Microbes iron/manganese oxidizing

As the demand for electron acceptors increases, facultative microbes can utilize oxidized forms of Mn(IV) and Fe(III) as electron acceptors during the catabolic breakdown of organic matter (Figure 5.31). Manganese and iron cycles in wetlands are discussed in detail in Chapter 10. The carbon flow during Mn and Fe reduction has not been studied as extensively as other electron acceptors (oxygen, nitrate, sulfate, and carbon dioxide). 

Such electron-switching means that, like iron, manganese can donate electrons to life and can form the basis of metabolic cycles in underwater sediments. While iron can only change from plus-two to plus-three, manganese can go all the way up to plus-four. This plus-four form of manganese is very reactive and will pull oxygen atoms from its environment to make a blanket of black manganese-oxide crust around the microbe colony. 

Microbiologically influenced corrosion is defined by the National Association of Corrosion Engineers as any form of corrosion that is influenced by the presence and/or activities of microorganisms. Although MIC appears to many humans to be a new phenomenon, it is not new to the microbes themselves. Microbial transformation of metals in their elemental and various mineral forms has been an essential part of material cycling on earth for billions of years. Some forms of metals such as reduced iron and manganese serve as energy sources for microbes, while oxidized forms of some metals can substitute for... 

The mechanisms of superoxide-dismuting activity of SODs are well established. Dismutation of superoxide occurs at copper, manganese, or iron centers of SOD isoenzymes CuZnSOD, MnSOD, or FeSOD. These isoenzymes were isolated from a variety of sources, including humans, animals, microbes, etc. In the case of CuZnSOD, dismutation process consists of two stages the one-electron transfer oxidation of superoxide by cupric form (Reaction (1)) and the one-electron reduction of superoxide by cuprous form (Reaction (2)). 

The physical location of Fe(III) and Mn(IV) oxides in the soil profile can also influence their utility as electron acceptors by microorganisms. As discussed before, oxidized forms of iron and manganese are insoluble/immobile and they are usually present in oxidized/aerobic portions of the soil. Thus, the decomposition of organic matter with Fe(III) and Mn(lV) as electron acceptors will be restricted to this zone. If this layer is disrupted as a result of organic matter loading, oxygen depletion may promote the reduction of Fe(III) and Mn(IV) oxide precipitates by facultative bacteria. Microbes growing directly on Fe(III) and Mn(IV) oxide mineral surfaces may solubilize and utilize them as electron acceptors. 

Some bacteria are involved directly in the oxidation or reduction of metal ions such as iron and manganese. Some microbes produce organic acids such as formic and succinic acid or mineral acids such as sulfuric acid. Some bacteria can oxidize sulphur or sulfide to sulfate or reduce sulfates to hydrogen sulfide (H2S) (41). 

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