Editors' Choice

Science  20 Jul 2007:
Vol. 317, Issue 5836, pp. 296
  1. GEOPHYSICS

    Electric Aftershocks

    1. Joanne Baker

    Earthquake ruptures are expected to generate electromagnetic activity within the surrounding rocks, but direct evidence for this effect has been lacking. Laboratory experiments on real rocks do generate currents due to fluid movement and piezoelectric effects, but they are weak and in the geological setting it is hard to disentangle them from anthropogenic signals or more ambient electronic noise. Park et al. report possible detection of a characteristic electrical signal using an electrode array placed on the San Andreas Fault at Parkfield, California. Electrical disturbances lasting 3 hours were picked up within 250 m of the fault immediately after a magnitude 6.0 earthquake that occurred in September 2004; signals of opposite polarity were subsequently detected after two magnitude 5.0 aftershocks. Although similar electromagnetic changes do occur on a daily basis in this area, the team argue that the localization, timing, and unusual polarity of their signals support association with the earthquake rupture process. They propose fluid movements as the most likely cause of the electrical signals, although they are unable to explain the rapid onset. No precursor signals were observed, so this technique may not ultimately help with earthquake prediction. — JB

    J. Geophys. Res. 112, 10.1029/2005JB004196 (2007).

  2. ECOLOGY/EVOLUTION

    Smaller Harvests Than Expected

    1. Andrew M. Sugden

    Leaf-cutting ants of the genus Atta are ubiquitous residents of neotropical forests. They construct large subterranean colonies and journey on trails across the forest floor and into the forest canopy, where they harvest leaf fragments that are carried back to the nest. The fragments nourish a mutualistic fungus that in turn provides protein and carbohydrate for the ant colony. Leaf-cutters have been widely assumed to be the dominant herbivores in the forests they inhabit, but supportive quantitative data for this assumption are sparse. Herz et al. first used a rapid and nondestructive method, involving the sampling of refuse deposited by ants outside their nests, as a proxy for measuring the daily harvest of leaves. Then they collected data from nearly 50 nests over 15 months in a Panamanian forest and calculated that the ants were actually responsible for only about 0.7% of total leaf consumption by all folivores (insects and vertebrates) in the forest. Even though these results indicate that the defoliation by leaf-cutters is more modest than previously thought, Urbas et al. found that herbivory by leaf-cutters in a Brazilian forest increased at the margins (versus the interiors) of forests that had been fragmented by human disturbance, thus amplifying environmental change at the forest edge. — AMS

    Biotropica 39, 476; 482; 489 (2007).

  3. BIOCHEMISTRY

    Surviving a Dry Spell

    1. Gilbert J. Chin

    Life (as we know it) is based on carbon, and one fortuitous factor is the compatibility of sugars and water. Glucose is readily soluble (at much higher concentrations than the building blocks of other biological polymers), easily handled by enzymes via its chemical functionalities, and benign (and perhaps even beneficial) in its interactions with other biochemicals. In considering the major circulating sugar in insects—trehalose, which is a head-to-head dimer of glucose—the extraordinary tolerance of Polypedilum vanderplanki larvae to dessication comes to mind. When the rock pools where these larvae live dry up, the larval fat body synthesizes trehalose and releases it into the hemolymph in order to protect tissue constituents as water is lost. When water becomes available again, dehydrated larvae undergo rehydration and resume their developmental progression into adult midges. Kikawada et al. have identified a trehalose transporter (called TRET1) in P. vanderplanki. They show that it is specific for trehalose versus maltose, sucrose, and lactose; they also show that it functions as a low-affinity, high-capacity facilitated transporter that can be expressed benignly in mammalian cells. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 104, 11585 (2007).

  4. COMPUTER SCIENCE

    Natural and Artificial Flavors

    1. David Voss

    Computer scientists have long worried that their field suffers from split personality disorder: is what they do mathematics or engineering? True, they work on problems such as writing software to carry out calculations on a machine, but they also grapple with the most abstract mathematical properties of computational procedures and the logic of algorithms. So the debate has raged: Is the field a science of the natural world or only a science of the artificial? Denning argues that computer science is decidedly a natural science. Information storage and processing have been found to be fundamental elements of many fields, from the biological data stored in DNA to the quantum information that is transmitted and modified as particles interact. In many areas, principles that transcend computing machines form a set of questions about the deep structure of computation. These questions, in turn, are driving innovative ways to teach computing, sometimes without using sophisticated computer gadgetry at all. The author concludes that the field encompasses a science of information processing in both natural and artificial systems. — DV

    Commun. ACM 50, 13 (2007).

  5. ECOLOGY/EVOLUTION

    Eats Roots and Leaves

    1. Andrew M. Sugden

    The understanding of food webs in soil has lagged behind that of above-ground or aquatic systems because of the bewildering complexity of soil organism communities and the sheer intractability of making observations and doing experiments in soil. It has long been thought that invertebrates in forest soils derive most of their carbon from leaf litter that falls from trees. Pollierer et al. used a construction crane to alter the isotopic ratio of 13C and 12C supplied (as CO2) to the canopy of a Swiss forest. They then reciprocally transferred the resultant leaf litter to neighboring forest areas that had experienced a normal isotopic ratio of CO2, and measured the isotopic ratios in the tissues of soil animals. The carbon isotopic ratio in the invertebrates more closely matched that of the tree roots rather than that of the leaf litter to which they were exposed, indicating that the diet of these animals derived primarily from root tissue and exudates as compared to fallen leaves (which therefore appear to be processed largely by microorganisms). If this pattern extends to other temperate forests, the configuration of below-ground food webs and patterns of carbon flux might have to be reconsidered. — AMS

    Ecol. Lett. 10, 729 (2007).

  6. CHEMISTRY

    Different Routes to a Cluster

    1. Phillip D. Szuromi

    In heterogeneous catalysis, the routes whereby molecules come and go from the active sites can substantially affect their reactivity. Röttgen et al. have examined a case where direct and indirect adsorption processes compete: the oxidation of CO over Pd clusters supported on MgO films grown on a metal substrate. The Pd clusters (either Pd8 or Pd30) were mass-selected before deposition, and by changing their surface coverage, the authors could vary the ratio of incoming CO that adsorbed directly on the cluster versus that arriving via diffusion from the support. Data and modeling revealed that for the Pd8 clusters, the reaction probability was the same whether the CO arrived directly or by diffusion, whereas for the Pd30 clusters, the CO supplied by reverse spillover from the support was less reactive than that impinging directly. The results highlight the subtleties of structure-dependent activation energies. — PDS

    J. Am. Chem. Soc. 129, 10.1021/ja068437f (2007).

  7. STKE

    Numb Cells Keep Moving

    1. John F. Foley

    Integrins are heterodimeric transmembrane receptors that bind to components of the extracellular matrix and are important for both cellular adhesion and migration. The clustering of activated integrins on the substrate-facing surface of the leading edge of a cell results in the recruitment of various proteins, including actin stress fibers, to form a focal adhesion complex (FAC). Cells move, in part, through the coordinated assembly and disassembly of focal adhesions at the leading edge of the cell. Numb is a cargo-specific adaptor protein that binds to several endocytic proteins, and Nishimura et al. examined the role of Numb in endothelial and epithelial cell cultures. In a wound-healing assay, Numb polarized toward the leading edge of migrating cells (just behind the lamellipodium), and immunostaining demonstrated that Numb and β-integrin colocalized at focal adhesions. Coimmunoprecipitation experiments revealed that Numb bound to the PAR (for partitioning defective) polarization complex PAR-3. This complex also localizes to the leading edge of polarized migrating cells. One component of this complex, atypical protein kinase C (aPKC) phosphorylated Numb in HeLa cells and, as a consequence, Numb no longer bound to integrins. The authors propose that Numb binds to free integrin molecules (rather than disrupting FACs) and recruits them to clathrin-coated structures to initiate integrin recycling, and that the localization and function of Numb are negatively regulated by aPKC. — JFF

    Dev. Cell 13, 15 (2007).