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Susan Glasauer Peter G Weidler Terry J Beveridge 《Geochimica et cosmochimica acta》2003,67(7):1277-1288
The reductive dissolution of FeIII (hydr)oxides by dissimilatory iron-reducing bacteria (DIRB) could have a large impact on sediment genesis and Fe transport. If DIRB are able to reduce FeIII in minerals of high structural order to carry out anaerobic respiration, their range could encompass virtually every O2-free environment containing FeIII and adequate conditions for cell growth. Previous studies have established that Shewanella putrefaciens CN32, a known DIRB, will reduce crystalline Fe oxides when initially grown at high densities in a nutrient-rich broth, conditions that poorly model the environments where CN32 is found. By contrast, we grew CN32 by batch culture solely in a minimal growth medium. The stringent conditions imposed by the growth method better represent the conditions that cells are likely to encounter in their natural habitat. Furthermore, the expression of reductases necessary to carry out dissimilatory Fe reduction depends on the method of growth. It was found that under anaerobic conditions CN32 reduced hydrous ferric oxide (HFO), a poorly crystalline FeIII mineral, and did not reduce suspensions containing 4 mM FeIII in the form of poorly ordered nanometer-sized goethite (α-FeOOH), well-ordered micron-sized goethite, or nanometer-sized hematite (α-Fe2O3) crystallites. Transmission electron microscopy (TEM) showed that all minerals but the micron-sized goethite attached extensively to the bacteria and appeared to penetrate the outer cellular membrane. In the treatment with HFO, new FeII and FeIII minerals formed during reduction of HFO-Fe in culture medium containing 4.0 mmol/L Pi (soluble inorganic P), as observed by TEM with energy-dispersive X-ray spectroscopy, selected area electron diffraction, and X-ray diffraction. The minerals included magnetite (Fe3O4), goethite, green rust, and vivianite [Fe3(PO4)2 · 8H2O]. Vivianite appeared to be the stable end product and the mean coherence length was influenced by the rate of FeIII reduction. When Pi was 0.4 mol/L under otherwise identical conditions, goethite was the only mineral observed to form, and less Fe2+ was produced overall. Hence, the ability of DIRB to reduce Fe (hydr)oxides may be limited when the bacteria are grown under nutrient-limited conditions, and the minerals that result depend on the vigor of FeIII reduction. 相似文献
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C. Beveridge 《Solar physics》2006,236(1):41-57
Magnetic topology is a powerful tool for constraining certain physical properties of a given magnetic configuration, including
the strengths and locations of current sheets, relative helicity and the magnetic free energy available for reconnection.
A critical feature of magnetic topology is the separator, a field line bordering several different regions of connectivity.
With existing methods, these field lines are at best computationally expensive and at worst impossible to find. A new method
is presented for finding the Minimal Separator Set, all of the separators that necessarily exist in a configuration, and to
use this information in combination with the optical analogy and a simulated annealing method to ‘cool’ an initial guess for
each separator into a good approximation. 相似文献
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A gap between the potential and practical realisation of adaptation exists: adaptation strategies need to be both climate-informed and locally relevant to be viable. Place-based approaches study local and contemporary dynamics of the agricultural system, whereas climate impact modelling simulates climate-crop interactions across temporal and spatial scales. Crop-climate modelling and place-based research on adaptation were strategically reviewed and analysed to identify areas of commonality, differences, and potential learning opportunities to enhance the relevance of both disciplines through interdisciplinary approaches. Crop-modelling studies have projected a 7–15% mean yield change with adaptation compared to a non-adaptation baseline (Nature Climate Change 4:1–5, 2014). Of the 17 types of adaptation strategy identified in this study as place-based adaptations occurring within Central America, only five were represented in crop-climate modelling literature, and these were as follows: fertiliser, irrigation, change in planting date, change in cultivar and area cultivated. The breath and agency of real-life adaptation compared to its representation in modelling studies is a source of error in climate impact simulations. Conversely, adaptation research that omits assessment of future climate variability and impact does not enable to provide sustainable adaptation strategies to local communities so risk maladaptation. Integrated and participatory methods can identify and reduce these sources of uncertainty, for example, stakeholder’s engagement can identify locally relevant adaptation pathways. We propose a research agenda that uses methodological approaches from both the modelling and place-based approaches to work towards climate-informed locally relevant adaptation. 相似文献
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Many dynamic phenomena in the solar corona are driven by the complex and ever-changing magnetic field. It is helpful, in trying to model these phenomena, to understand the structure of the magnetic field, i.e. the magnetic topology. We study here the topological structure of the coronal magnetic field arising from four discrete photospheric flux patches, for which we find that seven distinct, topologically stable states are possible; the changes between these are caused by six types of bifurcation. Two bifurcation diagrams are produced, showing how the changes occur as the relative positions and strengths of the flux patches are varied. A method for extending the analysis to higher numbers of sources is discussed. 相似文献
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Whiting events: biogenic origin due to the photosynthetic activity of cyanobacterial picoplankton 总被引：3，自引：0，他引：3
Thompson JB Schultze-Lam S Beveridge TJ Des Marais DJ 《Limnology and oceanography》1997,42(1):133-141
An annual whiting event occurs each year in late May to early June in Fayetteville Green Lake, New York. The initiation of this event correlates with exponential growth of the Synechococcus population within the lake. Synechococcus is the dominant (by approximately 4 orders of magnitude) autotrophic organism owing to the oligotrophic condition of the lake. The delta 13C values of the dissolved inorganic C range seasonally from -9.5% in winter to -6.2% in summer due to photosynthetic activity. Calcite precipitates principally in the microenvironment surrounding Synechococcus because of a photosynthetically driven alkalization process and the availability of the cells as nucleation sites. This calcite has a heavier delta 13C value (>4%) than does the dissolved inorganic C of the lake water owing to the cells' preferential uptake of 12C. A conceptual model suggests that photosynthetic activity and cell surface chemistry, together with the substantial surface area that arises from the great abundance of micron-sized cells, allow Synechococcus to dominate the annual whiting events in Fayetteville Green Lake. 相似文献
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The Sun’s magnetic field is the primary factor determining the structure and evolution of the solar corona. Here, magnetic
topology is used in combination with a Green’s function method to model the global coronal magnetic field with a spherical
photosphere. We focus on the case of three negative flux sources and one positive source, completing our previous categorisation
of the topological states and bifurcations that are present in quadrupolar configurations in a spherical geometry. Three fundamental
varieties of topological state are found, with three types of bifurcation taking one to the other. A comparison to the equivalent
results for a planar photosphere is then carried out, and the differences between the two cases are explained. 相似文献
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The photosphere possesses many small, intense patches of magnetic flux. Each of these patches (or sources) is connected magnetically through the corona to several sources of opposite polarity. An elemental flux loop consists of all of the flux joining one such source to another. We find that each source is connected to twenty other sources,
on average, and that the typical flux and diameter of elemental loops in the corona are 1016 Mx and 200 km; there are approximately 17 separators for each source. We also model a typical large-scale coronal loop consisting
of many elemental loops and determine its complex internal topology. Each upright null lies at the end of about 22 separatrices,
which tend to be clustered together in trunk-like structures, analogous to river-valleys in a geographical contour map. Prone
nulls correspond to saddle points, while their spines are analogous to watersheds. 相似文献