Dissimilatory metal reduction

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Introduction

Dissimilatory metal reduction is a process that is utilized by microbes to conserve energy through oxidizing organic or inorganic electron donors and reducing a metal or metalloid. Microbial metal reduction enables organisms to create electrochemical gradients, which provides the chemical energy required for growth. Metal reducing microorganisms are becoming a research focus due to their potential to facilitate bioremediation in areas that are contaminated with heavy metals or radionuclides. Furthermore, these organisms are integral for the development of microbial fuel cell. The mechanisms of dissimilatory metal reduction are significant in order to exploit its detoxifying and electric-generating advantage.

Fe(III) Reduction

Access to Electron Acceptor

Fe(III) reductions occur most often in soils, sediments and the subsurface[9]. The availability of soluble Fe(III) is limited in soil and sediments because the pH is above pH 4; the predominant form is low-solubility Fe(III) (hydr)oxide[9]. In order to access this insoluble solid-phase of iron, iron-respiring microorganisms utilize soluble electron shuttles and Fe(III)-chelating compound, and direct electron transfer via outer membrane enzyme, nanowires, or pili[3].

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The location of iron-respiring microbes in a three-dimensional biofilm has an impact on which Fe(III)-acquiring mechanism the microbes are going to engage in. Microbes, that are closer to the mineral surface, preferentially use extracellular membrane-bound enzyme to transfer electrons[[#References9]. Alternatively, microbes that are embedded in between the matrix, are more likely to transfer electrons through shuttles or nanowires[9].

Fe(III)-reducing Microorganisms

There are a number of microorganisms that are able to reduce Fe(III), for instance, Geobacter metallireducens and Shewanella putrefaciens[6].

Geobacter metallireducens

Geobacter metallireducens predominantly uses acetate, a fermentation product, as electron donor while it can also oxidize alcohol and fatty acid[6]. Theoretical calculation of the efficiency of iron-reduction coupled to fermentation, it is plausible to say that fermentation would contribute more electron to Fe(III) during the metabolism than oxygen-based respiration[6]. The energy generating reaction for Geobacter metallireducens is[6]:

                acetate - + 8 Fe(llI) + 4 H2O → 2 HCO3 + 8 Fe(lI) + 9 H+ 

Among the Geobacter family, the TCA cycle leads to complete oxidation of acetates or other electron donors and ATP is generated primarily through oxidative phosphorylation[5] . To transfer electrons to Fe(III), Geobacter metallireducens possesses a membrane-bound Fe(III)-reductase. It also produces soluble form and membrane-bound c-type cytochromes[6]. Another way to transfer electron to Fe(III) is extracellular transport of electrons to Fe(III) through microbial nanowires[8]. According to a knock-out mutation experiment of Geobacter sulfurreducens, the results indicate that mutants that lack conductive pili are not able to grow because they are not able to reduce Fe(III) oxides yet pili is not necessary for Fe(III) oxide attachment to cell[8].

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Shewanella putrefaciens

S. putrefaciens is another bacterium that is capable of Fe(III) reduction. Unlike Geobacter family, Shewanella, respirating anaerobically, utilizes substrate-level phosphorylation as a primary energy conservation mechanism to sustain growth[5]. Major organic electron donors for this particular bacterium are formate, lactate and pyruvate[6].

                formate- + 2 Fe(III) + H20 →  HCO3 + 2 Fe(II) + 2 H+;
                lactate - + 4 Fe(llI) + 2 H2O →  acetate - + HCO3 + 4 Fe(II) + 5 H+;
                pyruvate - + 2 Fe(lII) + 2 H2O → acetate - + HCO3 + 2 Fe(II) + 3 H+.

As indicated in the above reactions, lactate and pyruvate are not completely oxidized. Thus, it is expected that lactate and pyruvate only contribute a little to electron and carbon flow in Fe(III)-reducing environment[6]. In an anaerobic environment, the production of c-type cytochromes are stimulated in the outer membrane; If c-type cytochromes serve as a mediator for electron transfer to reduce insoluble Fe(III) oxides, this hypothesizes a possible mechanism for Fe(III) reduction in the absence of Fe(III) reductase identification[6]. Also, there is an association between proton translocation and electron transfer to Fe(III) because of the experimental finding of depressed pH value in the medium while incubating S. putrefaciens anaerobically[6]. In another study of S. oneidensis MR-1, the experimental finding suggests that the presence of different oxidation states of Fe on both the cell surface and in extracellular environment offer an insight on the Fe(III)-reducing mechanism; the researchers also conclude that surface contact-mediated electron transfer is significant[3].

Environmental Significance

The impact of Fe(III) reduction in soil and sediment is profound. For examples, its role in the removal of organic matter in a variety of sedimentary environments and decomposition of aromatic compounds[6]. Geobacter is also an electricigen which produces electricity by oxidizing organic compound and subsequently transferring electron to electrode. Furthermore, ferrous Fe is a strong reductant for a variety of contaminants including U(VI)[9]. Similarly, the mineralized product of Fe(III) reduction, magnetite, helps immobilize U(VI) through secondary formation of magnetite which U(VI) can incorporate itself into the magnetite[9].