Electron Acceptors in Microbial Systems

Microorganisms use these reduction reactions to consume the electrons generated by the oxidation of their energy-yielding substrate. Although this substrate can be any of a variety of substances, let us take for an example the oxidation of formaldehyde CH2O) to CO2 and HjO by the reaction  

This reaction can be coupled with any one of the electron accepting (reduction) reactions listed in Table 7-11. From the AG° values of these coupled reactions it can be deduced that the amount of free energy that a microorganism can obtain from the coupled redox reactions listed in Table 7-11 is in direct proportion to its pe value. 

TABLE 7-11 Electron Acceptance Reactions (Reduction Reactions) (Energy-Producing Reactions) in Aerobic and Anaerobic Systems 

When nitrate is utilized, the reaction to produce Naigi (denitrification) is favored over the reduction of NOg through NOg to NH4 (nitrate reduction) for the same reason it yields more usable energy to the organism that catalyzes it. For example, in a nitrifying activated sludge plant, there is 

Greenwood, Nitrification and Nitrate Dissimilation in Soil. Part 2. Effect of Oxygen Concentration, Plant and Soil, J7 365-378 (1962). 

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