Polycyclic aromatic hydrocarbons in the environment - Microbial degradation

The microbiology of the degradation of polycyclic aromatic hydrocarbons (PAH) is a rapidly progressing area. Research aims both at developing the knowledge of the fate of these ubiquitous and genotoxic pollutants in the environment and at prompting the utilization of microbiological processes for the remediation of industrial soils contaminated with these compounds. A review of present knowledge in this field is presented here. Degradative organisms are essentially bacteria and fungi. The biodegradation processes involved are aerobic and are initiated by oxygenases, Bacteria utilize two to four cycle-PAH as growth substrates, leading to their mineralization. In addition, lignolytic fungi, as well as bacteria, carry out a cometabolic attack of PAH. A survey of the various microorganisms degrading PAH and of the metabolic pathways involved is presented. The mechanisms by which these scarcely soluble compounds are made accessible to the degradative microorganisms is discussed. Studies completed with bacteria had shown that two distinct mechanisms are involved, namely transfer by dissolution in the aqueous phase and direct interfacial uptake. Another important aspect is the fate of PAH in terms of carbon balances during the bacterial degradation of individual PAH and of PAH mixtures. High levels of mineralization can be obtained, In the case of PAH mixtures such high mineralization levels are obtained using complex microflora involving a large participation of cometabolism. Progress achieved in recent years leads to give up considering PAH as intrinsically biodegradation-recalcitrant compounds. In the environment, an important parameter limiting PAH degradation is their accessibility to microorganisms.