Dissimilatory iron reduction

Iron (and manganese) ean be reduced enzymatically by various pathways, which are summarized in great detail by Lovley (1991). One of the prerequisites is the direct contact between the bacteria and solid phase ferric iron (Munch and Ottow 1982). The following given equations shall be considered as representative for a series of reactions within each group. 

For the case of fermentative Fe(III)-reduction it is important to note that during fermentation Fe(III) only serves as a minor sink for electrons and that organic substrates are primarily transformed to organic acids or alcohols. 

---

DiChristina TJ (1992) Effects of nitrate and nitrite on dissimilatory iron reduction by Shewanella putrefaciens 200. J Bacteriol 174 1891-1896. 

Hansel CM, Benner SG, Neiss J, Dohnalkova A, Kukkadapu RK, Fendorf S (2003) Secondary mineralization pathways induced by dissimilatory iron reduction of ferrihydrite under advective flow. Geochim Cosmochim Acta 67 2977-2992... 

Fig. 7.16 Pore water and extraction results from hemipelagic sediments off Uruguay (redrawn from Haese et al. 2000). Dissolution and precipitation of Fe is reflected by the easy reducible iron oxyhydroxide fraction whereas less reducible iron oxides soluble by subsequent citrate/dithionite/bicarbonate (CDB) extraction remain constant. A concurrent liberation of Mn and Fe indicates dissimilatory iron reduction and subsequent iron reoxidation by manganese oxides, which results in the build-up of Mn Under these conditions the actual dissimilatory iron reduction rate is higher than deduced from iron pore water gradients.

In fully marine systems siderite formation is probable to occur below the sulfate reduction zone where dissolved sulfide is absent, if reactive iron is still present and the Fe/Ca-ratio of pore water is high enough to stabilize siderite over calcite (Berner 1971). The coexistence of siderite and pyrite in anoxic marine sediments was shown by Ellwood et al. (1988) and Haese et al. (1997). Both studies attribute this observation to the presence of microenvironments resulting in different characteristic early diagenetic reactions next to each other within the same sediment depth. It appears that in one microenvironment sulfate reduction and the formation of pyrite is predominant, whereas at another site dissimilatory iron reduction and local supersaturation with respect to siderite occurs. Similarly, the importance of microenvironments has been pointed out for various other processes (Jorgensen 1977 Bell et al. 1987 Canfield 1989 Gingele 1992). 

Explain why bioturbation is important for the rate of dissimilatory iron reduction in sediments. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

Assimilation, as mentioned above, is unlikely to have an effect on corrosion as trace amounts of iron are required for it to occur, whereas dissimilatory iron reduction involves electron transfer to iron as part of both anaerobic fermentation or anaerobic respiration [40, 66, 67]. The inpact of fermentor iron reducers has not been studied in detail, perhaps because they do not reduce ferric iron as rapidly or extensively as anaerobic respitory IRB [112], However, Panter reported that fermentative IRB in submerged environments are encountered more frequently than the IRB that use ferric ion in anaerobic respiration [112]. Nonetheless, as mentioned earlier, it is not yet known whether the fermentative IRB could have a great contribution to corrosion. Most probably, then, the only remaining nominee for having an impact on corrosion would be the respiratory iron reducers. 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

Heijman CG, C Holliger, M A Glaus, RP Schwarzenbach, J Zeyer (1993) Abiotic reduction of 4-chloronitro-benzene to 4-chloroaniline in a dissimilatory iron-reducing enrichment culture. Appl Environ Microbiol 59 4350-4353. 

This requires a biomass which can be metabolized. The process usually involves enzymatic transfer of electrons by micro-organisms from the decomposing biomass (represented in the above equation as CH2O) to the Fe " in Fe " oxides. As seen from eq.16.3, reduction consumes protons and is, therefore, favoured, the lower the pH (see also Chap. 12). It usually takes place when all pores are filled with water (see reviews by Fischer, 1988 and Van Breemen, 1988). Biotic reduction of Fe oxides is now recognized as an important process in the oxidation (metabolism) of organic pollutants in soils by dissimilatory, iron-reducing bacteria. 

F. (2000) Dissimilatory iron-reducing bacteria can influence the reduction of carbon tetrachloride by iron metal. Environ. Sd. Techn. 34 2461-2464... 

Fendorf S. (2001) Iron promoted reduction of chromate by dissimilatory iron-reducing bacteria. Environ. Sd. Techn. 35 522-527... 

Sulfur isotopes can effectively be used to examine important geochemical processes associated with redox changes in sedimentary environments. The speciation of sulfur is strongly affected by redox potential, pH, productivity, microbial sulfate reduction, and iron availability (Berner, 1984). More details are provided on the sulfur cycle in chapter 12. In general, during microbial dissimilatory sulfate reduction there is fractionation of sulfur... 

Apart from microenvironments, an explanation for the concurrent dissimilatory sulfate and iron reduction was provided by Postma and Jakobsen (1996). They demonstrated that the stabilities of iron oxides are decisive with respect to iron and/or sulfate reduction assuming that the fermentative step and not the overall energy yield is overall rate limiting. Additionally, it shall be noted that the typical sulfate reducing bacteria Desulfovibrio desulfuricans was found to reduce iron oxide enzymatically contemporarily or optionally (Coleman et al. 1993). When only very small concentrations of as sole electron donor were available iron oxide instead of sulfate was used as electron acceptor by D. desulfuricans. 

Table 7.3 Summary of results quantifying the relative contribution of dissimilatory iron and manganese reduction for the decomposition of organic matter.

---