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putrefaciens, Desulfovibrio

aWith permission from Silvia et al. (2005) with data from Ehrlich (1996), Frankenberger and Lossi (1995), and Lovley (1993).

b NE, not enzymatic; AR, element is terminal electron acceptor in anaerobic respiration; D, detoxification; E, energy source.

aWith permission from Silvia et al. (2005) with data from Ehrlich (1996), Frankenberger and Lossi (1995), and Lovley (1993).

b NE, not enzymatic; AR, element is terminal electron acceptor in anaerobic respiration; D, detoxification; E, energy source.

did not conclusively demonstrate increases in cell yield. Shewanella oneidensis (formerly Alteromonas putrefaciens and then Shewanella putrefaciens) and Geobacter metallireducens were then unequivocally shown to conserve energy for growth through the reduction of Fe3+ or Mn4+, and more recently, numerous organisms that can grow using Fe3+ and Mn4+ as electron acceptors have been isolated. Most organisms that are known to grow through the reduction of Fe3+ or Mn4+ are relatives of Ge. metallireducens and include the genera Geobacter, Desul-furomonas, Desulfuromusa, and Pelobacter. With the exception of the last group, these organisms are able to completely oxidize a wide range of organic compounds, including acetate, when respiring using Fe3+ or Mn4+. Pelobacter are restricted to ethanol, lactate, formate, and H2.

The primary products of the metabolism of the fermentative Fe3+-reducing microorganisms are typical fermentation acids, alcohols, and H2. Most of the electron transfer to Fe3+ during the metabolism of sugars and amino acids results from the oxidation of the fermentation products, and acetate is considered to be the most important fermentation product in Fe3+-reducing environments. The Fe3+ reducer Ge. metallireducens oxidizes acetate by the reaction acetate- + 8 Fe3+ + 4 H2O ^ 2 HCO3 + 8 Fe2+ + 9 H+.

Ge. metallireducens oxidizes various other volatile fatty acids and simple alcohols. Sh. oneidensis can also conserve energy to support growth by coupling the oxidation of formate, which is oxidized to CO2, or lactate or pyruvate, which are incompletely oxidized, to the reduction of Fe3+:

formate3 + 2 Fe3+ + H2O ^ HCO3~ + 2 Fe2+ + 2 H+, lactate3 + 4 Fe3+ + 2 H2O ^ acetate3 + HCO3 + 4 Fe2+ + 5 H+, pyruvate3 + 2 Fe3+ + 2 H2O ^ acetate3 + HCO3 + 2 Fe2+ + 3 H+.

Ge. metallireducens can also completely oxidize a wide variety of monoaro-matic compounds, such as toluene, p-cresol, and phenol, to carbon dioxide with Fe3+ serving as the sole electron acceptor.

Dissimilatory Fe3+ reduction has a greater overall environmental impact than microbial reduction of any other metal. It has been implicated as an important process in the following phenomena: (1) organic matter decomposition in a variety of freshwater, estuarine, and marine sediments; (2) the decomposition of aromatic hydrocarbons in contaminated aquifers; (3) the control of the extent of methane formation in shallow freshwater environments; (4) the release of phosphate and trace metals into soil solution; (5) soil gleying; and (6) the corrosion of buried iron and steel pipes (see review by Lovley, 1991). Manganese reduction may also serve to (1) assist in the oxidation of organic matter in waters or sediments, (2) release dissolved Mn into groundwaters and sediments, and (3) release trace metals bound to Mn oxides.

Reduction of other metals. Several other metals are subject to microbially mediated reduction (Lloyd, 2003, Table 15.12). Several organisms have been isolated that can grow through dissimilatory reduction of As5+ (e.g., Sulfurospirillum arsenophilum, a microaerobic S-reducing bacterium isolated from an As-contaminated watershed). The reduction of AsO43 to AsO23 by the ArcC reductase enzyme also forms the basis for a microbial arsenic resistance mechanism. A wide range of facultative anaerobes are able to reduce Cr6+ to Cr3+, including E. coli, Pseudomonas spp., Sh. oneidensis, and Aeromonas spp. Obligate anaerobes are also able to reduce Cr enzymatically, and the reduction of Cr coupled with anaerobic growth has been observed in S-reducing bacteria.

Numerous microorganisms reduce Hg2+ to Hg0 for self-protection only, as little evidence exists that Hg reduction supports microbial growth. Reduction is linked to mercury-resistance (mer) operons providing a detoxification in which highly soluble Hg2+ is reduced to volatile Hg0. The reduction is for protection against the toxic metal rather than for the energy-conserving electron transport characteristics of other microbial reductions. The mercuric reductase is a flavin-containing disulfide that drives the following reaction:

Organisms active in reducing Hg include strains of Pseudomonas spp., enteric bacteria, S. aureus, Ac. ferrooxidans, group B Streptococcus, Streptomyces, and Cryptococcus. Other organisms such as Bacillus, Vibrio, Flavobacterium,

TABLE 15.13 Microbial Genera Known to Contribute to Methylation of Metals under Aerobic Conditions"

Genera Arsenic (As) Mercury (Hg) Selenium (Se) Lead (Pb)

Fungi

Genera Arsenic (As) Mercury (Hg) Selenium (Se) Lead (Pb)

Fungi

TABLE 15.13 Microbial Genera Known to Contribute to Methylation of Metals under Aerobic Conditions"

Aspergillus

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