Methane fermentation intermediates

Methane fermentation may be considered a reduction of C02 to CH4 accompanied by oxidation of any one of the intermediate oxidation states to C02. Since all of the latter have pE°(W) less than —6.2 (this for... 

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. 

Another conceptual approach to fermentation methanol from biomass is to convert biomass to acetate with suitable acetogenic populations, followed by direct conversion of acetate to methanol. This type of process does not involve intermediate methane and might possibly improve the operational characteristics of the process. The process can be represented by... 

Assessing biological activity at a field site based on monitoring data can be difficult. However, there are a number of monitoring parameters that can be indicative of halorespiration. First, the presence of methane in the groundwater indicates that fermentation is occurring and that the potential for halorespiration exists. Second, the transformation of PCE andTCE has been studied intensely and many researchers report that of the three possible DCE isomers, 1,1 -DCE is the least significant intermediate and that cis-1,2-DCE predominates over trans-1,2-DCE. Third, because chlorinated ethenes are 55 to 85% chlorine by mass, the... 

Carbohydrate conversion into new industrial intermediates and end-products like propanediol, methane or syngas needs either net energy input ( reduction equivalents ) or decarboxylation, which results in a significant and costly yield loss and lowers the CO2 fixation net gain. This balance loss is also a conceptual draw-back of present fuel ethanol fermentation from sucrose and starch hydrolysates. So far, the only bulk intermediates with no or minimal... 

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