Editors' Choice

Science  23 May 2008:
Vol. 320, Issue 5879, pp. 987

    A Steady Supply of Food

    The environmental and anthropogenic factors affecting leatherback turtle populations are generally unknown and cannot explain why populations are increasing in the Atlantic yet declining in the Pacific. By analyzing reproductive success rates, migratory patterns, and oceanographic variables, Saba et al. demonstrate that turtle populations are most likely regulated by the abundance of zooplankton where they forage, as measured by the net primary productivity (NPP) estimated from satellite data. NPP was shown to vary among oceanic regions and to be affected by regime shifts that typically enhance ocean productivity; however, both the total NPP and overall consistency in NPP over time were much lower in the Pacific regions examined than in the Atlantic. In turn, these differences in local resources appear to affect turtle size and overall egg production—larger females that have matured faster lay more eggs than smaller females. These differences, coupled with anthropogenic impact and the effects of climatic shifts on NPP, may explain the discrepancy in demography between the Atlantic and Pacific leatherback populations. — LMZ

    Ecology 89, 1414 (2008).


    Tracking Gyrating Memories

    Whereas the bits in present magnetic memory storage media are written using electrical current pulses in a write head, recent work has shown that the spin-polarized current in a ferromagnetic medium can impart a spin-transfer torque to the magnetization, resulting in its motion. Such a wired-up memory architecture would allow yet higher storage density and provide faster access times than present hard drives offer. Understanding how the spin current affects the dynamics will help in developing memory storage devices based on this effect. Bolte et al. provide a direct view of the dynamics of the process. They use time-resolved x-ray imaging to track the motion of the magnetization, showing how magnetic vortices in permalloy dots gyrate and jiggle in response to the injected spin-polarized current. The technique is sufficiently sensitive to uncover the various contributions to the dynamics, thereby offering the opportunity to fine-tune the process. — ISO

    Phys. Rev. Lett. 100, 176601 (2008).


    Does Saltwater Wobble?

    When ionic salts dissolve in water, the resultant solution tends to become more viscous than the pure liquid. This observation can be broadly understood based on a picture in which the web of hydrogen bonds holding the water molecules together rigidifies around solvated ions. However, the molecular details appear much more complicated, because time-resolved vibrational spectroscopy has suggested that most individual water molecules continue to rotate freely regardless of how much dissolved salt is present. Turton et al. explore this discrepancy using two related spectroscopic techniques to probe the molecular structure of aqueous salt solutions. The first, dielectric relaxation spectroscopy, is sensitive to individual molecules' orientations, and confirms the rotational freedom previously observed. The second technique, optical Kerr effect (OKE) spectroscopy, reflects polarizability and so is more sensitive to intermolecular changes that stem from translation. The OKE data reveal increasing translational restriction with rising salt concentration, as the ion solvation shells crowd against one another. The authors note that such a decoupling of rotation and translation is analogous to the jamming that occurs during transitions from a liquid to a glass upon supercooling. — JSY

    J. Chem. Phys. 128, 161102 (2008).


    Engineering a Healing Environment

    In respiratory conditions such as asthma and chronic obstructive pulmonary disease, the airways narrow, leading to impaired oxygen exchange. Tissue engineering affords one approach to reverse such damage: Endothelial cells embedded in a polymer matrix adopt normal morphologies and, when implanted close to sites of damage, promote vascular tissue repair without triggering an immune response. Zani et al. have applied this method by embedding into a denatured collagen matrix both the epithelial cells that line airways and the endothelial cells from surrounding tissue layers. Wrapping injured trachea in this cell-containing matrix enhanced recovery as measured by the size of the lumen and extent of the injury. Either cell type alone had beneficial effects, but both together synergized to improve luminal size and epithelial area more effectively. Measurements of cytokine and growth factor secretion from the endothelial and epithelial cells indicate that the two cell types are interdependent. Their crosstalk increases healing, presumably via secretion of repair-promoting factors, but because the cells are interspersed within the matrix, the normal cellular architecture of the tissue seems not to be required. — KK

    Proc. Natl. Acad. Sci. U.S.A. 105, 7046 (2008).


    P in All Its Guises

    Phosphorus is the rarest of the major biological nutrients on Earth, tending to be locked up primarily in a few rather insoluble minerals. Nonetheless, it plays a key role in biological processes and metabolism, feeds primary production in the oceans and plants on land, and is required to form bones, teeth, and some shells. Rampant soil fertilization and industrial use have conversely led to the problem of too much phosphorus in many rivers and estuaries, where it is difficult to remove.

    Filippelli considers the long-term geological cycling of phosphorus and how this critical element gets released to the environment over time or sequestered. Erosion enhances the release of phosphorus, which makes its way to the oceans. Records of past marine phosphorus concentrations and biological productivity suggest that the formation and uplift of the Himalayas may have thus fueled ocean productivity ∼20 million years ago; similar arguments have been proposed for glacial cycles. Further in the past, the relation between enhanced erosion and thick sequences of phosphorus marine rocks remains unclear. Today, the phosphorus cycle is dominated by human activities; we have doubled the natural riverine load globally. As discussed by Oelkers and Valsami-Jones, further use at this rate may soon run up against the limited geological supply. — BH

    Elements 4, 89; 83 (2008).


    A Specialist Repertoire

    Sequencing the genome of the fungus Trichoderma reesei has led to surprising questions about how it degrades biomass efficiently and whether it can be engineered for the commercial production of biofuels. Breaking down lignocellulose in plant cell walls requires cellulases and hemicellulases, but Martinez et al. found that T. reesei harbors fewer genes dedicated to cellulose digestion than do 13 other fungi that digest plant cell walls. Some of these genes are located in clusters, which may reflect a functional organization that allows efficient enzyme production. Moreover, genes encoding the secretory machinery of T. reesei are similar to those of budding yeast, consistent with its extraordinary ability to secrete 100 g of protein per liter. The absence of a wider variety of cellulose- degrading enzymes may be an opportunity to engineer industrial strains of T. reesei for economical processing of biomass feedstocks used in the production of biofuels. — LC

    Nat. Biotechnol. 26, 553 (2008).

  7. Science Signaling

    Lateral Receptor Transfer

    Although many malignant gliomas display EGFRvIII, an oncogenic mutant form of the epidermal growth factor receptor (EGFR), the mutant receptor may be expressed only by a subset of cells in the tumor. Al-Nedawi et al. found that a human glioma cell line (U373) transfected with EGFRvIII (U373vIII) produced more membrane-derived microvesicles than did the parent cell line. The U373vIII cell-derived microvesicles contained EGFRvIII, as well as the lipid raft marker flotillin-1. Unlike U373 cells, U373vIII cells formed subcutaneous tumors when injected into immunodeficient mice, and these tumors released EGFRvIII-containing microvesicles into the circulatory system. U373 cells that had been exposed to EGFRvIII-containing microvesicles showed increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). Pharmacological treatment to inhibit signaling by EGFR-family receptors attenuated this increase in ERK1/2 phosphorylation, as did treatment with annexin V (to prevent microvesicle uptake). EGFRvIII-containing microvesicles also stimulated other downstream events: release of vascular endothelial growth factor, levels of the anti-apoptotic protein Bcl-xL, and the ability of U373 cells to grow in soft agar (indicative of malignant transformation). The authors conclude that membrane microvesicles provide a pathway for transfer of the transformed phenotype. — EMA

    Nat. Cell Biol. 10, 619 (2008).

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