34,538 research outputs found
Structure and function of natural sulphide-oxidizing microbial mats under dynamic input of light and chemical energy
We studied the interaction between phototrophic and chemolithoautotrophic sulphide-oxidizing microorganisms in natural microbial mats forming in sulphidic streams. The structure of these mats varied between two end-members: one characterized by a layer dominated by large sulphur-oxidizing bacteria (SOB; mostly Beggiatoa-like) on top of a cyanobacterial layer (B/C mats) and the other with an inverted structure (C/B mats). C/B mats formed where the availability of oxygen from the water column was limited (<5 mu M). Aerobic chemolithotrophic activity of the SOB depended entirely on oxygen produced locally by cyanobacteria during high light conditions. In contrast, B/C mats formed at locations where oxygen in the water column was comparatively abundant (445 mu M) and continuously present. Here SOB were independent of the photosynthetic activity of cyanobacteria and outcompeted the cyanobacteria in the uppermost layer of the mat where energy sources for both functional groups were concentrated. Outcompetition of photosynthetic microbes in the presence of light was facilitated by the decoupling of aerobic chemolithotrophy and oxygenic phototrophy. Remarkably, the B/C mats conserved much less energy than the C/B mats, although similar amounts of light and chemical energy were available. Thus ecosystems do not necessarily develop towards optimal energy usage. Our data suggest that, when two independent sources of energy are available, the structure and activity of microbial communities is primarily determined by the continuous rather than the intermittent energy source, even if the time-integrated energy flux of the intermittent energy source is greater
Self-limited self-assembly of chiral filaments
The assembly of filamentous bundles with controlled diameters is common in
biological systems and desirable for the development of nanomaterials. We
discuss dynamical simulations and free energy calculations on patchy spheres
with chiral pair interactions that spontaneously assemble into filamentous
bundles. The chirality frustrates long-range crystal order by introducing twist
between interacting subunits. For some ranges of system parameters this
constraint leads to bundles with a finite diameter as the equilibrium state,
and in other cases frustration is relieved by the formation of defects. While
some self-limited structures can be modeled as twisted filaments arranged with
local hexagonal symmetry, other structures are surprising in their complexity.Comment: 5 pages, 5 figure
Untwisting of a cholesteric elastomer by a mechanical field
A mechanical strain field applied to a monodomain cholesteric elastomer will
unwind the helical director distribution. There is an analogy with the
classical problem of an electric field applied to a cholesteric liquid crystal,
but with important differences. Frank elasticity is of minor importance unless
the gel is very weak. The interplay is between director anchoring to the rubber
elastic matrix and the external mechanical field. Stretching perpendicular to
the helix axis induces the uniform unwound state via the elimination of sharp,
pinned twist walls above a critical strain. Unwinding through conical director
states occurs when the elastomer is stretched along the helical axis.Comment: 4 pages, RevTeX 3 style, 3 EPS figure
The perceived service quality, satisfaction and behavioural intent towards cellphone network service providers : a generational perspective
The importance of providing customers with quality services in order to satisfy them and encourage future purchases is well documented in literature. Yet, the cellphone network provider industry of South Africa seems to be riddled with service quality problems. This study aims to determine different generations’ perceived service quality of services and satisfaction levels with services provided by cellphone network service providers, as well as their behavioural intentions towards these providers. The study furthermore examines the correlations between these three constructs and uncovers significant differences between different generations. Data was collected from 2339 respondents by means of a self-administered questionnaire. Young Generation Y consumers perceive the service quality levels and service satisfaction levels of these providers as significantly lower than other generations. This is also true for their behavioural intent. Significant correlations also exist between the generations’ perceived level of service quality they experience, their perceived level of satisfaction with, and their behavioural intent towards providers. This implies that providers should strongly focus their efforts on satisfying the needs, and improving the service satisfaction of young Generation Y consumers in order to retain them in the future. Providers should also vary marketing strategies based upon the differences uncovered between the generations
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Shock recovery experiments confirm the possibility of transferring viable microorganisms from Mars to Earth
Extract from introduction: With regard to the impact and ejection phase we tested the case for the transfer of microorganisms from Mars to Earth. Using a high explosive set-up thin layers of bacterial endospores of Bacillus subtilis, of the lichen Xanthoria elegans and of the cyanobacterium Chroococcidiopsis sp. embedded between two plates of gabbro were subjected to 10, 20, 30, 40 and 50 GPa which is the pressure range observed in Martian meteorites [1]
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Life after shock: the mission from Mars to Earth
Extract from introduction: The minerals of the Martian meteorites collected so far indicate an exposure to shock waves in the pressure range of 5 to 55 GPa [1]. As terrestrial rocks are frequently inhabited by microbial communities, rocks ejected from a planet by impact processes may carry with them endolithic microorganisms, if microbial life existed/exists on this planet
A Microservice Infrastructure for Distributed Communities of Practice
Non-formal learning in Communities of Practice (CoPs) makes up a significant portion of today’s knowledge gain. However, only little technological support is tailored specifically towards CoPs and their particular strengths and challenges. Even worse, CoPs often do not possess the resources to host or even develop a software ecosystem to support their activities. In this paper, we describe a distributed, microservice-based Web infrastructure for non-formal learning in CoPs. It mitigates the need for central infrastructures, coordination or facilitation and takes into account the constant change of these communities. As a real use case, we implement an inquiry-based learning application on-top of our infrastructure. Our evaluation results indicate the usefulness of this learning application, which shows promise for future work in the domain of community-hosted, microservice-based Web infrastructures for learning outside of formal settings
Spacecraft charging and ion wake formation in the near-Sun environment
A three-dimensional (3-D), self-consistent code is employed to solve for the
static potential structure surrounding a spacecraft in a high photoelectron
environment. The numerical solutions show that, under certain conditions, a
spacecraft can take on a negative potential in spite of strong photoelectron
currents. The negative potential is due to an electrostatic barrier near the
surface of the spacecraft that can reflect a large fraction of the
photoelectron flux back to the spacecraft. This electrostatic barrier forms if
(1) the photoelectron density at the surface of the spacecraft greatly exceeds
the ambient plasma density, (2) the spacecraft size is significantly larger
than local Debye length of the photoelectrons, and (3) the thermal electron
energy is much larger than the characteristic energy of the escaping
photoelectrons. All of these conditions are present near the Sun. The numerical
solutions also show that the spacecraft's negative potential can be amplified
by an ion wake. The negative potential of the ion wake prevents secondary
electrons from escaping the part of spacecraft in contact with the wake. These
findings may be important for future spacecraft missions that go nearer to the
Sun, such as Solar Orbiter and Solar Probe Plus.Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasma
Decreasing initial telomere length in humans intergenerationally understates age-associated telomere shortening
Telomere length shortens with aging, and short telomeres have been linked to a wide variety of pathologies. Previous studies suggested a discrepancy in age-associated telomere shortening rate estimated by cross-sectional studies versus the rate measured in longitudinal studies, indicating a potential bias in cross-sectional estimates. Intergenerational changes in initial telomere length, such as that predicted by the previously described effect of a father's age at birth of his offspring (FAB), could explain the discrepancy in shortening rate measurements. We evaluated whether changes occur in initial telomere length over multiple generations in three large datasets and identified paternal birth year (PBY) as a variable that reconciles the difference between longitudinal and cross-sectional measurements. We also clarify the association between FAB and offspring telomere length, demonstrating that this effect is substantially larger than reported in the past. These results indicate the presence of a downward secular trend in telomere length at birth over generational time with potential public health implications
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