Microbial Reduction of Crystalline Iron(III) Oxides: Influence of Oxide Surface Area and Potential for Cell Growth

Quantitative aspects of microbial crystalline iron- (III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 x 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron (III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 X 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe (III)- reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8-18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)- reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe- (III) available for microbial reduction. Results suggest that the potential for cell growth and Fe (II) generation will be determined by the iron (III) oxide surface site concentration in the soil or sediment matrix

In Situ Immobilization of Uranium in Structured Porous Media via Biomineralization at the Fraction/Matrix Interface

The major objectives of the University of Alabama component of this project are to (1) characterize the chemical composition (mainly iron and uranium abundance and redox speciation) of FRC Area 2 sediments; (2) assess the potential for stimulation of microbial Fe(III) and U(VI) reduction in slurries of Area 2 sediments; and (3) analyze the response of microbial community structure to biostimulation, specifically with regard to the abundance and diversity of dissimilatory metal-reducing bacterial (DMRB) populations. As an inclusive working hypothesis, we anticipate that it will be possible to stimulate microbial metal reduction in Area 2 sediment depth strata containing substantial quantities of Fe(III) oxides and U(VI), and that a major enrichment in known DMRB (e.g. Geobacteraceae and related organisms) will take place in response to biostimulation. Information on rates of microbial metabolism, patterns of Fe/U biotransformation, and microbial community response obtained in the slurry experiments will be used to constrain preliminary numerical simulations of the field-scale biostimulation experiment, and to provide molecular (e.g. 16S rRNA/rDNA) targets for assessing the response of DMRB activity to in situ biostimulation. Our progress to date on the above objectives is summarized

In Situ Immobilization of Uranium in Structured Porous Media via Biomineralization at the Fracture/Matrix Interface – Subproject to Co-PI Eric E. Roden

Although the biogeochemical processes underlying in situ bioremediation technologies are increasingly well understood, field-scale heterogeneity (both physical and biogeochemical) remains a major obstacle to successful field-scale implementation. In particular, slow release of contamination from low-permeability regions (primarily by diffusive/dispersive mass transfer) can hinder the effectiveness of remediation. The research described in this report was conducted in conjunction with a project entitled “In Situ Immobilization of Uranium in Structured Porous Media via Biomineralization at the Fracture/Matrix Interface”, which was funded through the Field Research element of the former NABIR Program (now the Environmental Remediation Sciences Program) within the Office of Biological and Environmental Research. Dr. Timothy Scheibe (Pacific Northwest National Laboratory) was the overall PI/PD for the project, which included Scott Brooks (Oak Ridge National Laboratory) and Eric Roden (formerly at The University of Alabama, now at the University of Wisconsin) as separately-funded co-PIs. The overall goal of the project was to evaluate strategies that target bioremediation at interfaces between high- and low-permeability regions of an aquifer in order to minimize the rate of contaminant transfer into high-permeability/high fluid flow zones. The research was conducted at the Area 2 site of the Field Research Center (FRC) at Oak Ridge National Laboratory (ORNL). Area 2 is a shallow pathway for migration of contaminated groundwater to seeps in the upper reach of Bear Creek at ORNL, mainly through a ca. 1 m thick layer of gravel located 4-5 m below the ground surface. Hydrological tracer studies indicate that the gravel layer receives input of uranium from both upstream sources and from diffusive mass transfer out of highly contaminated fill and saprolite materials above and below the gravel layer. We sought to test the hypothesis that injection of electron donor into this layer would induce formation of a redox barrier in the less conductive materials above and below the gravel, resulting in decreased mass transfer of uranium out these materials and attendant declines in groundwater U(VI) concentration. Details regarding the planning, execution, and results of the in situ biostimulation experiment will be provided in separate peer-reviewed publications by the project PIs and colleagues. This report summarizes research activities conducted at The University of Alabama (2002-2005) and the University of Wisconsin (2005-2007) in support of the field experiment, which included (1) chemical and microbiological characterization of sediment cores from Area 2; (2) sediment slurry experiments with Area 2 materials which evaluated the biogeochemical response to ethanol amendment and the potential for U(VI) reduction; (3) analysis of the response of groundwater microbial communities to in situ biostimulation. In addition, biogeochemical reaction models of microbial metabolism in ethanol-stimulated sediments, developed based on sediment slurry experiments, are described

Influence of Aqueous and Solid-Phase Fe(II) Complexants on Microbial Reduction of Crystalline Iron(III) Oxides

The influence of aqueous (NTA and EDTA) and solidphase (aluminum oxide, layer silicates) Fe(II) complexants on the long-term microbial reduction of synthetic goethite by Shewanella alga strain BrY was studied. NTA enhanced goethite reduction by promoting aqueous Fe(II) accumulation, in direct proportion to its concentration in culture medium (0.01-5 mM). In contrast, EDTA failed to stimulate goethite reduction at concentrations e1 mM, and 5 mM EDTA enhanced the final extent of reduction by only 25% in relation to nonchelator controls. The minor effect of EDTA compared to NTA, despite the greater stability of the Fe(II)- EDTA complex, likely resulted from sorption of Fe(II)- EDTA complexes to goethite. Equilibrium Fe(II) speciation calculations showed that Fe(II)aq should increase with NTA at the expense of the solid-phase Fe(II) species, whereas the opposite trend was true for EDTA due to Fe(II)EDTA adsorption. The presence of aluminum oxide and layer silicates led to a variable but significant (1.5 to \u3e 3-fold) increase in the extent of goethite reduction. Speciation of Fe(II) verified the binding of Fe(II) by these solid-phase materials. Our results support the hypothesis that iron(III) oxide reduction may be enhanced by aqueous or solid-phase compounds which prevent or delay Fe(II) sorption to oxide and FeRB cell surfaces

Influence of Aqueous and Solid-Phase Fe(II) Complexants on Microbial Reduction of Crystalline Iron(III) Oxides

The influence of aqueous (NTA and EDTA) and solidphase (aluminum oxide, layer silicates) Fe(II) complexants on the long-term microbial reduction of synthetic goethite by Shewanella alga strain BrY was studied. NTA enhanced goethite reduction by promoting aqueous Fe(II) accumulation, in direct proportion to its concentration in culture medium (0.01-5 mM). In contrast, EDTA failed to stimulate goethite reduction at concentrations e1 mM, and 5 mM EDTA enhanced the final extent of reduction by only 25% in relation to nonchelator controls. The minor effect of EDTA compared to NTA, despite the greater stability of the Fe(II)- EDTA complex, likely resulted from sorption of Fe(II)- EDTA complexes to goethite. Equilibrium Fe(II) speciation calculations showed that Fe(II)aq should increase with NTA at the expense of the solid-phase Fe(II) species, whereas the opposite trend was true for EDTA due to Fe(II)EDTA adsorption. The presence of aluminum oxide and layer silicates led to a variable but significant (1.5 to \u3e 3-fold) increase in the extent of goethite reduction. Speciation of Fe(II) verified the binding of Fe(II) by these solid-phase materials. Our results support the hypothesis that iron(III) oxide reduction may be enhanced by aqueous or solid-phase compounds which prevent or delay Fe(II) sorption to oxide and FeRB cell surfaces

Carbon cycling in mesohaline Chesapeake Bay sediments 1: POC deposition rates and mineralization pathways

Organic carbon cycling in sediments at two locations in the mesohaline Chesapeake Bay was analyzed using available data on sediment sulfate reduction, sediment oxygen consumption, and particulate organic carbon (POC) deposition and burial. Estimates of POC deposition based on the sum of integrated sediment metabolism and POC burial compared well with direct estimates derived from chlorophyll-a collection rates in mid-water column sediment traps. The range of POC deposition estimates (15–31 mol C m−2 yr−1) accounted for a large fraction (36–74%) of average annual net primary production in the mesohaline Bay. The difference between rates of POC deposition and permanent burial indicated that 70–85% of deposited carbon is mineralized on the time scale of a year. Carbon mineralization through sulfate reduction accounted for 30–35% of average net primary production, and was likely responsible for 60–80% of total sediment carbon metabolism. Oxidation of reduced sulfur accounted for a large but quantitatively uncertain portion of SOC in mid-Bay sediments. Our results highlight the quantitative significance of organic carbon sedimentation and attendant anaerobic sediment metabolism in the carbon cycle of a shallow, highly productive estuary