B53C-1001 1340h
The Microbial Karst Sulfuric Acid Dynamo
The original model for sulfuric acid speleogenesis attributes limestone dissolution to the oxidation of gaseous H2S to sulfuric acid on limestone cave walls (Egemeier 1981). This model has recently been reexamined in Lower Kane Cave, Wyoming (USA), where the most intense limestone dissolution appears to be the result of microbial colonization of limestone surfaces below the water table (Engel et al. 2004). In contrast, sulfuric acid speleogenesis in the Frasassi Caves (Italy) is equally intense above and below the water table, and is mediated not only by sulfur-oxidizing bacteria but by a complex community of sulfur cycling microorganisms including diverse sulfate-reducing bacteria. The sulfate-reducing bacteria were identified in 16S rDNA clone libraries from both cave walls and cave stream biofilms. These findings suggest a new model for sulfuric acid speleogenesis in which a full range of oxidants and reductants available to indigenous sulfur-cycling microbial communities control the extent of sulfur recycling and sulfuric acid production at limestone surfaces.
B53C-1002 1340h
Electrostatic Interaction of Viruses With Heterogeneously Charged Surfaces Under Environmental Conditions
The electrostatic interactions of three viruses: Norwalk, MS2 and Qb, with environmentally relevant charged surfaces have been calculated as a function of fluid pH, ionic strength, and surface charge heterogeneity. The virus is modeled as a charged particle comprised of a shell of amino acid residues whose charges are a function of pH and which contains a spherical, charged, RNA core. The viral particle is placed in the vicinity of an infinite Gouy-Chapman (Poisson-Boltzmann) plane having surface charge density, s1, which contains an atomistic region of different charge density, s2, and of finite size. The atomistic region is treated at the Debye-Hückel level of theory. It is found that the electrostatic interaction of the three viruses to these planar surfaces is strongly dependent upon the ionic strength of the solution and the spatial distribution of amino acid residues in the viral coat.
B53C-1003 1340h
Structural analysis of solids formed by reacting U(VI) with Fe(0)
Structural elucidation of uranium associated with iron is critical in understanding the chemistry of the passivity layers. Uranyl (VI) solutions were reacted with zerovalent iron (0) powder as a function of molality and pH, and the resulting solids were characterized using SEM and EXAFS techniques. Lower pH (3-5) favored up to 99 percent solid formation from the solution U. The solid phase formation appeared to be a combination of partial reduction, adsorption and precipitation processes, depending on the pH condition. Schoepite at different hydration levels and also alpha- and beta- oxides were the predominant U solids. The morphology and structural coordination of the solids varied as a function of the boundary conditions. Results of these studies can be utilized in optimizing an efficient decontamination design of the corroded steels.
B53C-1004 1340h
Microbial Sulfur Cycling in an Acid Mine Lake
Geochemical dynamics of a tailings impacted lake in Northern Ontario were investigated over a three-year period, in which active pyrrhotite slurry disposal was initiated in year two. A strong seasonal trend of decreasing epilimnetic pH with significant diurnal acid production, pre-, during and post slurry deposition was observed with high rates observed compared to pre-slurry. Slurry deposition occurred at the surface of the lake and acted as a reaction stimulant for acid generation. Over the diurnal timescale investigated, the highest rates of acid production occurred not at the lake surface but within the metaliminetic region of the lake. This region was exemplified by strong decreasing oxygen gradients, and thus observed high rates of acid generation are more consistent with microbial pathways of sulfur oxidation than with abiotic, oxygen catalyzed pathways. Consistent with microbial catalysis, metalimnetic rates of acid generation were highest during June and July when microbial populations and metabolic rates were maximal. These results indicate that microbial oxidation of sulfur species play a major role in acid generation in this system. Further, observed rates of acid generation exceed those predicted by published abiotic rates of pyrrhotite oxidation, but are consistent with literature estimates of acid generation catalyzed by microbial activity. Acidithiobacilli accounted for up to 50% of the microbial community pre slurry, but were absent post slurry deposition. These results are the first to demonstrate quantitatively that microbial sulfur oxidation can play a predominant role in acid generation within mine tailings impacted systems. They further highlight the need to evaluate the more complex pathways by which microorganisms process sulfur as the conditions, controls and process rates differ from those observed for abiotic reactions.
B53C-1005 1340h
Dissolution Kinetics of Hausmannite in the Presence of the Siderophore, DFO-B
Siderophores are organic ligands with a very high affinity for Fe(III) that have been shown to dissolve iron-(III) bearing minerals, thus increasing the bioavailability of this nutrient. Recent work also has shown that siderophores are capable of forming stable Mn (III) complexes in aqueous environments and that they can promote the dissolution of Mn (III)-bearing minerals such as manganite ($\gamma$-MnOOH). The goal of this study was to investigate the dissolution kinetics of the mineral, hausmannite, in the presence of desferrioxamine B (DFO-B), a common trihydroxamate siderophore, synthesized by fungi and bacteria, which may play a role in the reduction of soluble Mn (III) to Mn$^2^+$. Hausmannite, a mixed-valence Mn (II, III) oxide mineral with the formula Mn$_3$O$_4$, is a proposed intermediate in the oxidation of Mn$^2^+$ to thermodynamically stable MnO$_2$. Hausmannite particles were synthesized and characterized by X-ray diffraction, BET specific surface area, and SEM imaging. The extent and rate of dissolution were measured as a function of pH and ligand concentration in batch reactors at 25 $\pm$ 3 $\deg$C. Total Mn concentrations were measured by ICP-AES, while complexed-Mn (III) was measured using UV-Vis spectrophotometry. In the absence of the ligand, the greatest amount of total Mn in solution was observed at pH $\approx$ 5. There was a sharp decrease in the extent of dissolution at pH $>$ 5, however, and no appreciable dissolution occurred at pH $>$ 8. In the presence of DFO-B, ligand-promoted dissolution dominated at pH $>$ 8, with more than 90 $%$ of the ligand complexed within 1 hour after addition. Below pH $\approx$ 8, a complex combination of reductive, ligand-promoted, and proton-promoted dissolution was observed. Our study revealed that complex mechanisms are involved in the DFOB-promoted dissolution of hausmannite, with several dissolution pathways occurring simultaneously. The results presented herein may have implications for mineral weathering, the biogeochemical cycling of Fe and Mn in the environment, and the availability of these nutrients to plants and microorganisms.
B53C-1006 1340h
Biological Alteration of Basaltic Glass With Altered Composition and Oxidation States
The ocean crust is an extreme and oligotrophic environment and yet recent studies have shown that reactions between oceanic crust and seawater are capable of supporting microbial life. We are specifically targeting volcanic glass as a source of energy and nutrients necessary to support endolithic microbial communities. A significant amount of chemical energy is available from the process of iron oxidation and our goal is to determine the ability of microorganisms to use Fe(II) as an energy source as well as liberate other essential nutrients from the host rock. In addition, microbes may oxidize Mn or use phosphate from glass. To explore the dependency of microbial life on these nutrients and energy sources, we produce basaltic glasses with varying Fe oxidation states and relative abundance of iron, manganese and phosphate and introduce them to microbial isolates and consortia both in the laboratory and in deep-ocean environments. The natural exposure experiments occur in a variety of settings including hydrothermal vents and cold deep seawater (Loihi Seamount), brines (Mediterranean), and basaltic flows at spreading ridges (EPR), when possible on submarine lava flows of recent and known age. Upon collection of the exposure experiments, we compare basalt colonizing microbial communities on our synthetic glasses with the in situ glass communities through a large culturing effort and molecular (t-RFLP) studies. So far we have produced a number of enrichment cultures and isolated several iron-oxidizing and manganese-oxidizing bacteria that were used to inoculate glasses in the laboratory. Laboratory experiments concentrate on biofilm formation and dissolution of the colonized glasses. Continued collection of exposure experiments on a yearly time-scale provides valuable information regarding spatial and temporal variations in microbial community diversity and structure. We have also analyzed the authigenic reaction products of seafloor, microbially mediated alteration of glass, and compared these results to the types of enrichment cultures found and the composition and oxidation state of the synthetic and natural glasses. We will present preliminary results on culturing, molecular structure and experimental approach in exposure experiments and glass preparation.
B53C-1007 1340h
Carbon Mineralization Pathways and Early Diagenesis in Lake Erie Sediments
In spite of the long-standing paradigm whereby organic matter degradation proceeds by redox reactions that consume oxidants in the order of free energy yield, diagenesis in marine and fresh water sediments often yield different results. The reasons for this are the highly variable absolute and relative abundances of electron acceptors and the different microbial populations found in freshwater environments. As contaminant availability and subsequent impact on aquatic ecosystems are directly linked to these transformations, it is important to understand the most important degradation pathways and their rates. To this end we have conducted chemical analyses of Lake Erie sediment pore-waters and a preliminary characterization of the vertical distribution of microbiological populations. Sediments were collected at four locations in the Central and Eastern basins of Lake Erie during cruises of the R/V LIMNOS in May and June of 2004 respectively. High-resolution vertical profiles of several redox-active species (O2, Fe2+, Mn2+, Fe3+ and S2-) have been obtained by voltammetry using Au/Hg amalgam micro-electrodes. These are the first high-resolution pore-water profiles obtained for multiple redox species using Au/Hg amalgam microelectrodes in the Great Lakes. These profiles show oxygen depletion to levels below detection (5 uM) at depths that range from <1 to 6 mm below the sediment-water interface. Frequently, there is up to 1 cm separation between the depth at which O2 became undetectable and the depth of the first measurable Mn2+. The vertical concentration profiles of Mn2+ and Fe2+ are highly variable between stations and seem to be related to the local bathymetry. Alternatively this variability may be related to the abundance of solid phase Mn and Fe at these sites. The presence of voltammetric peaks measured between -0.5 and -0.6 V, that are often attributed to dissolved organic Fe (III) species, could be produced as part of a strategy by Fe reducing microorganisms to render solid phase Fe (III) bioavailable. Mn2+ voltammetric peaks were shifted to potentials more negative than the -1.53 to -1.55 mV commonly observed in marine pore waters. This shift is consistent with previous studies in freshwaters and has been ascribed to Mn2+ complexation by organic ligands (e.g. Luther et al, 2003). However, this shift may be due to analytical artifacts associated with using a solid state Ag/AgCl reference electrode in low ionic strength solutions. Measurable sulphide in the first 5 cm below the sediment-water interface is sporadic which suggests that sulphate reduction occurs in micro-environments locally enriched in organic carbon. Preliminary cultivation-independent, microbiological analyses have revealed 16s rDNA clones that are closely related to known species capable of enzymatic reduction of Fe(III) and the dechlorination of organic compounds (e.g. Anaeromyxobacter dehalogenans). These organisms were vertically dispersed within several different core sections suggestive of an intriguing tie between diagenetic reactions and anthropogenic organic compound degradation in these sediments. Coupling high-resolution voltammetry and spatially resolved genomic tools to investigate the controls on sediment pore water chemistry holds a promising future for elucidating the controls on early diagenesis in freshwater ecosystems.
B53C-1008 1340h
Microbially Induced Reductive Dissolution of Trace Element-Rich Lacustrine Iron-Oxides
Iron (oxy)hydroxides are ubiquitous components of surfacial materials and are often the dominant redox buffering solid phases in soils and sediments. As a result, the geochemical behavior of these minerals has a profound influence on the global biogeochemical cycling of trace elements, including heavy metals and arsenic (As), in addition to nutrients such as, sulfur (S), carbon (C), nitrogen (N), and phosphorus (P). Understanding the behavior of trace elements and nutrients during biological and abiotic processes that effect iron (Fe) mineral phase transformations is paramount for predicting their distribution, mobility, and bioavailability in the environment. To evaluate the impact of dissimilatory Fe-reduction (DIR) on trace element mobility we have conducted batch incubations of Fe-rich lateritic lacustrine sediments. In contrast to mid-latitude lakes where Fe (oxy)hydroxides constitute only a small fraction of the total sediment, tropical lake sediments have been known to comprise up to 40-60 wt. % Fe-oxides. Under suboxic and nonsulphidogenic conditions it is likely that DIR plays a prominent role in early diagenesis and therefore may exert control on the fate and distribution of many trace elements in this environment (e.g. Crowe et al. 2004). In batch incubations conducted in a minimal media of similar composition to typical freshwater the lacustrine Fe-oxides were reductively dissolved at a rate very similar to pure synthetic goethite of similar surface area (measured by N2-BET). This is in contrast to the slower rates previously observed for trace element substituted Fe-oxides. These slower rates have been attributed to surface passivation by secondary Al and Cr mineral precipitation. We propose that these passivation effects may be offset in minimal media incubations by enhanced microbial metabolism due the presence of nutrients (P, Co and other metals) in the lacustrine Fe-oxides. These nutrients became available with progressive reduction as the nutrient bearing phases were dissolved. It was found that during DIR many trace elements (e.g. Ni, Mn, Co, Cr, P, and Si) were redistributed between the aqueous and solid phases. However trace element release was not congruent with Fe-oxide dissolution and the maximum aqueous concentrations of Mn, and Co were observed after less than three days of incubation. The rapid release of metals, particularly Co, Mn and Ni, suggest that these elements may be present in discrete phases more readily reduced than the bulk iron (oxy)hydroxides (e.g. MnO2). Cr is initially solubilized but is subsequently removed with progressive Fe reduction. This is consistent with the reduction of aqueous Cr (VI) to Cr (III) by Fe2+. Thus, in natural Fe-oxides there is potential for significant Fe2+ re-oxidation following the release of solid phase oxidants. In summary, our experiments suggest that in lacustrine environments Fe-oxides may be reductively dissolved at higher rates than predicted from laboratory experiments using single-phase pure iron (oxy)hydroxides. The release of the macronutrient phosphorus during DIR may enable sustained reduction in carbon rich anaerobic lake sediments. Furthermore, MnO2 may place a significant role in controlling trace element cycling even in very Fe-rich sediments.
B53C-1009 1340h
Ranking Geochemical Energy Availability in Hydrothermal Ecosystems
The energy available to hyperthermophilic microorganisms in hot springs can be theoretically estimated using thermodynamic calculations based on geochemical measurements. The relative abundance of different geochemical energy sources (the "ranking" of these reactions) in particular hot springs may provide one explanation for the differences in hot spring microbial communities and also facilitate the culture of ecologically-relevant microorganisms. Geochemical sampling of seven Yellowstone National Park hot springs was repeated five times from 1999 to 2004 with the intent to compare the geochemistry and geochemical energy available to microorganisms. These seven hot springs were located in three separate regions of Yellowstone National Park: three hot springs, including Obsidian Pool, were sampled in the Mud Volcano area; two in the Sylvan Springs area (Gibbon Meadows); and one each in Imperial Meadows and Sentinel Meadows (Lower Geyser Basin). The hot springs were 75 to $93\deg$C (with one $65\deg$C exception) and spanned the bulk of the pH range at Yellowstone (pH 1.8 to 7.6). Geochemical measurements made on hot springs included redox-active species containing C, N, O, H, S, and Fe; these species were measured by field spectrophotometry and ion chromatography of fluid samples and gas chromatographic analysis of gas samples. From these measurements chemical affinities were calculated for 179 inorganic reactions which encompass the suite of autotrophic energy sources potentially available in each pool. Composite affinities for each reaction were compiled for each of the seven primary pools. The composite for each pool was assembled from repeat measurements from the primary pool as well as nearby pools with similar geochemistry. Calculations show that over half of these inorganic reactions could provide enough energy for a microorganism to survive, based on the threshold value of energy required by {it E. coli} (20 kJ per mole of electron pairs). Some microorganisms, including those in syntrophic associations, may have a much lower energetic threshold (near 5 kJ per mole of electron pairs; Jackson and McInerney, 2002, Nature 415:454), which includes another quarter of the calculated reactions. Oxygen is the most energetic electron acceptor in all pools, but the relative ranking of the most energetic reactions changes from pool to pool. Iron-containing reactions are the most sensitive to pH variations. The subtle shifts in the ranking of the energy supplies lead to testable predictions about microbial community structure and function. Some highly energetic reactions correspond to unknown metabolisms; targeting these potential metabolisms with enrichment cultures could expand the range of known thermophiles.
http://geopig.asu.edu
B53C-1010 1340h
Tracking Changes in Ocean Oxygenation with Molybdenum Isotopes
The oxygenation of the oceans has varied through geologic time but the timing and extent of these variations are not well understood, nor do we have a good understanding of the connections between changes in atmospheric and ocean oxygenation. The development and refinement of ocean paleoredox proxies is therefore a high priority in unraveling the history of the atmosphere. Because of the connections between oxygen and the carbon cycle, such proxies are also important to understanding climate change on long timescales. The molybdenum (Mo) stable isotope system has emerged as a valuable new tool for the investigation of ocean paleoredox. To a first approximation, Mo enters the oceans via rivers and is removed by adsorption to Mn oxides in oxic sedimentary environments and via scavenging of Mo oxythiomolybdates in sulfidic settings. There is an isotopic contrast between these sinks of ca. 2\permil. Hence, the steady-state Mo isotope composition of the oceans can be considered to reflect the balance between fractionated Mo removal to Mn-oxides vs. near-quantitative extraction in sulfidic settings, with removal to Mn-oxides forcing a steady-state isotopic offset between average crustal Mo (0\permil) and seawater (ca. 1.6\permil) today. The source-seawater offset would have been smaller - i.e., seawater isotopically lighter - during extended periods of expanded ocean anoxia because a smaller fraction of the Mo removed would have been buried as fractionated (isotopically light) Mo associated with Mn-oxide bearing sediments. The Mo isotope system is particularly valuable because it can provide information about regional or global ocean redox, rather than only local redox, as a consequence of the long ocean residence time of this element. We will review the promise and limitations of this new proxy and present an update of our most recent findings. These include: evidence of enhanced ocean anoxia between 1.8 and 1.0 Ga despite rising atmospheric oxygen, consistent with other emerging evidence about the oceans during the "middle age" of Earth history; indications of modestly enhanced ocean anoxia during the mid-to-late Devonian; and data that may suggest short-lived episodes of ocean anoxia during the late Cretaceous preceding and following Ocean Anoxic Event 2.
B53C-1011 1340h
Formation and Stability of Manganese-Desferrioxamine B Complexes
Recent laboratory and field studies suggest that Mn(III) forms persistent aqueous complexes with high-affinity ligands, particularly those produced by microbes. Aqueous Mn(III) species thus may play a significant, as-yet largely unexplored role in biogeochemical processes. We determined stability constants for both Mn(II) and Mn(III) complexes with the common tri-hydroxamate siderophore, desferrioxamine B (DFOB). We found the thermodynamic stability constants of the species, MHDFOB$^{{it\x-2}}$ [M = Mn(II), {\it x} = 2; M = Mn(III), {\it x} = 3] to be K$_{Mn(II)}$ = 10$^{6.8 \pm 0.1}$ and K$_{Mn(III)}$ = 10$^{29.2 \pm 0.2}$ at 25$\deg$C. The Mn(III)HDFOB$^{+}$ complex is stable for pH in the range 7.0 - 11.3, but at pH $<$ 7.0, Mn(III)HDFOB$^{+}$ decays by internal electron transfer, yielding oxidized DFOB products and Mn$^{2+}$. For pH $>$ 11.3, the complex decays by disproportionation, yielding Mn$^{2+}$ and solid MnO$_{2}$. The Mn(III)HDFOB$^{+}$ complex may be formed by either the oxidation of aqueous Mn(II)-HDFOB complexes or the DFOB-promoted dissolution of solid manganese(III) oxides. The DFOB-promoted Mn(II) air-oxidation rate was found to be proportional to the concentration of Mn(II)-DFOB complexes. At pH $>$ 6.5, the dissolution of manganite ($\gamma$-MnOOH) in the presence of DFOB is predominantly a non-reductive ligand-promoted reaction whose rate is proportional to the adsorbed surface concentration of DFOB. At pH $<$ 6.5, Mn$^{2+}$ is the dominant species resulting from manganite dissolution, thus implicating a reductive dissolution pathway. The results of this study have broad implications for the biogeochemical cycling of manganese, redox-active elements, and siderophores in natural waters and soils.
B53C-1012 1340h
Structure, Properties, and Reactions of Biogenic Manganese Oxides Predicted by Density Functional Theory.
Biogenic manganese oxides are nanoparticles produced by a wide variety of microorganisms. They typically exhibit short-range order and have the characteristic 7.2 Angstrom layer spacing of the birnessite family of manganese oxides. Density functional theory(DFT) has been used to study the properties of the oxide structures. The effects of different spin states have been modeled; preliminary results show that different spin states have a large effect on the physical structure of the crystals. Biogenic manganese oxides contain Mn(IV) vacancy sites, leading to a negative structural charge. The reaction of cations and water with these vacancies also have been probed using DFT modeling.
B53C-1013 1340h
Bacterial control on the structure of As-Fe oxy-hydroxides in acid mine drainage.
Nano-crystalline or amorphous iron oxy-hydroxides are kinetically favored with respect to stable crystalline phases in low temperature environments. Therefore, they frequently occur as transient phases in Earth's surface environments. They exhibit very-high surface areas (few 100 cm2/g) and thus play a key role in the geochemical cycles of minor and trace elements, including toxic elements as arsenic. Natural low-temperature iron oxides also potentially host biological signatures since they can form through various biologically driven reactions. In the present communication, we compare the mineralogy and crystal chemistry of biogenic As-rich iron precipitates synthesized using various acidophilic bacterial strain isolated from an exceptionally arsenic-rich acid mine drainage [1]. XAS, XRD, SEM and TEM investigation of these highly reactive nano-minerals obtained in controlled conditions allows to better constrain their mechanisms of formation. Our data show that the enzymatic oxidation of Fe(II) and/or As(III) play a key role in controlling the nature of the mineral species precipitating in acid mine drainage. We show that the nature of mineral species forming from solutions can be directly determined by the metabolic activity of specific bacterial strains. This influence is thought to be primarily indirect, bacteria controlling the rate of Fe(II) and As(III) oxidation reactions, which in turn leads to various Fe(III) and As(V) super-saturation conditions. These latter parameters are crucial in controlling the structure of nano-crystalline As-Fe low temperature minerals. 1- Morin et al. (2003) Bacterial formation of tooeleite and mixed As(III)/(V)-Fe(III) gels in the Carnoules acid mine drainage, France. A XANES, XRD and SEM study. Environ. Sci. and Technol. 37,1705-1712.
B53C-1014 1340h
Manganese and Iron Interactions in Cave and Rock Varnish Communities
Microbial communities in arid land caves and surface desert environments interact with Fe and Mn, yielding deposits of intimately associated Fe- and Mn-oxides as a result. Although the geological setting and fundamental rock makeup may be similar in some cases, the environments of these two types of communities differ radically. The manner in which the organisms interact with the rock environment may reflect these differences in the resulting minerals, although the biological oxidation mechanisms of Mn and Fe may be similar. We are mapping the Mn and Fe deposition patterns in the mineral coatings in relation to the concentrations of organic carbon indicative of microbial presence, identifying minerals that are biogenic, along with isolating the perpetrators responsible and reproducing relevant minerals in the laboratory. We are also uncovering the underlying biodiversity as revealed by molecular phylogenetic techniques. The ultimate goal is to determine the degree of microbial responsibility for the secondary mineral deposits observed and the potential role of these communities in both dissolution of subsurface bedrock and deposition of surface oxide coatings. Synchrotron XRF and XRD data reveal differences in the mineralogy of the coatings. Lithiophorite is the predominate crystalline Mn-mineral in cave samples, although crystal size is small. TEM analyses show that the Mn-oxides range from amorphous or microcrystalline to exhibiting coherent crystalline lattices. XRF mapping indicates that Ni, Cu, Zn, As and Pb are associated with the Mn-oxides. In the desert surface oxide samples birnessite predominates the crystalline minerals. X-ray maps show a laminated structure with a complex and variable chemistry, including variability in trace elements such as Ni and Pb. DNA extraction of rock varnish samples, followed by the construction of clone libraries from community DNA, demonstrated the apparent predominance of cyanobacteria in rock varnish communities. The clone library sequences also revealed the presence of actinobacteria, chloroflexi, Alphaproteobacteria, and environmental isolates whose closest relatives were found in Hawaiian volcanic soils, thermal soils, and uranium wastes. Cultured isolates from both environments produce amorphous oxides followed by an array of minerals that undergo increasing crystallization over time (months to years) with live cultures but which cease when cultures are killed.
http://www.i-pi.com/~diana/slime