2,507 research outputs found
Constructions for cyclic sieving phenomena
We show how to derive new instances of the cyclic sieving phenomenon from old
ones via elementary representation theory. Examples are given involving objects
such as words, parking functions, finite fields, and graphs.Comment: 18 pages, typos fixed, to appear in SIAM J. Discrete Mat
Niche partitioning of bacterial communities in biological crusts and soils under grasses, shrubs and trees in the Kalahari
The Kalahari of southern Africa is characterised by sparse vegetation interspersed with microbe-dominated biological soil crusts (BSC) which deliver a range of ecosystem services including soil stabilisation and carbon fixation. We characterised the bacterial communities of BSCs (0–1 cm depth) and the subsurface soil (1–2 cm depth) in an area typical of lightly grazed Kalahari rangelands, composed of grasses, shrubs, and trees. Our data add substantially to the limited amount of existing knowledge concerning BSC microbial community structure, by providing the first bacterial community analyses of both BSCs and subsurface soils of the Kalahari region based on a high throughput 16S ribosomal RNA gene sequencing approach. BSC bacterial communities were distinct with respect to vegetation type and soil depth, and varied in relation to soil carbon, nitrogen, and surface temperature. Cyanobacteria were predominant in the grass interspaces at the soil surface (0–1 cm) but rare in subsurface soils (1–2 cm depth) and under the shrubs and trees. Bacteroidetes were significantly more abundant in surface soils of all areas even in the absence of a consolidated crust, whilst subsurface soils yielded more sequences affiliated to Acidobacteria, Actinobacteria, Chloroflexi, and Firmicutes. The common detection of vertical stratification, even in disturbed sites, suggests a strong potential for BSC recovery after physical disruption, however severe depletion of Cyanobacteria near trees and shrubs may limit the potential for natural BSC regeneration in heavily shrub-encroached areas
UV Finite Brane Decay
The decay of an unstable D-brane via closed string emission and open string
pair production is considered in subcritical string theory with a spacelike
linear dilaton. The decay rate is given by the imaginary part of the annulus,
which has ambiguities corresponding to the choices of incoming closed and open
string vacua. An exact expression for the full annulus diagram is computed with
a natural choice of incoming vacua. It is found that the ultraviolet
divergences present in critical string theory in both of these processes are
absent for any nonzero spacelike dilaton. Implications for the vexing issue of
the tachyon dust are discussed.Comment: 21 pages, 3 figures, minor changes, changed to JHEP styl
Thermostability in endoglucanases is fold-specific
<p>Abstract</p> <p>Background</p> <p>Endoglucanases are usually considered to be synergistically involved in the initial stages of cellulose breakdown-an essential step in the bioprocessing of lignocellulosic plant materials into bioethanol. Despite their economic importance, we currently lack a basic understanding of how some endoglucanases can sustain their ability to function at elevated temperatures required for bioprocessing, while others cannot. In this study, we present a detailed comparative analysis of both thermophilic and mesophilic endoglucanases in order to gain insights into origins of thermostability. We analyzed the sequences and structures for sets of endoglucanase proteins drawn from the Carbohydrate-Active enZymes (CAZy) database.</p> <p>Results</p> <p>Our results demonstrate that thermophilic endoglucanases and their mesophilic counterparts differ significantly in their amino acid compositions. Strikingly, these compositional differences are specific to protein folds and enzyme families, and lead to differences in intramolecular interactions in a fold-dependent fashion.</p> <p>Conclusions</p> <p>Here, we provide fold-specific guidelines to control thermostability in endoglucanases that will aid in making production of biofuels from plant biomass more efficient.</p
The influence of trees, shrubs, and grasses on microclimate, soil carbon, nitrogen, and CO2 efflux:Potential implications of shrub encroachment for Kalahari rangelands
Shrub encroachment is a well-documented phenomenon affecting many of the world's drylands. The alteration of vegetation structure and species composition can lead to changes in local microclimate and soil properties which in turn affect carbon cycling. The objectives of this paper were to quantify differences in air temperatures, soil carbon, nitrogen, and CO2 efflux under trees (Vachellia erioloba), shrubs (Grewia flava), annual and perennial grasses (Schmidtia kalahariensis and Eragrostis lehmanniana) collected over three seasons at a site in Kgalagadi District, Botswana, in order to determine the vegetation-soil feedback mechanism affecting the carbon cycle. Air temperatures were logged continuously and soil CO2 efflux was determined throughout the day and evening using closed respiration chambers and an infrared gas analyser. There were significant differences in soil carbon, total nitrogen, CO2 efflux, light and temperatures beneath the canopies of trees, shrubs and grasses. Daytime air temperatures beneath shrubs and trees were cooler compared to grass sites, particularly in summer months. Night time air temperatures under shrubs and trees were, however, warmer than at the grass sites. There was also significantly more soil carbon, nitrogen and CO2 efflux under shrubs and trees compared to grasses. Whilst the differences observed in soils and microclimate may reinforce the competitive dominance of shrubs and present challenges to strategies designed to manage encroachment they should not be viewed as entirely negative. Our findings highlight some of the dichotomies and challenges to be addressed before interventions aiming to bring about more sustainable land management can be implementedpublishersversionPeer reviewe
Kinetochore life histories reveal an Aurora-B-dependent error correction mechanism in anaphase
Chromosome mis-segregation during mitosis leads to aneuploidy, which is a hallmark of cancer and linked to cancer genome evolution. Errors can manifest as "lagging chromosomes" in anaphase, although their mechanistic origins and likelihood of correction are incompletely understood. Here, we combine lattice light-sheet microscopy, endogenous protein labeling, and computational analysis to define the life history of >10 kinetochores. By defining the "laziness" of kinetochores in anaphase, we reveal that chromosomes are at a considerable risk of mis-segregation. We show that the majority of lazy kinetochores are corrected rapidly in anaphase by Aurora B; if uncorrected, they result in a higher rate of micronuclei formation. Quantitative analyses of the kinetochore life histories reveal a dynamic signature of metaphase kinetochore oscillations that forecasts their anaphase fate. We propose that in diploid human cells chromosome segregation is fundamentally error prone, with an additional layer of anaphase error correction required for stable karyotype propagation
Matrix Cosmology
Exact time-dependent solutions of c=1 string theory are described using the
free fermion formulation. One such class of solutions describes draining of the
Fermi sea and has a spacetime interpretation as closed string tachyon
condensation. A second class of solutions, corresponding to droplets of Fermi
liquid orbiting in phase space, describes closed cosmologies which bounce
through singularities.Comment: 21 pages, 6 figures, v2: added references, minor additions and
correction
Black Hole Remnants and the Information Puzzle
Magnetically charged dilatonic black holes have a perturbatively infinite
ground state degeneracy associated with an infinite volume throat region of the
geometry. A simple argument based on causality is given that these states do
not have a description as ordinary massive particles in a low-energy effective
field theory. Pair production of magnetic black holes in a weak magnetic field
is estimated in a weakly-coupled semiclassical expansion about an instanton and
found to be finite, despite the infinite degeneracy of states. This suggests
that these states may store the information apparently lost in black hole
scattering processes.Comment: 16 pages, revision has 5 figures uuencode
Spatial organisation of fungi in soil biocrusts of the Kalahari is related to bacterial community structure and may indicate ecological functions of fungi in drylands
Biological soil crusts, or biocrusts, are microbial communities found in soil surfaces in drylands and in other locations where vascular plant cover is incomplete. They are functionally significant for numerous ecosystem services, most notably in the C fixation and storage due to the ubiquity of photosynthetic microbes. Whereas carbon fixation and storage have been well studied in biocrusts, the composition, function and characteristics of other organisms in the biocrust such as heterotrophic bacteria and especially fungi are considerably less studied and this limits our ability to gain a holistic understanding of biocrust ecology and function. In this research we characterised the fungal community in biocrusts developed on Kalahari Sand soils from a site in southwest Botswana, and combined these data with previously published bacterial community data from the same site. By identifying organisational patterns in the community structure of fungi and bacteria, we found fungi that were either significantly associated with biocrust or the soil beneath biocrusts, leading to the conclusion that they likely perform functions related to the spatial organisation observed. Furthermore, we showed that within biocrusts bacterial and fungal community structures are correlated with each other i.e., a change in the bacterial community is reflected by a corresponding change in the fungal community. Importantly, this correlation but that this correlation does not occur in nearby soils. We propose that different fungi engage in short-range and long-range interactions with dryland soil surface bacteria. We have identified fungi which are candidates for further studies into their potential roles in biocrust ecology at short ranges (e.g., processing of complex compounds for waste management and resource provisioning) and longer ranges (e.g., translocation of resources such as water and the fungal loop model). This research shows that fungi are likely to have a greater contribution to biocrust function and dryland ecology than has generally been recognised
A Note on Inflation with Tachyon Rolling on the Gauss-Bonnet Brane
In this paper we study the tachyonic inflation in brane world cosmology with
Gauss-Bonnet term in the bulk. We obtain the exact solution of slow roll
equations in case of exponential potential. We attempt to implement the
proposal of Lidsey and Nunes, astro-ph/0303168, for the tachyon condensate
rolling on the Gauss-Bonnet brane and discuss the difficulties associated with
the proposal.Comment: RevTex4, 5 pages, no figures, Minor clarifications added and
references updated, To appear in PR
- …