This Week in Science

Science  05 Mar 2010:
Vol. 327, Issue 5970, pp. 1175
  1. The Fall of the Dinosaurs


    According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earth's surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them.

  2. Sestrin and the Consequences of Aging

    The protein kinase TOR (target of rapamycin) plays key roles in the control of fundamental biological processes, including growth, metabolism, aging, and immune function. Sestrin proteins show increased abundance in response to stress and have been implicated in control of TOR activity. Lee et al. (p. 1223; see the Perspective by Topisirovic and Sonenberg) characterized Drosophila fruit flies lacking sestrins. Sestrins were implicated in a negative feedback loop in which the abundance of sestrins is controlled by TOR activity with sestrins concomitantly also inhibiting activity of TOR. Furthermore, flies lacking sestrins showed accumulation of fat, muscle degeneration, and heart abnormalities similar to those that plague aging humans with a sedentary life-style.

  3. Early Origin of Earth's Magnetic Field

    Earth's magnetic field protects us from stellar winds and radiation from the Sun. Understanding when, during the Earth's formation, the large-scale magnetic field was established is important because it impacts understanding of the young Earth's atmosphere and exosphere. By analyzing ancient silicate crystals, Tarduno et al. (p. 1238; see the Perspective by Jardine) demonstrate that the Earth's magnetic field existed 3.4 to 3.45 billion years ago, pushing back the oldest record of geomagnetic field strength by 200 million years. This result combined with estimates of the conditions within the solar wind at that time implies that the size of the paleomagnetosphere was about half of that typical today, but with an auroral oval of about three times the area. The smaller magnetosphere and larger auroral oval would have promoted loss of volatiles and water from the early atmosphere.

  4. Aging Snowball Earth

    Earth's glacial cycles have varied dramatically over time; at one point glaciers may have covered nearly the entire planet. Correlating various paleoclimate proxies such as fossil and isotope records from that time hinges on the ability to acquire precise age estimates of rocks deposited around the time of this so-called “Snowball Earth.” Macdonald et al. (p. 1241) report new high-precision U-Pb dates of Neoproterozoic strata in the Yukon and Northwest Territories, Canada, to calibrate the timing of carbon isotope variation in rocks from other locations around the globe. Based on the estimated past positions of where these rocks were deposited, glaciers probably extended to equatorial latitudes. The overlap with the survival and, indeed, diversification of some eukaryotes in the fossil record suggests that life survived in localized ecological niches during this global glaciation.

  5. Little Things Do Matter

    Gas-phase sulfuric acid is important during atmospheric particle formation, but the mechanisms by which it forms new particles are unclear. Laboratory studies of the binary nucleation of sulfuric acid with water produce particles at rates that are many orders of magnitude too small to explain the concentration of sulfuric acid particles found in the atmosphere. Sipilä et al. (p. 1243) now show that gas-phase sulfuric acid does, in fact, undergo nucleation in the presence of water at a rate fast enough to account for the observed abundance of sulfuric acid particles in the atmosphere. These particles, which contain 1 to 2 sulfuric acid molecules each, were not detectable previously, owing to their small size, with diameters as small as 1.5 nanometers.

  6. Ignition Set to Go


    One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys). Glenzer et al. (p. 1228, published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231, published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings.

  7. Bubble, Bubble, Warming and Trouble

    Vast quantities of methane are stored in ocean sediments, mostly in the form of clathrates, but methane is also trapped in submerged terrestrial permafrost that was flooded during the last deglaciation. There is thus concern that climate warming could warm ocean waters enough to release methane cryogenically trapped beneath the seabed, causing even more warming. Shakhova et al. (p. 1246; see the Perspective by Heimann) report that more than 80% of the bottom water, and more than 50% of the surface water, over the East Siberian Arctic Shelf, is indeed supersaturated with methane that is being released from the sub-sea permafrost, and that the flux to the atmosphere now is as great as previous estimates of that from the entire world ocean.

  8. Reexamining Glial Function


    In the last 20 years glial cells have been elevated from being considered as passive elements during neuronal transmission. By eliciting astroglial calcium rises, so-called gliotransmitters such as glutamate, ATP, or d-serine can be released and the activity of neighboring neurons modulated. However, this emerging picture has been challenged. Agulhon et al. (p. 1250; see the Perspective by Kirchhoff) reexamined these questions using two previously characterized mouse models. Calcium elevations induced selectively in astrocytes caused no change in multiple measures of synaptic activity. Furthermore, in mutant mice unable to elevate intracellular calcium, all synaptic measures were at wild-type levels. Astrocytic calcium signaling activity was thus not tied to the release of gliotransmitters and didn't affect synaptic transmission, short and long-term synaptic plasticity.

  9. Corpse-Sorting Machinery

    Phagocytosis of apoptotic cells is an integral part of the cell death program and plays critical roles in tissue remodeling, suppression of inflammation, and regulation of immune responses. The clearance of cell corpses requires their engulfment and subsequent degradation by phagocytic cells. During this process, receptors of the CED-1 family play a central role in recognizing cell corpses, transducing engulfment signals, and initiating the maturation of phagosomes containing apoptotic cell corpses. Retromer is a multisubunit protein complex conserved from yeast to mammals that mediates retrograde transport of transmembrane cargo from the endosome to the trans-Golgi network. Failure in recycling these proteins leads to their delivery to lysosomes where they are degraded. Chen et al. (p. 1261, published online 4 February) report that the Caenorhabditis elegans retromer complex is essential for the phagocytosis of CED-1 and thus for the clearance of apoptotic cells.

  10. Managing Crossovers

    In all sexual eukaryotes a special type of cell division called meiosis produces gametes or spores. For chromosomes to segregate properly to the daughter cells during meiosis, DNA crossovers must occur between every pair of homologous chromosomes. The position and number of these crossovers, which help to hold the homologous chromosomes together, is carefully controlled, in part by the condensin I complex. Youds et al. (p. 1254) show that in the nematode, Caenorhabditis elegans, crossovers are regulated at a second level by the anti-recombinase RTEL-1 (regulator of telomere elongation helicase–1). RTEL-1 prevents crossovers occurring too close to each other, and ensures that only one occurs per pair of homologous chromosomes.

  11. Carboxysomes in a Row

    The carboxysome is an organelle-like proteinaceous microcompartment that sequesters the enzymes of carbon fixation from the rest of the cytoplasm in cyanobacteria. Cyanobacterial carbon fixation is a major component of the global carbon cycle. Savage et al. (p. 1258) now show that carboxysomes are linearly arranged within the cytoplasm in a process that involves the bacterial cytoskeleton. This arrangement is important in carboxysome partitioning during cell division. When carboxysome partitioning is disrupted by interfering with the bacterial cytoskeleton, carbon fixation is impaired.

  12. Dropping a Notch

    Between 2000 and 2001, the concentration of water vapor in the stratosphere dropped by about 10%. Water vapor is an important greenhouse gas, so did the decrease affect climate and slow global warming? Solomon et al. (p. 1219, published online 28 January) used a combination of data and models to show that lower stratospheric water vapor probably has contributed to the flattening of global average temperatures since 2000, by acting to slow the rate of warming by about 25%. Furthermore, the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid warming, suggesting how important the concentration of stratospheric water vapor might be to climate.

  13. Episodic Rise

    Sea-level rise between the end of the Last Glacial Maximum and the beginning of the Holocene accelerated episodically, with the fastest rate occurring during meltwater pulse 1A (14,000 years ago) and perhaps during another interval called meltwater pulse 1B (11,300 years ago). However, existing records are not precise enough to confidently assess sea-level change during meltwater pulse 1B or the Younger Dryas cold event (12,900 to 11,600 years ago). Bard et al. (p. 1235, published online 14 January) present data from three cores drilled in coral reefs in Tahiti that allow a better quantification of sea-level changes between 14,000 and 9000 years ago. No significant change in the rate of sea-level rise could be detected for the period previously identified as meltwater pulse 1B; reducing the likelihood that such an episode occurred. Furthermore, sea level rose more rapidly at the start of the Younger Dryas but slowed during the rest of the cold event. These findings have implications for our understanding of the dynamics of deglaciation.