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during the process before organisms developed the ability to produce superoxide dismutases for this purpose. Since microorganisms cannot take up insoluble forms of Fe into their cells, and since the solubility of Fe3+ in soil solution is low, soil bacteria and fungi have developed the ability to produce Fe3+-chelating compounds called siderophores (Fig. 15.9). Siderophores help keep Fe3+ in solution and permit the uptake into microbial cells. After transport into the cell, the chelated Fe3+ is usually reduced enzymatically to Fe2+ and released from the siderophore, which itself can be released by the cell for further scavenging of iron.

Several other micronutrient metal elements are required by microorganisms, plants, and animals as part of the structure of a number of enzymes (Table 15.11). Manganese is required in photosynthesis, where it plays a role in the production of oxygen in photosystem II. It can also serve as an energy source for some bacteria and also as a terminal electron acceptor. Molybdenum is an essential part of a number of enzyme structures such as nitrogenase, nitrate reductase, and sulfite reductase and may also act as an effective inhibitor of sulfate reduction. Selenium is an essential component together with Mo in the structure of formate dehydrogenase in several bacterial species. Nonessential metal elements such as Hg, As, Pb, and Cd may not be required nutritionally, but many microorganisms have acquired the ability to alter these metals enzymatically in their environments as a means to combat potentially toxic concentrations. These elements also bioaccumulate in the tissues of higher animals and therefore pose potential human health risks.

TABLE 15.11 Some Micronutrients and Their Functions in Microorganisms"

Element

Soluble ionic form

Function in microbial metabolism

Fe

Fe2+, Fe3+

Present in cytochromes, ferredoxins, and other iron-sulfur proteins; cofactor of enzymes

Zn

Zn2+

Present in alcohol dehydrogenase, alkaline phosphatase, adolase, and RNA and DNA polymerase

Mn

Mn2+

Present in bacterial and mitochondrial superoxide dismutase and in photosystem II; cofactor of some enzymes

Mo

MoO4+

Present in nitrate reductase, nitrogenase, xanthine dehydrogenase, and formate dehydrogenase

Se

SeO3-

Present in glycine reductase and formate dehydrogenase

Co

Co2+

Present in coenzyme B12-containing enzymes

Cu

Cu2+

Present in cytochrome oxidase, nitrite reductase of denitrifying bacteria, and oxygenases

Ni

Ni2+

Present in urease, hydrogenase, and factor F430

" With permission from Gottschalk (1986).

" With permission from Gottschalk (1986).

MICROBIAL TRANSFORMATIONS Oxidation and Reduction

Biologically mediated reduction-oxidation reactions are often associated with energy production by the organism. Reduced inorganic compounds are oxidized by chemoautotrophs to generate electrons used in ATP production, while oxidized compounds are reduced during energy production under anaerobic conditions when the element in question acts as an alternative terminal electron acceptor.

Oxidation of Fe and Mn. Microorganisms are involved in the oxidation of Fe, but not all Fe oxidation is microbially mediated. Ferrous iron (Fe2+) is rapidly oxidized chemically in aerated solution at pH >5. It is therefore difficult to demonstrate conclusively the microbial role of Fe oxidation under neutral or alkaline pH conditions. The best evidence for microbially mediated Fe oxidation is from aci-dophilic bacteria operating at pH <5. The most commonly studied acidophilic iron-oxidizing organism is Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans). It is easily cultivated and has been the focus of extensive genetic and physiological studies (Rawlings, 2001). It is a gram-negative, motile rod that derives energy and reducing power from the oxidation of Fe2+, reduced forms of S, metal sulfides, H2, and formate. It uses CO2 as its C source and preferentially NH3-N as its N source, though it can also use NO3-N and some strains can fix N2. Ac. ferrooxidans is mesophilic, growing in the range of 15 to 42°C with an optimum in the range 30-35°C. Other acidophilic iron oxidizers include Leptospirillum ferroooxidans, Sulfolobus spp., and Acidianus spp. Leptos. ferrooxidans oxidizes iron for energy but cannot oxidize reduced S. The best known examples of extreme thermophilic iron-oxidizing bacteria are Sulfolobus acidocaldarius and Acidianus brierleyi (formerly Sulfolobus brierleyi), both of which belong to the

Archaea. Their temperature range is between 55 and 90°C, with an optimum in the range 70-75°C. Sulfur is their usual electron donor, but it can be replaced by Fe2+.

Gallionella ferruginea represents the strongest evidence for enzymatic oxidation of Fe under neutral pH conditions; however, it is equivocal. Gallionella are stalked bacteria that use Fe2+ as their electron donor and CO2 as their C source. The bean-shaped cells secrete colloidal Fe(OH)3, without an organic matrix. They are aerobic and occur in Fe-containing fresh or marine waters. Other bacteria that have been associated with Fe oxidation include sheathed bacteria such as Sphaerotilus, Leptothrix, and "Clonothrix." However, these bacteria are more likely iron-depositing bacteria rather than Fe oxidizers in that they bind already oxidized iron at their cell surfaces. The distinction between abiotic and microbially mediated Fe oxidation is difficult as the physicochemical properties of biogenic iron oxides are similar to those of their abiotic counterparts (Fortin and Langley, 2005), and therefore it remains difficult to determine the exact role of microorganisms in their formation.

Bacterial oxidation of Mn2+ occurs in both soils and sediments. On ocean bottoms, microorganisms are responsible for the formation of ferromanganese nodules. The chemical oxidation of Mn2+ occurs only above pH 8, and therefore Mn oxidation in neutral and acidic environments is microbially mediated. A large number of bacterial and fungal groups are participants. Three phylogenetically distinct organisms have been studied extensively for Mn oxidation: Leptothrix discophora, Pseudomonas putida strains MnB1 and GB-1 (later synonym for Arthrobacter siderocapsulatus), and a Bacillus sp. strain (Tebo et al., 2004). All three organisms oxidize Mn enzymatically on an exopolymer matrix surrounding the cell: Leptot. discophora on an extracellular sheath, Ps. putida on the outer membrane glycocalyx, and Bacillus on the exosporium (Fig. 15.10). Molecular genetic

FIGURE 15.10 Transmission electron micrographs of representatives of the three Mn-oxidizing bacteria (with permission from Tebo et al., 2004). Left, an unidentified Leptothrix sp.; center, spores of the marine Bacillus sp. strain SG-1; right, Pseudomonas putida strain MnB1.

techniques have revealed that all three organisms possess genes that are involved in Mn oxidation and that share sequence similarity with multicopper oxidase enzymes. Other Mn oxidizers, such as Leptothrix pseudoochracea, will oxidize dissolved Mn2+ with catalase enzymes that utilize hydrogen peroxide (H2O2).

Some rhizosphere bacteria oxidize Mn and deposit MnO2 on the outside of the root. Examination shows the root coated with a black precipitate. The easiest method of control is by genetic selection of the host plant, making it incompatible for the bacteria. Manganese toxicity from excess available Mn can occur in acidic soils. The precipitation of Mn in the filaments of a mycorrhizal fungus has been found to allow the growth of Mn-sensitive plants in such soils.

Reduction of Fe and Mn. Enzymatic Fe or Mn reduction occurs as part of anaerobic respiration in which the oxidized form serves as the dominant or exclusive terminal electron acceptor. It may also accompany fermentation in which the metal serves as a supplementary electron acceptor. Both processes are referred to as dis-similatory iron reduction. When Fe or Mn is reduced during cellular uptake for incorporation into cellular components, the process is referred to as assimilatory iron or manganese reduction. Dissimilatory reduction is the dominant process and will be the focus of this discussion.

Iron reduction in the form of anaerobic respiration is an important means of mineralization of organic matter in low-O2 environments where sulfate and nitrate occur in amounts insufficient to support sulfate or nitrate respiration. A wide range of Archaea and bacteria are able to conserve energy through the reduction of ferric iron (Fe3+) to ferrous iron (Fe2+), and many of the same organisms are also able to grow through the reduction of Mn4+ to Mn2+ (Table 15.12). The environmental significance of these processes is that Fe-reducing communities can be responsible for most of the organic matter oxidation in many subsurface environments, and several xenobiotics can be degraded under anaerobic conditions by Fe and Mn reducers. Historically, studies on the reduction of Fe3+ and Mn4+ focused on organisms that grow predominantly by fermentation of sugars with metals used as minor electron acceptors, typically <5% of the reducing equivalents used for metal reduction. Metal reduction through this form of metabolism

TABLE 15.12A Microbially Mediated Oxidation Reactions of Metals in Soils and Sediments and Examples of Organisms Involved"

Element

Half-reaction

Strategy'

Example organisms involved

Fe

2Fe2+ ^ 2Fe3+ + 2e-

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