Microbially Driven Redox Reactions in Anoxic Environments: Pathways, Energetics, and Biochemical Consequences

After consumption of molecular oxygen, anaerobic microbial communities can use a continuum of alternative electron acceptors such as nitrate, manganese oxides, iron oxides, sulfate or CO2, with decreasing spans of available free energy. The electron transfer to insoluble metal oxides or to partner organisms such as methanogens may require the employment of electron carrier systems such as soil organic matter or sulfur compounds, with consequences for the reaction kinetics. The redox potentials of the electron acceptor systems do not only influence the reaction energetics but may also determine the pathways of degradation, especially in the degradation of organic contaminants. These aspects are discussed with examples of aromatic compounds, in particular phenols and cresols. The results demonstrate that beyond the mere lack of oxygen availability also the redox potential of the electron acceptor system in play determines to a large extent the kinetics, energetics and biochemistry of anaerobic transformation processes.