Editors' Choice

Science  11 Jan 2013:
Vol. 339, Issue 6116, pp. 120
  1. Chemistry

    Acid to Acid

    1. Jake Yeston

    Hydration of sulfur trioxide (SO3) to sulfuric acid (H2SO4) in Earth's atmosphere gives rise to acid rain as well as particle formation. A sound mechanistic understanding of this reaction is thus central to the chemistry of climate and pollution mitigation. Although the necessary bonding rearrangements appear straightforward—S-O bond formation accompanied by proton transfer from water to any of the other oxygen atoms—the direct bimolecular reaction is in fact extremely slow. Theoretical and experimental studies have demonstrated that the assistance of a second, catalytic, water molecule is crucial to facilitate the proton transfer. Torrent-Sucarrat et al. now report, on the basis of extensive theoretical calculations, that a second sulfuric acid molecule ought to be an even more potent catalyst; in other words, the reaction is autocatalytic. The authors combine electronic structure calculations with a transition-state theory framework to predict rate-constant ratios, and then estimate the prevalence of this pathway depending on the relative background concentrations of water and sulfuric acid. The sulfuric acid–to–water ratio is too low for significant autocatalysis in the troposphere, but the pathway could play a pivotal role in the stratosphere as well as in aerosols, and perhaps in the atmosphere of Venus.

    J. Am. Chem. Soc. 134, 20632 (2012).

  2. Ecology

    Vital Variation

    1. Sacha Vignieri

    It is increasingly recognized that we are in the midst of a sixth mass extinction due to a suite of anthropogenic activities. How different species respond to these changes is varied. González-Suárez and Revilla ask whether there are clear patterns in susceptibility to extinction among mammals, using the database panTHERIA, which consists of data on over 4400 mammalian species. They examined morphological, ecological, and life-history traits as they relate to the threat of extinction. As might be expected, species with lower densities and slower rates of reproduction and maturation, such as larger mammals, experienced a greater risk of decline. However, greater variation in traits such as body size and litter size, among others, was associated with reduced risk of extinction, regardless of body size. This suggests that species that maintain variability at the population level may be better able to adapt to changing environmental conditions. These results can inform conservation efforts across an array of species and emphasize the importance of protecting natural variation in all its forms.

    Ecol. Lett. 10.1111/ele.12035 (2012).

  3. Physics

    Shrinking Atomic Clocks

    1. Ian S. Osborne

    Atomic clocks housed in national laboratories around the world provide a precise time signal on which our present high-technology lives depend. Precise synchronization allows for colossal amounts of data to be reliably transferred around networks of optic fibers at ever-increasing rates, as well as providing accurate navigation systems. With increasing demand for mobile devices, there is a requirement for these atom-based time pieces to shrink. Present chip-scale atomic-clock (CSAC) devices based on alkali atoms in a buffer gas are lightweight (<50 g) and consume little power (<150 mW) but tend to show frequency drift over time that requires frequent recalibration. Clocks based on trapped single atoms or ions should be more stable because the atoms are vacuum-packed and do not interact. However, these tend to be relatively large and power-hungry. Jau et al. have developed a miniature atomic clock based on trapped Yb ions that has size and power requirements similar to those of existing CSAC technology but also offers to match the long-term stability expected of the much larger trapping systems.

    Appl. Phys. Lett. 101, 253518 (2012).

  4. Physics

    Classifying Topological Phases

    1. Jelena Stajic

    One of the most exciting developments in condensed-matter physics over the past few years has been the prediction and discovery of new topological phases of matter. The most interesting of these, topological insulators, are characterized by surface states robust to perturbations that preserve time reversal symmetry (TRS). Recently, other symmetries, such as spatial inversion, have been demonstrated to be protective of topological states. Slager et al. have greatly expanded the theoretical framework for identifying new topological states in non-interacting fermionic systems by classifying states protected by different space group symmetries of the underlying crystal lattice. They found two broad classes of such phases, one where the protection comes from both TRS and space group symmetry, and another which is protected by lattice symmetry alone and would be considered topologically trivial under the TRS-based classification system. This led to the identification of 18 distinct phases in two dimensions and at least 70 in three dimensions. It is expected that the classification will guide future experimental efforts and inform the study of topological matter in the presence of interactions.

    Nat. Phys. 10.1038/NPHYS2513 (2012).

  5. Ecology

    Microbial Conversion

    1. Andrew M. Sugden

    Conversion of tropical forest to pasture has been widespread in the Amazon Basin in recent decades, a process that has been accompanied by a loss of diversity of flora and fauna and often by a reduction in soil fertility. However, how conversion affects soil microorganisms is largely unknown. Using DNA sequencing techniques, Rodrigues et al. compared the bacterial composition of forest and pasture soils at a site in Rondonia, Brazil. In pasture soils, there was a reduction in the diversity of taxa in the phylum Acidobacteria, organisms that are sensitive to increases in pH and soil carbon content, both of which occur after forest clearance. On the other hand, there was an increase in diversity within the phylum Firmicutes, which are tolerant of desiccation and greater extremes of temperature. At individual sample sites, the alpha diversity (number of taxa) of bacteria was higher in the pasture soils than in forest soils, but the beta diversity (variation between sites) of the pasture soils was significantly lower than in the forest. The lower beta diversity of the pasture soils implies a biotic homogenization of the soil microbiota after the conversion to pasture, and an eventual loss of overall diversity despite local increases in alpha diversity.

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1220608110 (2012).

  6. Biophysics

    Dying to Survive

    1. Valda Vinson

    Bacteria grown on surfaces often form dense communities, often with complex three-dimensional wrinkled structures, called biofilms. These are often problematic—for example, in industrial and medical settings—but can also be harnessed for uses such as waste remediation. Asally et al. showed that in Bacillus subtilis, wrinkling is caused by localized cell death that spatially focuses mechanical forces and may be a community-level stress response. Wilking et al., also working with B. subtilis, showed that the wrinkles make up the top of a network of channels that provide a system for enhanced transport of nutrients. As biofilms grow, nutrients are consumed by cells on the periphery, leaving bacteria in the center facing nutrient depletion. Such channels provide a conduit for nutrient flow and are most highly connected near the center of the biofilm. Flow through the channels was driven by spatial variation in evaporation and also by factors such as osmotic pressure gradients. Channels are internalized as the biofilm ages; however, the network remains permeable even in the late stages of biofilm growth, which suggests that the channels may be physiologically relevant throughout the biofilm life cycle.

    Proc. Natl. Acad. Sci. U.S.A. 109, 18891; 10.1073.pnas.1216376110 (2012).

  7. Bioengineering

    Vascular Glue

    1. Paula A. Kiberstis

    Atherosclerosis is characterized by the accumulation of plaque—lipid-laden inflammatory cells surrounded by a fibrous cap—in the inner lining of blood vessels. In so-called vulnerable plaques, the fibrous cap is at high risk of rupture, an event that results in blood clots that can potentially trigger a heart attack or stroke. The development of strategies that can stabilize and/or heal vulnerable plaques is a major goal of cardiovascular research.

    Inspired by the properties of underwater adhesives secreted by marine mussels, Kastrup et al. designed an adhesive hydrogel that can be painted on atherosclerotic plaques, with the idea that this “glue” might be useful both for localized deposition of drugs and for strengthening the fibrous cap to prevent its rupture. In a proof-of-concept study, gel containing dexamethasone, an antiinflammatory drug, was applied via a catheter to inflamed atherosclerotic plaques in the carotid arteries of mice. The gel remained adherent for over a month, and the plaque in the gel-treated mice displayed a thicker fibrous cap and a 25% reduction in a marker of inflammatory cells as compared with plaques in control mice.

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1217972110 (2012)