrocarbons eg., fluoranthene and pyrene, are degraded by cometabolism by bacteria such as Beijenrinckia and white rot fungi, Phanerochaete chrysosporium. One of the major problems in their degradation is their extremely low water solubility.
Anaerobic Degradation
Simple aromatics without hydroxyl,
carbonyl or carboxyl substituents were thought not to be degraded under anaerobic
conditions. However, Benzene, toluene etc are also degraded
in anaerobic methanogenic conditions, as in composts, aquifers and sediments.
Xylenes, ethyl benzenes also are degraded under similar conditions. For ring
hydroxylation, oxygen is provided by water in these conditions
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