This Week in Science

Science  15 Dec 2006:
Vol. 314, Issue 5806, pp. 1220
  1. El Niño Integrated


    The El Niño-Southern Oscillation, or ENSO, is the most energetic of all of the large-scale, quasi-periodic, ocean-atmosphere climate oscillations that happen on human time scales. Although ENSO occurs in and above the Pacific Ocean, nearly every region of Earth is affected by changes in weather, ecosystems, and the global carbon cycle. McPhaden et al. (p. 1740) review the physics of ENSO, and its most important environmental and socioeconomic impacts, in order to understand it as an “integrating” concept in earth science.

  2. Rubidium-Rich Stars

    When stars get old, they swell into giants. At the bottom of their convecting zones, heavy elements, such as rubidium and strontium, are produced through the s process by slow nuclear reactions involving the capture of neutrons. In hot stars that are several times more massive than the Sun, theory predicts that the neutrons generated by conversion of 22Ne to 25Mg should produce large amounts of the long-lived isotope 87Ru, but little of this material has been observed. García-Hernández et al. (p. 1751, published online 9 November; see the Perspective by Boothroyd) have found the missing Rb in 60 asymptotic giant branch stars in our Galaxy. These stars have levels of 87Ru that are 10 to 100 times greater than that of the Sun but are only three to eight times more massive. The discovery points to metallicity differences in nucleosynthesis reactions in the late stages of the evolution of intermediate-mass stars, and also has implications for isotopic anomalies observed in some presolar grains found in meteorites.

  3. Stalking the Quantum Spin Hall Effect

    In the quantum spin Hall effect (QSHE), coupling between the orbital and spin angular momentum of an electron on the edges of a bulk insulating state creates a conduction state that allows charge flow in only one direction. It does not require any external magnetic field (whose presence would break time-reversal symmetry), and it has been suggested that the helical edge states could conduct without dissipation. Although graphene exhibits characteristics of this state, its small energy gap complicates experimental observations. Bernevig et al. (p. 1757; see the Perspective by Kane and Mele) present theoretical work proposing that HgTe/CdTe quantum-well structures should be a more robust QSHE system, and present a outline of how the effect could be experimentally detected.

  4. Coaxial Organic Photoconductor

    In an organic photovoltaic devices, separate electron donor and acceptor layers harvest charge carriers created by absorption of photons. Yamamoto et al. (p. 1761; see the Perspective by Würthner) have self-assembled coaxial nanotubes from molecules that contain a large electron-donating aromatic core (hexabenzocoronene) on which is appended, via a long, flexible linker, an electron-accepting trinitrofluoronene group. These nanowires are 16 nanometers in diameter and several micrometers in length. When cast as films on electrodes, the nanotubes show large changes in conductivity (on-off ratios in excess of 104) when irradiated with ultraviolet-visible light. The nanotubes avoid formation of a nonconducting charge-transfer complex that was seen in an alternate microfiber morphology of these same molecules.

  5. Grow, Stamp, Stack


    The assembly of complex electronic and communication circuitry requires the assembly of several layers of dissimilar materials, and methods such as wafer bonding and epitaxial growth can deposit layers of thin films for the production of complex three-dimensional structures. However, for preformed nanomaterials, such carbon nanotubes or semiconductor nanowires, or materials with limited thermal stability, alternative assembly strategies will be needed for their controlled deposition. Ahn et al. (p. 1754) have produced nanometer-scale semiconducting materials and then printed them with an additive stamping process. The printing process works well on both rigid and flexible substrates and produces high-quality, robust electronic systems.

  6. Mitochondrial Footprint of Human Migrations

    Modern humans are thought to have dispersed out of Africa along a single southern path—from the Horn of Africa across Bab-el-Mandeb (the Gate of Tears) to the Arabian Gulf and then along the coasts of the Indian Ocean to Southeast Asia and Australasia, a migration route consistent with the known distribution of mitochondrial DNA (mtDNA) genetic markers. Curiously, haplotypoes M1 and U6, which are closely related of some of the predominantly Asian halogroups, are specifically found in North and East Africa. Olivieri et al. (p. 1767) sequenced a wide-ranging series of M1 and U6 mtDNA genomes and found that populations bearing these markers must have arisen in southwestern Asia and then returned to North and East Africa some 40,000 to 45,000 years ago, at a time when changes in climate created fragmented deserts in these areas. Ancestral M1 and U6 populations apparently moved through the Levant when populations from this area were moving into Europe.

  7. Plankton Extinctions in Acidic Oceans

    At the Paleocene-Eocene Thermal Maximum (∼55 million years ago), rapid increases in atmospheric CO2 levels in excess of 1000 parts per million by volume (about three times current levels) raised global temperatures by more than 5°C and caused marine and terrestrial extinctions. Gibbs et al. (p. 1770) used several high-resolution cores to examine the effects of this event on plankton and found little ecological basis for the extinctions. Most of the taxa that went extinct were rare taxa, and these did so rapidly, within the first 10,000 years or so of the event. Despite any effects of the high CO2 levels on acidifying the oceans, there was no preferential extinction of plankton relying on calcium carbonate structures.

  8. P[acman] P[romotes] T[ransformation]

    About 20 years ago, the development of transgenic methods that used P element transformation greatly facilitated gene analysis in Drosophila melanogaster. However, the method limits the size of DNA fragments for insertion and can only target specific sites in the genome. Venken et al. (p. 1747, published online 30 November) now develop a tool, termed P[acman], that allows site-pecific insertion of DNA fragments of more than 100 kilobases. The method will facilitate structural and functional analyses of any Drosophila gene and will permit tagging of proteins in vivo.

  9. Injectable Virulence


    Little is known about the molecular determinants of virulence in eukaryotic pathogens like Toxoplasma gondii and malaria. Progress has been hampered by inefficient genetic tools, large genomes, and complex life cycles. Using forward genetic analysis, Taylor et al. (p. 1776) and Saeij et al. (p. 1780) show that a few clustered genes on a single chromosome control the dramatic difference seen in the virulence of natural lineages of the parasite T. gondii. The most important of these genes encodes a conserved serine/threonine kinase that is injected into the host cell. Although this process is reminiscent of type III secretion in bacteria, it is mechanistically and evolutionarily distinct.

  10. Quieting the Brain at Birth

    Birth entails a multitude of transitions. Studying rats, Tyzio et al. (p. 1788) have identified yet one more, a link between oxytocin exposure and the switch in how certain brain neurons fire. The neuro-transmitter GABA (γ-aminobutyric acid) is usually excitatory in fetal brain neurons but inhibitory once they mature. Exposure to oxytocin during parturition causes a switch from excitation to inhibition in GABA signaling. This quieting of neuronal activity may serve to protect the brain against transient hypoxia during birth.

  11. Climbing the Gradient

    During chemotaxis, cells respond to tiny changes in the concentration of chemoattractant molecules and move toward their source. Chen et al. (p. 1792; see the Perspective by Linden) show that in addition to receptors for the chemoattractant peptide N-formyl-Met-Leu-Phe (fMLP), human neutrophils use two other receptor systems to promote appropriate cell migration. Neutrophils exposed to a concentration gradient of fMLP released adenosine triphosphate (ATP) at the leading edge of the cell. The released ATP appeared to act in an autocrine manner and stimulated purinergic receptors to provide a signal required for proper orientation of the cell.

  12. Invisible Distraction

    An invisible, subthreshold stimulus can influence visual processing and behavior. However, subthreshold stimulus processing in the brain is not understood. Tsushima et al. (p. 1786; see the Perspective by Stoerig) discovered that when a subthreshold stimulus is task-irrelevant, it activates the visual cortex more than suprathreshold stimuli and severely disrupts task performance.

  13. Frigid Old Faithful?

    The south pole of Saturn's moon Enceladus harbors a subterranean hotspot and spouts a plume of enriched water vapor. One possible source for this plume would be an underlying pool of warm liquid water. Kieffer et al. (p. 1764) describe an alternative model that does not require liquid water. Instead, heat deforms a crust of clathrate tens of kilometers thick to release the gas. This model is consistent with the composition of the gas plume and is evidence of ongoing tectonics at the south pole.

  14. Totting Up Top Fish

    The status of top predators, such as tuna and shark in ocean fisheries, is controversial, with some reports suggesting biomass declines of as much as 90% as a result of fishing pressure. Sibert et al. (p. 1773) synthesize all available data on eight stocks of large predatory fish in the Pacific Ocean and reached rather different conclusions. The authors place reductions in the biomass of predator species since 1950 to between 9% to 64%, and neither the average size, nor the trophic status of fish in the population has changed. Despite these lower estimates of the effects of fishing pressure, more effective management is still required to ensure that Pacific fisheries are sustainable.

  15. Deep Hot Nitrogen Fixation

    In the deep ocean, high-temperature life is restricted to chemolithotrophic systems or secondary processing of organic material originally formed by photosynthesis at the surface. Mehta and Baross (p. 1783; see the Perspective by Capone) have isolated a methanogenic archaeon living near hydrothermal vents that fixes nitrogen at 92°C, which is 28°C higher than the current known upper temperature limit for this process. This organism possesses an ancient nitrogenase that can function at these high temperatures and pressures. As well as biotechnological application, this discovery has important implications for the extent of life in the subsurface, which is likely to be limited by biologically available nitrogen.