Taking stock of SLSN and LGRB host galaxy comparison using a complete sample of LGRBs

Long gamma-ray bursts (LGRBs) and superluminous supernovae (SLSNe) are both explosive transients with very massive progenitor stars. Clues about the nature of the progenitors can be found by investigating environments in which such transients occur. While studies of LGRB host galaxies have a long history, dedicated observational campaigns have only recently resulted in a high enough number of photometrically and spectroscopically observed SLSN hosts to allow statistically significant analysis of their properties. In this paper we make a comparison of the host galaxies of hydrogen-poor (H-poor) SLSNe and the Swift/BAT6 sample of LGRBs. In contrast to previous studies we use a complete sample of LGRBs and we address a special attention to the comparison methodology and the selection of SLSN sample whose data have been compiled from the available literature. At intermediate redshifts (0.3 < z < 0.7) the two classes of transients select galaxies whose properties (stellar mass, luminosity, star-formation rate, specific star-formation rate and metallicity) do not differ on average significantly. Moreover, the host galaxies of both classes of objects follow the fundamental metallicity relation and the fundamental plane of metallicity. In contrast to previous studies we show that at intermediate redshifts the emission line equivalent widths of the two populations are essentially the same and that the previous claims regarding the higher fraction of SLSN hosts among the extreme emission line galaxies with respect to LGRBs are mostly due to a larger fraction of strong-line emitters among SLSN hosts at z < 0.3, where samples of LGRB hosts are small and poorly defined.Comment: 7 pages, 4 figures, accepted to Astronomy & Astrophysic

High-cycle fatigue strength of a pultruded composite material

Dealing with composites in polymeric matrix, the pultruded ones are among the more suitable for large production rates and volumes. For this reason, their use is increasing also in structural applications in civil and mechanical engineering. However, their use is still limited by the partial knowledge of their fatigue behaviour; in many applications it is, indeed, required a duration of many millions of cycles, while most of the data that can be found in literature refer to a maximum number of cycles equal to 3 millions. In this paper a pultruded composite used for manufacturing structural beams is considered and its mechanical behaviour characterized by means of static and high-cycle fatigue tests. The results allowed to determine the S-N curve of the material and to assess the existence of a fatigue limit. Observations at the scanning electronic microscope (SEM) allowed to evaluate the damage mechanisms involved in the static and fatigue failure of the material

Sensitivity analysis of cohesive zone model parameters to simulate hydrogen embrittlement effect

For many steels and alloys used in engineering field, the presence of atomic hydrogen in working environment can produce a deleterious effect. In fact, when this small element penetrates into the material lattice induces a drastically decrease of the mechanical properties. This process is known as hydrogen embrittlement. This complex phenomenon involves chemical and physical factors that are strictly dependent on the microstructure of the material. Some examples are hydrogen diffusivity, solubility of hydrogen into the material and concentration related not only to the interstitial lattice sites (NILS) but also to the traps sites that is the most difficult part to quantify. The present work starts from the development of 2D finite elements cohesive zone model reproducing a toughness test of a high-strength low carbon steel, AISI 4130 operating in hydrogen-contaminated environment. With three consequent steps of simulations, the model implements diffusion and crack propagation analyses using cohesive elements. The embrittlement effect of hydrogen is considered by decreasing the cohesive law (TSL), which expresses the constitutive response of the material to the fracture behavior, based on the total hydrogen concentration. It includes NILS and traps sites. Aim of the work is a sensitivity analysis of the parameters included into the model. In particular, the influence of the hydrogen diffusion coefficient as well as the initial concentration set to calculate the total hydrogen concentration at the crack tip are taken into account. Both a comparison of the values used in the model with literature data and a critical discussion of the results obtained by the sensitivity analysis will be presented

Design and characterization of a biomimetic composite inspired to human bone

Many biological materials are generally considered composites, made of relatively weak constituents and with a hierarchical arrangement, resulting in outstanding mechanical properties, difficult to be reached in man-made materials. An example is human bone, whose hierarchical structure strongly affects its mechanical performance, toughness in particular, by activating different toughening mechanisms occurring at different length scales. At microscale, the principal toughening mechanism occurring in bone is crack deflection. Here, we study the structure of bone and we focus on the role of the microstructure on its fracture behaviour, with the goal of mimicking it in a new composite. We select the main structural features, the osteons, which play a crucial role in leading to crack deflection, and we reproduce them in a synthetic composite. The paper describes the design, manufacturing and characterization of a newly designed composite, whose structure is inspired to the Haversian structure of cortical bone, and that of a classic laminate developed for comparative reasons. We conclude with a critical discussion on the results of the mechanical tests carried out on the new composite and on the comparative laminate, highlighting strengths and shortcomings of the new biomimetic material

Mechanics of collagen-hydroxyapatite model nanocomposites

Bone is a hierarchical biological composite made of a mineral component (hydroxyapatite crystals) and anorganic part (collagen molecules). Small-scale deformation phenomena that occur in bone are thought tohave a significant influence on the large scale behavior of this material. However, the nanoscale behaviorof collagen–hydroxyapatite composites is still relatively poorly understood. Here we present a molec-ular dynamics study of a bone model nanocomposite that consist of a simple sandwich structure ofcollagen and hydroxyapatite, exposed to shear-dominated loading. We assess how the geometry of thecomposite enhances the strength, stiffness and capacity to dissipate mechanical energy. We find that H-bonds between collagen and hydroxyapatite play an important role in increasing the resistance againstcatastrophic failure by increasing the fracture energy through a stick-slip mechanism

High-cycle fatigue strength of a pultruded composite material

Dealing with composites in polymeric matrix, the pultruded ones are among the more suitable for large production rates and volumes. For this reason, their use is increasing also in structural applications in civil and mechanical engineering. However, their use is still limited by the partial knowledge of their fatigue behaviour; in many applications it is, indeed, required a duration of many millions of cycles, while most of the data that can be found in literature refer to a maximum number of cycles equal to 3 millions. In this paper a pultruded composite used for manufacturing structural beams is considered and its mechanical behaviour characterized by means of static and high-cycle fatigue tests. The results allowed to determine the S-N curve of the material and to assess the existence of a fatigue limit. Observations at the scanning electronic microscope (SEM) allowed to evaluate the damage mechanisms involved in the static and fatigue failure of the material

BIOREMEDIATION OF A POLYCHLORINATED BIPHENYL (PCB) POLLUTED SITE: DEGRADING POTENTIAL OF SOIL MICROBIOTA AND EXPLOITATION OF PLANT-BACTERIA INTERACTIONS FOR ENHANCED RHIZOREMEDIATION

The release of xenobiotic chemicals into the environment has dramatically increased over the last century following industrialization, with a consequent impact on the ecosystems and human health. Polychlorinated biphenyls (PCB), in particular, are among the twelve chlorinated organic compound families initially listed as persistent organic pollutants (POPs) by the Stockholm Convention on POPs. PCB, due to their chemical properties and high stability, have been widely used by industries in the twentieth century as dielectric and coolant fluids. Despite their production has been banned since the 1970s-1980s, these pollutants contaminate soils and waters and affect the ecosystems worldwide, being widespread global contaminants. Due to their high lipophilicity, PCB are recalcitrant to biodegradation, persist in the environment and bioaccumulate in the lipids of animals and humans, biomagnifying in the food web. It has been proved that PCB have relevant toxic effects on human health, including carcinogenic activity. The remediation of PCB-contaminated soils represents therefore a primary issue for our society; nonetheless, the available physical-chemical technologies have strong environmental and economic impact and are unsuitable for in situ soil remediation in extended contaminated areas. Rhizoremediation is a type of phytoremediation that relies on the capability of soil microbes responding to plant biostimulation, to degrade pollutants. This strategy appears as the most suitable for the detoxification of large-scale PCB-polluted soils. Among soil contaminants, rhizoremediation of PCB is specifically relying on the positive interactions between plants and microorganisms in the rhizosphere. In fact, several organic aromatic compounds released through root deposition can promote the activation of the biphenyl catabolic pathway that is responsible for the microbial oxidative PCB metabolism, thereby improving the overall PCB degradation performance in aerobic conditions in soil. Moreover, plant-growth promoting (PGP) microorganisms selected in the rhizosphere can sustain plant growth under stressed conditions typical of polluted soils, in turn enhancing the plant biostimulation. Nevertheless, the efficiency of this biotechnology in situ has been poorly assessed in the scientific literature, since the upscaling from laboratory to greenhouse conditions to the field was rarely implemented. Site-specific environmental conditions still represent a major challenge for an efficient in situ rhizoremediation intervention, especially when it comes to understand how the pollution fingerprint affects the autochthonous degrading bacterial populations and whether these are able to establish positive interactions with the introduced plant species. This PhD project focused on the Site of National Priority (SIN) Caffaro, a large site located in Northern Italy historically polluted by chlorinated POPs and metals. Aim of the work was to study the phylogenetic and functional diversity of the soil microbiota, assessing the correlation between diversity and pollutant profiles as a proxy to evaluate the biodegradation potential in the SIN Caffaro soil. A further aim of the thesis was to focus on the plant rhizosphere microbiome in order to setup in situ rhizoremediation strategies by evaluating the plant species with the higher potential for biostimulation. The soil microbiome of three former agricultural fields within the SIN Caffaro was investigated with molecular ecology \u201316S rRNA metagenomic sequencing and DNA fingerprinting- and biochemical \u2013fluorescein hydrolyses- approaches. The results revealed that the bacterial communities\u2019 structure, their phylogenetic diversity and the soil microbial activity were related with the soil physical and chemical parameters, both along the soil depth profile and across the surface of the area of collection. These findings suggest the adaptation of the microbial communities to the high xenobiotics concentrations in the soil, possibly resulting in PCB biodegradation abilities. To assess the natural attenuation potential of autochthonous rhizosphere bacteria, we studied bacterial communities along a soil gradient from the non-vegetated to the root-associated soils of three different plant species spontaneously established in the most polluted field within the SIN Caffaro. The overall bacterial community structure of the non-vegetated and root-associated soil fractions was described by 16S rRNA metagenomic sequencing, and a collection of rhizobacteria isolates able to use biphenyl as unique carbon source was assayed for plant growth promotion (PGP) traits and bioremediation potential. The three plant species differentially affected the structure of the bacterial communities in the root-associated soil fractions, establishing the well known so-called rhizosphere effect. Nonetheless, the similar phylogenetic composition of the communities in all the soil fractions and the ubiquitous presence of the degrading potential, assessed by the presence of the bphA gene in the soil metagenome, leads to speculate that the soil contamination was one of the drivers for the enrichment of populations potentially able to sustain the process of natural attenuation. In vitro screening showed that biodegradation and PGP potential were widespread in the rhizosphere cultivable microbiome and the results of in vivo test on model plants suggested that two Arthrobacter sp. strains could be further investigated as bioenhancers on plant species of interest for rhizoremediation. To assess the rhizoremediation potential of different plant species and soil treatments, a microcosm-scale experiment was set up with the SIN Caffaro soil in greenhouse conditions, and the biostimulation effect was studied on the soil microbiota at different sampling times for 24 months. Plant species and treatments were identified basing upon an extensive literature screening, aimed to select the species/treatments that in previous studies showed to be effective in PCB rhizoremediation. The results of bacterial DNA fingerprinting and biochemical analysis of the soil surrounding the plant roots revealed that all the plants, when compared with unplanted control microcosms, significantly changed the bacterial communities\u2019 structure and stimulated the overall degrading activity in the soil. The stimulation of the soil microbiota leading to i) a shift in phylogenetic composition and ii) an increase in the organic matter hydrolytic activity, which may contribute to enhance PCB bioavailability and in turn their degradation in the polluted soil, is an indication of a potential positive rhizoremediation effect. The results need nevertheless to be substantiated by chemical analysis which will confirm the effective decrease of pollutants in soil surrounding roots and/or a change in the pollutants fingerprint. From the biostimulated soil a collection of Actinobacteria strains displaying in vitro biodegradation and PGP-related traits was also obtained. Three strains belonging to the genus Rhodococcus were in particular characterized for their PCB degradation capacity and for the ability to promote Arabidopsis thaliana growth and root development under laboratory conditions. Since A. thaliana root exudates previously showed to promote PCB degradation by a Rhodoccoccus bacterial isolate, these results open future research perspectives on the investigation of plant-bacteria interaction for PCB rhizoremediation. Overall, the results disclosed the existence of a PCB natural attenuation potential within the autochthonous microbial communities of the SIN Caffaro soil, and pointed out that rhizoremediation could be an effective strategy to enhance soil detoxification. Further research should be focused to better characterise the degrading microbiome inhabiting the soil and to identify the best plant-treatment combination with the support of chemical investigations assessing the rate of PCB removal from soil. Also, in vivo test with PCB-degrading and PGP bacterial strains are required to assess their potential as bioaugmentation tools to sustain a rhizoremediation intervention

Mutant cycles at CFTR's non-canonical ATP-binding site support little interface separation during gating

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel belonging to the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily. ABC proteins share a common molecular mechanism that couples ATP binding and hydrolysis at two nucleotide-binding domains (NBDs) to diverse functions. This involves formation of NBD dimers, with ATP bound at two composite interfacial sites. In CFTR, intramolecular NBD dimerization is coupled to channel opening. Channel closing is triggered by hydrolysis of the ATP molecule bound at composite site 2. Site 1, which is non-canonical, binds nucleotide tightly but is not hydrolytic. Recently, based on kinetic arguments, it was suggested that this site remains closed for several gating cycles. To investigate movements at site 1 by an independent technique, we studied changes in thermodynamic coupling between pairs of residues on opposite sides of this site. The chosen targets are likely to interact based on both phylogenetic analysis and closeness on structural models. First, we mutated T460 in NBD1 and L1353 in NBD2 (the corresponding site-2 residues become energetically coupled as channels open). Mutation T460S accelerated closure in hydrolytic conditions and in the nonhydrolytic K1250R background; mutation L1353M did not affect these rates. Analysis of the double mutant showed additive effects of mutations, suggesting that energetic coupling between the two residues remains unchanged during the gating cycle. We next investigated pairs 460-1348 and 460-1375. Although both mutations H1348A and H1375A produced dramatic changes in hydrolytic and nonhydrolytic channel closing rates, in the corresponding double mutants these changes proved mostly additive with those caused by mutation T460S, suggesting little change in energetic coupling between either positions 460-1348 or positions 460-1375 during gating. These results provide independent support for a gating model in which ATP-bound composite site 1 remains closed throughout the gating cycle