Oxidation-reduction reactions methane fermentations

Oxidation—reduction reactions Many reactions in wastewater treatment including organic oxidation, methane fermentation, nitrification, and denitrification are mediated by bacteria. They are important in the solubilisation of iron and manganese. 

The reactions unique to the pathway for Methanosarcina thermophila are shown in Figure 11.2 and Table 11.3. In the pathway, the carbon-carbon bond of acetate is cleaved, followed by reduction of the methyl group to methane with electrons originating from oxidation of the carbonyl group to carbon dioxide thus the pathway is a true fermentation. 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

Bechamp (15) in 1867 was the first to describe methane production from a simple fermentation product, ethanol, and to attribute it to a microbial fermentation. The second product he found to be formed from ethanol was caproate, now known to be produced by Clostridium kluyveri (4). Thus, carbon-carbon bonds are not only destroyed early in the fermentation chain, they can also be reductively synthesized if the reaction is paired with the energy yielding oxidation of another substrate (c/. initial fermentations of acetate and ethanol and also of H2, Figure 1). Several successive fermentations may then be required to convert these products to methane and CO2. 

The over-all reduction of CO2 to CH4 is expected to be a spontaneous process that goes through the reduction levels of formate, formaldehyde, and methanol with only a limited, perhaps early requirement for activation by ATP. At the lowest reduction stages, extra ATP may even be generated. In the fermentation of methanol by M. barkeri which utilizes only the last reduction step for methane formation (Reaction 12) somewhat more than 1 mole of ATP appears to be generated for each mole of CH3OH oxidized to CO2, judging from cell yields (14). 

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