Acetate dissimilation

Stable hydrogen (50) and carbon (6 C) Isotopic compositions of methane gas bubbles formed In the sediments of several shallow aquatic environments were measured and found to range from -346 /oo to -263°/oo and from -75.0°/oo to -51.5°/oo, respectively. Evaluation of the <5D data with a previously published model Implies that acetate dissimilation accounts for about 50% to 802 of the total methane production. 6D-CH, and are... 

Figure 2 Is a plot of 6 D-H2O versus 6 D-CH for samples obtained from littoral zone sediments of several freshwater lakes and from several shallow (1 m or less water depth) areas of the Tampa Bay estuary. Also shown on Figure 2 are lines that describe predicted 6D-H2O/6D-CH Isotopic pairs resulting from varying the relative contributions to methane production of the acetate dissimilation and CO2 reduction pathways. This model was originally proposed by Woltemate et al. (22) and used In that study to estimate that methyl group transfer (from acetate or other methyl group donors such as methanol) was responsible for about 76% of total methane production In the sediments of Wurmsee, a shallow lake near Hannover, FRG.

Stable hydrogen and carbon Isotopic compositions of biogenic methane produced In the sediments of several freshwater and estuarine environments have been measured and Interpreted using a previously published model. The results Infer that acetate dissimilation Is the dominant methanogenlc pathway In these sediments, accounting for about 50 to 80% of the total methane production, with CO2 reduction responsible for the remainder. In general, 6D-CH and are Inversely correlated. Implying... 

In contrast, dissimilation of acetate may take place by reversal of the pathway used by organisms snch as Clostridium thermoaceticum for the synthesis of acetate from COj. In the degradation of acetate, the pathway involves a dismutation in which the methyl group is successively oxidized via methyl THF to COj while the carbonyl group is oxidized via bound carbon monoxide. Snch THF-mediated reactions are of great importance in the anaerobic degradation of pnrines, which is discussed in Chapter 10, Part 1. 

The formation of oxaloacetic acid by dehydrogenation implies that this acid may be dissimilated by two mechanisms. It is known (62), (114) that oxaloacetic acid is subject to decarboxylation under acid conditions, and that higher pH is favorable to its stability. Thus, alkaline media enable the add to remain unchanged long enough to be split, yielding acetate and oxalate, while acidic media cause decarboxylation. 

Despite the lower rate in the presence of inhibitor, radioactively labelled palmitate was added to a CHCla-inhibited system. Table IV shows that butyrate was formed at a faster rate than formate, acetate, or propionate. The fact that butyrate was now one of the major end products of palmitate dissimilation indicates that secondary reactions involving acetate and/or propionate were probably serving to remove hydrogen produced during dissimilation since methanogenesis was inhibited in these experiments. This was partially verified by the findings that radioactively labelled acetate was converted to formate and butyrate at faster rates in inhibited than in uninhibited sludge. It is also possible that formation of butyrate indicates some alternative to -oxidation as a dissimilatory reaction. Acetate itself was formed from C02 in the presence or absence... 

Three important natural succinate producers, Actinobacillus succinogenes, Mannheimia succiniciproducens, and Basfia succiniciproducens, all members of the Pasteurellaceae family within the y-proteobacteria, were isolated from mmen contents (Guettler et al., 1999 Lee et al., 2002 Kuhnert et al., 2010). All three species are capnophilic (thrive in the presence of high concentrations of carbon dioxide), facultative anaerobic bacteria which share a similar fermentative metabolism resulting in the production of succinate as major product accompanied by pyruvate dissimilation to lactate, acetate, formate, and in some cases to ethanol as minor products. 

Gunsalus and Sagers (1958), in studying this sequence for C. acidiurici, propose that dissimilation of glycine proceeds either by condensation with FH4 and oxidation of the adduct, or else by way of serine to pyruvate and thence oxidative decarboxylation to acetate and COs. Formate and CO2 are in equilibrium, thus providing another cyclic mechanism for oxidation of Cl units. A similiar reaction sequence is stated to hold for the dissimilation of ycine by the anaerobe Diplococcua glyeinophilus. Glyoxylic acid is inert in these systems. 

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