Drop In Growth Rate Research Articles

Adaptive evolution of carbapenem-resistant hypervirulent Klebsiella pneumoniae in the urinary tract of a single patient

The emergence of carbapenem-resistant hypervirulent Klebsiella pneumoniae (CR-hvKp) is a growing concern due to its high mortality and limited treatment options. Although hypermucoviscosity is crucial for CR-hvKp infection, the role of changes in bacterial mucoviscosity in the host colonization and persistence of CR-hvKp is not clearly defined. Herein, we observed a phenotypic switch of CR-hvKp from a hypermucoviscous to a hypomucoviscous state in a patient with scrotal abscess and urinary tract infection (UTI). This switch was attributed to decreased expression of rmpADC, the regulator of mucoid phenotype, caused by deletion of the upstream insertion sequence ISKpn26. Postswitching, the hypomucoid variant showed a 9.0-fold decrease in mice sepsis mortality, a >170.0-fold reduction in the ability to evade macrophage phagocytosis in vitro, and an 11.2- to 40.9-fold drop in growth rate in normal mouse serum. Conversely, it exhibited an increased residence time in the mouse urinary tract (21 vs. 6 d), as well as a 216.4-fold boost in adhesion to bladder epithelial cells and a 48.7% enhancement in biofilm production. Notably, the CR-hvKp mucoid switch was reproduced in an antibiotic-free mouse UTI model. The in vivo generation of hypomucoid variants was primarily associated with defective or low expression of rmpADC or capsule synthesis gene wcaJ, mediated by ISKpn26 insertion/deletion or base-pair insertion. The spontaneous hypomucoid variants also outcompeted hypermucoid bacteria in the mouse urinary tract. Collectively, the ISKpn26-associated mucoid switch in CR-hvKp signifies the antibiotic-independent host adaptive evolution, providing insights into the role of mucoid switch in the persistence of CR-hvKp.

The impact of polystyrene nanoplastics (PSNPs) on physiological and biochemical parameters of the microalgae Spirulina platensis

Nanoplastics, as emerging pollutants, have harmful effects on living organisms and the environment, the mechanisms and extent of which remain unclear. Microalgae, as one of the most important biological groups in the food chain and sensitive environmental indicators to various pollutants, are considered a suitable option for investigating the effects of nanoplastics. In this study, the effects of polystyrene nanoplastics on the growth rate, dry weight, chlorophyll a and carotenoid levels, proline, and lipid peroxidation in the Spirulina platensis were examined. Three concentrations of 0.1, 1, and 10 mg L-1 of PSNPs were used alongside a control sample with zero concentration, with four repetitions in one-liter containers for 20 days under optimal temperature and light conditions. Various analyses, including growth rate, dry weight, proline, chlorophyll a and carotenoid levels, and lipid peroxidation, were performed. The results indicated that exposure to PSNP stress led to a significant decrease in growth rate, dry weight, and chlorophyll a and carotenoid levels compared to the control sample. Furthermore, this stress increased the levels of proline and lipid peroxidation in Spirulina platensis. Morphological analysis via microscopy supported these findings, indicating considerable environmental risks associated with PSNPs.

Culturing the sea cucumber Holothuria poli in open-water integrated multi-trophic aquaculture at a coastal Mediterranean fish farm

The survival and growth of the sea cucumber Holothuria poli were assessed during a 12-month field study when cultured at a commercial fish farm in Malta as part of an integrated multi-trophic aquaculture (IMTA) system. Sea cucumbers were cultured directly below a fish cage at 0 m, E0, at 10 m (E10) and 25 m (E25) distances from the cage and at two reference sites (R1 and R2) located over 800 m away from the fish farm. Mass mortalities were recorded at E0 within the first month of the study due to smothering by settled wastes. All individuals died at one of the reference sites, R1, by the end of the study. After deducting missing sea cucumbers, survival rates at E10 (23%) and E25 (33%) from the fish cage were similar to the remaining reference site (R2) (27%). Stocking density and physical disturbances to the sea cucumber cage setup were also probable cause for the low survival rates. The relative weight gain (RWG) and specific growth rates (SGR) of H. poli varied significantly between sites close to the fish farm and the reference site. The SGR of H. poli at E10 (0.18 ± 0.02% day−1) and E25 (0.20 ± 0.01% day−1) was positive over the whole study period while no average growth was recorded at the reference site (−0.04 ± 0.07% day−1) over the same period. Differences in RWG and SGR were recorded throughout the study. The overall growth observed in H. poli by January was followed by a drop in growth rate across all sites and an increase in SGR at E25 in July. Slower growth rates were observed as water temperature approached 15 °C. The results indicated that the sediments near the commercial fish cage provided an enriched source of food that supported significantly better growth in H. poli. This suggests that H. poli in IMTA might have the potential to uptake organic farm waste and increase aquaculture production, albeit with important considerations for setup design and stocking density.

Electron acceleration at oblique angles via stimulated Raman scattering at laser irradiance >10^{16}Wcm^{-2}μm^{2}.

The generation of hot, directional electrons via laser-driven stimulated Raman scattering (SRS) is a topic of great importance in inertial confinement fusion (ICF) schemes. Little recent research has been dedicated to this process at high laser intensity, in which back, side, and forward scatter simultaneously occur in high energy density plasmas, of relevance to, for example, shock ignition ICF. We present an experimental and particle-in-cell (PIC) investigation of hot electron production from SRS in the forward and near-forward directions from a single speckle laser of wavelength λ_{0}=1.053μm, peak laser intensities in the range I_{0}=0.2-1.0×10^{17}Wcm^{-2} and target electron densities between n_{e}=0.3-1.6%n_{c}, where n_{c} is the plasma critical density. As the intensity and density are increased, the hot electron spectrum changes from a sharp cutoff to an extended spectrum with a slope temperature T=34±1keV and maximum measured energy of 350 keV experimentally. Multidimensional PIC simulations indicate that the high energy electrons are primarily generated from SRS-driven electron plasma wave phase fronts with k vectors angled ∼50^{∘} with respect to the laser axis. These results are consistent with analytical arguments that the spatial gain is maximized at an angle which balances the tendency for the growth rate to be larger for larger scattered light wave angles until the kinetic damping of the plasma wave becomes important. The efficiency of generated high energy electrons drops significantly with a reduction in either laser intensity or target electron density, which is a result of the rapid drop in growth rate of Raman scattering at angles in the forward direction.

Manganese Depletion Leads to Multisystem Changes in the Transcriptome of the Opportunistic Pathogen Streptococcus sanguinis.

Streptococcus sanguinis is a primary colonizer of teeth and is typically considered beneficial due to its antagonistic relationship with the cariogenic pathogen Streptococcus mutans. However, S. sanguinis can also act as an opportunistic pathogen should it enter the bloodstream and colonize a damaged heart valve, leading to infective endocarditis. Studies have implicated manganese acquisition as an important virulence determinant in streptococcal endocarditis. A knockout mutant lacking the primary manganese import system in S. sanguinis, SsaACB, is severely attenuated for virulence in an in vivo rabbit model. Manganese is a known cofactor for several important enzymes in S. sanguinis, including superoxide dismutase, SodA, and the aerobic ribonucleotide reductase, NrdEF. To determine the effect of manganese depletion on S. sanguinis, we performed transcriptomic analysis on a ΔssaACB mutant grown in aerobic fermentor conditions after the addition of the metal chelator EDTA. Despite the broad specificity of EDTA, analysis of cellular metal content revealed a decrease in manganese, but not in other metals, that coincided with a drop in growth rate. Subsequent supplementation with manganese, but not iron, zinc, or magnesium, restored growth in the fermentor post-EDTA. Reduced activity of Mn-dependent SodA and NrdEF likely contributed to the decreased growth rate post-EDTA, but did not appear entirely responsible. With the exception of the Dps-like peroxide resistance gene, dpr, manganese depletion did not induce stress response systems. By comparing the transcriptome of ΔssaACB cells pre- and post-EDTA, we determined that manganese deprivation led to altered expression of diverse systems. Manganese depletion also led to an apparent induction of carbon catabolite repression in a glucose-independent manner. The combined results suggest that manganese limitation produces effects in S. sanguinis that are diverse and complex, with no single protein or system appearing entirely responsible for the observed growth rate decrease. This study provides further evidence for the importance of this trace element in streptococcal biology. Future studies will focus on determining mechanisms for regulation, as the multitude of changes observed in this study indicate that multiple regulators may respond to manganese levels.

Vegetative growth of 28 genotypes of Coffea canephora at 850 meters of altitude

Coffea canephora plants grown at low or elevated temperatures undergo changes in photosynthetic processes, which have a direct impact on vegetative growth. This species shows plants with distinct stress tolerance mechanisms and a great genetic diversity; however, there are no studies concerning the study of altitude on plant growth for these genotypes. The objective of this study was to evaluate the growth rates of orthotropic and plagiotropic branches from 28 Coffea canephora genotypes, ca. 3 years of age, cultivated at 850 m of altitude. The experimental design was a randomized complete block design with four repetitions. Each repetition was compounded by five plants. The period of the experimental evaluation was from 01/23/2013 to 07/07/2013. We selected and identified the orthotropic and plagiotropic branches, which were evaluated bimonthly according to their length (mm), assessing the growth rates. The results were associated with climatic factors that occurred during the experimental period. The genetic material of Coffea canephora evaluated in the study shows genetic diversity to the characteristics growth rate of orthotropic and plagiotropic branches, thus, it is possible to select genotypes with potential for cultivation at 850 m of altitude. In general, genotypes of Coffea canephora exhibited variations in growth rates, mainly in response to changes in temperature, with a sharp and constant drop in growth rates between the period from 05/05/2013 to 05/ 23/2013, when the recorded temperatures were always below 13 °C. Genotypes A1, NV2 and NV1 can be pointed as the ones which showed the best vegetative growth results at 850 meters of altitude.

Estimating actual COVID-19 case numbers using cumulative death count-A method of measuring effectiveness of lockdown of non-essential activities: a South African case study.

IntroductionEstimating the number of SARS-CoV-2 infected individuals at any specific time point is always a challenge due to asymptomatic cases, the incubation period and testing delays. Here we use an empirical analysis of cumulative death count, transmission-to-death time lag, and infection fatality rate (IFR) to evaluate and estimate the actual cases at a specific time point as a strategy of tracking the spread of COVID-19.MethodsThis method mainly uses death count, as COVID-19 related deaths are arguably more reliably reported than infection case numbers. Using an IFR estimate of 0.66%, we back-calculate the number of cases that would result in the cumulative number of deaths at a given time point in South Africa between 27 February and 14 April. We added the mean incubation period (6.4 days) and the onset-to-death time lag (17.8 days) to identify the estimated time lag between transmission and death (25 days, rounded up). We use the statistical programming language R to analyze the data and produce plots.ResultsWe estimate 28,182 cases as of 14 April, compared with 3,465 reported cases. Weekly growth rate of actual cases dropped immediately after lockdown implementation and has remained steady, measuring at 51.2% as of 14 April. The timing of drop in growth rate suggests that South Africa’s infection prevention strategy may have been effective at reducing viral transmission.ConclusionEstimating the actual number of cases at a specific time point can support evidence-based policies to reduce and prevent the spread of COVID-19. Non-reported, asymptomatic, hard to reach and, mild cases are possible sources of outbreaks that could emerge after lockdown. Therefore, close monitoring, optimized screening strategy and prompt response to COVID-19 could help in stopping the spread of the virus.

Growth Defects and Loss-of-Function in Synthetic Gene Circuits.

Synthetic gene circuits perturb the physiology of their cellular host. The extra load on endogenous processes shifts the equilibrium of resource allocation in the host, leading to slow growth and reduced biosynthesis. Here we built integrated host-circuit models to quantify growth defects caused by synthetic gene circuits. Simulations reveal a complex relation between circuit output and cellular capacity for gene expression. For weak induction of heterologous genes, protein output can be increased at the expense of growth defects. Yet for stronger induction, cellular capacity reaches a tipping point, beyond which both gene expression and growth rate drop sharply. Extensive simulations across various growth conditions and large regions of the design space suggest that the critical capacity is a result of ribosomal scarcity. We studied the impact of growth defects on various gene circuits and transcriptional logic gates, which highlights the extent to which cellular burden can limit, shape, and even break down circuit function. Our approach offers a comprehensive framework to assess the impact of host-circuit interactions in silico, with wide-ranging implications for the design and optimization of bacterial gene circuits.

Growth Rate Modulation Enables Coexistence in a Competitive Exclusion Scenario Between Microbial Eukaryotes

Coexistence usually are exceeding the explicable rate by competitive exclusion principle. Since the pioneer Gause, many studies have used protist microcosm systems to study competitive exclusion. We explored a two-species system with the testate-amoebae: (Arcella intermedia and Pyxidicula operculata), where competitive exclusion is expected to occur. We determined their growth curves individually and under competitive interaction. We used a state-space model to represent system dynamics and calculated posterior population sizes simulating competition dynamics. Contrarily to our expectation, Arcella and Pyxidicula showed similar growth rates (1.37 and 1.46 days–1 respectively) and only different carrying capacity (1,997 and 25,108 cells cm–2 respectively). The maximum number of cells of both species when growing in competition was much lower if compared to the monospecific cultures (in average, 73% and 80% less for Arcella and Pyxidicula respectively). However, our competition experiments always resulted in coexistence. According to the models, the drop in growth rates and stochasticity mainly explains our coexistence results. We propose that a context of ephemeral resources can explain these results. Additionally, we propose generating factors of stochasticity as intraspecific variation, small population effects, toxicity of waste products and influence of the bacterial community.

Multiple Optimal Phenotypes Overcome Redox and Glycolytic Intermediate Metabolite Imbalances in Escherichia coli pgi Knockout Evolutions.

A mechanistic understanding of how new phenotypes develop to overcome the loss of a gene product provides valuable insight on both the metabolic and regulatory functions of the lost gene. The pgi gene, whose product catalyzes the second step in glycolysis, was deleted in a growth-optimized Escherichia coli K-12 MG1655 strain. The initial knockout (KO) strain exhibited an 80% drop in growth rate that was largely recovered in eight replicate, but phenotypically distinct, cultures after undergoing adaptive laboratory evolution (ALE). Multi-omic data sets showed that the loss of pgi substantially shifted pathway usage, leading to a redox and sugar phosphate stress response. These stress responses were overcome by unique combinations of innovative mutations selected for by ALE. Thus, the coordinated mechanisms from genome to metabolome that lead to multiple optimal phenotypes after the loss of a major gene product were revealed.IMPORTANCE A mechanistic understanding of how microbes are able to overcome the loss of a gene through regulatory and metabolic changes is not well understood. Eight independent adaptive laboratory evolution (ALE) experiments with pgi knockout strains resulted in eight phenotypically distinct endpoints that were able to overcome the gene loss. Utilizing multi-omics analysis, the coordinated mechanisms from genome to metabolome that lead to multiple optimal phenotypes after the loss of a major gene product were revealed.

Very Low Temperature Epitaxy of Heavily In-situ Phosphorous Doped Ge Layers and High Sn Content GeSn Layers

Combining in-situ phosphorous doping and tensile strain yield a direct bandgap-like behavior in germanium. The electrons given by the n-type impurities to the strained Ge crystalline lattice then occupy the L valleys of the conduction band, exalting fast, direct transitions between the Γ valley of the conduction band and the valence band. Using low temperatures (e.g. 400°C) and relatively high pressures (100 Torr, typically) enabled us to promote P incorporation in Ge:P layers [1]. With the conventional precursors used (GeH4 and PH3), we could not go to temperatures lower than 400°C, however. We have recently quantified the interest of using digermane (Ge2H6) in order to enhance the Ge growth rate at temperatures 425°C and lower [2] (by 5 to 10 orders of magnitude compared to GeH4). We have thus used digermane and phosphine (PH3) to grow Ge:P layers at 350°C, 100 Torr on top of slightly p-type {Ge Strain Relaxed Buffer / Si (001) substrate} stacks. The F(Ge2H6)/F(H2) Mass-Flow Ratio (MFR) and the growth duration were fixed (1.98x10-4 and 30 minutes, respectively). The process parameter varying was the PH3 mass-flow. We have first of all evidenced a Ge:P growth rate increase with the PH3 mass-flow (Figure 1), from 5 nm.min-1 for intrinsic Ge up to 11 nm.min-1 for the highest PH3 mass-flow probed. This could be due to (i) the catalysis of H desorption from the surface by P atoms, freeing sites for growth or/and (ii) a local surface temperature increase, caused by exothermal reactions [3]. All samples were golden and mirror-like under grazing light. Note that the situation was somewhat different at 400°C, 100 Torr with a GeH4 + PH3 chemistry. A transition from smooth/mirror-like to rough/milky surfaces and a sudden Ge:P growth rate drop were then evidenced for F(PH3)/F(H2) above 10-3 [4]. Conventional ω-2θ scans around the (004) XRD order were performed on those Ge:P layers. The resulting profiles are provided in Figure 2. For the highest two PH3 mass-flows probed, we obtained well-defined, intense Ge:P layer peaks (together with Pendellösung fringes on each side for the highest doping). We were thus able, from their angular position, to calculate the associated substitutional P concentrations, 2.7x1020 and 6.5x1020 cm-3. We have then performed SIMS depth profiling and four points probe measurements in order to gain access to the atomic and electrically active P concentrations in our Ge:P samples (Figure 3). As expected, the atomic P concentration monotonically increases with the PH3 mass-flow, from 9x1018 cm-3 for the lowest PH3 flow, up to 5x1020 cm-3 for the highest one. This increase slows down for F(PH3)/F(H2) MFRs above 2x10-6, however (in-line with Fig. 1 Ge:P growth rate evolution). The substitutional P concentrations (from XRD) measurements, are close to the atomic values. The vast majority of the P atoms thus seem to be substitutional. The evolution of the P+ ion concentration with the PH3 flow is somewhat peculiar. At first, it increases with the phosphine flow. The highest P+ concentration, 7.5x1019 cm-3, was obtained for F(PH3)/F(H2) = 1.25x10-6. This value is nearly 4 times higher than the value obtained at 400°C with GeH4 (~2x1019 cm-3). For higher PH3 flows, we then have a definite decrease of the P+ ions concentration. This could be due to an increase of point defects in the Ge:P crystal lattice.Finally, we have used AFM (Figure 4) in order to confirm that the Ge:P layers were smooth and featureless; the surface root mean square (rms) roughness and Z range associated with 20×20 μm² images of the most highly doped layer of the series were equal to 0.8 and 7.3 nm only, respectively. Small islands (density close to 1.6x107 cm−2) were by contrast present on the surface of intrinsic Ge layers.The high phosphorous-doping obtained here is very interesting to boost the optical performances of Ge. Another method in order to obtain a direct bandgap in group-IV semiconductors is to alloy Ge with Sn. GeSn layers have recently been shown to have a direct bandgap provided that the Sn content is high enough (12%) and the built-in strain low. This in mind, we will present some data on high Sn content (in the 6% to 10% range), compressively-strained GeSn layers we have grown at 325°C, 100 Torr on Ge SRBs (thanks to Ge2H6 and SnCl4). [1] J.M. Hartmann et al., J. Cryst. Growth 347 (2012) 37. [2] J. Aubin et al., J. Cryst. Growth, accepted (April 2016); doi: 10.1016/j.jcrysgro.2016.04.018. [3] D. Grützmacher et al., ECS Trans. 64 (11) (2014) 85. [4] J.M. Hartmann et al., Thin Solid Films 602 (2016) 13. Figure 1

The Effect of the Bi Precursors, (CH3)3Bi and (C2H5)3Bi, on the Metal‐Organic Vapor Phase Epitaxy of GaAs1‐yBiy Films

This study compares the effectiveness of two Bi sources, trimethyl bismuth (TMBi) and triethyl bismuth (TEBi), in the growth of GaAs1‐yBiy thin films. Through optimization of the growth conditions, GaAs1‐yBiy‐GaAs heterostructures with high lateral homogeneity of Bi are grown. TEBi results in a lower carbon concentration than is typical of methyl‐based compounds when used at low growth temperatures. These findings suggest the process of Bi incorporation proceeds more efficiently using the TEBi precursor. These studies do not show noticeable growth rate reduction under the dilute flows of TMBi and TEBi, however at high Bi source flux, a growth rate drop was observed for both sources.

Systems-level response to point mutations in a core metabolic enzyme modulates genotype-phenotype relationship.

Linking the molecular effects of mutations to fitness is central to understanding evolutionary dynamics. Here, we establish a quantitative relation between the global effect of mutations on the E. coli proteome and bacterial fitness. We created E. coli strains with specific destabilizing mutations in the chromosomal folA gene encoding dihydrofolate reductase (DHFR) and quantified the ensuing changes in the abundances of 2,000+ E. coli proteins in mutant strains using tandem mass tags with subsequent LC-MS/MS. mRNA abundances in the same E. coli strains were also quantified. The proteomic effects of mutations in DHFR are quantitatively linked to phenotype: the SDs of the distributions of logarithms of relative (to WT) protein abundances anticorrelate with bacterial growth rates. Proteomes hierarchically cluster first by media conditions, and within each condition, by the severity of the perturbation to DHFR function. These results highlight the importance of a systems-level layer in the genotype-phenotype relationship.

Screening of lactic acid bacteria for their potential as microbial cell factories for bioconversion of lignocellulosic feedstocks.

BackgroundThe use of fossil carbon sources for fuels and petrochemicals has serious impacts on our environment and is unable to meet the demand in the future. A promising and sustainable alternative is to substitute fossil carbon sources with microbial cell factories converting lignocellulosic biomass into desirable value added products. However, such bioprocesses require availability of suitable and efficient microbial biocatalysts, capable of utilizing C5 sugars and tolerant to inhibitory compounds generated during pretreatment of biomass. In this study, the performance of a collection of lactic acid bacteria was evaluated regarding their properties with respect to the conversion of lignocellulosic feedstocks. The strains were examined for their ability to utilize xylose and arabinose as well as their resistance towards common inhibitors from pretreated lignocellulosic biomass (furan derivatives, phenolic compounds, weak acids).ResultsAmong 296 tested Lactobacillus and Pediococcus strains, 3 L. pentosus, 1 P. acidilactici and 1 P. pentosaceus isolates were found to be both capable of utilizing xylose and arabinose and highly resistant to the key inhibitors from chemically pretreated lignocellulosic biomass. When tested in broth with commonly found combinations of inhibitors, the selected strains showed merely 4%, 1% and 37% drop in growth rates for sugarcane bagasse, wheat straw and soft wood representatives, respectively, as compared to Escherichia coli MG1655 showing decreased growth rates by 36%, 21% and 90%, respectively, under the same conditions.ConclusionThe study showed that some strains of Lactobacilli and Pediococci have the potential to be used as production platforms for value-added products from pretreated lignocellulosic biomass. Selected Lactobacilli and Pediococci strains were able to tolerate the key inhibitors in higher concentrations compared to E.coli; in addition, as these isolates were also capable of fermenting xylose and arabinose, they constitute good candidates for efficient lignocellulosic feedstock bioconversions.

Nonlinear MHD simulations of the gravitational ballooning mode close to marginal stability

The ballooning mode is thought to play a key role in the mechanism which drives the problematic edge localized modes in tokamak plasmas, and possibly in other plasma eruptions, such as in the magnetosphere. We investigate the essential nonlinear physics of this mode by simulating an instability driven by gravity in a slab of magnetized plasma; magnetic field curvature plays a similar role in toroidal confinement systems.Full ideal magnetohydrodynamics (MHD) simulations are performed, which exhibit three distinct phases of the mode's evolution: (I) a linear phase that agrees well with the predictions of linear ideal MHD; (II) a non-linear regime where the instantaneous growth rate evolves, and is somewhat lower than the linear value and (III) an explosive plasma eruption. These regimes are characterized and compared with the predictions of analytic nonlinear theory, considering cases that are close to and far from marginal stability. Evidence of a subcritical instability is demonstrated in phase II where, provided the mode's amplitude is large enough, it can develop into an eruption even for values of gravity that give linear stability. The drop in growth rate during phase II depends on the strength of the linear drive, demonstrating that the initial phases of the nonlinear evolution are also dependent on the strength of the linear drive. To extend the calculations deeper into the nonlinear regime a four-field reduced MHD model is developed, which reproduces the same features as the full ideal MHD system.

Is the middle-income trap about to be sprung?

In spite of respective growth for the Chinese economy, some fear it will fall into the middle-income trap and see its growth rate drop. This article examines factors for such a slowdown in China including undervalued exchange rates, high investment rates, and impacts of unfavorable demographic conditions. Finally, the author discusses prospects for slowdown in all the BRICS countries.

Influence of bubbles on grain growth in ice

Numerical static grain growth simulations of ice with air bubbles as a second phase show a significant drop in grain-growth rate compared to bubble-free ice. The magnitude of this drop in growth rate is dependent on the bubble boundary mobility, the volume fraction of air, the average bubble size and the bubble size distribution. The rate of grain growth decreases at first, as the microstructure evolves towards a steady state. Only then does grain growth follow the expected linear increase of mean grain area with time. In experiments, this decrease in growth rate could erroneously be interpreted as growth with a deviating growth exponent.

Africa and the Global Economic Crisis: Impacts, Policy Responses and Political Economy

Abstract: The global financial and economic crisis, which hit Africa with a lag, has caused serious setbacks to the continent's growth momentum and likely jeopardized hard‐won development gains of recent years. Even though most African countries are likely to avoid recession, the sharp drop in growth rates can have serious, long‐term consequences, especially for poor and vulnerable Africans. Furthermore, the speed of the continent's recovery and longer‐term growth rates remain uncertain. What is, however, remarkable is that unlike in previous crises (as the energy crisis in the 1970s), this time round the response of most African governments has been to continue with the prudent macroeconomic policies of the past decade, and it is these policies that are giving these countries the fiscal space with which to cushion the impact of the global crisis and avoid an even deeper recession. The emergency rescue packages that some countries introduced are also carefully designed to maintain reform momentum in the medium term. At the same time, some countries have used the crisis as an opportunity to accelerate reforms, leaving them in a better position to take advantage of a recovery in the global economy. Thus, while the global crisis has wreaked havoc on African economies, the policy response of most governments demonstrates that the overall policy environment in Africa remains sound. This paper provides three hypotheses to explain this policy response, which stands in some contrast with African governments’ responses to previous crises. First, the prudent macroeconomic stance taken by these countries is simply a reflection of their access to capital markets and thin domestic financial markets. Second, the experience of the past decade, where prudent macroeconomic policies resulted in countries being better able to respond to the crisis, created an environment where governments were reluctant to deviate too far from their macroeconomic stance, lest they be left vulnerable during the next crisis. And third, that there has been a shift in the public's attitude towards economic reforms, and African policymakers were reflecting this shift in their policy responses to the crisis. The paper concludes that if this third hypothesis is the most likely one, it bodes well for Africa's future, despite the global crisis. For it means that economic reforms in Africa will continue and will unlikely be reversed, even with a sluggish global economy, because the public's mind‐set has shifted.

Coldwater RAS in an Arctic charr farm in Northern Norway

Use of coldwater recirculating aquaculture systems (RAS) are still very rare in Norway, and only two farms are producing Arctic charr. This project took place in one of these commercial Arctic charr farms; Villmarksfisk AS in Bardu, Northern Norway.The farm gets its make-up water from ground water that holds 5°C year around. Temperature in the rearing water varies between 7.5°C (“low”) to 12°C (“high”) through the year. The biological filter in the RAS seems to work stable at both “low” and “high” temperatures, including after incidents when feeding has been stopped for a day and started on top again the day after. Such an extreme change in loading was measured as a 70% increase in TAN concentration, with only minor changes in nitrite levels recorded. The biofilter also kept the nitrite stable and low in spite of diurnal variation in TAN excretion at normal feeding regimes (every day in three periods at day-time).The farming concept is to stock the farm with wild caught juvenile fish for on-growing to market size fish (0.75–1.00kg). A drop in growth rate during early autumn has been a main concern for the farm. This may reflect a seasonal shift in growth potential, sometimes referred to as “autumn depression”. Interestingly, there is little sign of seasonal changes in the growth of hatchery-produced fish tested in the farm. Sampling of water quality through the seasons in tanks holding fish undergoing such growth depressions indicate that TAN excretion is much higher per kg feed used in the wintertime than in the springtime. This observation corresponds with the lack of weight gain during wintertime despite that the fish is feeding. Thus, feed conversion calculations indicate that feed utilization also varies with season reaching its nadir during this period.Both challenges concerning RAS in cold water and strongly reduced growth in autumn and winter time, have been investigated from 2005 to 2007 in a project financed by the Norwegian research council and the partners in the agricultural framework agreement.

Cumulative damage model based on two-mode fatigue damage bounds

Based on earlier work by the author, a macro-model of damage accumulation is developed that circumvents crack tip micro-mechanics through introduction of two-mode damage curves. In such model, the actual microcrack configuration is replaced by an ‘equivalent’ crack while the actual heterogeneous medium is replaced by an effective homogeneous and isotropic medium in which the crack is embedded. By the simultaneous presence of slip and opening-mode damage during each cycle, the two-mode damage curves reflect conditions at a crack tip in the effective medium. Such curves are obtained from conditions imposed at four discrete microstructure barriers. With all barriers established in terms of the fatigue limit. The fatigue process is thus divided into three stages, the analogues of the three creep stages under constant stress. Two length scales are defined from order of magnitude considerations: a macroscale defining a representative volume element (RVE) and a microscale defining a much smaller volume of grain size order; both volumes surrounding the ‘equivalent’ crack tip. The presence of a sufficiently large number of grains of random size and orientation makes the RVE representative of the medium as a whole; in such a medium, the location and orientation of the ‘equivalent’ crack becomes immaterial. While the RVE is dominated by monotonic slip, the microvolume represents a region of cyclic slip; both volumes surround the crack tip, in line with fracture mechanics concepts. This model is then applied to cumulative damage and bounds on mean residual lifetime are obtained for two-stage high–low (H–L) and low–high (L–H) cycling. The presence of microstructure barriers proves essential to damage irreversibility in L–H cycling, leading to a division of the entire region behind the S-N curve into three regions dominated by part or by all three fatigue stages. A statistical analysis based on the two-mode bounds leads to a median residual lifetime and to bounds on the variance. It is argued that the variance can only be bracketed by the present model but not uniquely defined, unless ‘instantaneous’ cyclic hardening is introduced as an additional random variable. Bounds on crack growth rate under constant amplitude cycling are also derived that effectively coincide in the macrocrack regime, thus explaining the small crack rate scatter observed. An experimental method is proposed whereby long fatigue lifetimes, including the fatigue limit, can be indirectly determined from a crack growth rate drop that occurs at the transition from a predominantly slip to a predominantly opening-mode interval of crack growth.