Microbial Transformations of Aromatic Pollutants

Abstract

Over geological time microorganisms have evolved catabolic enzyme systems for the metabolism of naturally-occurring aromatic compounds and laboratory studies have revealed certain general features of aromatic metabolism. Thus, oxygen plays a central role in the hydroxylation and fission of the aromatic nucleus. Bacteria incorporate both atoms of molecular oxygen into aromatic hydrocarbons to form cis-dihydrodiols which then undergo further oxidation to catechols. In contrast mammals and fungi incorporate one atom of oxygen into aromatic hydrocarbons to form reactive arene oxides which undergo enzymatic hydration to form dihydrodiols with a trans-stereochemistry. The presence of two hydroxyl groups on the aromatic nucleus is a prerequisite for enzymatic fission of the ring. The hydroxyl substituents may be ortho or para to each other. Subsequent metabolic sequences vary depending on the organism and the site of ring cleavage. Molecules synthesized by man are degraded if they bear a structural relationship to naturally-occurring compounds. These features are discussed in relation to the microbial transformations of PCB's and chlorinated benzoic acids.