Microbial degradation of aromatic compounds — from one strategy to four

Until recently, it was though that the microbial degradation of aromatic compounds was absolutely dependent on the use of molecular oxygen for ring cleavage. However, as described here by Fuchs and colleagues, microorganisms have other ways of breaking down these compounds when oxygen is scarce or absent. Aromatic compounds are both common growth substrates for microorganisms and prominent environmental pollutants. The crucial step in their degradation is overcoming the resonance energy that stabilizes the ring structure. The classical strategy for degradation comprises an attack by oxygenases that hydroxylate and finally cleave the ring with the help of activated molecular oxygen. Here, we describe three alternative strategies used by microorganisms to degrade aromatic compounds. All three of these methods involve the use of CoA thioesters and ring cleavage by hydrolysis. However, these strategies are based on different ring activation mechanisms that consist of either formation of a non-aromatic ring-epoxide under oxic conditions, or reduction of the aromatic ring under anoxic conditions using one of two completely different systems.