Anaerobic benzene degradation by bacteria

Abstract

Benzene is a widespread and toxic contaminant. The fate of benzene in contaminated aquifers seems to be primarily controlled by the abundance of oxygen: benzene is aerobically degraded at high rates by ubiquitous microorganisms, and the oxygen-dependent pathways for its breakdown were elucidated more than 50 years ago. In contrast, benzene was thought to be persistent under anoxic conditions until 25 years ago. Nevertheless, within the last 15 years, several benzene-degrading cultures have been enriched under varying electron acceptor conditions in laboratories around the world, and organisms involved in anaerobic benzene degradation have been identified, indicating that anaerobic benzene degradation is a relevant environmental process. However, only a few benzene degraders have been isolated in pure culture so far, and they all use nitrate as an electron acceptor. In some highly enriched strictly anaerobic cultures, benzene has been described to be mineralized cooperatively by two or more different organisms. Despite great efforts, the biochemical mechanism by which the aromatic ring of benzene is activated in the absence of oxygen is still not fully elucidated; methylation, hydroxylation and carboxylation are discussed as likely reactions. This review summarizes the current knowledge about the 'key players' of anaerobic benzene degradation under different electron acceptor conditions and the possible pathway(s) of anaerobic benzene degradation.

Figure 1

Proposed degradation pathway for sulfate‐dependent syntrophic benzene mineralization, based on previous observations (Vogt et al., 2007; Kleinsteuber et al., 2008; Herrmann et al., 2010; J. Rakoczy, K.M. Schleinitz, N. Müller, H.‐H. Richnow, unpubl. data). T‐RFLP and SIP data suggest that the Cryptanaerobacter/Pelotomaculum (CP) phylotype assimilates the majority of benzene within the consortium. During benzene oxidation to acetate by the CP phylotype, hydrogen is formed and consumed by various sulfate reducers, driving the syntrophic process. Benzene‐derived acetate might be consumed by several ecophysiologically different organisms, but especially by an epsilonproteobacterium. The majority of the produced carbon dioxide is thought to be formed from the acetate released by the CP phylotype. Small amounts of acetate are aceticlastically converted to methane.

Figure 2

Possible anaerobic benzene activation steps and further transformation reactions to benzoyl‐CoA as central metabolite (modified from Foght, 2008 and Laban et al., 2010). (a) Methylation; (b) hydroxylation; (c) carboxylation. Benzoyl‐CoA can be further reduced by ATP‐dependent or ‐independent benzoyl‐CoA reductases (Kung et al., 2009).