This Week in Science

Science  29 Jun 2001:
Vol. 292, Issue 5526, pp. 2393
  1. Electrons Dance in the Spotlight

    Application of a magnetic field polarizes the spin of an electron in the direction of the magnetic field. The net moment of an electronic ensemble can rotate in response to a varying magnetic field, and methods such as nuclear magnetic resonance can take advantage of this effect to probe the electronic environment of many materials. In semiconductors, however, the lifetime of conduction electrons is on the time scale of the shortest magnetic field pulses and so other techniques are required. Gupta et al. (p. 2458; see the cover and the news story by Service) show that an optical tipping pulse can induce an effective magnetic field that causes the precessional dynamics of the magnetic moment to be altered in a controlled and reversible manner. The femtosecond length scales of these optical pulses are so short that electrons could be probed thousands of times in their coherent lifetimes, thus opening applications in quantum computing.

  2. C Changes

    Carbon-14, which has a half-life of about 5700 years, occurs naturally on Earth only because it is continuously produced in the atmosphere through nuclear reactions caused by cosmic rays. Large variations in the atmospheric abundance of 14C are possible if there are changes in its production rate or how it is cycled between the ocean, atmosphere, and land. Beck et al. (p. 2453; see the Perspective by Bard) analyzed a stalagmite from the Bahamas and show that large variations in the atmospheric abundance of 14C, greater than could have been caused by modulation of the production rate, occurred during the past 45,000 years. This finding is important for carbon dating because accurate dates depend on precise values for the initial abundance of 14C, and for understanding how ocean circulation and carbon sequestration may have varied during that interval.

  3. Slab Remnant

    Earth's crust is recycled into the mantle at subduction zones; this process not only produces earthquakes but also affects the thermal, chemical, and mechanical structure of the mantle. Chen and Brudzinski (p. 2475; see the Perspective by Green) analyzed thousands of earthquakes along the Tonga subduction zone in the southwest Pacific Ocean near Fiji and inferred the presence of a subhorizontal remnant of a subducted slab of oceanic crust between depths of 400 to 660 kilometers. This remnant slab is from an ancient subduction zone and is detached from the current Tonga subduction zone. This remnant-slab model explains the odd pattern of seismicity, provides a barrier for subduction of the Tonga slab into the lower mantle, and alters the thermal and rheological properties of the upper mantle in this region.

  4. Boron Extends Carbon's Reach

    In most of its saturated compounds, carbon is bound to four other atoms in a tetrahedral arrangement. Unsaturated carbon compounds have planar structures but have coordination numbers of two or three. Wang and Schleyer (p. 2465) report density-functional calculations which suggest that whole families of hyparenes—hypercoordinated aromatic or anti-aromatic molecules—may be possible. In these structures, carbon is bound to five other atoms in a planar arrangement. The key to their stability is the incorporation of boron into the molecules. No experimental observations of such molecules have been reported to date, but if the borocarbon species detected by mass spectrometry can be isolated and identified, hyparenes may be found.

  5. Thrown into the Mix

    Phase separation at the nanoscale can lead to rich morphologies. Such effects are often seen in diblock copolymers, which can tie together otherwise immiscible polymers, and in mixtures of homopolymers and nanoparticles. What happens when a diblock copolymer is mixed with nanoparticles? Thompson et al. (p. 2469) have combined two existing theories, one used to successfully describe polymer thermodynamics, and a second used to describe particle ordering in colloidal systems, to model a diblock copolymer-nanoparticle mixture and predict what kind of morphologies could form. The model not only reproduces results of some recent experiments and simulations, but also predicts a number of novel morphologies. Conditions are also found where the nanoparticles self-assemble into nanowires or nanosheets.

  6. Caching Zinc

    Cells of the bacterium Escherichia coli contain a total concentration of zinc of about 0.2 millimolar (mM). This abundant metal functions as a cofactor in many enzymes within the cell, so Outten and O'Halloran (p. 2488) were interested to know how much of the element was available free for binding to proteins. They calibrated the zinc-binding properties of two zinc sensor proteins that control synthesis of proteins that regulate zinc uptake or efflux from the cell. These proteins responded to a very narrow and extraordinarily low concentration range for zinc. Zinc binding was saturated at concentrations of 10−15 M. Given the volume of a bacterial cell of about 2 × 10−15 liters, one atom per cell would be present at a concentration 1 × 10−9 M. Because the zinc sensors respond at concentrations that are six orders of magnitude lower, the authors conclude that there are no persistent free zinc ions in the cytoplasm. They propose that chaperone or trafficking proteins may mediate exchange of the metal between proteins, as they do for copper ions.

  7. No Need for Males

    Most multicelled animals are diploid (having pairs of chromosomes), but examples are known where the males are haploid (their somatic cells have half the normal chromosome number) but the females are diploid. Weeks et al. (p. 2479; see the Perspective by Otto and Jarne) have found a mite in Brazilian coffee plantations that has haploid females. Brevipalpus phoenicis apparently has no need for males because it reproduces asexually, but males can be recovered. An endosymbiotic bacterium—no relation to Wolbachia, which is known to feminize other insects—has feminized the haploid males. If female mites are treated with antibiotics, about half their progeny become male. These findings suggest that B. phoenicis females very recently shed diploidy to become a haploid asexual species.

  8. NO to Night Lights

    The enchanting bursts of light that fireflies emit during courtship depend on a biochemical reaction that occurs in an abdominal light-producing organ, aptly named the lantern. Trimmer et al. (p. 2486; see the news story by Pennisi) report that regulation of the flash-producing reaction relies on both oxygen and the free-radical gas, nitric oxide (NO). NO is produced within the lantern and increases the flash rate and locomotion behavior in flies. Upon reaching the mitochondria, NO may alter respiration to increase the level of oxygen, which is known to be the immediate molecular trigger for light production.

  9. A Twist on Nitrogen Fixation

    We rely on prokaryotes to fix atmospheric nitrogen, and now Lilburn et al. (p. 2495) have added a new group to the list, the spirochetes. This family comprises a large and ubiquitous group of free-living, parasitic, and symbiotic organisms, and finding a new metabolic capacity among them fills a substantial gap in our knowledge of global nitrogen budgets. The survey started with spirochetes in termites, as it was known that nitrogen fixation supplied up to 60% of the total nitrogen in termite biomass, but there was poor evidence for the organisms responsible. Moreover, the reliance on nitrogen fixation is variable and may be regulated, in the termite host at least, by the presence of oxygen and the relative availability of more readily useable nutrients.

  10. Instant Gratification Versus Delayed Reward

    In order to obtain a large reward, one sometimes has to make sacrifices or reject the easier option of an immediately available but lesser reward. However, a number of individuals do not seem to see this connection and always go for the instant gratification. To study the neuroanatomical basis of this behavior, Cardinal et al. (p. 2499) made excitotoxic lesions in several brain areas of the rat. They found that lesioning the core of the nucleus accumbens initiated a persistent impulsive choice for food rewards. However, selectively lesioning the anterior cingulate cortex or the medial prefrontal cortex, two major afferents to the nucleus accumbens, did not change the animals' choice performance compared to sham-operated controls.

  11. Restoring LTP

    Studies with knockout mice lacking an AMPA receptor subunit (glutamate receptor A, or GluR-A) indicate that after the induction of long-term potentiation (LTP), increased transmission at hippocampal synapses is established and maintained by an increased response of AMPA receptors. In this preparation, the lack of AMPA receptors at nonsynaptic sites has been linked to the absence of LTP. Mack et al. (p. 2501) used a conditional knockout preparation of AMPA receptors tagged with green fluorescent protein to show that the GluR-A subunit is essential for pairing-induced LTP at mature hippocampal synapses.

  12. Rapidly Rising Melts

    At a subduction zone boundary, water-rich sediments and rocks are dragged down into the lower crust and mantle. As these rocks are heated, they melt and will buoyantly rise to the surface and eventually erupt along volcanic centers. Recent isotopic analyses of these subduction zone lavas indicated that these melts rise rapidly from depth to the surface. Hall and Kincaid (p. 2472) conducted laboratory experiments of a subduction zone to determine why the melts rise so rapidly. Shear flow along the subduction zone coupled with the rise of multiple buoyant diapirs created a low-density and low-viscosity conduit that allowed the rapid rise of melt.

  13. Turning On to Nutrients

    Until recently, the nutrient-poor open ocean was considered the marine equivalent of a desert. Kolber et al. (p. 2492; see the Perspective by Fenchel) have isolated and cultured a distinct type of facultative photosynthetic bacteria from several sites in various oceans, which use carotenoids rather than bacteriochlorophyll for light harvesting, and assessed their abundance, distribution, and ecological contribution. This type of bacterium is usually found in environments rich in organic matter. Studies in culture now show that these ubiquitous Erythrobacter only resort to photosynthesis when nutrients are in short supply. The authors suggest that the facultative and conventional obligate microbial phototrophs coexist in a mutually dependent way, and that the facultative phototrophs can regulate the expression of their photosynthetic apparatus in response to nutrient levels.

  14. Pollen Protein Profiles

    The proteins on the surface of a pollen grain are likely participants in regulating effective pollination and self-incompatibility. Mayfield et al. (p. 2482) have now surveyed the larger proteins found on the surface of the Arabidopsis pollen grain. Fragmentary peptide sequences from those proteins above a size cut-off yielded enough information to identify the corresponding genes. Many of the pollen coat proteins were of two types—lipases (lipid cleaving proteins) and oleosins (lipid-binding proteins). The genes encoding the lipases and oleosins were clustered within the Arabidopsis genome. The allelic diversity and gene clustering of these two gene families may well facilitate the maintenance of species specificity in reproduction.

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