This study reports virally mediated mortality rates of Phaeocystis globosa single cells in the southern North Sea during 2 consecutive spring blooms (2003 and 2004). An adapted dilution method was ...used to estimate viral lysis and microzooplankton grazing simultaneously. Parallel dilution experiments were performed with 30 kDa ultrafiltrate (virus and grazer-free diluent) and 0.2 mum filtered seawater (grazer-free, but virus-containing diluent). Specific viral lysis rates were calculated from the difference in P. globosa growth rates between the 2 dilution series after 24 h incubation under natural conditions. The validity of this method was tested using a culture of P. globosa infected with a known P. globosa virus (PgV). The field data show that virally induced mortality can be a substantial loss factor for P. globosa single cells (up to 0.35 d super(-1)), comparable to that due to microzooplankton grazing (up to 0.4 d super(-1)). Viral lysis was the major cause of total P. globosa cell lysis. Assuming no loss due to sinking, viral lysis accounted for 5 to 66% of the total mortality of P. globosa single cells. Viral lysis and total putative PgV abundance increased concomitantly with abundance of P. globosa single cells whilst the increase in infective PgV was delayed. This delay may be caused by the formation of transparent exopolymeric particles that are generated when P. globosa colonies disrupt and are known to passively adsorb viruses. Viruses and microzooplankton were shown to be major controlling agents of P. globosa single cells, although their relative significance varied over the course of the bloom and between years.
In most aquatic bacterial communities, it is possible to discriminate bacterial cells with a high nucleic acid content (HNA) from those with a low nucleic acid content (LNA) by flow cytometry. The ...distribution of leucine incorporation rate per cell (specific activity) within the fraction of bacteria with a high apparent nucleic acid content was investigated in coastal seawater using flow cytometry and cell sorting techniques. Bacterial cells from a natural seawater sample were first labeled with tritiated leucine, stained with a fluorescent nucleic acid stain and then sorted based on their fluorescence and scatter signals, assuming that fluorescence was proportional to the cellular nucleic acid content and scatter signals to biovolume. Our results clearly demonstrated that specific activity was heterogeneously distributed within the group of HNA cells and increased with both scatter and fluorescence signals. This shows that important differences in terms of cell-specific rates of activity exist within the HNA group and that specific activity is positively correlated with both the nucleic acid content of cells and their biovolume. We hypothesized that this heterogeneity may be partly due to the structure of bacterial communities. Therefore, further investigations were made by analyzing the nucleic acid content and scatter properties of a set of 10 species isolated from the same coastal area. Our results confirm that both fluorescence and scatter values vary greatly between species at a given growth stage and between growth stages for a given species. Variations reported at different growth stages suggest that both parameters are sensitive to the metabolic activity of individual cells and confirm the positive relationship reported in this study between scatter and fluorescence parameters and specific activity within the HNA cellular fraction of natural communities. Interestingly, it was shown that HNA and LNA cells were present in late stationary phase cultures. This suggests that LNA cells found in natural communities may be dead or dying cells.
During the last 10 years, the dye 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) has been used to determine the in situ number of “active” bacteria in different ecosystems. A part of this success is ...due to a simple protocol, which does not require sophisticated equipment. However, it has not been established whether the method determines viable cells, e.g. those capable of growth and cell division, as opposed to cells that are active in the sense of having some detectable metabolic activity. In this study, the number of CTC-positive cells through the growth stages of
Escherichia coli was estimated and compared to counts of the total number of bacteria, the culturability (CFU counts) and respiratory activity (CO
2 evolution). There was a good correlation between the number of CTC-positive cells and the CFU count, regardless of the growth phase. However, CTC could still be reduced by a large part of the population during the first hours of stationary phase even if the bacteria were no longer releasing CO
2. Thus, the reduction of CTC is a good estimator for cell viability, rather than cell activity. Additionally, a review of the literature showed that there is presently no standardized protocol for using CTC, which makes difficult at present the comparison of active bacterial numbers in different samples from different sites.
Conventional organic salts represent a new paradigm in many areas of research. Despite their great potential, an improvement in their physicochemical properties requires the chemical modification of ...their intrinsic structure. Thus, an efficient pathway was developed for the preparation of polyfunctional imidazolium monomers incorporating aromatic rings and terminal epoxides which presented a real synthetic challenge. In this work, we describe the reactivity of various oxidizing agents to develop a strong, clean and powerful methodology to generate epoxidized salts. Various reaction conditions for the formation of the epoxides were investigated such as the role of the cation and the counterion as well as the influence of an aromatic and/or aliphatic linker chain. Finally, we have evaluated the thermal properties of these new polyfunctional salts by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
•A fatigue crack growth model is proposed based on plastic CTOD.•The effect of stress state was predicted qualitatively.•The effect of stress ratio was predicted qualitatively for plane stress and ...plane strain states.•The effect of overloads and load blocks was predicted qualitatively.•The relation between crack tip plastic deformation and da/dN varies with the material.
The plastic CTOD range, δp, which quantifies crack tip plastic deformation, is used to study fatigue crack propagation instead of the classical ΔK elastic parameter. A da/dN-δp model was defined for the 304L stainless steel. da/dN was obtained experimentally in standard CT specimens while δp was predicted numerically using the finite element method. The numerical models replicated the experimental tests in terms of geometry, material properties and loading. The da/dN-δp model was used to predict the effects of stress state, load ratio and variable amplitude loading. All the results obtained respect the trends observed in literature, attesting that the da/dN-δp model is able to produce good qualitative predictions. Additionally it was found that crack closure is a main phenomenon affecting δp, and therefore da/dN, but there is also an effect of mean stress.
In this work, new (multi)functional-dedicated polymer materials were designed and processed from the copolymerization between novel imidazolium ionic liquid monomer (ILM) with a conventional ...polyetheramine denoted Jeffamine D230. First, a facile and robust synthetic route was investigated in order to design polyfunctional imidazolium monomers bearing an aromatic ring and two epoxy functions at the end of aliphatic chains. Then, the main mechanisms of epoxy opening leading to polymerization with different kinetics were modeled through the reaction between a monofunctional epoxy and aliphatic mono- and diamines by using “in situ” NMR spectroscopy. Finally, the monomer molecular structure-network architecture-physical properties relationships of the resulting IL-modified epoxy networks were investigated. As a consequence, epoxy networks with a glass transition temperature of 55 °C and with enhanced properties such as thermal stability (>300 °C), storage modulus of 700 MPa at room temperature, and an ionic conductivity (4 × 10–4 S m–1 for 70 °C) combined with an hydrophobic character of their surface (33 mJ m–2) were prepared.
•Predict three dimensional crack front shape under fatigue loading.•Demonstrate the efficiency of stress fields’ method to calculate stress intensity factor.•Demonstrate the influence of plasticity ...induced crack closure on the crack front shape.•Establish the limitations of the use of stress fields’ method beyond small scale yielding.
The objective of this study is to develop a numerical tool using the commercial software, Abaqus and Python, to predict the fatigue crack front shape while taking into account the influence of plasticity induced crack closure on crack propagation in three Dimensional (3D) structures. In this aim, a 3D model of a compact tension specimen made out of stainless steel 304L, and subjected to a constant loading scheme, is proposed. The crack propagation is considered to be driven by the stress fields developed in the vicinity of the crack tip and thus by the stress intensity factor K. Two parallel simulations are used: an elastic simulation intends to calculate the local maximum stress intensity factor while the other, an elasto-plastic one, aims at obtaining the plastic wake and the resulting crack closure load. The results of both simulations are combined in order to constitute the effective stress intensity factor range, which is in turn used, along with Paris law, to calculate the crack propagation along the thickness. The local crack advancements obtained allow to construct the new crack front. Finally, a node release technique is used with geometry remeshing to issue new iterations with new boundary conditions that respond to the changes in the crack front. The procedure is repeated until the stabilization of the effective stress intensity factor values all along the specimen thickness is reached. The results obtained are compared with previously issued experimental results, showing very good results in small scale yielding and beyond that a large dependency on the plastic zone size developed in the neighborhood of the crack front.
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•New access to enantioenriched non-proteogenic phenylalanine derivatives.•Enantioselective decarboxylative protonation reaction of amidohemimalonates esters.•Significant interaction ...of the hemimalonate N-protecting group with the catalyst.•Catalytic efficiency of thioureas, squaramides and bis-cinchona squaramides.
Access to enantioenriched non-proteogenic phenylalanine derivatives is described using the enantioselective decarboxylative protonation reaction of amidohemimalonate esters catalysed by various cinchona-based compounds. This study compares the catalytic efficiency as well as the enantioselectivity induced by three types of common organocatalysts, namely thioureas, squaramides and bis-cinchona squaramides. One of the main outcome of this work is the observation of a significant influence of the N-protecting group of the hemimalonate on its interaction with the catalyst. This methodology carried out under mild conditions exhibits good substrate scope and functional group tolerance. A substoichiometric amount of catalyst can also be used in certain cases while affording good yields and selectivities.
This paper proposes an overview of some critical aspects of fatigue crack propagation in metallic materials mainly concentrated on the near-threshold domain. The role of predominant influencing ...parameters including intrinsic parameters as alloy composition and microstructure, or extrinsic factors as loading condition, crack geometry, crack closure, atmosphere environment is particularly discussed. The first section of the paper is devoted to basic mechanisms of fatigue crack propagation after correction for crack closure, with a modeling framework including the intrinsic propagation stages in high vacuum considered as a reference inert environment, and the environmentally-assisted crack propagation regimes identified in gaseous atmosphere. The second section of the paper is dedicated to application of this basic framework to analyze three critical examples of crack propagation including small cracks, propagation at low temperature and ultra-slow crack propagation at ultrasonic frequency.
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