744 research outputs found
Dynamical streams in the solar neighbourhood
The true nature of the Hyades and Sirius superclusters is still an open
question. In this contribution, we confront Eggen's hypothesis that they are
cluster remnants with the results of a kinematic analysis of more than 6000 K
and M giants in the solar neighbourhood. This analysis includes new radial
velocity data from a large survey performed with the Coravel spectrometer,
complemented by Hipparcos parallaxes and Tycho-2 proper motions (Famaey et al.
2004). A maximum-likelihood method, based on a bayesian approach, has been
applied to the data, in order to make full use of all the available data
(including less precise parallaxes) and to derive the properties of the
different kinematic subgroups. Two such subgroups can be identified with the
Hyades and Sirius superclusters. Stars belonging to them span a very wide range
of age, which is difficult to account for in Eggen's scenario. These groups are
thus most probably "dynamical streams" related to the dynamical perturbation by
spiral waves rather than to cluster remnants.
In this scenario, the Hyades and Ursa Major clusters just happen to be in the
Hyades and Sirius streams, which are purely dynamical features that have
nothing to do with the remnants of more massive primordial clusters. This
mechanism could be the key to understanding the presence of an old metal-rich
population, and of many exoplanetary systems in our neighbourhood. Moreover, a
strong spiral pattern seems to be needed in order to yield such prominent
streams. Since spiral structure is usually baryonic, this would leave very
little room for dark matter. This may be an indication that the era of the
dark-matter paradigm explaining the dynamics of the Galaxy may come to an end,
and is being superseded by modified gravity.Comment: 5 pages, 1 figure, to appear in The Three Dimensional Universe with
GAIA, eds M. Perryman & C. Turo
Microlensing Events from Measurements of the Deflection Angle
Microlensing events are now regularly being detected by monitoring the flux
of a large number of potential sources and measuring the combined magnification
of the images. This phenomenon could also be detected directly from the
gravitational deflection, by means of high precision astrometry using
interferometry. Relative astrometry at the level of 10\muas may become
possible in the near future. The gravitational deflection can be measured by
astrometric monitoring of a bright star having a background star within a small
angular separation. This type of monitoring program will be carried out for the
independent reasons of discovering planets from the angular motion they induce
on the nearby star around which they are orbiting, and for measuring
parallaxes, proper motions and orbits of binary stars. We discuss three
applications of the measurement of gravitational deflections by astrometric
monitoring: measuring the mass of the bright stars that are monitored,
measuring the mass of brown dwarfs or giant planets around the bright stars,
and detecting microlensing events by unrelated objects near the line of sight
to the two stars. We discuss the number of stars whose mass could be measured
by this procedure. We also give expressions for the number of expected
microlensing events by unrelated objects, which could be stars, brown dwarfs,
or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter
Feeding cessation alters host morphology and bacterial communities in the ascidian Pseudodistoma crucigaster
11 páginas, 2 tablas, 7 figurasBackground: Ascidians can associate with abundant and diverse consortia ofmicrobial symbionts, yet these
communities remain unexamined for the majority of host ascidians and little is known about host-symbiont interactions.
Methods: We coupled electron microscopy and 16S rRNA gene tag pyrosequencing to investigate the bacterial
communities associated with the colonial ascidian Pseudodistoma crucigaster, a species endemic to theMediterranean Sea
that has a life cycle with two phases: actively-filtering (active) and non-filtering (resting) forms.
Results: Resting colonies exhibited a reduced branchial sac (feeding apparatus) and a thickened cuticle. Electron
microscope images also suggested higher abundance of colonizing microorganisms on surfaces of resting colonies.
Accordingly, bacterial sequences associated with environmental sources (sediment and biofilms, >99 % similarity) were
detected exclusively in resting colonies. Bacterial communities of P. crucigaster colonies (active and resting) were
dominated by 3 core taxa affiliated (>94 % similarity) with previously described symbiotic Alphaproteobacteria in marine
invertebrates. Shifts in rare bacteria were detected when ascidians entered the resting phase, including the appearance of
strictly anaerobic lineages and nitrifying bacterial guilds.
Conclusions: These findings suggest that physical (thickened cuticle) and metabolic (feeding cessation) changes
in host ascidians have cascading effects on associated bacteria, where modified oxygen concentrations and
chemical substrates for microbial metabolism may create anaerobic microhabitats and promote colonization by
environmental microorganisms.This research was funded by the Marie Curie International Reintegration
Grant FP7-PEOPLE-2010-RG 277038 within the 7th European Community
Framework Program, the Spanish Government projects MARSYMBIOMICS
CTM2013-43287-P and CHALLENGEN CTM2013-48163, and the Catalan
Government grant 2014SGR-336 for Consolidated Research Groups.Peer reviewe
A simulation method for fatigue-driven delamination in layered structures involving non-negligible fracture process zones and arbitrarily shaped crack fronts
Most of the existing methods for fatigue-driven delamination are limited to
two-dimensional (2D) applications or their predictive capabilities have not
been validated in three-dimensional (3D) problems. This work presents a new
cohesive zone-based computational method for simulating fatigue-driven
delamination in the analysis of 3D structures without crack migration. The
method accurately predicts fatigue propagation of non-nelgigible fracture
process zones with arbitrarily shaped delamination fronts. The model does not
require any kind of fitting parameter since all the input parameters are
obtained experimentally from coupon tests. The evaluation of the energy release
rate is done using two new techniques recently developed by the authors (the
growth driving direction and the mode-decomposed J-integral) leading to an
accurate prediction of delamination propagation under mixed-mode and
non-self-similar growing conditions. The new method has been implemented as a
UEL for Abaqus and validated against an experimental benchmark case with
varying crack growth rate and shape and extension of the fracture process zone.Comment: 37 pages, 14 figures, 7 table
Ultrastructure, molecular phylogenetics and chlorophyll a content of novel cyanobacterial symbionts in temperate sponge hosts
Marine sponges often harbor photosynthetic symbionts that may enhance host metabolism and ecological success, yet little is known about the factors that structure the diversity, specificity, and nature of these relationships. Here, we characterized the cyanobacterial symbionts in two congeneric and sympatric host sponges that exhibit distinct habitat preferences correlated with irradiance: Ircinia fasciculata (higher irradiance) and Ircinia variabilis (lower irradiance). Symbiont composition was similar among hosts and dominated by the sponge-specific cyanobacterium Synechococcus spongiarum. Phylogenetic analyses of 16S-23S rRNA internal transcribed spacer (ITS) gene sequences revealed that Mediterranean Ircinia spp. host a specific, novel symbiont clade ("M") within the S. spongiarum species complex. A second, rare cyanobacterium related to the ascidian symbiont Synechocystis trididemni was observed in low abundance in I. fasciculata and likewise corresponded to a new symbiont clade. Symbiont communities in I. fasciculata exhibited nearly twice the chlorophyll a concentrations of I. variabilis. Further, S. spongiarum clade M symbionts in I. fasciculata exhibited dense intracellular aggregations of glycogen granules, a storage product of photosynthetic carbon assimilation rarely observed in I. variabilis symbionts. In both host sponges, S. spongiarum cells were observed interacting with host archeocytes, although the lower photosynthetic activity of Cyanobacteria in I. variabilis suggests less symbiont-derived nutritional benefit. The observed differences in clade M symbionts among sponge hosts suggest that ambient irradiance conditions dictate symbiont photosynthetic activity and consequently may mediate the nature of host-symbiont relationships. In addition, the plasticity exhibited by clade M symbionts may be an adaptive attribute that allows for flexibility in host-symbiont interactions across the seasonal fluctuations in light and temperature characteristic of temperate environments
Application of flow cytometry analysis to elucidate the impact of scale-down conditions in Escherichia coli cultivations P. Gil Salvador 2013 Award in Bioengineering category. (November 22, 2013 in the Annual General Assembly of the AIQS)
Inhomogeneities appear in large-scale fed-batch bioprocesses;especially when the distribution of the feedingsolution and of the oxygen is uneven. In order to studythe consequences of these heterogenic conditions on theculture, scale-down bioreactor experiments have beendesigned. These studies have revealed that cells exposedto oscillatory conditions are affected on various cellularlevels of regulation. However, not many of these studieshave been performed on the observation of the behaviourof the single cell level of Escherichia coli. Therefore, flowcytometry (FCM) is chosen as analytical tool to study thecellular viability in the scale-down approaches of a twocompartment reactor (Two-CR) and a newly establishedthree compartment reactor (Three-CR). An optimization ofdifferent staining methods applied in these experiments isalso performed, since staining procedures for flow cytometrystudies of bacterial populations are still not well-established yet
A game of russian roulette for a generalist dinoflagellate parasitoid: Host susceptibility is the key to success
© 2016 Alacid, Park, Turon, Petrou and Garcés. Marine microbial interactions involving eukaryotes and their parasites play an important role in shaping the structure of phytoplankton communities. These interactions may alter population densities of the main host, which in turn may have consequences for the other concurrent species. The effect generalist parasitoids exert on a community is strongly dependent on the degree of host specificity. Parvilucifera sinerae is a generalist parasitoid able to infect a wide range of dinoflagellates, including toxic-bloom-forming species. A density-dependent chemical cue has been identified as the trigger for the activation of the infective stage. Together these traits make Parvilucifera-dinoflagellate hosts a good model to investigate the degree of specificity of a generalist parasitoid, and the potential effects that it could have at the community level. Here, we present for the first time, the strategy by which a generalist dinoflagellate parasitoid seeks out its host and determine whether it exhibits host preferences, highlighting key factors in determining infection. Our results demonstrate that in its infective stage, P. sinerae is able to sense potential hosts, but does not actively select among them. Instead, the parasitoids contact the host at random, governed by the encounter probability rate and once encountered, the chance to penetrate inside the host cell and develop the infection strongly depends on the degree of host susceptibility. As such, their strategy for persistence is more of a game of Russian roulette, where the chance of survival is dependent on the susceptibility of the host. Our study identifies P. sinerae as a potential key player in community ecology, where in mixed dinoflagellate communities consisting of hosts that are highly susceptible to infection, parasitoid preferences may mediate coexistence between host species, reducing the dominance of the superior competitor. Alternatively, it may increase competition, leading to species exclusion. If, however, highly susceptible hosts are absent from the community, the parasitoid population could suffer a dilution effect maintaining a lower parasitoid density. Therefore, both host community structure and host susceptibility will determine infectivity in the field
Stable populations in unstable habitats: temporal genetic structure of the introduced ascidian Styela plicata in North Carolina
14 páginas, 3 tablas, 4 figuras.The analysis of temporal genetic variability is
an essential yet largely neglected tool to unveil and predict
the dynamics of introduced species. We here describe
the temporal genetic structure and diversity over time of
an introduced population of the ascidian Styela plicata
(Lesueur, 1823) in Wilmington (North Carolina, USA,
34°08′24″N, 77°51′44″W). This population suffers important
salinity and temperature changes, and in June every
year we observed massive die-offs, leaving free substratum
that was recolonized within a month. We sampled 12–14
individuals of S. plicata every 2 months from 2007 to 2009 (N = 196) and analyzed a mitochondrial marker (the gene
cytochrome oxidase subunit I, COI) and seven nuclear
microsatellites. Population genetic analyses showed similar
results for both types of markers and revealed that most
of the genetic variation was found within time periods.
However, analyses conducted with microsatellite loci also
showed weak but significant differences among time periods.
Specifically, in the samplings after die-off episodes
(August–November 2007 and 2008) the genetic diversity
increased, the inbreeding coefficient showed prominent
drops, and there was a net gain of alleles in the microsatellite
loci. Taken together, our results suggest that recruits
arriving from neighboring populations quickly occupied
the newly available space, bringing new alleles with them.
However, other shifts in genetic diversity and allele loss
and gain episodes were observed in December–January and
February–March 2008, respectively, and were apparently
independent of die-off events. Overall, our results indicate
that the investigated population is stable over time and
relies on a periodic arrival of larvae from other populations,
maintaining high genetic diversity and a complex interplay
of allele gains and losses.This research was supported
by a grant from the United States–Israel Binational Science Foundation
(BSF), Jerusalem, Israel (number 2014025), the Spanish Government
project CTM2013—48163—and the Catalan Government Grant
2014SGR-336 for Consolidated Research Groups.Peer reviewe
Till death do us part: Stable sponge-bacteria associations under thermal and food shortage stresses
Sporadic mass mortality events of Mediterranean sponges following periods of anomalously high temperatures or longer than usual stratification of the seawater column (i.e. low food availability) suggest that these animals are sensitive to environmental stresses. The Mediterranean sponges Ircinia fasciculata and I. oros harbor distinct, species-specific bacterial communities that are highly stable over time and space but little is known about how anomalous environmental conditions affect the structure of the resident bacterial communities. Here, we monitored the bacterial communities in I. fasciculata (largely affected by mass mortalities) and I. oros (overall unaffected) maintained in aquaria during 3 weeks under 4 treatments that mimicked realistic stress pressures: control conditions (13°C, unfiltered seawater), low food availability (13°C, 0.1 µm-filtered seawater), elevated temperatures (25°C, unfiltered seawater), and a combination of the 2 stressors (25°C, 0.1 µm-filtered seawater). Bacterial community structure was assessed using terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene sequences and transmission electron microscopy (TEM). As I. fasciculata harbors cyanobacteria, we also measured chlorophyll a (chl a) levels in this species. Multivariate analysis revealed no significant differences in bacterial T-RFLP profiles among treatments for either host sponge species, indicating no effect of high temperatures and food shortage on symbiont community structure. In I. fasciculata, chl a content did not significantly differ among treatments although TEM micrographs revealed some cyanobacteria cells undergoing degradation when exposed to both elevated temperature and food shortage conditions. Arguably, longer-term treatments (months) could have eventually affected bacterial community structure. However, we evidenced no appreciable decay of the symbiotic community in response to medium-term (3 weeks) environmental anomalies purported to cause the recurrent sponge mortality episodes. Thus, changes in symbiont structure are not likely the proximate cause for these reported mortality events
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
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