2,612 research outputs found
Natural Transfer of Viable Microbes in Space from Planets in the Extra-Solar Systems to a Planet in our Solar System and Vice-Versa
We investigate whether it is possible that viable microbes could have been
transported to Earth from the planets in extra-solar systems by means of
natural vehicles such as ejecta expelled by comet or asteroid impacts on such
planets. The probabilities of close encounters with other solar systems are
taken into account as well as the limitations of bacterial survival times
inside ejecta in space, caused by radiation and DNA decay. The conclusion is
that no potentially DNA/RNA life-carrying ejecta from another solar system in
the general Galactic star field landed on Earth before life already existed on
Earth, not even if microbial survival time in space is as long as tens of
millions of years. However, if the Sun formed initially as a part of a star
cluster, as is commonly assumed, we cannot rule out the possibility of transfer
of life from one of the sister systems to us. Likewise, there is a possibility
that some extra-solar planets carry life that originated in our solar system.
It will be of great interest to identify the members of the Sun's birth cluster
of stars and study them for evidence for planets and life on the planets. The
former step may be accomplished by the GAIA mission, the latter step by the SIM
and DARWIN missions. Therefore it may not be too long until we have
experimental knowledge on the question whether the natural transfer of life
from one solar system to another has actually taken place.Comment: 25 pages, 1 table, accepted to Ap
Proof of ultra-violet finiteness for a planar non-supersymmetric Yang-Mills theory
This paper focuses on a three-parameter deformation of N=4 Yang-Mills that
breaks all the supersymmetry in the theory. We show that the resulting
non-supersymmetric gauge theory is scale invariant, in the planar
approximation, by proving that its Green functions are ultra-violet finite to
all orders in light-cone perturbation theory.Comment: 13 pages, 1 figure; v2: minor correction
Chronic impacts of invasive herbivores on a foundational forest species: a wholeâtree perspective
Forests make up a large portion of terrestrial plant biomass, and the longâlived woody plants that dominate them possess an array of traits that deter consumption by forest pests. Although often extremely effective against native consumers, invasive species that avoid or overcome these defenses can wreak havoc on trees and surrounding ecosystems. This is especially true when multiple invasive species coâoccur, since interactions between invasive herbivores may yield nonâadditive effects on the host. While the threat posed by invasive forest pests is well known, longâterm field experiments are necessary to explore these consumerâhost interactions at appropriate spatial and temporal scales. Moreover, it is important to measure multiple variables to get a âwholeâplantâ picture of their combined impact. We report the results of a 4âyr field experiment addressing the individual and combined impacts of two invasive herbivores, the hemlock woolly adelgid (Adelges tsugae) and elongate hemlock scale (Fiorinia externa), on native eastern hemlock (Tsuga canadensis) in southern New England. In 2011, we planted 200 hemlock saplings into a temperate forest understory and experimentally manipulated the presence/absence of both herbivore species; in 2015, we harvested the 88 remaining saplings and assessed plant physiology, growth, and resource allocation. Adelgids strongly affected hemlock growth: infested saplings had lower above/belowground biomass ratios, more needle loss, and produced fewer new needles than control saplings. Hemlock scale did not alter plant biomass allocation or growth, and its coâoccurrence did not alter the impact of adelgid. While both adelgid and scale impacted the concentrations of primary metabolites, adelgid effects were more pronounced. Adelgid feeding simultaneously increased free amino acids local to feeding sites and a ~30% reduction in starch. The cumulative impact of adelgidâinduced needle loss, manipulation of nitrogen pools, and the loss of stored resources likely accelerates host decline through disruption of homeostatic sourceâsink dynamics occurring at the wholeâplant level. Our research stresses the importance of considering longâterm impacts to predict how plants will cope with contemporary pressures experienced in disturbed forests
Individual and nonâadditive effects of exotic sapâfeeders on root functional and mycorrhizal traits of a shared conifer host
Forest pests drive tree mortality through disruption of functional traits linked to nutrient acquisition, growth and reproduction. The impacts of attack by individual or multiple aboveâground herbivores on root functional traits critical to tree health have received little attention. This is especially true for exotic herbivores, organisms often found in disturbed forests. We excavated wholeâroot systems from eastern hemlock (Tsuga canadensis) individuals experimentally infested with hemlock woolly adelgid (HWA: Adelges tsugae) and elongate hemlock scale (EHS: Fiorina externa) individually, or in combination, for periods of 2 and 4 years. Belowâground root biomass, functional traits and storage nutrients were measured to assess impacts of herbivory. We also quantified ectomycorrhizal fungal (EMF) colonisation of fine roots and used cultureâindependent methods to examine EMF diversity. Trees infested with HWA had a greater root mass fraction (root to total biomass ratio), although feeding had no observable effects on root functional traits (e.g. specific root length) or on resource allocation to roots. HWA feeding did significantly reduce EMF colonisation of hemlock fine roots, though surprisingly, EMF diversity and that of other fungal associates were unaffected. In contrast to HWA, EHS (alone or in conjunction with HWA) feeding had no observable effect on belowâground traits or EMF colonisation alone; however, its presence mediated HWA effects when trees were coâinfested. Simultaneous infestation within the same year yielded significant reductions in EMF colonisation, whereas prior EHS attack weakened HWA effects. Our results collectively suggest that prior EHS attack dampens the impact of HWA on belowâground functional traits. This highlights how the timing and sequence of herbivore arrival can alter plantâmediated interactions between herbivores and their effects on aboveâbelowâground linkages and associated tree health
AnĂĄlise dos co-movimentos entre os mercados de capitais do Brasil e dos EUA
In this article copula theory is used to analyze the co-movements between the Brazilian and American capital markets. To formulate an effective asset allocation strategy, it is important to understand extreme events â both positive (booms) and negative (crashes) â and their effects on markets. The market indexes used are the Ibovespa and the S&P 500, covering the period from March 2001 to April 2007. We tested the adherence to the log-returns of the main copulas found in the financial literature, using the following criteria: log-likelihood, Akaike information criterion and Bayesian information criterion. The results show that the symmetrized Joe-Clayton copula is most suitable to model the dependence structure between the log-returns of the Ibovespa and the S&P500. This work differs from some previous ones (e.g., Mendes & Moretti, 2005 and Canela & Collazo, 2005) because we take into account the modeling of dynamic copulas, as introduced by Patton (2006). Finally, from the tail-dependence indexes over time, it can be concluded that the occurrence of crashes in the American market tends to affect the Brazilian market more than does the occurrence of booms.Neste artigo, a teoria de cĂłpulas Ă© utilizada para analisar os co-movimentos entre os mercados de capitais do Brasil e dos EUA. Na finalidade de implementação de uma estratĂ©gia de alocação de ativos Ă© importante entender os eventos extremos â tanto os positivos (boom) como os negativos (crashes) â e seus efeitos sobre os mercados. Os Ăndices de mercado usados sĂŁo o IBOVESPA e o S&P 500 cobrindo o perĂodo de 03/2001 a 04/2007. A aderĂȘncia aos log-retornos das principais cĂłpulas encontradas na literatura financeira Ă© avaliada. Os seguintes critĂ©rios foram escolhidos: o Log-likelihod, o critĂ©rio de informação de Akaike e o critĂ©rio de informação bayesiano. Os resultados mostram que a cĂłpula de Joe-Clayton simetrizada Ă© a mais adequada para modelar a estrutura de dependĂȘncia entre os log-retornos do IBOVESPA e os do S&P500. Este trabalho difere de alguns estudos jĂĄ realizados [e.g. Mendes e Moretti(2005) e Canela e Collazo(2005)], pois leva em consideração a modelagem de cĂłpulas dinĂąmicas introduzida por Patton (2006). Finalmente, atravĂ©s dos Ăndices de dependĂȘncia caudal ao longo do tempo, pode-se concluir que a ocorrĂȘncia de eventos extremos negativos (crashes) no mercado norte-americano tende a afetar mais o mercado brasileiro quando da comparação da ocorrĂȘncia dos eventos extremos positivos (booms)
Old World megadroughts and pluvials during the Common Era
Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other âOld Worldâ climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the âOld World Drought Atlasâ (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability
Modelling human embryoid body cell adhesion to a combinatorial library of polymer surfaces
Designing materials to control biology is an intense focus of biomaterials and regenerative medicine research. Discovering and designing materials with appropriate biological compatibility or active control of cells and tissues is being increasingly undertaken using high
throughput synthesis and assessment methods. We report a relatively simple but powerful machine-learning method of generating modelsthat link microscopic or molecular properties of polymers or other materials to their biological effects. We illustrate the potential of these methods by developing the first robust, predictive, quantitative, and purely computational models of adhesion of human embryonic stem cell embryoid bodies (hEB) to the surfaces of 496-member polymers
Lateglacial and Holocene climate and environmental change in the northeastern Mediterranean region: Diatom evidence from Lake Dojran (Republic of Macedonia/Greece)
The juncture between the west-east and north-south contrasting Holocene climatic domains across the Mediterranean is complex and poorly understood. Diatom analysis of Lake Dojran (Republic of Macedonia/Greece) provides a new insight into lake levels and trophic status during the Lateglacial and Holocene periods in the northeastern Mediterranean. Following a very shallow or even desiccated state at the core base at ca. 12,500 cal yr BP, indicated by sedimentological and hydro-acoustic data, diatoms indicate lake infilling, from a shallow state with abundant benthos to a plankton-dominated relatively high lake level and eutrophic state thereafter. Diatom-inferred shallowing between ca. 12,400 - 12,000 cal yr BP and a very low lake level and eutrophic, oligosaline state between ca. 12,000 - 11,500 cal yr BP provide strong evidence for Younger Dryas aridity. The earliest Holocene (ca. 11,500 - 10,700 cal yr BP) was characterised by a high lake level, followed by a lake-level reduction and increased trophic level between ca. 10,700-8,500 cal yr BP. The lake was relatively deep and exhibited peak Holocene trophic level between ca. 8,500-3,000 cal yr BP, becoming shallow thereafter. The diatom data provide more robust evidence and strengthen previous lake-level interpretation based on sedimentological and geochemical data during the earliest, mid and late Holocene, and also clarify previous uncertainty in interpretation of Lateglacial and early-Holocene lake-level change. Our results are also important in disentangling regional climate effects from local catchment dynamics during the Holocene, and to this end we exploit extant regional palynological evidence for vegetation change in the highlands and lowlands. The importance of seasonality in driving Holocene climate change is assessed by reference to the summer and winter latitudinal temperature gradient (LTG) model of Davis and Brewer (2009). We suggest that increased precipitation drove the high lake level during the earliest Holocene. The early- Holocene low lake level and relatively high trophic state may result climatically from high seasonality of precipitation and locally from limited, nutrient-rich catchment runoff. We argue that the mid- Holocene relatively deep and eutrophic state was driven mainly by local vegetation succession and associated changes in catchment processes, rather than showing a close relationship to climate change. The late-Holocene shallow state may have been influenced by a temperature-induced increase in evaporative concentration, but was coupled with clear evidence for intensified human impact. This study improves understanding of Lateglacial and Holocene climate change in the northeastern Mediterranean, suggests the important role of the LTG on moisture availability during the Holocene, and clarifies the influence of catchment processes on palaeohydrology
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