Thermodynamics methanogenic bacteria

In essence, SO reducers in this case are fermenting bacteria that depend on H2-consuming methanogens to maintain a low H2 partial pressure, thereby making fermentation thermodynamically feasible. In addition, methanogenic bacteria can consume the acetate produced by the SO reducers. 

Figure 6.3 shows a mass balance model for sulphur speciations. Once other more thermodynamically amenable electron acceptors have been depleted, sulphate is used by sulphate reducing bacteria, such as Desulfovibrio sp. This occurs until the sulphate concentration falls to a point where it is outcompeted for the organic substrate by methanogenic bacteria. The sulphide produced by sulphate reduction is then removed or recycled by different processes depending on the environment of deposition. 

The thermodynamic yield of the various anaerobic pathways is related to their affinity for substrates. Sulfate-reducing bacteria outcompete methanogens by maintaining H2 concentrations below the threshold for the process to be thermodynamically feasible (Lovley et al., 1982), and Fe(III) reducers do the same to SOU reducers. In fact, the equilibrium concentration of H2 can be used to predict the dominant metabolic pathways in a given environment (Lovley and Goodwin, 1988 Lovley et al., 1994a). 

Scholten J. C. M., Murrell J. C., and Kelly D. P. (2003) Growth of sulfate-reducing bacteria and methanogenic archaea with methylated sulfur compounds a commentary on the thermodynamic aspects. Arc/i. Microbiol. 179, 135-144. 

---