This Week in Science

Science  11 Sep 2009:
Vol. 325, Issue 5946, pp. 1315
  1. Assessing the Arctic

    The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering.

  2. Efficient Carbon Nanotube Photodiodes

    A single photon absorbed in a single-walled carbon nanotube device can generate multiple unbound particles carrying an electric charge. Gabor et al. (p. 1367) report that in such a device at low temperatures, excitation with light of increasing energy leads to well-defined stepwise increases in current. Interestingly, because of the unique band structure of carbon nanotubes, this behavior is analogous to particle-antiparticle creation commonly observed in high-energy particle physics. These observations point to the promise of investigations in other nanoscale carbon systems, such as graphene, and could lead to numerous applications, including highly sensitive photon detection and ultra-efficient photovoltaics.

  3. Mapping Out an Entropic Landscape

    CREDIT: ROST ET AL.

    Quantum critical points are continuous phase transitions occurring near absolute zero displaying interesting properties that may have valuable applications. Experimental thermodynamic information on quantum critical systems is sparse, partly because the better known systems are tuned using hydrostatic pressure, and thermodynamic measurements are difficult to perform under such conditions. Rost et al. (p. 1360, published online 6 August 2009; see the Perspective by Fisk) built and calibrated bespoke apparatus for a thermodynamic study of strontium ruthenate using a magnetic field as the tuning parameter. The specific heat and magnetocaloric measuremements were combined to map an “entropy landscape” of the quantum criticality and phase formation. This technique should offer a way to probe other materials to reveal unusual properties, such as novel metallic states and superconductivity.

  4. Himalayan-Tibetan Underplate

    The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Nábělek et al. (p. 1371) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath.

  5. It's a Gas

    Many antibiotics, including beta-lactams, aminoglycosides, and quinolones, kill bacteria (at least in part) by oxidative stress. Gusarov et al. (p. 1380) show that nitric oxide (NO) produced by bacterial NO synthases (bNOS) protects bacteria, including Staphylococcus aureus and Bacillus anthracis, against toxic agents they may encounter in the soil or in host organisms. Thus, bNOS activity is specifically induced in response to antibiotics and, in turn, activates the expression of another key antioxidant enzyme: superoxide dismutase. Hence, NO-mediated antibiotic resistance not only operates by direct chemical modification of toxic molecules, but also alleviates oxidative stress caused by naturally occurring antibiotics.

  6. Blowing Away Coal

    China is the world's largest carbon dioxide producer and the world's second-largest producer of electrical power, 80% of which it generates by burning coal. An affordable, carbon-free source of electrical power generation would thus constitute an important way for China to reduce its CO2 emissions and other environmental impacts of fossil-fuel burning. McElroy et al. (p. 1378, see the cover) show that there is enough wind in China to generate electricity to supply the nation's entire projected demand for 2030 (about twice what is used now) at reasonable prices per kilowatt-hour.

  7. Making Mother of Pearl

    Nacre is an iridescent layer of calcium carbonate lining the inside of shells of marine mollusks and is commonly known as “mother of pearl.” It is composed of layers of uniformly oriented crystals of aragonite (a metastable form of calcium carbonate) separated by layers of organic matrix. How the ordered structure of aragonite layers is achieved has been unclear. Suzuki et al. (p. 1388, published online 13 August 2009; see the Perspective by Kröger) identified two acidic matrix proteins (Pif 97 and Pif 80) that regulate nacre formation in the Japanese pearl oyster. The proteins appear to form a complex in which Pif 80 binds to aragonite and Pif 97 binds to other macromolecules in the organic matrix.

  8. Cycling Around

    CREDIT: FRANKENBERG ET AL.

    Water vapor is the most important greenhouse gas, and clouds are one of the most important components of climate, but the global hydrological cycle is still poorly-enough understood that the atmospheric cycling of water and cloud formation are inadequately represented in global climate models. As the transformation from liquid into vapor tends to deplete water of the isotope deuterium, Frankenberg et al. (p. 1374) were able to use satellite measurements of global “heavy” water abundances to provide a deeper understanding of atmospheric water dynamics.

  9. MITE-y Jumps

    Tranposable elements are genetic elements ubiquitous to most genomes, and their expansion, loss, and potential movement are major components of genome evolution. A type of noncoding transposable element in plants, known as a MITE (minature inverted repeat transposable element), shows evidence of recent expansion within specific rice genomes. However, these elements lack the necessary transposase to help them move. Yang et al. (p. 1391; see the Perspective by González and Petrov) found that the expansion of MITEs occurs because they exploit the activity of other, unrelated, transposons and consequently identified the molecular determinants that allowed mobilization. Surprisingly, the element coding for the transposase repressed its own movement unless the MITE was present. These findings may explain why some genetic elements can replicate within the genome and reach high copy numbers while others do not.

  10. Seeing Double

    CREDIT: BLEICHERT ET AL.

    A particular set of ubiquitous small (nucleolar) ribonucleoproteins are important for optimal ribosome function and protein synthesis. Bleichert et al. (p. 1384) used electron microscopy and single-particle analysis to investigate the structure of an archaeal version that contains the small RNA (sRNA) and all the associated core proteins. Unexpectedly, this ribonucleoprotein is a homodimer, formed of two sRNAs and four copies of each of the core proteins. This dimer is likely to be the enzymatically active form, as mutations disrupting di-sRNP formation inhibited activity.

  11. Maintaining Germline Stem Cells

    Spermatogonial stem cell pools in postnatal testes have to be maintained to continuously generate spermatozoa. It has been difficult to identify these stem cells in vivo, because of their small numbers and lack of appropriate molecular markers, but now Sada et al. (p. 1394) show that the RNA-binding protein NANOS2 is expressed in a small subset of spermatogonia that behave as self-renewing stem cells in intact testes. By a combinatorial use of loss- and gain-of-function studies, NANOS2 was found to be essential for the maintenance of the immature state of spermatogonial stem cells by supporting their self-renewing properties and by suppressing differentiation.

  12. Making Myelin

    The myelin sheath insulates neurons and facilitates rapid axonal conduction, and its disruption is characteristic of some neurological disorders, such as multiple sclerosis. Axonal signals stimulate Schwann cells to form myelin in peripheral nerves, but the mechanism is not completely known. By characterizing a mutation identified in zebrafish, Monk et al. (p. 1402; see the Perspective by Meijer) show that Gpr126, a member of the G protein–coupled receptor superfamily, is essential for myelin formation. It appears that Gpr126 acts as a receptor for axonal signals to elevate cAMP levels in Schwann cells and trigger myelination.

  13. Minimizing Brain Energy Consumption

    How much energy is actually required to generate neuronal activity and information processing? By combining direct recordings at physiological temperatures from mossy fiber axons in rat brain slices with modeling and simulation approaches, Alle et al. (p. 1405; see the Perspective by Magistretti) found that the regenerative action potentials in nonmyelinated axons of mammalian hippocampus are remarkably energy efficient. The data indicate a surprisingly minor contribution of action potentials to the entire energy expenditure of neural information processing.

  14. A Lower Tunnel

    Among the peculiarities inherent in quantum mechanics is the ability of particles to tunnel through barriers that they lack the energy to surmount classically, as happens during radioactive decay. Strong laser fields can liberate electrons in this way from atoms and molecules. Akagi et al. (p. 1364) elegantly confirm that tunneling is not limited to the highest-energy electrons in a system by mapping the energy and momentum of both the ejected electron and positive ion produced when an intense laser pulse impinges on hydrogen chloride. When the molecule adopts specific orientations relative to the laser field, tunneling occurs from lower-lying states, as well as the highest-energy occupied orbital. This raises the possibility of tunneling microscopy capable of imaging the electronic structure of single molecules.

  15. Crystal Clear

    Guanine nucleotide exchange factors stimulate exchange of guanine diphosphate (GDP) for guanine triphosphate (GTP) and activate small guanosine triphosphatases (GTPases) required for the regulation of many biological processes. Yang et al. (p. 1398) provide a detailed picture of the complete catalytic cycle by which a guanine nucleotide exchange factor, DOCK9, activates the small GTPase, Cdc42. A small region of DOCK9 appears to sense whether GTP or GDP is bound to Cdc42 in a mechanism that is distinct from that observed for other GTPases.

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