A VLA Study of Newly-Discovered Southern Latitude Non-Thermal Filaments in the Galactic Center: Radio Continuum Total-intensity and Spectral Index Properties

The non-thermal filament (NTF) radio structures clustered within a few hundred parsecs of the Galactic Center (GC) are apparently unique to this region of the Galaxy. Recent radio images of the GC using MeerKAT at 1 GHz have revealed a multitude of faint, previously unknown NTF bundles (NTFBs), some of which are comprised of as many as 10 or more individual filaments. In this work we present Very Large Array (VLA) observations at C- and X-bands (4 - 12 GHz) at arcsecond-scale resolutions of three of these newly-discovered NTFBs, all located at southern Galactic latitudes. These observations allow us to compare their total-intensity properties with those of the larger NTF population. We find that these targets generally possess properties similar to what is observed in the larger NTF population. However, the larger NTF population generally has steeper spectral index values than what we observe for our chosen targets. The results presented here based on the total-intensity properties of these structures indicate that the NTFs are likely all formed from Cosmic Rays (CRs). These CRs are either generated by a nearby compact source and then diffuse along the NTF lengths or are generated by extended, magnetized structures whose magnetic field undergoes reconnection with the NTF magnetic field.Comment: 18 pages, 13 figures, 2 tables. Submitted to ApJ for peer-revie

Selective Permeability of Carboxysome Shell Pores to Anionic Molecules

Carboxysomes are closed polyhedral cellular microcompartments that increase the efficiency of carbon fixation in autotrophic bacteria. Carboxysome shells consist of small proteins that form hexameric units with semipermeable central pores containing binding sites for anions. This feature is thought to selectively allow access to RuBisCO enzymes inside the carboxysome by HCO_3– (the dominant form of CO_2 in the aqueous solution at pH 7.4) but not O_2, which leads to a nonproductive reaction. To test this hypothesis, here we use molecular dynamics simulations to characterize the energetics and permeability of CO_2, O_2, and HCO_3– through the central pores of two different shell proteins, namely, CsoS1A of α-carboxysome and CcmK4 of β-carboxysome shells. We find that the central pores are in fact selectively permeable to anions such as HCO_3–, as predicted by the model

A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties

The Radio Arc is one of the brightest systems of non-thermal filaments (NTFs) in the Galactic Center, located near several prominent HII regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S-, C-, and X-bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow sub-filamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from -1000 to -5800 rad m$\rm^{-2}$, likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large scale helical segments near the Radio Arc could be components of such a source causing these changes in intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.Comment: PDF should be 24 pages with 13 figure

Exploitation status of infralittoral abalone (Haliotis midae) and alikreukel (Turbo sarmaticus) in the southern section of the Eastern Cape coast, South Africa

Intertidal size-frequency distributions and densities for Haliotis midae and Turbo sarmaticus were examined at 10 sites experiencing varying pressure of human exploitation along the southern section of the Eastern Cape coast, South Africa. Target species' densities and maximum sizes were related both to the numbers of collectors on the shore and to indirect indicators of exploitation such as number of households in the vicinity and distance to the nearest beach access point. For both species, there was variation in density (P < 0.05) and size (P < 0.05) among sites, with densities ranging between 0.03-2.23 m^(-2) and 0.07-4.93 m^(-2) for H. midae and T. sarmaticus, respectively. Maximum sizes ranged between 49.4-153.5 mm (H. midae) and 28.3-104.4 mm (T. sarmaticus) shell length. Population parameters such as mean maximum size and total density were significantly negatively related to exploitation indicators for both species. In addition, densities of sexually mature and legal-size individuals of T. sarmaticus were significantly negatively related to the number of households. However, only for H. midae were densities of subadults significantly negatively related to the number of collectors, suggesting that reproduction of abalone may be suppressed at the most exploited sites. Exploitation of T. sarmaticus tends to be localized near population centres, whereas H. midae is collected over a larger range of sites. Overall, T. sarmaticus is less affected by exploitation than H. midae

Organization, Structure, and Assembly of alpha-Carboxysomes Determined by Electron Cryotomography of Intact Cells

Carboxysomes are polyhedral inclusion bodies that play a key role in autotrophic metabolism in many bacteria. Using electron cryotomography, we examined carboxysomes in their native states within intact cells of three chemolithoautotrophic bacteria. We found that carboxysomes generally cluster into distinct groups within the cytoplasm, often in the immediate vicinity of polyphosphate granules, and a regular lattice of density frequently connects granules to nearby carboxysomes. Small granular bodies were also seen within carboxysomes. These observations suggest a functional relationship between carboxysomes and polyphosphate granules. Carboxysomes exhibited greater size, shape, and compositional variability in cells than in purified preparations. Finally, we observed carboxysomes in various stages of assembly, as well as filamentous structures that we attribute to misassembled. shell protein. Surprisingly, no more than one partial carboxysome was ever observed per cell. Based on these observations, we propose a model for carboxysome assembly in which the shell and the internal RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) lattice form simultaneously, likely guided by specific interactions between shell proteins and RuBisCOs. (C) 2009 Elsevier Ltd. All rights reserved

Stable photoinduced metal-organic nanosheet blue phosphor for white light emission

Organic-based phosphors are promising alternatives to rare-earth metal based white light-emitting diodes (LEDs). Integrating phosphors into organic LED systems, however, is constrained by limited long-term chemical- and photo-stability, particularly for blue emitters, along with fabricability. Coumarins are common blue emitters for red-green-blue (RGB) or blue-yellow white light emission (WLE) systems but are susceptible to photodimerization and decay, both supressing fluorescence. This work employs a guest@host architecture to entrap 7-methoxycoumarin (MC) in metal-organic framework nanosheets (MONs) of the 2D phase of ZIF-7 (Z7-NS). This prevented photodimerization due to the arrangement of dye molecules within the nanosheet spacings. Favourably, upon ultraviolet exposure (365 nm), a new intense broader emission band forms irreversibly, derived from charge-transfer behaviour of the MC chromophores. This resulted in intense blue emission with 4.2 × improved photoluminescence quantum yield (PLQY) (Φ = 26.6 %) compared to molecular MC. Contrastingly, encapsulation of MC in 3D ZIF-7 (MC@Z7) also prevented photodimerization, but only exhibited purple-blue low-intensity, low PLQY MC monomeric fluorescence. The fabricability of MC@Z7-NS was demonstrated by inkjet printing blue emitting thin-films that showed improved homogeneity and quantum yield over MC@Z7. Finally, an RGB triple guest@Z7-NS was synthesised and optimised over 30 samples to produce WLE with ideal CIE coordinates of (0.33, 0.34), high PLQY of 65.08 %, and tuneable cool-warm temperatures. Combined, the work demonstrates the remarkable potential of guest@MON systems for improved WLE OLEDs

Peptidoglycan Remodeling and Conversion of an Inner Membrane into an Outer Membrane during Sporulation

Two hallmarks of the Firmicute phylum, which includes the Bacilli and Clostridia classes, are their ability to form endospores and their “Gram-positive” single-membraned, thick-cell-wall envelope structure. Acetonema longum is part of a lesser-known family (the Veillonellaceae) of Clostridia that form endospores but that are surprisingly “Gram negative,” possessing both an inner and outer membrane and a thin cell wall. Here, we present macromolecular resolution, 3D electron cryotomographic images of vegetative, sporulating, and germinating A. longum cells showing that during the sporulation process, the inner membrane of the mother cell is inverted and transformed to become the outer membrane of the germinating cell. Peptidoglycan persists throughout, leading to a revised, “continuous” model of its role in the process. Coupled with genomic analyses, these results point to sporulation as a mechanism by which the bacterial outer membrane may have arisen and A. longum as a potential “missing link” between single- and double-membraned bacteria

The Structure of Isolated Synechococcus Strain WH8102 Carboxysomes as Revealed by Electron Cryotomography

Carboxysomes are organelle-like polyhedral bodies found in cyanobacteria and many chemoautotrophic bacteria that are thought to facilitate carbon fixation. Carboxysomes are bounded by a proteinaceous outer shell and filled with ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the first enzyme in the CO_2 fixation pathway, but exactly how they enhance carbon fixation is unclear. Here we report the three-dimensional structure of purified carboxysomes from Synechococcus species strain WH8102 as revealed by electron cryotomography. We found that while the sizes of individual carboxysomes in this organism varied from 114 nm to 137 nm, surprisingly, all were approximately icosahedral. There were on average ~250 RuBisCOs per carboxysome, organized into three to four concentric layers. Some models of carboxysome function depend on specific contacts between individual RuBisCOs and the shell, but no evidence of such contacts was found: no systematic patterns of connecting densities or RuBisCO positions against the shell's presumed hexagonal lattice could be discerned, and simulations showed that packing forces alone could account for the layered organization of RuBisCOs

Organization, Structure, and Assembly of α-Carboxysomes Determined by Electron Cryotomography of Intact Cells

Carboxysomes are polyhedral inclusion bodies that play a key role in autotrophic metabolism in many bacteria. Using electron cryotomography, we examined carboxysomes in their native states within intact cells of three chemolithoautotrophic bacteria. We found that carboxysomes generally cluster into distinct groups within the cytoplasm, often in the immediate vicinity of polyphosphate granules, and a regular lattice of density frequently connects granules to nearby carboxysomes. Small granular bodies were also seen within carboxysomes. These observations suggest a functional relationship between carboxysomes and polyphosphate granules. Carboxysomes exhibited greater size, shape, and compositional variability in cells than in purified preparations. Finally, we observed carboxysomes in various stages of assembly, as well as filamentous structures that we attribute to misassembled shell protein. Surprisingly, no more than one partial carboxysome was ever observed per cell. Based on these observations, we propose a model for carboxysome assembly in which the shell and the internal RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) lattice form simultaneously, likely guided by specific interactions between shell proteins and RuBisCOs