Matilde M. Urrutia

Influence of Biogenic Fe(II) on Bacterial Crystalline Fe(III) Oxide Reduction

Bacterial crystalline Fe(III) oxide reduction has the potential to significantly influence the biogeochemistry of anaerobic sedimentary environments where crystalline Fe(III) oxides are abundant relative to poorly crystalline (amorphous) phases. A review of published data on solid-phase Fe(III) abundance and speciation indicates that crystalline Fe(III) oxides are frequently 2- to S 10-fold more abundant than amorphous Fe(III) oxides in shallow subsurface sediments not yet subjected to microbial Fe(III) oxide reduction activity. Incubation experiments with coastal plain aquifer sediments demonstrated that crystalline Fe(III) oxide reduction can contribute substantially to Fe(II) production in the presence of added electron donors and nutrients. Controls on crystalline Fe(III) oxide reduction are therefore an important consideration in relation to the biogeochemical impacts of bacterial Fe(III) oxide reduction in subsurface environments. In this paper, the influence of biogenic Fe(II) on bacterial reduction of crystalline Fe(III) oxides is reviewed and analyzed in light of new experiments conducted with the acetate-oxidizing, Fe(III)-reducing bacterium (FeRB) Geobacter metallireducens . Previous experiments with Shewanella algae strain BrY indicated that adsorption and/or surface precipitation of Fe(II) on Fe(III) oxide and FeRB cell surfaces is primarily responsible for cessation of goethite ( f -FeOOH) reduction activity after only a relatively small fraction (generally < 10%) of the oxide is reduced. Similar conclusions are drawn from analogous studies with G. metallireducens . Although accumulation of aqueous Fe(II) has the potential to impose thermodynamic constraints on the extent of crystalline Fe(III) oxide reduction, our data on bacterial goethite reduction suggest that this phenomenon cannot universally explain the low microbial reducibility of this mineral. Experiments examining the influence of exogenous Fe(II) (20 mM FeCl 2 ) on soluble Fe(III)-citrate reduction by G. metallireducens and S. algae showed that high concentrations of Fe(II) did not inhibit Fe(III)-citrate reduction by freshly grown cells, which indicates that surface-bound Fe(II) does not inhibit Fe(III) reduction through a classical end-product enzyme inhibition mechanism. However, prolonged exposure of G. metallireducens and S. algae cells to high concentrations of soluble Fe(II) did cause inhibition of soluble Fe(III) reduction. These findings, together with recent documentation of the formation of Fe(II) surface precipitates on FeRB in Fe(III)-citrate medium, provide further evidence for the impact of Fe(II) sorption by FeRB on enzymatic Fe(III) reduction. Two different, but not mutually exclusive, mechanisms whereby accumulation of Fe(II) coatings on Fe(III) oxide and FeRB surfaces may lead to inhibition of enzymatic Fe(III) oxide reduction activity (in the absence of soluble electron shuttles and/or Fe(III) chelators) are identified and discussed in relation to recent experimental work and theoretical considerations.

Bacterial clay authigenesis: a common biogeochemical process

Abstract Transmission electron microscopic (TEM) analyses of freshwater biofilms and bacterial cells, grown in experimental culture, have shown that these microorganisms are commonly associated with fine-grained (Fe, Al)-silicates of variable composition. The inorganic phases develop in a predictable manner, beginning with the adsorption of cationic iron to anionic cellular surfaces, supersaturation of the proximal fluid with Fe3+, nucleation and precipitation of a precursor ferric hydroxide phase on the cell surface, followed by reaction with dissolved silica and aluminum and eventually the growth of an amorphous clay-like phase. Alternatively, colloidal species of (Fe, Al)-silicate composition may react directly with either the anionic cellular polymers or adsorbed iron, depending on their net charge. Over time, these hydrous precursors may dehydrate and convert to more stable crystalline phases. Because microbial biofilms are expansive and highly reactive surfaces at the sediment–water interface, coupled with their ability to bind soluble components and form solid inorganic phases, they should influence the chemical composition of the overlying aqueous microenvironment, and ultimately contribute to the makeup of river bottom sediment.

Microbial and surface chemistry controls on reduction of synthetic Fe(III) oxide minerals by the dissimilatory iron‐reducing bacterium Shewanella alga

The role of Fe(II) biosorption and the effect of medium components on the rate and long‐term extent of Fe(III) oxide reduction (FeRed) by a dissimilatory Fe(III)‐reducing bacterium (Shewanella alga strain BrY) were examined in batch culture experiments. Introduction of fresh S. alga cells into month‐old cultures in which Fe(III) reduction had ceased resulted in further reduction of synthetic amorphous Fe(III) oxide, hematite, and two forms of goethite (Gt). Fresh S. alga cells were also able to reduce a substantial amount of synthetic Gt that had been partly or completely saturated with sorbed Fe(II). Cells that had been precoated with Fe(II) showed a reduced rate and capacity for FeRed. These results indicated that biosorption of Fe(II) had a major impact on FeRed. S. alga cells were shown to have an Fe(II) sorption capacity of ∼0.1 mmol g−1, compared with ∼0.25 mmol g−1 determined for the synthetic Gt. Sorption experiments with component mixtures indicated that direct interaction between cells and oxide...

Bacterial reductive dissolution of crystalline Fe(III) oxide in continuous-flow column reactors

ABSTRACT Bacterial reductive dissolution of synthetic crystalline Fe(III) oxide-coated sand was studied in continuous-flow column reactors in comparison with parallel batch cultures. The cumulative amount of aqueous Fe(II) exported from the columns over a 6-month incubation period corresponded to (95.0 ± 3.7)% (n = 3) of their original Fe(III) content. Wet-chemical analysis revealed that only (6.5 ± 3.2)% of the initial Fe(III) content remained in the columns at the end of the experiment. The near-quantitative removal of Fe was visibly evidenced by extensive bleaching of color from the sand in the columns. In contrast to the column reactors, Fe(II) production quickly reached an asymptote in batch cultures, and only (13.0 ± 2.2)% (n = 3) of the Fe(III) oxide content was reduced. Sustained bacterial-cell growth occurred in the column reactors, leading to the production and export of a quantity of cells 100-fold greater than that added during inoculation. Indirect estimates of cell growth, based on the quantity of Fe(III) reduced, suggest that only an approximate doubling of initial cell abundance was likely to have occurred in the batch cultures. Our results indicate that removal of biogenic Fe(II) via aqueous-phase transport in the column reactors decreased the passivating influence of surface-bound Fe(II) on oxide reduction activity, thereby allowing a dramatic increase in the extent of Fe(III) oxide reduction and associated bacterial growth. These findings have important implications for understanding the fate of organic and inorganic contaminants whose geochemical behavior is linked to Fe(III) oxide reduction.

Reductive immobilization of U(VI) in Fe(III) oxide-reducing subsurface sediments: Analysis of coupled microbial-geochemical processes in experimental reactive transport systems

Although the fundamental microbiological and geochemical processes underlying the potential use of dissimilatory metal-reducing bacteria (DMRB) to create subsurface redox barriers for immobilization of uranium and other redox-sensitive metal/radionuclide contaminants are well-understood (Lovley et al., 1991; Gorby and Lovley, 1992; Lovley and Phillips, 1992; Lovley, 1995; Fredrickson et al., 2000; Wielinga et al., 2000; Wielinga et al., 2001), several fundamental scientific questions need to be addressed in order to understand and predict how such treatment procedures would function under in situ conditions in the subsurface. These questions revolve around the dynamic interactions between hydrologic flux and the coupled microbial-geochemical processes which are likely to occur within a redox barrier treatment zone.

Advanced Experiment Analysis of controls on Microbial FE(III) Oxide Reduction

Understanding factors which control the long-term survival and activity of Fe(III)-reducing bacteria (FeRB) in subsurface sedimentary environments is important for predicting the ability of these organisms to serve as agents for bioremediation of organic and inorganic contaminants. This project seeks to refine our quantitative understanding of microbiological and geochemical controls on bacterial Fe(III) oxide reduction and growth of FeRB, using laboratory reactor systems which mimic to varying degrees the physical and chemical conditions of subsurface sedimentary environments. Methods for studying microbial Fe(III) oxide reduction and FeRB growth in experimental systems which incorporate advective aqueous phase flux are being developed for this purpose. These methodologies, together with an accumulating database on the kinetics of Fe(III) reduction and bacterial growth with various synthetic and natural Fe(III) oxide minerals, will be applicable to experimental and modeling studies of subsurface contaminant transformations directly coupled to or influenced by bacterial Fe(III) oxide reduction activity.