This Week in Science

Science  08 Mar 2013:
Vol. 339, Issue 6124, pp. 1121
  1. Earth's Ingredients


    What was the composition of the earliest terrestrial starting blocks? The answer lies in understanding how Earth's interior separated into mantle and core components. Siebert et al. (p. 1194, published online 10 January) performed a series of high pressure and temperature experiments to track how chromium and vanadium, which have a slight affinity for iron, partition into metal and silicate fractions. Combined with accretionary models, the data suggest that Earth accreted under the same relatively oxidizing conditions under which the most common types of meteorites formed. Transferring oxygen in the form of FeO from the mantle to the core could have gradually reduced the mantle to its present-day oxidation state.

  2. Characterizing Quantum Measurement

    In the classical world, the result of measurement is often viewed as independent of the experimental apparatus. In the quantum world, however, the act of measurement has an effect through processes such as back-action, entanglement, and decoherence. Looking at the scattering of single electrons from a single ion and the evolution of the spin state of that ion, Glickman et al. (p. 1187) probed how these processes are intertwined and characterize how the result of quantum measurement emerges from the system's interaction with its environment.

  3. S-T-R-E-T-C-H Me

    Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. Hao et al. (p. 1191; see the Perspective by Zhou) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite.

  4. Exceptional Now

    The climate has been warming since the industrial revolution, but how warm is climate now compared with the rest of the Holocene? Marcott et al. (p. 1198) constructed a record of global mean surface temperature for more than the last 11,000 years, using a variety of land- and marine-based proxy data from all around the world. The pattern of temperatures shows a rise as the world emerged from the last deglaciation, warm conditions until the middle of the Holocene, and a cooling trend over the next 5000 years that culminated around 200 years ago in the Little Ice Age. Temperatures have risen steadily since then, leaving us now with a global temperature higher than those during 90% of the entire Holocene.

  5. Bees Get That Caffeine "Buzz"

    Caffeine improves memory in humans, millions of whom find that their daily dose enhances clarity, focus, and alertness. The human relationship with caffeine is relatively recent, however, and thus its impact on our brains is likely a by-product of its true ecological role. Caffeine occurs naturally in the floral nectar of Coffea and Citrus plants. Wright et al. (p. 1202; see the Perspective by Chittka and Peng) found that caffeine presented at naturalistic levels significantly improved the ability of bees to remember and locate a learned floral scent and potentiated the responses of neurons involved in olfactory learning and memory.

  6. Hot, Toxic Eukaryote

    Unusually, the single-celled eukaryote red alga, Galdieria sulphuraria, can thrive in hot, acidic springs. This organism is endowed with extraordinary metabolic talents and can consume a variety of strange carbohydrates, as well as turn on photosynthesis when the food runs out. Schönknecht et al. (p. 1207; see the Perspective by Rocha) discerned from phylogenetic analysis of its genome that during its evolution, G. sulphuraria appears to have commandeered at least 75 bacterial and archaeal genes by horizontal gene transfer and then applied gene expansion to boost its metabolic repertoire.

  7. It's a SIRT

    Intense attention has focused on the SIRT1 deacetylase as a possible target for anti-aging drugs. But unexpected complications in assays of SIRT1 activity have made it unclear whether compounds thought to be sirtuin-activating compounds (STACs) are really direct regulators of the enzyme. Further exploration of these effects by Hubbard et al. (p. 1216; see the Perspective by Yuan and Marmorstein) revealed that interaction of SIRT1 with certain substrates allows activation of SIRT1 by STACs and identified critical amino acids in SIRT1 required for these effects. Mouse myoblasts reconstituted with SIRT1 mutated at this amino acid lost their responsiveness to STACs.

  8. On the Origin of Tumor Tregs

    The tumor microenvironment is often seeded with regulatory T cells (Tregs), which inhibit antitumor immunity. Using mice with genetically driven prostate cancer, Malchow et al. (p. 1219; see the Perspective by Joshi and Jacks) found a population of Tregs that were enriched in the prostate of tumor-bearing mice. Surprisingly, these cells were also present in female mice and were found to be specific, not for a tumor-specific antigen, but rather for an antigen normally expressed in the prostate. Prostate antigen–specific Tregs arose in the thymus and their selection was dependent on Aire, a protein that drives the expression of tissue-specific antigens in the thymus. Thus, Tregs that seed tumors likely arise in the thymus, are not necessarily tumor-specific, and are recruited and/or expand in an organ when a tumor arises.

  9. Still Living in Their Mother's House

    Dispersal, movement away from an organism's natal site, is a critical stage in the development of a juvenile into an adult and several drivers of this process have been proposed. A classic hypothesis put forward by Hamilton and May argues that individuals disperse away from kin in order to avoid competing with similar phenotypes. Many species have been shown to follow this pattern, and it is generally accepted that most organisms disperse well away from the sites where they were born. However, not all species follow this pattern and some display a distinct tendency to remain within their mother's home range. Hoogland (p. 1205) studied populations of three species of prairie dogs over more than three decades and found that the presence of close kin in these highly social rodents actually led young animals to remain on their natal site, whereas dispersal away from it occurred almost exclusively when close female kin were absent. This suggests that sociality may in fact result in individuals remaining with their close kin in order to benefit from cooperation and that longer-distance dispersal may instead occur when the opportunity for cooperation does not exist.

  10. Limits in Percolation Models

    Slight changes in the number of connections within a network that form at random (for example, connections in social networks) can lead to a huge increase in connectivity, a phenomenon termed "explosive percolation." These percolation transitions are often studied with Erdös and Rényi models, in which edges connecting pairs of vertices in a network are added randomly or according to a rule. Whether these transitions are continuous in nature has been the subject of several recent studies. Cho et al. (p. 1185; see the Perspective by Ziff) examined the effect of avoiding bridge bonds that create a spanning cluster (one that completes the percolation path) on the continuity of transitions for a d-dimensional lattice (up to six dimensions). Analytical arguments and numerical studies reveal a critical value for the number of bonds m below which the percolation transition is continuous and above which it is discontinuous. The critical value depends on d and on the fractal dimension of the bridge bonds of the clusters.

  11. Antibiotic Mechanisms Revisited

    Several recent studies have suggested that bactericidal antibiotics kill cells by a common mechanism involving reactive oxygen species (ROS). Two groups tested this hypothesis using diverse experiments, with both finding that quinolone, lactam, and aminoglycoside antibiotics had similar efficacy for killing in the presence or absence of oxygen (or nitrate). Liu et al. (p. 1210) saw no increase in hydrogen peroxide production in antibiotic-exposed cells and found no association between antibiotic exposure and the expected symptoms of oxidative damage, such as the breakdown of iron-sulfur clusters in enzymes or of hydroxyl radical injuries to DNA. Similarly, Keren et al. (p. 1213) found no correlation between the production of ROS, inferred from hydroxyphenyl fluorescein dye measurements, and bacterial survival, nor was there any significant protective effect engendered by thiourea. The results do not support a common mode of action for bactericidal antibiotics mediated by ROS.

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