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Chapter 6 : Mineralogical Controls on Microbial Reduction of Fe(III) (Hydr)oxides
Category: Applied and Industrial Microbiology; Environmental Microbiology
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This chapter discusses the current state of knowledge regarding the geochemical controls on the initial and sustained reduction of Fe(III) minerals. It introduces the major controlling factors for microbial Fe(III) reduction. Cultured model dissimilatory metalreducing bacteria (DMRB) show diminished abilities to reduce the more crystalline phases, only reducing a small fraction of the potentially available Fe(III). The chapter summarizes the current understanding of the geochemical and mineralogical constraints on microbial Fe(III) (hydr)oxides reduction with the realization that the field is far from a resolution or consensus on what factors ultimately control the transfer of electrons to Fe(III) mineral surfaces. Recent investigations have clearly illustrated differential expression of proteins involved in metal respiration in the common DMRB Geobacter sp. More sophisticated techniques and experimental approaches, including surface-sensitive and time-resolved spectroscopy need to be used to accurately interrogate the molecular environment. The use of poised anodes as surrogate minerals holds promise in addressing the role of reduction potential in electron transfer processes. Ultimately, unraveling the enigma of the microbe- mineral interface will require a multidisciplinary approach requiring an appreciation of the physics and chemistry of mineral surfaces, the enzymatic and nonenzymatic pathways responsible for electron transfer, and the ecology of metal-reducing microbes within a complex mineral framework.
Structures of Fe(III) (hydr)oxides commonly used in microbial reduction experiments. (A) Goethite structure composed of octahedral double chains linked through corners. H atoms not shown. (B) Lepidocrocite structure composed of octahedral double chains in corrugated layers. The layers are cross-linked through edges. H atoms not shown. (C) Hematite structure composed of octahedra linked through edge- and corner-sharing as well as face-sharing along the c axis. Unit cell outlined in dashed black line. 10.1128/9781555817190.ch6.f1
Redox potentials of various Fe couples compared with other couples of relevance in groundwater and contaminated systems (modified from Brooks et al.  and Thamdrup et al.  ). A variety of U species are included to illustrate the role of complexation on reduction potential of soluble complexes. Temperature = 25°C, pH = 7, [Fe2+] = [Mn2+] = [NO3 –] = 10 μM, [U(VI)] = 50 μM, [Ca2+] = 5 mM, [HCO3 –] = 28.1 to 28.7 mM, [SO4 2–] = 10 mM, [HS–] = 1 μM, PN2 = 1 atm. Fe(III) clay potentials presented for SWa-1 with [Na+] = 100 μM and either mre1 = 0.02 (Eh = 420 mV) or mre1 = 0.70 (E = 110 mV) ( Favre et al., 2006 ). NTA, nitrilotriacetic acid. 10.1128/9781555817190.ch6.f2
Surface area-normalized reduction rates (mol liter–1 m–2 min–1) for various Fe(III) (hydr)oxides by G. sulfurreducens ( Cutting et al., 2009 ). Hematite (H), goethite (G), lepidocrocite (L), feroxyhyte (Fh), akaganeite (Ak), Schwertmannite (S), and two-line ferrihydrite (Fer2) were formed using different synthesis procedures to generate phases varying in size, morphology, surface area, and crystallinity. Reprinted from Cutting et al. (2009) with permission from Elsevier. 10.1128/9781555817190.ch6.f3
Relationship between the solubility product (*Kso) of various Fe(III) (hydr)oxides and the maximum initial Fe(III) reduction rate per cell (νmax) of S. putrefaciens strain 200R ( Bonneville et al., 2009 ). The solubility products were measured for each phase using a dialysis bag technique under acidic conditions (pH 1 to 2.5) at 25°C and defined as *Kso = a Fe3+ · a H+ –n . Reprinted from Bonneville et al. (2009) with permission from Elsevier. 10.1128/9781555817190.ch6.f4
Mean force adhesion upon retraction of S. oneidensis MR-1 embedded on cantilevers and Fe(III) oxide single crystal faces ( Neal et al., 2005 ). Pairwise comparisons between the means across the three cantilevers indicated significant differences between the two magnetite faces and between the magnetite and hematite faces. Figure reprinted from Neal et al. (2005) under the open access license agreement. 10.1128/9781555817190.ch6.f5
Maximum initial Fe(III) reduction rate (mmol liter–1 day–1) as a function of Al substitution. Trend lines extended to project Fe(III) reduction rates at higher Al levels. Equivalent Fe(III) reduction trends and crossover point were obtained when Fe(III) (hydr)oxides were provided as a slurry or precipitated onto quartz sand. Modified from Ekstrom et al., 2010 . 10.1128/9781555817190.ch6.f6
Physicochemical properties of Fe(III) (hydr)oxides commonly used in bioreduction experiments