Sulfur oxidation, energy yields

In true fermentation, the free energy drop between substrate (say glucose) and anaerobic end products is always modest by comparison with respiration, because fermentation is never based on electron transfer chains coupled to phosphorylation. Rather, true fermentations depend upon a variety of oxidation-reduction reactions involving organic compounds, C02, molecular hydrogen, or sulfur compounds. All these reactions are inefficient in terms of energy yield (moles ATP per mole substrate fermented), and, therefore, the mass of cells obtainable per mole of substrate is much smaller than with respiratory-dependent species. 

Type (a) is illustrated by the reaction of sulfur with ozone (Scheme 1), in which the first step is the excitation of the sulfur oxide SO. The latter transfer energy to the uranyl ion inside its coordination sphere. The yield of U02 excitation in this reaction is close to unity. 

The fractionation associated with the disproportionation of elemental sulfur has been explored for a variety of pure and enrichment cultures of organisms derived from both freshwater and marine environments (Canfield and Thamdrup 1994 Canfield et al. 1998a Habicht et al. 1998). For thermodynamic reasons sulfide concentration must be kept low (<1 mM Thamdrup et al. 1993) for elemental sulfur disproportionation to yield sufficient energy to support growth of the organism. Accordingly, cultures are generally enriched in iron oxides which buffer the sulfide concentration to low levels. A summary of the available results is presented in Table 4. 

Among the types of coal listed in Table 22.3, anthracite has the highest carbon content and consequently yields the most energy per mass when burned. Bituminous coal also contains a relatively high amount of carbon but has in addition high levels of sulfur, which results in increased formation of sulfur oxides when this type of coal is burned. Sulfur oxides are the pollutants that create acid rain (see Sections 3.6 and 15.12). 

When coal, oil, and gas are burned for energy in power plants and in industries, sulfur dioxide is produced. Sulfur dioxide combines with water in the atmosphere to produce sulfurous acid (H2SO3). Subsequent oxidation in the presence of oxygen (air) yields sulfuric acid (H2SO4). 

There are three possible types of three-electron bonds. Oxidation of a u bond leads to a cation-radical with a, u three-electron bond. This bond contains no antibonding electrons, and the total bond strength exceeds that of a double bond by the energy of half a n bond. Olefins can acquire the 2a—In bond on one-electron oxidation, the bond constructed from the electrons 2a and In. Oxidation of organic disulfides, RSSR, to their cation-radicals (RSSR) yields species in which the unpaired electron from the oxidized sulfur interacts with the unbound p-electron pair of the second sulfur (Glass 1999). This establishes a 2n-In bond on top of the already existing o bond. The overall bond strength of this five-electron (2a—2n-In ) bond also exceeds that of the normal... 

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