This Week in Science

Science  26 Aug 2005:
Vol. 309, Issue 5739, pp. 1242
  1. Ecological Community Structure Emerges


    Efforts to understand ecological community structure and function have been hampered by debates about the shape of species numerical abundance and species resource-use curves. Much confusion has arisen from a combination of two factors: limited data and limited power to detect differences between model fits. Connolly et al. (p. 1363) overcome both limitations by applying information theory model-selection procedures to a large data set of tropical corals and reef fishes. Both resource-use and numerical abundance distributions are well characterized by a log-normal distribution. The distribution shape emerges at markedly different scales for resource-use and numerical abundance distributions. The scales at which log-normal distributions of numerical abundance become apparent are similar for two groups of organisms that differ markedly in dispersal and demography (corals and fishes). The large scale at which relative abundance patterns emerge indicates that the scale and scope of coral reef conservation strategies are inadequate, highlighting the need integration and networking of Marine Protected Areas regionally, across national boundaries.

  2. Echoes of Phenol

    Multipulse nuclear magnetic resonance (NMR) techniques have long been used to study chemical equilibria in solution on time scales approaching microseconds. The advantage of NMR is that the induced nuclear spin dynamics used to obtain rates do not perturb the underlying chemistry of the system. Zheng et al. (p. 1338, published online 4 August 2005; see the Perspective by Dlott) show that an infrared analog of multipulse NMR, termed vibrational echo correlation spectroscopy, can raise the time resolution for such studies by more than six orders of magnitude. They quantify the picosecond time scale for association and dissociation of phenol-benzene complexes by relying on molecular vibrations, rather than nuclear spins, to track ensembles of exchanging molecules in solution.

  3. Turning Slightly Faster

    Several studies during the past 10 years suggest that Earth's inner core is rotating faster than the rest of the planet, but other studies have challenged these interpretations. Confirming super-rotation is important for understanding Earth's angular momentum and the generation of the magnetic field in the fluid outer core. Zhang et al. (p. 1357; see the news story by Kerr) have now analyzed 18 seismic doublets—nearly identical earthquakes that occur in the same place but separated by several to up to 35 years. A systematic offset in seismic waves that pass through the inner core demonstrate that it is indeed rotating faster than the rest of the planet by about 0.009 second per year.

  4. Singlet-Triplet States in the Mix


    The coupling of spins between adjacent quantum dots can form the basis of a quantum logic gate. However, recent work has shown that dots grown on GaAs also experience a large and random background field caused by the nuclear spins in the substrate, which leads to the spins losing their memory and mixing between spin-singlet and spin-triplet states. Koppens et al. (p. 1346) provide a comprehensive study of the extent of this effect and show how decoherence can be mitigated to some degree by tuning the coupling strength between the dots or polarizing the background nuclear spins.

  5. Superconductivity Makes a Reentrance

    How ferromagnetism and superconductivity can coexist in some metals has not been clear. Lévy et al. (p. 1343; see the Perspective by Mackenzie and Grigera) report that the superconducting ferromagnet, URhGe, enters a second superconducting phase at high magentic fields that are well above the region where superconductivity is destroyed. Magnetic torque and transport measurements suggest that superconductivty in this material is mediated by rotation of the magnetization.

  6. Close Encounters on the Catalytic Kind

    The enzyme nitrogenase catalyzes the reduction of atmospheric N2 to ammonia (NH3), which requires the input of six electrons (in addition to three protons). The electrons are carried on the iron-sulfur cluster of the Fe-protein and transferred to the MoFe-protein (where N2 reduction takes place) in a reaction that depends on the hydrolysis of adenosine triphosphate (ATP). Tezcan et al. (p. 1377) provide three crystal structures of the complex of the Fe-protein and MoFe-protein in three distinct nucleotide states: (i) with no nucleotide bound, (ii) with adenosine diphosphate bound, and (iii) with an ATP analog bound. Taken together, these snapshots show that electron transfer is greatly facilitated as the Fe-protein crawls through the three nucleotide states, where the ATP state is the only one that allows for a sufficiently close approach of the iron-sulfur cluster and the recipient P cluster of the MoFe-protein.

  7. Predator Diversity in the Oceans

    The diversity of large ocean predators will vary in relation to temperature and ocean productivity. Worm et al. (p. 1365; published online 28 July 2005; see the cover) used extensive data sets from fisheries records to determine how the diversity of large predatory species (tuna and billfishes) varies throughout the world's oceans. Overall, there has been a decline in diversity during the past 50 years. The detailed analysis reveals peaks in diversity that occur at intermediate latitudes. Temperature and dissolved O2 were the primary environmental factors that correlated with (and may cause) the peaks in diversity. The abundance of zooplankton is highest in these “hot spot” regions, which suggests that diversity peaks may be similar for organisms throughout the marine food chain.

  8. Stem Cell Research sans Embryos?

    The use of human embryos has triggered considerable societal debate about human embryonic stem (hES) cell research. Cowan et al. (p. 1369) describe an alternative method of deriving hES cells that may ultimately eliminate the need for human embryos and oocytes. Experimentally induced fusion of human adult somatic cells with hES cells in culture produces hybrid cells that are transcriptionally “reprogrammed” back to the embryonic state. If future experiments indicate that this reprogrammed state is retained after removal of the pluripotential ES cell nucleus (currently a formidable technical hurdle), the hybrid cells theoretically could be used for the production of genetically tailored hES cell lines.

  9. Results from the Canopy


    To understand the effects of rising levels of atmospheric CO2 levels on trees, there have been a number of free-air CO2 enrichment (FACE) experiments in recent years, mostly in young plantations. Körner et al. (p. 1360; see the news story by Pennisi) assessed the responses of mature trees in a nearnatural temperate forest using a system that delivered CO2 to the crowns of 35-meter-tall trees. After 4 years, different tree species had different responses to higher CO2, but one common response was a lack of sustained growth stimulation. Thus, carbon appears to pass through the system at a greater rate when CO2 levels are higher.

  10. Regulated Regulation in Immune Responses

    The activity of regulatory T cells (Treg) is responsible for controlling aberrant immune responses and autoimmunity, but these cells represent a potential barrier to certain types of therapeutic manipulation, such as in cancer immunotherapy. Peng et al. (p. 1380) provide evidence that part of human Treg control may be mediated directly by an innate signaling protein. Clones of human Treg cells, as well as isolated, naturally occurring Treg cells, expressed Toll-like receptor (TLR) 8. Ligands that could activate this receptor reversed the suppressive activity of these cells in culture, as well as in a mouse tumor model. Control over Treg activity via TLR signals may open new avenues for inhibiting unwanted immune suppression during cancer immunotherapy.

  11. Secrets of Malaria Invasion

    The parasite that causes cerebral malaria, Plasmodium falciparum, can switch the host receptors used for invasion of human red blood cells. This property has been known for more than 10 years but the underlying mechanism has been unclear. Using microarrays and gene knockouts, Stubbs et al. (p. 1384) have identified the PfRh4 gene as responsible for switching. This mechanism would be important for the parasite population to avoid host immune responses and erythrocyte polymorphisms, and has important implications for vaccine design.

  12. Millions of Microbe Species, If Left Alone

    The traditional methods of calculating diversity by identifying and counting organisms fail for microbes. We do not know how many species of microbes there are, even to within a few orders of magnitude. Gans et al. (p. 1387; see the Perspective by Curtis and Sloan) used a method based on historical data for DNA reassociation kinetics. The results are startling: The method suggests that there are about one million species in a pristine environment; most of which are quite rare. This number represents an increase of two orders of magnitude compared to first estimates. The work also highlights the dramatic effects of pollution on diversity: the presence of toxic metals extirpates the rare species leading to loss of 99% of the original diversity.

  13. Site Survey

    One use of open metal-organic framework (MOF) compounds is for gas storage. Some MOFs have pore sizes that are sufficiently large that molecules might have several different binding sites within the framework. Rowsell et al. (p. 1350) have obtained x-ray diffraction data for single crystals of a particular MOF compound, which is made up of Zn4O(CO2)6 units linked by phenylene groups, that had Ar or N2 absorb its pores at 30 kelvin. They identified five primary and three secondary absorption sites for these weakly binding gases with the framework, whose pores range from 12 to 15 angstroms in diameter.

  14. From Gradient to Spindle

    What is the physiological role of morphogenetic gradients in the formation of the mitotic spindle? Caudron et al. (p. 1373; see the Perspective by Clarke) confirm by experiment and modeling that chromosomes produce an extended RanGTP-importin-β interaction gradient that reflects the release of nuclear localization signal-containing proteins at different distances. This results in the formation of concentration gradients of activities that account for spatial regulation of microtubule nucleation and stabilization. These gradients are required for proper spatial coordination of spindle assembly.

  15. SUMO Wrestles Circadian Clock Components

    Core components of the mammalian circadian clock are transcription factors whose expression is controlled by feedback loops embedded in a complex regulatory circuitry. Cardone et al. (p. 1390; published online 18 August 2005) report that posttranslational modification of BMAL1, an essential component of the circadian clock, adds another level of complexity to controlling the clock mechanism. BMAL1 is covalently modified by SUMO in the mouse liver, which increases its rate of decay. SUMOylation of BMAL1 follows a daily cycle with timing that parallels its circadian activation. The modification is also induced by CLOCK, the heterodimerization partner of BMAL1. Expression of a mutated, non-SUMOylated form of BMAL1 altered rhythmic gene expression, indicating that modification by SUMO is important to the oscillatory function of the circadian clock.

  16. A Sharp and Slippery Turn

    One explanation for the variation in frictional forces along different directions on the same surface is that commensurability between the sliding surfaces can lead to intimate interlocking and high friction, whereas incommensurability should lead to lower friction. Park et al. (p. 1354) compared frictional forces on the twofold symmetric surface of an aluminum-nickel-cobalt quasicrystal. This surface is periodic in one direction but is aperiodic in the normal direction. They avoided the effects of adhesive forces by passivating their atomic force microscope tip with alkane thiols so that only hydrocarbon chains interacted with the surface. Friction along the aperiodic direction was eight times lower than in the periodic direction. They argue that dissipation forces, acting either through the formation of electron-hole pairs or though surface vibrations, are much different in the aperiodic direction for this surface.