+ 2CO2 + 2ATP [24]

C6Hi2O6, glucose; C3H5O3-PO3H2, glyceraldehyde 3-phosphate; C3H4O4-2(PO3H2), 1,3-bisphosphoglycerate; C3H3O3-PO3H2, phosphoenolpyruvate; C3H4O3, pyruvae; C2H6O, ethanol. "Not all intermediates are represented.

fermentation is shown in detail in Table 9.15 and several fermentation systems are summarized below (multiple arrows indicate missing steps): Ethanol fermentation by yeasts

Lactic acid homofermentation by bacteria

Glucose ^ 2 lactate + 2ATP

Lactic acid heterofermentation by bacteria

Glucose ^ ethanol + lactate + CO2 + 1ATP H2 production, e.g., butyrate fermentation by bacteria

Taking ethanol fermentation as an example, the average oxidation state of the C atoms in glucose is 0 (C6H12O6). Four electrons are removed from each of 2 C atoms to generate 2CO2 (oxidation state of C + 4), which are distributed by transferring 2 electrons to each of 4 atoms of C to form 2 ethanol molecules (2C2H6O; oxidation state of C - 2). The more reduced and the more oxidized moieties can be seen in each of the above examples except for lactic acid fermentation, in which subsequent reduction of one product may yield two identical molecules. In other cases, electrons may combine with H+ to form H2 using a pyruvate-ferredoxin oxidoreductase (Gottschalk, 1986) as in butyrate fermentation above.

Why bother with all this if energy as ATP is the goal? The reason is that formation of phosphorylated intermediates needed for substrate level phosphoryla-tion requires oxidation of the organic substrate. Such oxidation removes e- + H+, which must go somewhere. With no external electron acceptor, a vast array of internal mechanisms is used to relocate the e- + H+, hence the great diversity of fermentation systems and of fermentation intermediates within anaerobic ecosystems. Extent of oxidation, energy yield, and, hence, growth under such conditions is several fold lower than under aerobic conditions.

Methanogens are the strictest anaerobes normally found in nature and use a limited array of substrates: H2 + CO2, formate, methanol, methylamines, and acetate. These substrates are formed during fermentation or converted from fermentation products in anaerobic systems. Two groups of organisms produce methane. The first is strictly chemolithotrophic organisms that grow on H2 and CO2. They are fascinating in their ability to produce all their needs for energy and C from H2 and CO2 alone. Clearly some of the reducing equivalents from the H2 are used for CO2 reduction as well as for energy generation. The reaction is CO2 + H2 ^ CH4 + HOH. The free energy change is -136 kJ per reaction (Gottschalk, 1986). H2 can be produced by reactions such as fermentation of butyric acid to produce acetate according to C4H8O2 + 2HOH ^ 2C2H4O2 + 2H2. This is an endothermic reaction, but with a free energy change of +48.1 kJ per reaction under standard conditions (Gottschalk, 1986). Such a thermodynamically unfavorable free energy change would suggest that H2 could not be released. Ecosystems have adapted, however. Thermodynamically the actual free energy change is related to the difference between equilibrium concentration and the current concentration. Rapid utilization of H2 by methanogens (among others) in close proximity to H2 producers keeps the H2 concentration exceedingly low compared to the equilibrium concentration thereby driving the above reaction to the right. The net free energy change for the combined reactions is negative and hence thermodynamically favorable. Hence, syntrophic associations of organisms are important in soils. The second group of organisms that produces methane is chemoorganotrophic; they produce CH4 from substrates such as methanol, acetate, or methylamines, which contain methyl groups. For methanol fermentation to CH4 the overall reaction is 4CH3OH ^ 3CH4 + CO2 + 2HOH. Acetate is the most common and important, and its conversion to methane is written simply as C2H4O2 ^ CH4 + CO2. In the presence of SO43 acetate is oxidized to CO2 rather than being split to CH4 and CO2. Here we see the preference of SO43 as a terminal electron acceptor over methane fermentation. The arrangement of anaerobic and aerobic microsites close to each other in soils facilitates commensalistic associations that favor the use of fermentation products either in methanogenesis or by aerobic or anaerobic chemoorganotrophs.

how can the microbial contributions be viewed in a simplified and unified concept?

Two principles are fundamental to this simplified concept. They are:

1. The mechanisms by which soil organisms achieve their major functions are centered on the supply and interconversions of diverse forms of energy.

2. These mechanisms are also responsible for most other biological transformations observed in soils.

Transformations mediated by soil organisms result from their search for energy. Energy for soil organisms is obtained by passing electrons from e3 donors to e3 acceptors to produce ATP. Flow of e3 between donors and receptors changes the oxidation states of elements. These donors and acceptors form multiple interconnected oxidation-reduction couples, which lead to cycles through which electrons flow. Because of the central role of oxidation-reduction reactions, O2 availability would be expected to be, and is, is a major control on how these interconnected oxidation-reduction couples operate. Electron donors and acceptors in each couple are often different elements. Consequently these flows of e3 unite cycles of elements, alter mobility and functions of elements, and regulate soil biological transformations. Flow of electrons among these cycles unites activities of extremely diverse groups of soil organisms. Therefore, one can understand most soil biological transformations as a simple framework of interconnected cycles of e3 .

Such a conceptualization is a simple and robust way to unite the myriad details about transformations mediated by soil organisms. An electron cycle model accommodates the full spectrum of understanding from detailed presentations of reactions to global biogeochemical cycles and earth history. It further makes predictions about what we might yet find in nature concerning mineral transformations by soil organisms.

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook

Post a comment