Low Initial Density Research Articles

Pathogenicity and antibiotic susceptibility of two bacterial pathogens associated with the clam Tapes decussatus in some Egyptian fisheries

Bivalve aquaculture is generally influenced by bacterial pathogens that cause high mortality-related losses in hatcheries. Six bacterial phenotypes had been previously isolated as the most predominant microorganisms in carpet shell clam ( Tapes decussates ) samples collected from natural beds in representative Egyptian clam fisheries. The main aims of this study were to investigate the nature of the pathogenic strains that significantly affect clam survival and test their susceptibility to commercially available antibiotics. Based on their 16S rRNA sequences and some biochemical features, two potent clam pathogens were emerged; one of which is gram-positive and the other is gram-negative. The former isolate was identified as Micrococcus luteus and the latter as Vibrio alginolyticus . Experimental challenges with the two bacterial pathogens introduced at different initial cell concentrations (2.5 X 10 4 - 7.5 X 10 4 cfu ml -1 ) showed markedly diverse clam mortality results. However, the pathogenic interaction of M. luteus with clam survival was generally higher than that of V. alginolyticus . When introduced at a relatively low initial cell density, the infectious records of V. alginolyticus were significantly increased suggesting that the expression of its key virulence factors is mainly triggered as a response to host contact. Antibiotic susceptibility tests suggested chloramphenicol and tetracycline as markedly effective agents that can be used to control the spread of these two bacterial pathogens in aquaculture applications.

Efficient soft x-ray sources from laser-irradiated gold foam targets with well-controlled impurities

As an important x-ray source, enhancement of x-ray emissions from laser-produced plasmas is significant for various applications. Due to less expanding kinetic loss, gold foam with low initial density can have an enhanced x-ray conversion efficiency compared with solid-density gold. However, low-Z impurities within gold foam targets will diminish the enhancement remarkably, and should be tightly controlled. This paper presents an experimental study of a high brightness laser plasma soft x-ray source, based on a g gold foam target with negligible impurities irradiated by nanosecond laser pulses with power density around 3 × 10 W at the Shenguang II laser facility. A conversion efficiency, from multi-eV to multi-keV, of % is achieved in the x-ray emissions—about 21% relative enhancement compared with a solid-density gold target, and the highest conversion efficiency for Au foam planar targets yet. Good agreement has been achieved between the semi-analytical model prediction and the experimental results.

Exploring the fatigue strength improvement of Cu-Al alloys

As a significant scientific problem directly impacting on the long-term safety of engineering materials and facilities, the improvement of fatigue strength under fully-reversed cycling was comprehensively explored in this study. Advantageous material characteristics for the improvement of fatigue strength were summarized from the achievements of the previous researches, followed by a new attempt to combine them in material design. As the model material, α-Cu-Al alloys with clean ultrafine-grains as well as large proportions of twin boundaries were thus produced, which exhibited a notable fatigue strength improvement (up to 155% higher than the coarse-grained counterparts and 40% higher than the counterparts produced by severe plastic deformation). Furthermore, a general principle briefly summarized as localized fatigue damage reduction was proposed based on the analysis of the optimizing methods including microstructure optimization and composition optimization. Accordingly, several recommended features to obtain such high fatigue strength materials were finally listed for further anti-fatigue design, such as uniform grains with small size and stable boundaries; low initial dislocation density, and proper alloying composition.

A large-strain radial consolidation theory for soft clays improved by vertical drains

A system of vertical drains with combined vacuum and surcharge preloading is an effective solution for promoting radial flow, accelerating consolidation. However, when a mixture of soil and water is deposited at a low initial density, a significant amount of deformation or surface settlement occurs. Therefore, it is necessary to introduce large-strain theory, which has been widely used to manage dredged disposal sites in one-dimensional theory, into radial consolidation theory. A governing equation based on Gibson's large-strain theory and Barron's free-strain theory incorporating the radial and vertical flows, the weight of the soil, variable hydraulic conductivity and compressibility during the consolidation process is therefore presented.

Fast-charging high-energy lithium-ion batteries via implantation of amorphous silicon nanolayer in edge-plane activated graphite anodes

As fast-charging lithium-ion batteries turn into increasingly important components in forthcoming applications, various strategies have been devoted to the development of high-rate anodes. However, despite vigorous efforts, the low initial Coulombic efficiency and poor volumetric energy density with insufficient electrode conditions remain critical challenges that have to be addressed. Herein, we demonstrate a hybrid anode via incorporation of a uniformly implanted amorphous silicon nanolayer and edge-site-activated graphite. This architecture succeeds in improving lithium ion transport and minimizing initial capacity losses even with increase in energy density. As a result, the hybrid anode exhibits an exceptional initial Coulombic efficiency (93.8%) and predominant fast-charging behavior with industrial electrode conditions. As a result, a full-cell demonstrates a higher energy density (≥1060 Wh l−1) without any trace of lithium plating at a harsh charging current density (10.2 mA cm−2) and 1.5 times faster charging than that of conventional graphite.

Characterization and Optimization of a Fermentation Process for the Production of High Cell Densities and Lipids Using Heterotrophic Cultivation of Chlorella protothecoides

Abstract This article reports an in-depth characterization and optimization of a fermentation process with the aim of increasing the production of high cell densities as a prerequisite for high productivity of lipids. The freshwater microalgae Chlorella protothecoides was grown heterotrophically in a bioreactor on aerated and stirred cultivation medium with glucose as a carbon source and yeast extract as a nitrogen source. The detailed effect of carbon to nitrogen (C/N) ratio on growth and lipid content of C. protothecoides was studied. Results show that with a C/N ratio lower than 12, the medium was carbon-limited, favoring cell proliferation, and under nitrogen-starvation conditions, (higher C/N ratios), lipid accumulation was enhanced. A two-stage, fed-batch process was developed and achieved a maximum of 255.3 g/L of biomass over 228 h of fermentation, from a low initial cell density (< 2 g/L). The specific growth rate, measured when C/N ratio was low, was 0.078 h−1. The lipid productivity was 16.7 g/...

Effect of recrystallization on hot deformation mechanism of TA15 titanium alloy under uniaxial tension and biaxial gas bulging conditions

To investigate the effect of recrystallization on hot deformation mechanism of TA15 titanium alloy, rolled sheet and laser-welded tubes before and after recrystallization annealing were tested by uniaxial tensile tests and biaxial gas bulging respectively. The results show that both of the initial rolled sheet and the as-welded tube consisted of equiaxed α microstructure with high initial dislocation density. During uniaxial tensile tests, a quasi-steady flow state was achieved at 800°C, 0.001s−1, which demonstrated the best formability with a maximum elongation of 536%. Significant recrystallization occurred at the early stage of tensile deformation and the grain size was found to decrease first and then increase. Significant recrystallization also occurred at the early stage of the biaxial gas bulging of the as-welded tube. During the bulging of the annealed tube consisting of equiaxed α with low initial dislocation density, dynamic recrystallization (DRX) was the main microstructure evolution characteristic, but most of the DRX occurred at the middle-late stage. The relatively coarse equiaxed α and increasing fraction of low angle grain boundaries during deformation may restrict the occurrence of grain boundary sliding (GBS), which resulted in the relatively worse formability of the annealed tube. However, recrystallization which happened widely at the early stage, increased the fraction of high angle grain boundaries, refined the microstructure, promoted the occurrence of GBS and enhanced the formability of the initial rolled sheet and as-welded tube. It was also found that the stress state had no effect on the DRX mechanism for TA15 at 800°C, which was confirmed to be discontinuous dynamic recrystallization under both uniaxial tension and biaxial gas bulging conditions.

Decoupling recombination mechanisms and trap state localization in direct bandgap semiconductors using photoluminescence decay

In this work, we show that extraction of the true bulk lifetime from the biexponential decay that follows from low initial carrier density photoluminescence decay experiments is not generally possible, and introduce new models to enable extraction of the bulk lifetime in the case where the initial carrier density exceeds the doping level. From measurements with high initial carrier density, we establish quasi-equilibrium between localized and free carrier states and accurately measure the bulk lifetime. Using our new models, we measure the time constants associated with localization processes as well as nonradiative and radiative bulk recombination in our GaAs double heterostructures grown with molecular beam epitaxy from experiments with varied excitation strength providing initial carrier densities that range from around 1014 to 1017 cm–3. We demonstrate that this approach can be applied to lightly doped (1016 cm–3) materials where the strongest excitation yields initial carrier densities that exceed the doping level. In our n-type sample, we report lifetime values of (22.7 ± 0.1) ns for bulk recombination, (73 ± 1) ns for trap-capture, (51 ± 2) ns for trap-emission, and (63 ± 2) ns for trap-decay, with a low-level injection effective radiative efficiency of (27.5 ± 0.7)%. In our p-type sample, we report lifetime values of (78.9 ± 0.3) ns for bulk recombination, (77.5 ± 0.7) ns for trap-capture, (530 ± 10) ns for trap-emission, and (177 ± 4) ns for trap-decay, with a low-level injection effective radiative efficiency of (47.0 ± 0.8)%. In comparison with the long and short lifetimes extracted from the biexponential decay with weak excitation, the mean bulk lifetime measured with strong excitation was (33 ± 2)% and (53 ± 1)% longer than the short lifetime, and (68 ± 4)% and (103 ± 3)% shorter than the long lifetime in our n-type and p-type samples, respectively. In our n-type sample, the extracted low-level injection nonradiative lifetime was (33 ± 1) ns, and it was observed to remain constant with the injection level. In our p-type sample, the high-level injection nonradiative lifetime was measured to be (30 ± 30)% larger than the low-level injection nonradiative lifetime of (140 ± 2) ns.

The Response of Heterotrophic Prokaryote and Viral Communities to Labile Organic Carbon Inputs Is Controlled by the Predator Food Chain Structure.

Factors controlling the community composition of marine heterotrophic prokaryotes include organic-C, mineral nutrients, predation, and viral lysis. Two mesocosm experiments, performed at an Arctic location and bottom-up manipulated with organic-C, had very different results in community composition for both prokaryotes and viruses. Previously, we showed how a simple mathematical model could reproduce food web level dynamics observed in these mesocosms, demonstrating strong top-down control through the predator chain from copepods via ciliates and heterotrophic nanoflagellates. Here, we use a steady-state analysis to connect ciliate biomass to bacterial carbon demand. This gives a coupling of top-down and bottom-up factors whereby low initial densities of ciliates are associated with mineral nutrient-limited heterotrophic prokaryotes that do not respond to external supply of labile organic-C. In contrast, high initial densities of ciliates give carbon-limited growth and high responsiveness to organic-C. The differences observed in ciliate abundance, and in prokaryote abundance and community composition in the two experiments were in accordance with these predictions. Responsiveness in the viral community followed a pattern similar to that of prokaryotes. Our study provides a unique link between the structure of the predator chain in the microbial food web and viral abundance and diversity.

Long-term annealing of high purity aluminum single crystals: New insights into Harper-Dorn creep

Single crystals of 99.999% and 99.9999% pure aluminum were annealed at high elevated temperatures (0.98Tm) for relatively long times of up to one year, the longest in the literature. Remarkably, the dislocation density remains relatively constant at a value of about 109m−2 over a period of one year. The stability suggests some sort of “frustration” limit. This has implications towards the so-called “Harper-Dorn creep” that generally occurs at fairly high temperatures (e.g. > 0.90Tm) and very low stresses. It is possible that ordinary five-power-law creep occurs within the tradition Harper-Dorn regime with very low initial dislocation densities in aluminum. Higher initial dislocation densities, such as with this annealing study, may lead to Harper-Dorn (Newtonian) creep.

Underwater spark discharge with long transmission line for cleaning horizontal wells

A transmission line is discussed for application in an underwater spark-discharge technique in the cleaning of a horizontal well by incorporating a power-transmission model into the simulation. The pulsed-spark-discharge technique has been proposed for clogged-well rehabilitation, because it removes incrustations that are attached to well screens by using strong pressure waves that are generated by the rapid expansion of a spark channel. To apply the pulsed-spark-discharge technique to the cleaning of horizontal wells, the coaxial cable between the pulsed power supply and the spark gap as a load needs to be extended to a few hundred meters. Prior to field application, pulsed-spark-discharge experiments were conducted and the role of the transmission line was examined using an improved simulation model. In the model, a non-linear interaction of the spark channel and the capacitor bank is described by the pulse-forming action of the coaxial cable. Based on the accurate physical properties of the water plasma, such as the equation of state and electrical conductivity within the region of interest, the amount of energy contributed to the development of a shock wave was evaluated. The simulation shows that if the initial conditions of the spark channel are the same, no further reduction in strength of the pressure wave occurs, even if the cable length is increased above 50 m. Hence, the degraded peak pressure that was observed in the experiments using the longer cable is attributed to a change in the initial condition of the spark channel. The parametric study suggests that the low initial charging voltage, the high ambient water pressure, and the long cable length yield the low initial spark-channel density, which results in a reduced peak pressure. The simulation of line charging is presented to discuss the principle of disturbing the pre-breakdown process by an extended cable.

Atomistic investigation of homogeneous nucleation in undercooled liquid

Although nucleation, a fundamental physical phenomenon in nature, has long been studied by simulations and experiments, our knowledge of this process is still quite limited. Herein, the atomistic pathways of homogeneous nucleation are studied using the phase-field crystal model. We find that nucleation is of one-step type for low initial densities (solid volume fraction), whereas two-step nucleation (TS) dominates in other cases. For the TS process, the fraction and the lifetime of metastable intermediate phases will increase with increasing density, and these metastable phases can significantly accelerate the nucleation process. By calculating the nucleation barriers, we investigated the origin of the appearance of the metastable phases and the mechanism of two-step nucleation.

Growth of Grapevine Rootlings in Soil from a Field Nursery Naturally Infested with Meloidogyne incognita and Rhizoctonia solani

Grapevine rootlings (V. vinifera cv. Colombard and V. champini cv. Ramsey) were grown in pots containing soil from a field nursery naturally infested with Meloidogyne incognita and Rhizoctonia solani. Vine growth in untreated soil with or without dilution using clean sand was compared with growth in soil subjected to a range of treatments including: disinfestation either by aerated steaming or by fumigation with methyl bromide, amendment with 1 % (w/w) corn-meal, or treatment with fungicides and/or fenamiphos. Vines grown in infested soil suffered root rot caused by R. soiani and galling caused by M. i11cog11ita. Both disinfestation treatments effectively prevented root rotting and galling, and increased the growth of Colombard but not Ramsey. Fenamiphos increased the growth of the nematode-susceptible Colombard but not Ramsey at a low initial population density of three juveniles per 200 g of soil. Nematode reproduction on Ramsey in the shadehouse and in the field nursery on other rootstocks regarded as being highly resistant to root-knot nematode was higher than previously reported with other South Australian Meloidogyne spp. populations. Soil dilution increased Colombard growth and amendment with corn-meal reduced nematode reproduction on both cultivars. Rhizoctonia solani caused root rot in both cultivars but quintozene and tolclofos-methyl increased root growth only of Ramsey. These fungicides and pencycuron reduced the severity of root rot; tolclofos-methyl was particularly effective in reducing the frequency of isolation of R. solani from roots. Potassium phosphite did not reduce root rot or increase root growth. Quintozene, tolclofos-methyl and pencycuron also inhibited nematode reproduction but these effects were not consistently observed in both cultivars. Most isolates of R. solani from grapevine roots belonged to anastomosis group 4. Field observations suggested that galled roots were more prone to infection by R. solani than ungalled roots.

Effect of slaking on direct shear behaviour of crushed mudstones

This study investigates the effects of wetting and drying cycles, known as slaking, on the strength-displacement characteristics of crushed mudstone. A series of direct shear tests was conducted by simulating cyclic wetting and drying under different stress conditions using a modified direct shear apparatus. The effects of the stress ratio, the density of specimen, the initial water content before wetting, the slaking index, and the number of wetting and drying cycles on the test results was investigated. Experiments were also performed on less-slakable materials, including crushed sand stone, silica sand and glass beads, to compare the results with the crushed mudstone. Considerable creep displacement on the crushed mudstones was observed during both drying and wetting phases under the constant shear stress conditions. The creep displacement during the drying was more significant than during the wetting phase. The creep displacement accumulated with progressive wetting and drying cycles. The drying-induced displacement was observed when the water content became smaller than the amount of water absorption of the mudstone specimens. Correspondingly, a gradual decrease of the peak stress ratio was observed with the number of wetting and drying cycles. In contrast to the mudstones, the influence of cyclic wetting and drying on the crushed sand stone, silica sand and glass beads is almost negligible. A higher slaking index, a lower water content before wetting, and lower initial density accelerate the slaking of mudstones.

Inoculative release strategies of Macrolophus pygmaeus Rambur (Hemiptera: Miridae) in tomato crops: population dynamics and dispersal

Macrolophus pygmaeus Rambur (Hemiptera: Miridae) is a key biological control agent in greenhouse tomato crops. In the present study, we describe the population dynamics of M. pygmaeus after release during two generations in semi-commercial greenhouses in order to optimize biocontrol programs. We tested the effect of the number of weekly supplementary food applications consisting of a mixture of Ephestia kuehniella Zeller and Artemia franciscana Kellogg in a tomato crop on population numbers of M. pygmaeus at low and high initial release densities of the predator. Also, the effect of supplementary feeding on the predator’s dispersal was studied. Larger population densities of M. pygmaeus were obtained when food was supplied for a longer period. However, we observed fruit damage by M. pygmaeus at high densities resulting from too frequent food applications. Also, dispersal was slowed down as the number of supplementary food applications increased. Distributing M. pygmaeus over more plants at release results in higher total population densities. The optimal inoculative release strategy of M. pygmaeus is a trade-off between high population densities and fruit damage, fast or slow dispersal throughout the greenhouse and the number of release plants and work/costs related to the supplementation of food. The optimal strategy to overcome negative effects like fruit damage, slow dispersal and potential cannibalism proved to be a weekly provision of supplementary food during 6–8 weeks, with an initial release density strategy of 20 M. pygmaeus adults per plant. These results contribute to a more sustainable tomato production. A reliable and efficient inoculative release strategy for the key predator M. pygmaeus enhances the biocontrol potential and is of great value for tomato growers.

Rapid recovery of a population of the cryptic and evolutionarily distinct Hochstetter's Frog, Leiopelma hochstetteri, in a pest‐free environment

SummaryNew Zealand's native frogs (genus Leiopelma) display a very high degree of endemism and belong to a distinctive and ancient evolutionary lineage. All four extant species are considered to be threatened or endangered, but the mechanisms behind their decline are poorly understood. Many of the potential factors causing population declines are confounded for the two species living on the mainland; in particular, habitat degradation and the presence of introduced mammalian pests have been difficult to disentangle in the past. Here we present data on a population of Hochstetter's Frog (Leiopelma hochstetteri) living in a pest‐free mainland sanctuary, the 3363‐ha Maungatautari Ecological Island. Survey data from this population shows a fourfold increase in numbers between 2009 and 2012 from a very low initial density. Our results suggest that for Hochstetter's Frog at least, and possibly the other mainland species of Leiopelma, introduced mammals are a major driving factor in population declines. Given that the frog population at Maungatautari was discovered by serendipity when planning for the pest‐free sanctuary was already well underway, this also highlights the conservation value of large pest‐free areas and ecosystem‐level management.

Supernova enrichment of planetary systems in low-mass star clusters

The presence and abundance of short lived radioisotopes (SLRs) $^{26}$Al and $^{60}$Fe in chondritic meteorites implies that the Sun formed in the vicinity of one or more massive stars that exploded as supernovae (SNe). Massive stars are more likely to form in massive star clusters ($>$1000 M$_{\odot}$) than lower mass clusters. However, photoevaporation of protoplanetary discs from massive stars and dynamical interactions with passing stars can inhibit planet formation in clusters with radii of $\sim$1 pc. We investigate whether low-mass (50 - 200 M$_{\odot}$) star clusters containing one or two massive stars are a more likely avenue for early Solar system enrichment as they are more dynamically quiescent. We analyse $N$-body simulations of the evolution of these low-mass clusters and find that a similar fraction of stars experience supernova enrichment than in high mass clusters, despite their lower densities. This is due to two-body relaxation, which causes a significant expansion before the first supernova even in clusters with relatively low (100 stars pc$^{-3}$) initial densities. However, because of the high number of low mass clusters containing one or two massive stars, the absolute number of enriched stars is the same, if not higher than for more populous clusters. Our results show that direct enrichment of protoplanetary discs from supernovae occurs as frequently in low mass clusters containing one or two massive stars (>20 M$_{\odot}$) as in more populous star clusters (1000 M$_\odot$). This relaxes the constraints on the direct enrichment scenario and therefore the birth environment of the Solar System.

Physiological responses to acid stress by Saccharomyces cerevisiae when applying high initial cell density.

High initial cell density is used to increase volumetric productivity and shorten production time in lignocellulosic hydrolysate fermentation. Comparison of physiological parameters in high initial cell density cultivation of Saccharomyces cerevisiae in the presence of acetic, formic, levulinic and cinnamic acids demonstrated general and acid-specific responses of cells. All the acids studied impaired growth and inhibited glycolytic flux, and caused oxidative stress and accumulation of trehalose. However, trehalose may play a role other than protecting yeast cells from acid-induced oxidative stress. Unlike the other acids, cinnamic acid did not cause depletion of cellular ATP, but abolished the growth of yeast on ethanol. Compared with low initial cell density, increasing initial cell density reduced the lag phase and improved the bioconversion yield of cinnamic acid during acid adaptation. In addition, yeast cells were able to grow at elevated concentrations of acid, probable due to the increase in phenotypic cell-to-cell heterogeneity in large inoculum size. Furthermore, the specific growth rate and the specific rates of glucose consumption and metabolite production were significantly lower than at low initial cell density, which was a result of the accumulation of a large fraction of cells that persisted in a viable but non-proliferating state.

High concentration CO2 sequestration by using microalgae in staged cultivation

The acclimated fresh water microalga Chlorella vulgaris has the excellent growing capability in the process of staged cultivation (SC). In this study, the alga solutions with low (106/mL) and high (107/m) cell densities showed different abilities of growth with different levels of CO2–air mixture gases[5%, 10%, 15% (v/v)]. The SC process can improve the algal mass productivity rapidly by the aerating increased concentrations of CO2 of mixed gases into the algae solution in a short time. The results showed that the cell density was improved to 1.72 g L−1 within 7 days, showing 3.1, 1.48, and 1.70 times improvement compared to that in the biomass from the culture with separate 5, 10, and 15% CO2–air mixture gases. Besides, there were not the obvious distinctions on the contents of biochemicals (carbohydrate, protein, and lipid) from these algae. This study proved that SC process was an efficient method to improve biomass productivity of Chlorella sp. in a short time with low initial cell density. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1862–1867, 2016

Effects of initial density, nutrient, and water level regime on the seedling survival and growth of Typha orientalis Presl

Seedlings’ successful establishment is of importance in the preemption process of pioneers in wetlands. Although Typha orientalis Presl has been reported as a pioneer in Asia countries, studies on the seedling phase of T. orientalis are not available yet. A mesocosm experiment was conducted to understand the effects of biotic (initial density) and abiotic (nutrient and water level regime) factors on the seedling survival and growth of T. orientalis. Most seedlings survived under low initial density (93.8%) and eutrophic (95.5%) rather than high initial density (64.3%) and ombrotrophic (62.5%). Seedlings under low initial density, eutrophic, and flooded conditions showed relatively higher growth in shoot height. The final number of ramets showed an adverse tendency compared to the survival rate and shoot height particularly depending on the water level regime. T. orientalis compensated its biomass production with producing less but longer shoots under the flooded condition, indicating the phenotypic plasticity of T. orientalis as a deep water species. However, the compensation seemed to be guaranteed only under the condition of sufficient nutrients. Asian T. orientalis seemed not to be a pioneer but a weak-competitor not only in mature plant stage but also in juvenile seedling stage unless sufficient nutrients are guaranteed.