368 research outputs found
Microbial life in the open ocean: a universe of tiny cells separated by empty space
Marine microbes are fundamental components of food webs and the biogeochemical cycles that
maintain the habitability of the planet. In the oligotrophic open ocean, these microscopic
organisms live in a dilute environment separated from other cells by large distances at the
microscale while surrounded by very few essential nutrient molecules. For ubiquitous submicron sized and non-motile microbes, cellular growth requirements for hundreds of millions (or
more) of nutrient molecules are sustained predominantly by rapid molecular diffusion.
Characterizing the interactions of cells and molecules in the “empty space” of the ocean remains
central to understanding the drivers and consequences of oceanic biogeochemical cycles
The Health Status of a Population estimated: The History of Health State Curves
Following the recent publication of our book on Exploring the Health State of
a Population by Dynamic Modeling Methods in The Springer Series on Demographic
Methods and Population Analysis (DOI 10.1007/978-3-319-65142-2) we provide this
brief presentation of the main findings and improvements regarding the Health
State of a Population. (See at: http://www.springer.com/gp/book/9783319651415).
Here the brief history of the Health State or Health Status curves for
individuals and populations is presented including the main references and
important figures along with an illustrated Poster (see Figure 13 and
http://www.smtda.net/demographics2018.html). Although the Survival Curve is
known as long as the life tables have introduced, the Health State Curve was
calculated after the introduction of the advanced stochastic theory of the
first exit time. The health state curve is illustrated in several graphs either
as a fit curve to data or produced after a large number of stochastic
realizations. The Health State, the Life Expectancy and the age at mean zero
health state are also estimated. Keywords: Health State and Survival Curves,
Health status of a population, First exit time stochastic theory, stochastic
simulations of health state, Age at Maximum Curvature, Healthy Life Expectancy
and HALE, Standard Deviation, Health State Curves, Maximum human lifespan and
other.Comment: 11 pages, 13 figure
Structure formation in active networks
Structure formation and constant reorganization of the actin cytoskeleton are
key requirements for the function of living cells. Here we show that a minimal
reconstituted system consisting of actin filaments, crosslinking molecules and
molecular-motor filaments exhibits a generic mechanism of structure formation,
characterized by a broad distribution of cluster sizes. We demonstrate that the
growth of the structures depends on the intricate balance between
crosslinker-induced stabilization and simultaneous destabilization by molecular
motors, a mechanism analogous to nucleation and growth in passive systems. We
also show that the intricate interplay between force generation, coarsening and
connectivity is responsible for the highly dynamic process of structure
formation in this heterogeneous active gel, and that these competing mechanisms
result in anomalous transport, reminiscent of intracellular dynamics
Understanding the limits to generalizability of experimental evolutionary models.
Post print version of article deposited in accordance with SHERPA RoMEO guidelines. The final definitive version is available online at: http://www.nature.com/nature/journal/v455/n7210/abs/nature07152.htmlGiven the difficulty of testing evolutionary and ecological theory in situ, in vitro model systems are attractive alternatives; however, can we appraise whether an experimental result is particular to the in vitro model, and, if so, characterize the systems likely to behave differently and understand why? Here we examine these issues using the relationship between phenotypic diversity and resource input in the T7-Escherichia coli co-evolving system as a case history. We establish a mathematical model of this interaction, framed as one instance of a super-class of host-parasite co-evolutionary models, and show that it captures experimental results. By tuning this model, we then ask how diversity as a function of resource input could behave for alternative co-evolving partners (for example, E. coli with lambda bacteriophages). In contrast to populations lacking bacteriophages, variation in diversity with differences in resources is always found for co-evolving populations, supporting the geographic mosaic theory of co-evolution. The form of this variation is not, however, universal. Details of infectivity are pivotal: in T7-E. coli with a modified gene-for-gene interaction, diversity is low at high resource input, whereas, for matching-allele interactions, maximal diversity is found at high resource input. A combination of in vitro systems and appropriately configured mathematical models is an effective means to isolate results particular to the in vitro system, to characterize systems likely to behave differently and to understand the biology underpinning those alternatives
Electron quantum metamaterials in van der Waals heterostructures
In recent decades, scientists have developed the means to engineer synthetic
periodic arrays with feature sizes below the wavelength of light. When such
features are appropriately structured, electromagnetic radiation can be
manipulated in unusual ways, resulting in optical metamaterials whose function
is directly controlled through nanoscale structure. Nature, too, has adopted
such techniques -- for example in the unique coloring of butterfly wings -- to
manipulate photons as they propagate through nanoscale periodic assemblies. In
this Perspective, we highlight the intriguing potential of designer
sub-electron wavelength (as well as wavelength-scale) structuring of electronic
matter, which affords a new range of synthetic quantum metamaterials with
unconventional responses. Driven by experimental developments in stacking
atomically layered heterostructures -- e.g., mechanical pick-up/transfer
assembly -- atomic scale registrations and structures can be readily tuned over
distances smaller than characteristic electronic length-scales (such as
electron wavelength, screening length, and electron mean free path). Yet
electronic metamaterials promise far richer categories of behavior than those
found in conventional optical metamaterial technologies. This is because unlike
photons that scarcely interact with each other, electrons in subwavelength
structured metamaterials are charged, and strongly interact. As a result, an
enormous variety of emergent phenomena can be expected, and radically new
classes of interacting quantum metamaterials designed
Salespeople’s Renqing Orientation, Self-esteem, and Selling Behaviors: An Empirical Study in Taiwan
The purpose of this study was to investigate how salespeople’s renqing orientation and self-esteem jointly affect their selling behavior.
Data were obtained from a survey of salespeople from 17 pharmaceutical and consumer-goods companies in Taiwan (n = 216).
Salespeople’s renqing orientation (i.e., their propensity to adhere to the accepted norm of reciprocity) compensates the negative effect of self-esteem on their selling behaviors, such as adaptive selling and hard work.
Our study results underscore the critical role of the character trait of renqing orientation in a culture emphasizing a norm of reciprocity. Therefore, it would be useful to consider a strategy of recruiting salespeople with either a high self-esteem or a combination of high renqing orientation and low self-esteem.
The existing literature of industrial/organizational psychology and marketing primarily relies on constructs that are derived from Western cultural contexts. However, the present paper extended these literatures by investigating the possible joint effects of self-esteem with a trait originated from the Chinese culture on salespeople’s selling behaviors
Global Change Could Amplify Fire Effects on Soil Greenhouse Gas Emissions
Background: Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.[br/]
Methodology/Principal Findings: We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO(2) concentration, precipitation and nitrogen supply on soil nitrous oxide (N(2)O) emissions in a grassland ecosystem. We examined the responses of soil N(2)O emissions, as well as the responses of the two main microbial processes contributing to soil N(2)O production - nitrification and denitrification - and of their main drivers. We show that the fire disturbance greatly increased soil N(2)O emissions over a three-year period, and that elevated CO(2) and enhanced nitrogen supply amplified fire effects on soil N(2)O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO(2) and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots. [br/]
Conclusions/Significance: Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence
The deuteron: structure and form factors
A brief review of the history of the discovery of the deuteron in provided.
The current status of both experiment and theory for the elastic electron
scattering is then presented.Comment: 80 pages, 33 figures, submited to Advances in Nuclear Physic
Spontaneous Reaction Silencing in Metabolic Optimization
Metabolic reactions of single-cell organisms are routinely observed to become
dispensable or even incapable of carrying activity under certain circumstances.
Yet, the mechanisms as well as the range of conditions and phenotypes
associated with this behavior remain very poorly understood. Here we predict
computationally and analytically that any organism evolving to maximize growth
rate, ATP production, or any other linear function of metabolic fluxes tends to
significantly reduce the number of active metabolic reactions compared to
typical non-optimal states. The reduced number appears to be constant across
the microbial species studied and just slightly larger than the minimum number
required for the organism to grow at all. We show that this massive spontaneous
reaction silencing is triggered by the irreversibility of a large fraction of
the metabolic reactions and propagates through the network as a cascade of
inactivity. Our results help explain existing experimental data on
intracellular flux measurements and the usage of latent pathways, shedding new
light on microbial evolution, robustness, and versatility for the execution of
specific biochemical tasks. In particular, the identification of optimal
reaction activity provides rigorous ground for an intriguing knockout-based
method recently proposed for the synthetic recovery of metabolic function.Comment: 34 pages, 6 figure
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