Metals microbial reduction

Lovley DR. 1995. Microbial reduction of iron, manganese, and other metals. Adv Agr 54 175-231. 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

An actual contribution of humic substances to metal oxide reduction in natural systems has not been demonstrated, and there are processes such as adsorption or decomposition that could limit their effectiveness. Kostka et al. (2002a) observed that AQDS additions elicited a larger increase in Fe(III) reduction by S. oneidensis growing on ferrihydrite than smectite clay minerals. This suggests that the influence of humic substances may depend on soil or sediment mineralogy. Nevertheless, there is ample evidence to suggest that a portion of the anaerobic metabolism that was previously attributed to direct enzymatic Fe(III) and Mn(IV) reduction was actually none-nzymatic reduction by microbially reduced humic substances. 

Microbial reduction of certain metals to a lower redox state may result in reduced mobility and toxicity (Lovley, 2001 Finneran et al., 2002a). Such processes may accompany other metal precipitation mechanisms [e.g., in sulfate-reducing bacterial systems where reduction of Cr(VI) can be a result of indirect... 

Other energy-producing reactions of organisms involve the reduction of oxygen to water, the reduction of nitrate to ammonia and nitrogen gas, the reduction of sulfate to sulfide, and the reduction of carbon dioxide to methane. All of these reactions can exert a profound effect on water quality especially when it is realized that the affected chemical species also engage in many other chemical reactions. For example, the sulfide ion forms precipitates with many heavy metals. The microbial reduction of sulfate to sulfide could be accompanied by a reduction in the dissolved heavy metal content in a natural water. 

The biological function of thioneins is, most likely, the storage and detoxification of heavy metals. Apparently, the accumulation of metals is associated with intense reduction, localization, and adhesion on the outer membrane of metal particles formed. During the adaptation period, the cells can acquire a plasmid imparting resistance to the metal. This ability is realized through the increased rate of metal ions reduction. Thus natural selection may result in the formation of metal-tolerant microbial strains. ... 

Lloyd, J.R. 2003. Microbial reduction of metals and radionuclides. FEMS Microbiology Reviews 27(2-3) 411-425. 

Thus, it appears that in situ immobilization of toxic metals and radionuclides by microbial reduction is a plausible pathway for contaminant stabilization (Lov-ley Phillips, 1992) however, mineral surfaces and the production of biogenic materials may dictate the effectiveness of bacterially mediated metal-reduction processes or the operating mechanism (Tripathi, 1984 Hsi Langmuir, 1985 Zachara et al., 1989 Mesuere Fish, 1992 Kent et al., 1994, 1995 Chisholm-Brause et al., 1994 McKinley et al., 1995 Weng et al., 1996). Thus, there exists a need to study the intricate coupling of microbiological and geochemical processes on contaminant reduction. 

Because systems are normally not designed for use with this type of fluid, certain aspects should be reviewed with the equipment and fluid suppliers before a decision to use such fluids can be taken. These are compatibility with filters, seals, gaskets, hoses, paints and any non-ferrous metals used in the equipment. Condensation corrosion effect on ferrous metals, fluid-mixing equipment needed, control of microbial infection together with overall maintaining and control of fluid dilution and the disposal of waste fluid must also be considered. Provided such attention is paid to these designs and operating features, the cost reductions have proved very beneficial to the overall plant cost effectiveness. 

Additional hypotheses for their mechanism of action have more recently been proposed. It is well known that proanthocyanidins are able to complex metals through their ortho-diphenol groups. This property is often viewed as imparting negative traits (e.g., reduction of the bioavailability of essential mineral micronutrients, especially iron and zinc) [87]. Since iron depletion causes severe limitation to microbial growth, their ability to bind iron has been suggested as one of the possible mechanisms explaining the antimicrobial activity of proanthocyanidins [88] (Table 1). 

---