Diversity and distribution of anaeromyxobacter strains in a uranium-contaminated subsurface environment with a nonuniform groundwater flow.

Versaphilic Anaeromyxobacter dehalogenans strains implicated in hexavalent uranium reduction and immobilization are present in the fractured saprolite subsurface environment at the U.S. Department of Energy Integrated Field-Scale Subsurface Research Challenge (IFC) site near Oak Ridge, TN. To provide insight into the in situ distribution of Anaeromyxobacter strains in this system with a nonuniform groundwater flow, 16S rRNA gene-targeted primers and linear hybridization (TaqMan) probes were designed for Oak Ridge IFC Anaeromyxobacter isolates FRC-D1 and FRC-W, along with an Anaeromyxobacter genus-targeted probe and primer set. Multiplex quantitative real-time PCR (mqPCR) was applied to samples collected from Oak Ridge IFC site areas 1 and 3, which are not connected by the primary groundwater flow paths; however, transport between them through cross-plane fractures is hypothesized. Strain FRC-W accounted for more than 10% of the total quantifiable Anaeromyxobacter community in area 1 soils, while strain FRC-D1 was not detected. In FeOOH-amended enrichment cultures derived from area 1 site materials, strain FRC-D1 accounted for 30 to 90% of the total Anaeromyxobacter community, demonstrating that this strain was present in situ in area 1. The area 3 total Anaeromyxobacter abundance exceeded that of area 1 by 3 to 5 orders of magnitude, but neither strain FRC-W- nor FRC-D1-like sequences were quantifiable in any of the 33 area 3 groundwater or sediment samples tested. The Anaeromyxobacter community in area 3 increased from <10(5) cells/g sediment outside the ethanol biostimulation treatment zone to 10(8) cells/g sediment near the injection well, and 16S rRNA gene clone library analysis revealed that representatives of a novel phylogenetic cluster dominated the area 3 Anaeromyxobacter community inside the treatment loop. The combined applications of genus- and strain-level mqPCR approaches along with clone libraries provided novel information on patterns of microbial variability within a bacterial group relevant to uranium bioremediation.

Changes in bacterial and archaeal community structure and functional diversity along a geochemically variable soil profile.

Spatial heterogeneity in physical, chemical, and biological properties of soils allows for the proliferation of diverse microbial communities. Factors influencing the structuring of microbial communities, including availability of nutrients and water, pH, and soil texture, can vary considerably with soil depth and within soil aggregates. Here we investigated changes in the microbial and functional communities within soil aggregates obtained along a soil profile spanning the surface, vadose zone, and saturated soil environments. The composition and diversity of microbial communities and specific functional groups involved in key pathways in the geochemical cycling of nitrogen, Fe, and sulfur were characterized using a coupled approach involving cultivation-independent analysis of both 16S rRNA (bacterial and archaeal) and functional genes (amoA and dsrAB) as well as cultivation-based analysis of Fe(III)-reducing organisms. Here we found that the microbial communities and putative ammonia-oxidizing and Fe(III)-reducing communities varied greatly along the soil profile, likely reflecting differences in carbon availability, water content, and pH. In particular, the Crenarchaeota 16S rRNA sequences are largely unique to each horizon, sharing a distribution and diversity similar to those of the putative (amoA-based) ammonia-oxidizing archaeal community. Anaerobic microenvironments within soil aggregates also appear to allow for both anaerobic- and aerobic-based metabolisms, further highlighting the complexity and spatial heterogeneity impacting microbial community structure and metabolic potential within soils.

Speciation of uranium in sediments before and after in situ biostimulation.

The success of sequestration-based remediation strategies will depend on detailed information, including the predominant U species present as sources before biostimulation and the products produced during and after in situ biostimulation. We used X-ray absorption spectroscopy to determine the valence state and chemical speciation of U in sediment samples collected at a variety of depths through the contaminant plume at the Field Research Center at Oak Ridge, TN, before and after approximately 400 days of in situ biostimulation, as well as in duplicate bioreduced sediments after 363 days of resting conditions. The results indicate that U(VI) in subsurface sediments was partially reduced to 10-40% U(IV) during biostimulation. After biostimulation, U was no longer bound to carbon ligands and was adsorbed to Fe/Mn minerals. Reduction of U(VI) to U(IV) continued in sediment samples stored under anaerobic condition at < 4 degrees C for 12 months, with the fraction of U(IV) in sediments more than doubling and U concentrations in the aqueous phase decreasing from 0.5-0.74 to < 0.1 microM. A shift of uranyl species from uranyl bound to phosphorus ligands to uranyl bound to carbon ligands and the formation of nanoparticulate uraninite occurred in the sediment samples during storage.

Chromate reduction and retention processes within arid subsurface environments.

Chromate is a widespread contaminantthat has deleterious impacts on human health, the mobility and toxicity of which are diminished by reduction to Cr(III). While biological and chemical reduction reactions of Cr(VI) are well resolved, reduction within natural sediments, particularly of arid environments, remains poorly described. Here, we examine chromate reduction within arid sediments from the Hanford, WA site, where Fe(III) (hydr)oxide and carbonate coatings limit mineral reactivity. Chromium(VI) reduction by Hanford sediments is negligible unless pretreated with acid; acidic pretreatment of packed mineral beds having a Cr(VI) feed solution results in Cr(III) associating with the minerals antigorite and lizardite in addition to magnetite and Fe(II)-bearing clay minerals. Highly alkaline conditions (pH > 14), representative of conditions near high-level nuclearwaste tanks, result in Fe(II) dissolution and concurrent Cr(VI) reduction. Additionally, Cr(III) and Cr(VI) are found associated with portlandite, suggesting a secondary mechanism for chromium retention at high pH. Thus, mineral reactivity is limited within this arid environment and appreciable reduction of Cr(VI) is restricted to highly alkaline conditions resulting near leaking radioactive waste disposal tanks.