Kinetics of Mn(II) oxidation by Leptothrix discophora SS1

The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O₂, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O₂ and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria.     

Probing mid-mantle heterogeneity using PKP coda waves

We investigate the utility of PKP coda waves for studying weak scattering from small-scale heterogeneity in the mid-mantle. Coda waves are potentially a useful probe of heterogeneity in the mid-mantle because they are not preferentially scattered near the CMB, as PKP precursors are, but are sensitive to scattering at all depths. PKP coda waves have not been used for this purpose historically because of interference with other late-arriving energy due to near-surface resonance and scattering. Any study of deep mantle scattering using coda waves requires the removal of near-surface effects from the data. We have analyzed 3624 recordings of PKP precursors and coda made by stations in the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN). To study the range and time dependence of the scattered waves, we binned and stacked envelopes of the recordings. We have considered precursors that arrive within a 20 s window before PKP and coda waves in a 60 s window after PKP. The PKP scattered waves increase in amplitude rapidly with range as predicted by scattering theory. At ranges below 125°, we predict and observe essentially no scattered energy preceding PKP. Coda amplitudes at these ranges are independent of range and provide an estimate of energy due to near-surface effects that we can expect at all ranges. We use the average coda amplitude at ranges from 120 to 125° to correct coda amplitudes at other ranges. PKP coda waves show a strong dependence on time and range and are clearly influenced by scattering in the lower mantle. PKP coda waves, however, do not provide a tighter constraint on the vertical distribution of mantle heterogeneity than is provided by precursors. This is due, in part, to relatively large scatter in coda amplitudes as revealed by a resampling analysis. Modeling using Rayleigh–Born scattering theory and an exponential autocorrelation function shows that PKP coda amplitudes are not highly sensitive to the vertical distribution of heterogeneity in the mantle. To illustrate this we consider single-scattering in two extreme models of mantle heterogeneity. One allows heterogeneity just at the CMB; the other includes heterogeneity throughout the mantle. The amplitudes of precursors are tightly constrained by our stack and support our earlier conclusion that small-scale heterogeneity is uniformly distributed throughout the lower mantle. The best-fit model includes 8 km scale length heterogeneity with an rms velocity contrast throughout the mantle of 1%.     

Development and Governmentality

The subjectivity of individuals, the so‐called speakers and hearers of political discourse, who actually, or even ideally, populate a state, needs to be understood in terms of enunciative modalities ‐ the statuses, sites, and positions ‐ of their existence as political subjects. Enunciative modalities refer to the ways a discursive practice is attached to bodies in space (Clifford, 2001:56).Governmental thought territorializes itself in different ways… We can analyze the ways in which the idea of a territorially bounded, politically governed nation state under sovereign authority took shape… One can trace anomalous governmental histories of smaller‐scale territories… and one can also think of these [as] spaces of enclosure that governmental thought has imagined and penetrated… how [does it] happen that social thought territorializes itself on the problem of [for example] the slum in the nineteenth century (Rose, 1999:34–36)?     

Measurement and interpretation of molecular-level forces of interaction between the siderophore azotobactin and mineral surfaces

The forces of interaction were measured between the siderophore azotobactin and the minerals goethite (α-FeOOH) and diaspore (α-AlOOH) in aqueous solution using force microscopy. Azotobactin, a pyoverdin-type siderophore, was covalently linked to a hydrazide terminated atomic force microscope tip using a standard active ester protein coupling technique. Upon contact with each mineral surface, the adhesion force between azotobactin and goethite was two to three times the value observed for the isostructural Al-equivalent diaspore. The affinity for the solid iron oxide surface reflected in the force measurements correlates with the specificity of azotobactin for aqueous ferric iron. Further, the adhesion force between azotobactin and goethite significantly decreases (4 nN to 2 nN) when small amounts of soluble iron (0.1 μM FeCl₃ · 6H₂O) are added to the system at pH 3.5 suggesting a significant specific interaction between the chelating reactive center of azotobactin and the mineral surface. Changes in the force signature with pH and ionic strength were fairly predictable when considering mineral solubility, the charge character of the mineral surfaces, the molecular structure of azotobactin, and the intervening solution. For example, azotobactin-goethite adhesion values were consistently smaller at pH 3.5 relative to the forces at pH 7. At the lower pH, the large number of protons and the increase in the mineral solubility provides additional electron acceptors (e.g., H⁺ and Fe³⁺(aq)) that are free to compete for the basic oxygen chelating sites in the azotobactin structure. It is believed that this competition disrupts siderophore affinity for the surface resulting in decreased adhesion values.     

The effects of pH, ionic strength, and iron-fulvic acid interactions on the kinetics of non-photochemical iron transformations. II. The kinetics of thermal reduction

A combination of flow-injection analysis and kinetic analysis was used to examine the speciation of iron(II) and iron(III) in fulvic acid solutions as a function of pH, ionic strength, and time. This methodology was used to follow a shift in iron speciation from faster to slower reacting species over a timescale of several days. This speciation data shows that both iron(II) and iron(III)-fulvic acid complexes are important iron species in humic-containing natural waters and that their amounts and their rates of transformation to colloidal iron are controlled primarily by the kinetics of thermal (dark) reduction and iron(II) oxidation. The kinetic analysis methodology also yielded the rate constants for the thermal reduction of iron by the fulvic acid. These rate constants decrease with increasing pH and are independent of ionic strength. While thermal reduction was found to be too slow to produce large amounts of steady state iron(II) at circumneutral pH, it does provide a mechanism for iron redox cycling in the absence of photochemical or biochemical processes.