Microbial degradation of phenol and phenolic derivatives

The available technologies for the abatement of phenol from water and gaseous streams are briefly reviewed, and the recent advancements summarized. Separation technologies such as distillation, liquid-liquid extraction with different solvents, adsorption over activated carbons and polymeric and inorganic adsorbents, membrane pervaporation and membrane-solvent extraction, have been discussed. Destruction technologies such as non-catalytic, supercritical and catalytic wet air oxidation, ozonation, non-catalytic, catalytic and enzymatic peroxide wet oxidation, electrochemical and photocatalytic oxidation, supercritical wet gasification, destruction with electron discharges as well as biochemical treatments have been considered. As for the abatement of phenol from gases, condensation, absorption in liquids, adsorption on solids, membrane separation, thermal, catalytic, photocatalytic and biological oxidation have also been considered. The experimental conditions and the performances of the different techniques have been compared.

The biodegradation of aromatic hydrocarbons by fungi has traditionally been considered to be of a cometabolic nature. Recently, however, an increasing number of fungi isolated from air biofilters exposed to hydrocarbon-polluted gas streams have been shown to assimilate volatile aromatic hydrocarbons as the sole source of carbon and energy. The biosystematics, ecology, and metabolism of such fungi are reviewed here, based in part on re-evaluation of a collection of published hydrocarbon-degrading isolates obtained from authors around the world. Incorrect or outdated identifications in original publications are corrected by ribosomal DNA sequence analysis. The data show that many volatile-hydrocarbon-degrading strains are closely related to, or in some cases clearly conspecific with, the very restricted number of human-pathogenic fungal species causing severe mycoses, especially neurological infections, in immunocompetent individuals. Neurochemistry features a distinctive array of phenolic and aliphatic compounds that are related to molecules involved in the metabolism of aromatic hydrocarbons. Hence, there may be physiological connections between hydrocarbon assimilation and certain patterns of mammalian infection.

Ring-cleaving dioxygenases catalyze the oxygenolytic fission of catecholic compounds, a critical step in the aerobic degradation of aromatic compounds by bacteria. Two classes of these enzymes have been identified, based on the mode of ring cleavage: intradiol dioxygenases utilize non-heme Fe(III) to cleave the aromatic nucleus ortho to the hydroxyl substituents; and extradiol dioxygenases utilize non-heme Fe(II) or other divalent metal ions to cleave the aromatic nucleus meta to the hydroxyl substituents. Recent genomic, structural, spectroscopic, and kinetic studies have increased our understanding of the distribution, evolution, and mechanisms of these enzymes. Overall, extradiol dioxygenases appear to be more versatile than their intradiol counterparts. Thus, the former cleave a wider variety of substrates, have evolved on a larger number of structural scaffolds, and occur in a wider variety of pathways, including biosynthetic pathways and pathways that degrade non-aromatic compounds. The catalytic mechanisms of the two enzymes proceed via similar iron-alkylperoxo intermediates. The ability of extradiol enzymes to act on a variety of non-catecholic compounds is consistent with proposed differences in the breakdown of this iron-alkylperoxo intermediate in the two enzymes, involving alkenyl migration in extradiol enzymes and acyl migration in intradiol enzymes. Nevertheless, despite recent advances in our understanding of these fascinating enzymes, the major determinant of the mode of ring cleavage remains unknown.