Biochemistry of anaerobic biodegradation of aromatic compounds

Abstract

Aromatic compounds comprise the second largest group of natural products (Fig. 1); in addition a variety of xenobiotics are man-made aromatic pollutants. They are metabolized by microorganisms following two fundamentally different strategies: Under aerobic conditions, aromatic compounds are transformed by monooxygenases and dioxygenases into a few central intermediates such as catechol, protocatechuate and gentisate. These dihydroxylated compounds are suitable for an oxidative cleavage of the aromatic ring (the pertinent aerobic literature will not be covered here but see Chapter 11 by Smith). Under anoxic conditions, aromatic compounds have to be transformed by other means than by oxygenases. It is now proven that numerous low molecular weight aromatic compounds are degraded by different groups of anaerobic bacteria (early reports by Tarvin and Buswell 1934; Dutton and Evans 1967) and that the aromatic ring structures are reductively attacked, as proposed by the late W.C. Evans (Dutton and Evans 1969; Balba and Evans 1977; Evans 1977; Williams and Evans 1975; Evans and Fuchs 1988). It has to be pointed out, however, that there is no evidence for a significant anaerobic degradation of polymeric high molecular weight aromatics such as lignins, which represent probably more than half of the aromatic compounds (Hackett et al. 1977; Zeikus 1980; Young and Frazer 1987).