Understanding the dynamics of metals and radionuclides in soil environments is necessary for evaluating risks to pristine sites. An iron-rich creek soil of a former uranium-mining district (Ronneburg, Germany) showed high porewater concentrations of heavy metals and radionuclides. Thus, this study aims to (i) evaluate metal dynamics during terminal electron accepting processes (TEAPs) and (ii) characterize active microbial populations in biostimulated soil microcosms using a stable isotope probing (SIP) approach. In biostimulated soil slurries, concentrations of soluble Co, Ni, Zn, As, and unexpectedly U increased during Fe(III)-reduction. This suggests that there was a release of sorbed metals and As during reductive dissolution of Fe(III)-oxides. Subsequent sulfate-reduction was concurrent with a decrease of U, Co, Ni, and Zn concentrations. The relative contribution of U(IV) in the solid phase changed from 18.5 to 88.7% after incubation. The active Fe(III)-reducing population was dominated by delta-Proteobacteria (Geobacter) in (13)C-ethanol amended microcosms. A more diverse community was present in (13)C-lactate amended microcosms including taxa related to Acidobacteria, Firmicutes, delta-Proteobacteria, and beta-Proteobacteria. Our results suggested that biostimulated Fe(III)-reducing communities facilitated the release of metals including U to groundwater which is in contrast to other studies.

Toxic metals can be immobilized on surface sorption sites of soil Fe(III) minerals or can be included in the mineral structure (4, 29). Fe(III)-reducing bacteria (FeRB) can facilitate the release of these metals by reductive dissolution of Fe(III) oxides (9, 17) and bioreduction of Fe(III) oxide-bound trace metals (42). This release might enhance metal stress, suggesting that metal tolerance should be an important attribute for FeRB. Acidophilic FeRB can tolerate millimolar concentrations of Cd, Cu, Ni, and Zn (12), which might be a prerequisite to survival in habitats where low pH facilitates high metal solubility. In contrast, neutrophilic FeRB like Shewanella spp. tolerate only M concentrations (34, 36). Geobacter spp. have not been tested to the best of our knowledge, but metal tolerance proteins are expressed during growth in uranium-contaminated sediments, which might be connected to metal resistance (19).Near Ronneburg (Thuringia, Germany), uranium mining caused severe environmental contamination with metals and radionuclides (20). In creek bank alluvial soils of the Gessenbach, a main drainage system of upstream mining sites (41), high heavy metal concentrations occur both in solid phase and in the pore water of a ground-and surface water-influenced, oxidized, iron-rich Btlc horizon of a Luvic Gleysol. We demonstrated the solubilization of Co, Ni, Zn, As, and U in Btlc soil microcosms during biostimulated microbial Fe(III) reduction that was associated with the activity of microorganisms related to Delta-and Betaproteobacteria, Acidobacteria, and Firmicutes (7). The aims of this study were to (i) determine the heavy metal fraction of the solid phase, which could be released during reductive dissolution of Fe(III) oxides, (ii) estimate the effect of heavy metals on the activity of FeRB in the Gessenbach creek bank soil, and (iii) identify metal-tolerant FeRB, because the permanent exposure to contaminants during the last 50 years should have promoted metal tolerance.Soil geochemistry. Putative binding forms of heavy metals in the Btlc soil solid phase were determined by sequential extraction (8, 43) in samples collected in August 2006. Metal concentrations were analyzed with either ICP-MS (inductively coupled plasma-mass spectrometry) or ICP-OES (optical emission spectrometry) (8). Most metals (20 to 40%) and even 80% of As in Btlc soil were detected in fraction 5, which is representative for amorphous Fe(III) oxides (Fig. 1). A considerable amount of uranium (30%) was recovered in the specifically adsorbed fraction, whereas only Zn and Ni were primarily recovered in the mobile fraction. Zn and Ni also dominated the heavy metal pore water concentration of the creek bank soil, which was sampled monthly from June to November 2007 (7). Pore water heavy metal and As concentrations always peaked in the Btlc horizon (see Fig. S1 in the supplemental material) and reached maximum concentrations of 38.6, 16.4, 3.9, 1.5, 0.6, and 0.3 M for Zn, Ni, Al, Cu, Co, and Cd, respectively. Pore water Fe(II) concentrations, ...

Previous studies dealing with bacterial identification by means of Raman spectroscopy have demonstrated that micro-Raman is a suitable technique for single-cell microbial identification. Raman spectra yield fingerprint-like information about all chemical components within one cell, and combined with multivariate methods, differentiation down to species or even strain level is possible. Many microorganisms may accumulate high amounts of polyhydroxyalkanoates (PHA) as carbon and energy storage materials within the cell and the Raman bands of PHA might impede the identification and differentiation of cells. To date, the identification by means of Raman spectroscopy have never been tested on bacteria which had accumulated PHA. Therefore, the aim of this study is to investigate the effect of intracellular polymer accumulation on the bacterial identification rate. Combining fluorescence imaging and Raman spectroscopy, we identified polyhydroxybutyrate (PHB) as a storage polymer accumulating in the investigated cells. The amount of energy storage material present within the cells was dependent on the physiological status of the microorganisms and strongly influenced the identification results. Bacteria in the stationary phase formed granules of crystalline PHB, which obstructed the Raman spectroscopic identification of bacterial species. The Raman spectra of bacteria in the exponential phase were dominated by signals from the storage material. However, the bands from proteins, lipids, and nucleic acids were not completely obscured by signals from PHB. Cells growing under either oxic or anoxic conditions could also be differentiated, suggesting that changes in Raman spectra can be interpreted as an indicator of different metabolic pathways. Although the presence of PHB induced severe changes in the Raman spectra, our results suggest that Raman spectroscopy can be successfully used for identification as long as the bacteria are not in the stationary phase.

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