Fermentation, carbon cycle

In keeping with our perspective that the modem cycling of methane can be viewed as a subcycle of the CO2 carbon cycle, we represent microbial methane production by the fermentation of glucose ... 

A partial and abbreviated scheme showing the interrelationship between the methane bacteria and other representatives of the anaerobic carbon cycle is listed in Figure 1. The heavy arrows indicate methane fermentations by individual species or perhaps in some cases by closely dependent symbiotes. The remaining reactions of Figure 1 are catalyzed by propionibacteria, clostridia, butyribacteria, and other anaerobes. The general references of Wood (13), Barker (4), and Stadtman (14) may be consulted for further details and additional fermentations. 

Box 2 Heterogenity of carbon cycle in bottom sediments (after Fenchel et al, 1998) The carbon cycle, in both lake and sea sediments, is predominantly heterotrophic, but there are some aspects of autotrophy. The DOM produced from hydrolysis of fine particulate organic matter (FPOM) is processed by a number of oxidative and fermentative processes in aqueous sediments. The oxidants are O2, N03, NHa" ", Fe +, S04 , and CO2 in sequence from the top of the sediment downward. The DOC component of DOM has limited possibilities it can be oxidized by one of the listed oxidants or it can leave the sediment unoxidized. This is an obvious conclusion, but it has some interesting connotations, for example, the proportion of C oxidized by O2, etc. and the determination of the factors influencing this proportion. Clearly, the quantity of DOC will determine the depth of O2 penetration and the extent to which O2 can participate in C oxidation. An equally important factor is the depth at which DOC is produced by hydrolysis of POM the deeper the site of POM hydrolysis, the more likely will be the anoxic processing of the soluble products. Another very important factor is the degree to which HS is free to diffuse in marine sediments. If HS can diffuse to the sediment surface and react with O2, the depth of O2 penetration will be greatly reduced. 

The nature of the pressure cycle fermenter was mentioned earlier (figure 1.9). Not only does it neatly facilitate mass transfer of oxygen into solution and mass transfer of carbon dioxide out of solution, but less energy is used than that required by mechanical stirrers. Furthermore there is less difficulty in maintaining sterility. 

Respiratory, or oxidative, metaboHsm produces more energy than fermentation. Complete oxidation of one mol of glucose to carbon dioxide and water may produce up to 36 mol ATP in the tricarboxyHc acid (TCA) cycle or related oxidative pathways. More substrates can be respired than fermented, including pentoses (eg, by Candida species), ethanol (eg, by Saccharomjces), methanol (eg, by Hansenu/a species), and alkanes (eg, by Saccharomjces lipoljticd). 

Active Carbon. The process of adsorbiag impurities from carbon dioxide on active carbon or charcoal has been described ia connection with the Backus process of purifyiag carbon dioxide from fermentation processes. Space velocity and reactivation cycle vary with each appHcation. The use of active carbon need not be limited to the fermentation iadustries but, where hydrogen sulfide is the only impurity to be removed, the latter two processes are usually employed (see Carbon, activated carbon). 

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

The metabolic pathway for bacterial sugar fermentation proceeds through the Embden-Meyerhof-Paranas (EMP) pathway. The pathway involves many catalysed enzyme reactions which start with glucose, a six-carbon carbohydrate, and end with two moles of three carbon intermediates, pyruvate. The end pyruvate may go to lactate or be converted to acetyl CoA for the tricarboxylic acid (TCA) cycle. The fermentation pathways from pyruvate and the resulting end products are shown in Figures 9.7 and 9.8. 

Microbial insecticides are very complex materials in their final formulation, because they are produced by fermentation of a variety of natural products. For growth, the bacteria must be provided with a source of carbon, nitrogen, and mineral salts. Sufficient nutrient is provided to take the strain of choice through its life cycle to complete sporulation with concomitant parasporal body formation. Certain crystalliferous bacilli require sources of preformed vitamins and/or amino acids for growth. Media for growing these bacilli may vary from completely soluble, defined formulations, usable for bench scale work, to rich media containing insoluble constituents for production situations (10,27). Complex natural materials such as cottonseed, soybean, and fish meal are commonly used. In fact, one such commercial production method (25) is based on use of a semisolid medium, a bran, which becomes part of the final product. 

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