Fermentation and Methane Formation

In settings that do not contain oxygen or alternative electron acceptors, methane production is favored. Methane production is the final step in anaerobic decomposition of organic matter. 

Carbon in CH4 can be derived from C02 or fermentation of organic matter. The process of methane generation is a complex multistep process, by several species of bacteria, which results in the following general reaction  

Fermentation-methane reactions produce only approximately 20% of the energy produced by aerobic reactions. 

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The anaerobic decomposition of organic matter by fermentation, methanogenesis (methane formation) and sulfate respiration is exemplified in Table 3.2. 

In the case where the anaerobic processes take place under conditions where consumption of Ss by the sulfate-reducing biomass and the fermenting biomass must be considered, Equation (7.10) expresses the total anaerobic hydrolysis rate. This equation is based on the assumption that methane formation in sewers without sediment normally can be neglected (Section 3.2.2). 

Table 11.1. Reactions common to the C02-reduction and acetate fermentation pathways for methane formation and enzymes that catalyze them.

Whiticar MJ, Faber E, SchoeU M (1986) Biogenic methane formation in marine and freshwater environments CO2 reduction vs. acetate fermentation-isotopic evidence. Geochim Cosmochim Acta 50 693-709... 

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. 

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). 

The degradation of organic matter via fermentative pathways to small organic molecules such as lactate, butyrate, propionate, acetate, formate, Hj and CO is very important in marine sediments, since these compounds are the main substrates for sulfate reduction and partly for methane formation. 

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