This Week in Science

Science  10 Mar 2006:
Vol. 311, Issue 5766, pp. 1341
  1. Ecosystem Effects of Climate Change


    Contemporary climate changes affect the geographical distribution of a number of species of terrestrial and marine organisms. Grebmeier et al. (p. 1461) observed responses to climate change in an entire ecosystem, the northern Bering Sea. This ecosystem is relatively shallow, with a rich benthic prey source that supports bottom-feeding marine mammals and seabirds that are hunted by local human populations. During the past decade, there has been a geographic displacement of marine mammal population distributions northward, a reduction of benthic prey populations, an increase in pelagic fish, a reduction in sea ice, and an increase in air and ocean temperatures.

  2. Unraveling Chemical Collisions

    Gas-phase spectroscopy and accompanying theoretical computations have been used to resolve two long-standing puzzles in the interplay of electronic and nuclear molecular motion in chemical reactions (see the Perspective by Zare). Yin et al. (p. 1443) probed the impact of electronic state on the unimolecular dissociation of formaldehyde (H2CO) into H and HCO products. Their results suggest that bond scission in the ground state produces rapidly rotating HCO, whereas dissociation in the excited triplet state yields vibrationally excited HCO. Qiu et al. (p. 1440) studied a bimolecular reaction: collision of an F atom with H2 to yield HF and H. At a specific collision energy, the experiments and theory point to a transient complex, termed a Feshbach resonance, in which the colliding partners vibrate several times before rearranging to products.

  3. Clay and Atmospheric Oxygen

    The oxygen content of Earth's atmosphere increased dramatically and permanently during the Neoproterozoic and has remained high since then, which suggests that the mechanisms underlying this increase must have included some irreversible change in the global biogeochemical cycle. Kennedy et al. (p. 1446, published online 2 February; see the Perspective by Derry) hypothesize that oxygenation of the atmosphere resulted from an increase in the rate of burial of organic carbon caused by the accelerated production of clays. In shallow marine environments, clays retard the oxidation of organic matter and facilitate their burial. The authors use this insight, along with mineralogical and geochemical evidence of an increase in clay deposition in the Neoproterozoic, to show how the stepwise transition from a low-O2 atmosphere to one with abundant O2 could have occurred.

  4. Scandinavian Deglaciation

    The Scandinavian Ice Sheet, the second largest Northern Hemisphere ice sheet at the end of the last glacial period, must have contributed significantly to glacial-interglacial sea level and regional climate changes. However, the timing of the decay of the Scandinavian Ice Sheet remains poorly constrained. Rinterknecht et al. (p. 1449) present a suite of cosmogenic 10Be ages and radiocarbon dates of glacial deposits that define more precisely the timing of major fluctuations of the southern margin of the Scandinavian Ice Sheet in central and eastern Europe.

  5. Exposed Cometary Ice


    Exposed deposits containing water ice have been found on the surface of the comet 9P/Tempel 1. Images obtained by Sunshine et al. (p. 1453, published online 2 February) with cameras on board the Deep Impact spacecraft reveal several patches that are bluer than the rest of the surface. Absorption features in infrared spectra confirm the presence of water ice in these spots and suggest it is present in aggregates of grains that are tens of micrometers in size. The deposits are relatively impure and contain only a few percent water ice and are too small in area to be the main source of water vapor that outgases from the nucleus.

  6. Rodent Resurrection

    When the new species of rodent Laonastes was described last year, it attracted broad attention because it was claimed as a representative of an entirely new family of living mammals. Dawson et al. (p. 1456) compared Laonastes with the Diatomyidae, a poorly known group of rodents from the Oligocene and Miocene of Asia. Anatomical comparisons of a new fossil Miocene diatomyid with Laonastes confirmed that Laonastes is actually a living member of this “extinct” clade. Hence, Laonastes “resurrects” a clade of mammals that was formerly thought to have been extinct for more than 10 million years.

  7. Invasive Chain Reaction

    Biological invasions by exotic species are a leading threat to native biodiversity and entail enormous monetary costs. In a meta-analysis of field studies from a wide range of ecosystems, Parker et al. (p. 1459) challenge the hypothesis that invasive exotic plants become a problem in their adoptive lands because they left their coevolved herbivores behind. Instead, herbivores in the invaded communities are better able to resist invaders than do the enemies of those plants in their original home. By the same token, introduced herbivores are harder on native plants in lands they invade than on introduced plants, including those with which they coevolved. Thus, the replacement of native with exotic herbivores triggers an invasional “meltdown” whereby one exotic species facilitates invasions by others.

  8. Redox Stages in Respiration


    In bacteria and mitochondria, a flavin cofactor within complex I of the membrane accepts reducing equivalents, converts some of the energy into a proton gradient, and passes electrons onward via a quinone carrier to other membrane-bound enzymes. Sazanov and Hinchliffe (p. 1430, published online 9 February) describe the crystal structure of the eight-subunit hydrophilic portion (the part outside the membrane) of respiratory complex I from Thermus thermophilus and describe the environments of the flavin and the nine iron-sulfur clusters that transport the electrons from the dihydronicotinamide adenine dinucleotide (NADH) binding site into the hydrophobic (proton-pumping) domain of the complex. They propose that the outermost cluster accepts the second electron from the flavin, which helps to reduce the generation of potentially deleterious reactive oxygen species.

  9. Closing Nisin's Rings

    Nisin, an antimicrobial peptide widely used as a food preservative, is part of a group of posttranslationally modified peptides known as lantibiotics, which are characterized by thioether structures. Nisin contains five thioether rings of varying size formed by the enzyme NisC. Li et al. (p. 1464; see the Perspective by Christianson) have reconstituted the nisin cyclization process in vitro and determined the x-ray crystal structure of the NisC enzyme. NisC is structurally similar to mammalian farnesyl transferases with an active-site zinc ion that activates nucleophilic cysteine residues during cyclization.

  10. Global Problems in Protein Folding in Polyglutamine Diseases?

    A number of distinct, seemingly unrelated mechanisms have been proposed for polyglutamine, or trinucleotide repeat diseases, which include spinocerebellar ataxia type 3. These mechanisms include disregulation of transcription, protein degradation, and mitochondrial function, as well as activation of apoptosis. Gidalevitz et al. (p. 1471, published online 9 February; see the Perspective by Bates) have taken a genetic approach and find that polyglutamine expansions in Caenorhabditis elegans cause global perturbation in protein folding. This progressive disturbance of protein folding may provide an explanation for the multitude of cellular pathways affected in conformational diseases.

  11. Prevention Is Cheaper Than Treatment

    In strategies to fight the AIDS epidemic, considerable emphasis has been placed on treatment options and costs. Stover et al. (p. 1474, published online 2 February) have evaluated the cost-effectiveness of prevention approaches on the basis of UNAIDS/WHO predictions of prevalence. By their calculations, roughly 30 million new infections could be prevented between 2005 and 2015 if a package of 15 prevention approaches targeting sexual transmission and transmission among injecting drug users were used in 125 low-and middle-income countries. These averted infections translated into dramatic savings because of the diminished needs for treatment and care.

  12. Mixing Scents

    How are odors represented in the higher processing areas of the brain? Zou and Buck (p. 1477) compared the responses of mouse olfactory cortical neurons to binary mixtures of odorants versus their individual components. They monitored neuronal activity in the anterior piriform cortex of the same animals in response to individual odors and mixtures. The technique used enabled the authors to monitor neuronal activity in response to two temporally segregated experiences. The results suggest that olfactory cortical neurons receive convergent input from multiple odorant receptors and that a subpopulation may require such convergent input for activation.

  13. Pushy Protons

    Titanium dioxide semiconductors and their derivatives are under vigorous study as catalysts for the photogeneration of hydrogen from water. To model the complex dynamics at the liquid-solid interface, Li et al. (p. 1436) irradiated methanol-coated TiO2 to produce transiently solvated electrons and monitored their relaxation back to the bulk TiO2 with two-photon photoemission spectroscopy. The electron dynamics were coupled to that of the methanol nuclei; increased electron lifetimes were seen for CH3OD versus CH3OH. Density functional calculations implicate a proton-coupled mechanism in the back transfer of electrons to the TiO2 in a manner analogous to the charge transport mechanisms found in light-harvesting proteins.

  14. Making Molecular Motors Work Together

    Cooperativity between motor proteins can allow physiological functions that are not available with single motors. However, investigating how cooperativity relates to function is challenging. Diehl et al. (p. 1468) engineered model systems using bacterial expression of artificial proteins to produce polymeric scaffolds that assemble kinesin motors. They could then control the number of motors, the intermotor distance, and the nature of elastic coupling. Microtubule gliding velocities were enhanced in multimotor assemblies but, in contrast to collections of unorganized motors, were not influenced by the elasticity of the scaffold.

  15. Extending Genetic Interaction Maps

    Although there are elegant interaction maps available for yeast and Drosophila, it will become more difficult to find complete, high-quality data for higher organisms. Zhong and Sternberg (p. 1481; see the Perspective by Eddy) have developed a computational approach to cross-species data integration and have used it to generate a map of Caenorhabditis elegans genetic interactions. To test the robustness of their predictions, RNA interference analysis was used to verify novel interactors predicted for let-60 and itr-1.

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