This Week in Science

Science  18 Aug 2006:
Vol. 313, Issue 5789, pp. 888
  1. Light and Hydrogen


    Soon after the universe formed, it was filled with hydrogen atoms, yet today almost all the diffuse hydrogen between galaxies is ionized. Barkana (p. 931) reviews how and when the first stars and black holes lit up and ionized primordial hydrogen gas throughout the universe. Some understanding has come from computer simulations of the change that show the ionization is patchy and happens first in the densest regions of space. However, a full picture must await a new generation of radio telescopes that will map out this key epoch. Stars must exceed a certain size if they are to burn hydrogen through fusion, and Richer et al. (p. 936; see the news story by Bhattacharjee) have identified this fundamental mass limit in a deep census of globular cluster stars in our Milky Way taken with the Hubble Space Telescope. They also see a characteristic change in the color of white dwarfs in the cluster caused by the onset of molecular hydrogen formation in their atmospheres. Both effects had been predicted by theorists, and this experimental confirmation helps improve our understanding of the physics of low-mass stars and white dwarfs.

  2. Assessing Wildfire Activity

    Understanding the underlying causes of the increases in wildfire activity in the western United States during the last several decades will impact how to manage the risk that wildfires pose. Westerling et al. (p. 940, published online 6 July with the Perspective by Running; see the cover) compiled a comprehensive time series of large forest wildfires in the western United States for the period from 1970 to 2003, and compared those data with corresponding observations of climate, hydrology, and land surface conditions. Wildfire activity increased suddenly in the mid-1980s. Hydroclimate and fires are closely related, and climate variation has been the primary cause of the increase in fires during the period of their study, although land use changes can also be important. Longer springs and summers that could result as the world warms will continue to lengthen the fire season and continue to cause more large wildfires.

  3. Stimulated Atomic Hopping

    The tip of the scanning tunneling microscope can be used to pick up atoms and move them on surfaces, as well as induce motion through electronic excitations produced by the tunneling electrons. Stroscio et al. (p. 948) assembled short chains of Cu atoms terminated by a Co atom on a Cu(111) surface and analyzed the hopping induced by tunneling electrons of the Co atom between different sites at the end of the chain, which manifested itself as low-frequency “telegraph” noise. Density functional calculations help explain why the tip location that maximizes this hopping is not directly over the Co atom and how the barrier for motion increases with Cu chain length.

  4. An Open Arrangement


    The adsorption of organic molecules on close-packed surfaces of transition metals often leads to the formation of complete monolayers, but intermolecular forces such as hydrogen bonding can cause molecules to form ordered structures such as rows that leave areas uncovered. Pawin et al. (p. 961) report an example where competing interactions create a honeycomb network that has open pores with a diameter of 50 angstroms. The network formed by very low coverage of anthraquinone adsorbed on the Cu(111) surface has openings that are about five molecular diameters. The structure appears to balance hydrogen-bonding contacts, which facilitate the formation of molecular rows, but which compete with intermolecular repulsive forces.

  5. Graphene Sheets on the Double

    Single sheets of graphene can display unusual and potentially useful electronic properties, and theoretical work on coupled bilayer systems has indicated that a controllable gap may be induced if there is an asymmetry between the layers, which could be induced either by doping with atoms or application of an external electric field. Ohta et al. (p. 951) have used angle-resolved photoemission spectroscopy to determine the band structure of graphene bilayers in which asymmetry was induced by doping one sheet with adsorbed potassium atoms. The authors confirm that such control over the energy gap between the valence and conduction bands is possible.

  6. Emulsions on Demand

    Surfactants are widely used to stabilize emulsions in products, such as cosmetics, whose constituents would otherwise fail to mix. Many industrial processes, however, have multiple steps that require separating emulsion components after reaction or transport. Liu et al. (p. 958) show that amidine molecules bearing long hydrophobic tails can be cycled reversibly between surfactant and nonsurfactant forms. Room-temperature treatment of the amidines with an atmosphere of CO2 produces bicarbonate salts that stabilize aqueous-hydrocarbon emulsions. Bubbling of air through the system at 65°C reverses the reaction and breaks the emulsion. In the absence of CO2, the amidines act as effective de-emulsifiers of aqueous-crude oil suspensions.

  7. Revisiting Vietnam's Psychological Toll

    The magnitude of the Vietnam War's psychological toll on U.S. soldiers has been a subject of heated debate since 1988, when two major government-funded studies reported widely divergent rates of posttraumatic stress disorder (PTSD) in Vietnam veterans. Interest in this question has intensified as comparisons are now being made between the Vietnam War and the ongoing conflict in Iraq. Dohrenwend et al. (p. 978; see Perspective by McNally) have reexamined PTSD rates in Vietnam War veterans using improved diagnostic methods and military records (rather than self-reports) to document exposure to war zone stress. Their analysis revealed a lifetime PTSD rate of 18.7%, in between the two previous estimates (of 30.9% and 14.7%). An even stronger dose-response relation seen between war-related stress exposure and PTSD confirms that the war's psychological toll was real and substantial.

  8. Nailing the Axoneme

    Cilia and flagella are motile appendages that project from eukaryotic cells that play roles in motility and sensing in a variety of organisms and tissues. Nicastro et al. (p. 944) present cryoelectron tomography of frozen-hydrated, eukaryotic flagella to reveal structural features of life-like axonemes at ∼4 nanometer resolution that are important for axoneme function.

  9. Mixed Bouquets


    Flower color in plants is often selected through pollinator preference. Intermediate colors, when they arise in hybrids between two closely related species, are often selected against. Whibley et al. (p. 963; see the Perspective by Kramer and Donohue) investigated the genetic basis of flower color differences between closely related species of snapdragon. By analyzing a hybrid zone involving two color morphs, they identified three loci underlying color variation. Modeling of the genotypic space of color variation was used to map species into this space. The colors of flowers found in the hybrid zone occupied a distinct position in this space, one that is presumably less fit. These findings increase our understanding of adaptation in natural populations and suggest a new way of thinking about transitions between adaptive peaks.

  10. p53 and Tumor Angiogenesis

    The tumor suppressor protein, p53, transcriptionally activates genes that control cell cycle arrest, apoptosis, and other cellular processes that help to prevent tumor development. Teodoro et al. (p. 968) now show that p53 appears to keep tumors in check by activating the gene encoding α(II) collagen prolyl-4-hydroxylase. This enzyme is required for the extracellular release of collagen-derived peptides, such as endostatin and tumstatin, that are potent inhibitors of tumor angiogenesis. The p53 gene is inactivated in many human cancers, presumably leading to reduced production of endogenous antiangiogenic peptides that defend against tumor growth.

  11. Aging and Cancer

    Is there a link between organismal aging and cancer? Pinkston et al. (p. 971) address this question in a worm model of aging and tumor development and find that different signaling pathways implicated in the aging process also control tumorigenesis. Mutant worms with long life spans appear immune to the life-shortening effects of tumors because of enhanced defense mechanisms, including increased apoptosis and decreased cell proliferation within the tumors. Signaling pathways that control longevity may have coevolved with tumor suppressive mechanisms.

  12. Out of Defects, Order

    The molecules in a nematic liquid crystal will on average align in the same overall direction, but the presence of a secondary material will cause this overall alignment to curve or deviate, based on the interactions between the liquid crystal molecules and the secondary material. Muševič et al. (p. 954) use the ordering of a nematic liquid crystal mixed with colloidal particles to drive the crystallization of the colloidal particles. The formation of ordered two-dimensional colloidal structures is driven by defects present at the nematic-colloid interface. By using laser tweezers to position the colloidal particles, the authors could create ordered structures that have not been obtained by other methods.

  13. Diversity Produces Productivity

    What is the significance of genotypic diversity within populations for biodiversity and the functioning of ecosystems? Crutsinger et al. (p. 966) present experimental evidence that increases in genotypic diversity in a plant species, goldenrod, in a North American old-field system results in increased net primary productivity and arthropod species diversity. These field effects are of similar magnitude to those seen in experimental manipulations of plant species diversity.

  14. Gradual Channel Changes

    The voltage-gated potassium channel Kv2.1 is abundantly expressed in many mammalian central neurons, and its hyperactivity is associated with epileptic seizures and ischemic insults. Modification of the channel by phosphorylation and dephosphorylation alters its gating, but attempts to identify candidate phosphorylation sites have been confounded by the unusually large number of predicted phosphorylation sites. Park et al. (p. 975) have used an unbiased mass spectrometric approach to identify multiple in vivo phosphorylation sites on Kv2.1. Mutational analysis of these sites reveal that variable phosphorylation of the channel results in graded changes in the gating of Kv2.1 ionic currents. Such a graded regulatory mechanism could play a key homeostatic and neuroprotective role.

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