In aerobic phenol degradation, the highly reactive molecular oxygen is used for the initial attack on the aromatic ring as well as for the final ring cleavage step. A similarly reactive co-substrate is not available in anaerobic pathways. Therefore, anaerobic aromatic pathways use different mechanisms to degrade and cleave the compounds. In contrast to aerobic pathways, the reactions involved in anaerobic aromatic metabolism are largely reductive modifications of the substrate. In particular, the actual dearomatizing reaction in the different anaerobic pathways proceeds by reduction of the aromatic ring to non-aromatic cyclohexane-derivatives.
Pure cultures of anaerobically respiring bacteria can completely oxidize phenol to carbon dioxide using nitrate, ferric iron or sulfate as terminal electron acceptor (van Schie and Young, 2000). At least two phenol degradation pathways have been suggested to occur under methanogenic conditions (Karlsson et al., 2000). The most studied anaerobic pathway is phenol transformation via the "benzoyl-CoA" pathway. Not surprisingly, the efficiency of anaerobic pathways is not as high as in aerobic pathways. Hence, most bioremediation efforts directed at phenol removal invariably involve aerobic processes.
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