This Week in Science

Science  31 Oct 2003:
Vol. 302, Issue 5646, pp. 741
  1. An Old Unstable Ocean

    The “snowball” conditions of the Neoproterozoic may have been the coldest in Earth's history. Between 750 and 550 million years ago, essentially the entire planet was periodically covered with ice, although it is not clear why such extreme glaciations occurred only during that era. Ridgwell et al. (p. 859; see the Perspective by Archer) use a coupled model of the atmosphere-ocean-sediment carbon cycle to develop an explanation for the severity of Neoproterozoic ice ages. They found that an ocean without pelagic calcareous plankton—like that of the Neoproterozoic, because calcifying coccolithophores and foraminifera did not evolve until the early Phanerozoic—would be subject to intense swings of the carbon cycle, with low atmospheric CO2, and thus cold conditions, occurring when sea level fell.

  2. Going with the Grain Boundary

    High-resolution electron microscopy can achieve angstrom-level resolution, but imaging of light atoms is still challenging. Recently, oxygen vacancies in single crystals of perovskite ceramics were identified. Zhang et al. (p. 846) have now resolved oxygen atoms in grain boundaries, a problem of considerable interest because grain boundaries affect a number of material properties. They observed atom-spacing changes at the boundaries and expanded metal-oxygen bonding. These observations validate recent density functional theory calculations on the grain-boundary structure.

  3. The Fundamentals of Fragility

    Many materials that normally crystallize can form amorphous glasses if they are cooled rapidly. Empirically, the ease with which a glass will form has been described by its “fragility,” which can be quantified by measuring how fast its viscosity changes as it approaches the glass transition temperature Tg. Scopigno et al. (p. 849) show that the fragility can be correlated to the vibrational properties of the molecules well below Tg, which links the empirical fragility index to the interatomic interactions.

  4. Thinned Out

    Was the collapse of two large sections of the Larsen Ice Shelf in Antarctica during the past decade directly related to climate warming? Shepherd et al. (p. 856; see the news story by Kaiser) report satellite elevation measurements that show evidence of a widespread mass imbalance of the Larsen Ice Shelf, and conclude that ocean-driven melting has progressively thinned the ice shelf at its base. Their analysis shows that the shelf has thinned for at least 9 years, beginning prior to the well-documented collapse events of 1995 and 2002, and that this thinning may have played a pivotal role. They conclude that the remaining Larsen-C section will reach a similar condition in ∼100 years. These results identify a possible direct link between the observed climate warming and the breakup of ice shelves at the Antarctic Peninsula.

  5. Iceman Origins

    The Alpine Iceman, a well-preserved mummy with clothing and hunting-gathering equipment from the Neolithic-Copper Age of Europe, was recovered from a glacier along the Alpine watershed between Italy and Austria. The Iceman was about 46 years old when he died and lived about 5200 years ago. Müller et al. (p. 862; see the news story by Holden) have determined the Iceman's origin and migration from isotopic analyses of tooth enamel and dentine, cortical and trabecular bone, and contents of the intestine. The Iceman lived south of the Alpine watershed within a range of about 60 kilometers for his entire life, but he spent his early childhood at a different site within this range from where he spent most of his adulthood.

  6. Transcription of the Arabidopsis Genome

    Although the genome of the model plant Arabidopsis thaliana has been sequenced, it will be crucial for the research community to have all of the transcriptional units identified. Yamada et al. (p. 842) used two strategies to provide such a map. They produced both a set of full-length complementary DNAs and a set of high-density oligonucleotide arrays that “tiled” the entire genome. Using these tools, evidence was produced for the expression of previously hypothetical genes. In addition, a large fraction of genes were seen to have antisense expression. Finally, the work revealed the existence of transcriptional hotspots within centromeres, the region of the chromosome responsible for faithful chromosome segregation during cell division.

  7. Unmasking Silenced DNA in Neurons

    Methylation of DNA is thought to be important for long-term silencing of gene expression. Chen et al. (p. 885) and Martinowich et al. (p. 890) now show that DNA methylation is a critical regulator of acute neuronal gene expression in response to depolarization (see the Perspective by Klose and Bird). In both studies, MeCP2, a protein that specifically binds methylated DNA, associated with the promotor of the gene encoding brain-derived neurotrophic factor (BDNF) in unstimulated neurons, but was released once depolarization-induced signaling occurred. Chen et al. show that such release correlates with increased expression of the BDNF gene and appears to be associated with regulated phosphorylation of MeCP2. Martinowich et al. suggest that activity-dependent expression of the BDNF gene may result from release of a repression complex that contains MeCP2, histone deacetyase 1, and the corepressor mSin3A. The proposed role of MeCP2 in learning and memory may help to explain why mutations in MeCP2 leads to Rett syndrome, a human disease that causes abnormal brain development and progressive neurological decline.

  8. Predation Drives Lemming Population Dynamics

    Within the study of rodent population dynamics, it has been long debated whether predator-prey dynamics alone can account for high-latitude cycles in lemmings and voles, or whether dynamics of food plant availability and quality plays a key role in cycles. Gilg et al. (p. 866; see the Perspective by Hudson and Bjørnstad) bring together a remarkable set of long-term data (15 years, or three full 4-year cycles) for collared lemmings and their four vertebrate predators (stoat, fox, owl, and skua) in their 75-square-kilometer study site in Greenland. No evidence was found for food limitation of the lemmings at this site, even at peak population densities.

  9. Making Bigger DNA Helices

    Many nucleic acid analogs have been reported, but most of these represent modifications of the DNA backbone. Liu et al. (p. 868) describe a genetic pairing system with the bases expanded by the width of a benzene ring (or about 25%). Expanded adenine and thymine bases paired with the appropriate unmodified bases through normal hydrogen bonding. The expanded-diameter double helices were more stable than the Watson-Crick helix, probably because of enhanced base stacking. Pairing of four expanded bases with the four natural bases could potentially yield eight base pairs for information coding.

  10. Neurons Hold the Key

    The prion protein, PrP, is universally expressed in multiple cell types and is linked to the establishment and propagation of so-called prion diseases, such as new variant Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle. Mallucci et al. (p. 871; see the news story by Couzin) show that neuronal PrP depletion in mice with established neuroinvasive prion disease reverses early spongiform neuropathology, prevents neuronal loss, and completely halts progression to clinical disease. Thus, even in a background of cells still producing the pathological form of PrP, neurons lacking PrP do not undergo the pathological changes associated with prion disease.

  11. Organ Transplantation: Extending a Brief Encounter

    The need for improved specificity and lower toxicity in immune-suppressive drugs for organ transplantation has focused attention on the intracellular signaling pathways that regulate the immune responses of lymphocytes. Among these, the Janus kinase 3 (JAK3) has particular potential, because it mediates signals by several key cytokines, which each utilize a common gamma chain. Changelian et al. (p. 875) developed a highly selective, low-molecular-weight inhibitor of JAK3 that significantly extended organ allograft survival in primate and mouse organ transplant models after oral administration.

  12. Perceptions and Illusions

    Imaging studies have revealed the underlying causes of two different illusions (see the Perspective by Eysel). A motion stimulus (for example, a moving pattern on a screen) can cause the perceived location of a nearby stationary stimulus to be shifted toward the direction of motion. Using functional magnetic resonance imaging, Whitney et al. (p. 878) found that in early visual processing areas, the retinotopic location of the region activated by the stationary stimulus shifts in the opposite direction of motion. In contrast to what one would have predicted, the activation site shifts in the opposite direction to the perceived location. In the tactile funneling illusion, stimulating the tips of two adjacent fingers results in the perception of a single, stronger point of stimulation between the two fingers. Chen et al. (p. 881) used optical imaging to investigate the neural basis for this illusion in the primary somatosensory cortex (area SI). Consistent with the perception, area SI contains a focus of activity at the perceived site, which is greater than in control conditions, whereas neural activity represented at the two actual stimulation sites is less than when either site is stimulated alone. Thus, the location of the neural focus in SI explains the perceived location of the illusion, whereas the pattern of increases and decreases of activity across the three sites (two actual, one illusory) explains the perceived intensity.

  13. Air-Sea CO2 Fluxes in the Pacific

    A number of large-scale, quasi-periodic ocean-atmosphere processes that occur in the North Pacific Ocean, such as El Niño-Southern Oscillation and the Pacific Decadal Oscillation (PDO), exert an important influence on the exchange of CO2 between the ocean and the atmosphere. The PDO underwent major shifts in 1977 and 1989, but the effects of these shifts on CO2 chemistry have not yet been identified. Takahashi et al. (p. 852) describe the changes in CO2 chemistry in Pacific equatorial waters associated with the 1989 PDO phase shift, using 100,000 measurements of the partial pressure of CO2 (pCO2) in seawater made between 1980 and 2002. The sea-air pCO2 difference during the Pacific equatorial zone appears to have changed by as much as 30 to 40% during the past two decades, coincidently with the 1989 PDO phase shift, implying that the sea-to-air CO2 flux varied by a similar proportion. This result implies a decadal change of 10 to 15% of the mean global ocean CO2 uptake of 2 petagrams of carbon per year.

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