This Week in Science

Science  27 Jan 2006:
Vol. 311, Issue 5760, pp. 432
  1. Frozen Forms

    CREDIT: DEVILLE ET AL./MATERIAL SCIENCES DIVISION, LBNL

    When water freezes, it can form hexagonal plates that grow at different rates in different directions, and impurities can become trapped at the water-ice interfaces. Deville et al. (p. 515; see the Perspective by Halloran) exploited these effects to fabricate porous materials from concentrated ceramic powder suspensions, which could also be backfilled with a second material to make composites. The colloidal particles could then be etched away to leave a porous structure composed of the second material such as alumina. Using nacre and bone as their inspirations, the authors show how they can replicate these complex composite materials.

  2. Above-Ground Resources

    With fossil-fuel supplies steadily waning, recent research has focused on using plant-derived materials as a renewable substitute (see the Editorial by Koonin). Ragauskas et al. (p. 484) review progress in this area, ranging from plant genetics research for enhancing supply to enzymatic and other catalytic methods for breaking down the biomass into practical fuels and fine chemical precursors. Some of the economic challenges and benefits of changing the production infrastructure on such a large scale are also addressed. Ethanol is a renewable resource already in use as a liquid fuel, but its production from corn and cellulose is energy intensive, and some analyses have found that the overall process uses more energy than it creates. Farrell et al. (p. 506) rigorously analyzed a variety of relevant investigations, and found that the studies reporting negative net energy values are flawed. All of the studies show that current corn ethanol technologies reduce petroleum use significantly relative to gasoline. However, new production methods are needed if fuel ethanol is to reduce greenhouse gas emissions significantly.

  3. Unbalanced Superfluidity

    The pairing of fermions lies at the heart of superconductivity in metals and superfluidity in helium-3, where the spin populations are generally equal. Exotic pairing states are expected to arise for imbalanced spin populations, such as in the pairing of quark matter in neutron stars and in strongly magnetized superconductors, but such systems are difficult to realize experimentally. The availability of cold atom clouds of mixed atomic spin states has allowed the crossover regime between Bose-Einstein condensates of molecules and Bardeen-Cooper-Schrieffer superfluids to be probed experimentally. Two studies now address the quantum nature and the phase transition of interacting Fermi gases of lithium-6 in which unbalanced populations of two different spins states are prepared (see the 23 December 2005 news story by Cho). Zwierlein et al. (p. 492, published online 22 December 2005) examined the condensate fraction and superfluidity as a function of spin imbalance and found that superfluidity is remarkably stable against population imbalance. Partridge et al. (p. 503, published online 22 December 2005) detail the spatial structure and polarization of the mixed spin system.

  4. Tracking a Turn to the Left

    Semiconducting single-walled carbon nanotubes (SWNTs) exhibit band-gap fluorescence in the near-infrared, and the dielectric environment surrounding the SWNT can modulate the band-gap energy. Heller et al. (p. 508) show that this effect is sensitive enough to distinguish whether DNA wrapped around SWNTs is in the native B form or has been shifted to lower energies when the DNA adopts the left-handed Z form in the presence of divalent metal ions such as mercury or cobalt. These shifts were seen for the several different SWNT species present in a buffer solution and were used to detect micromolar levels of Hg2+ in highly scattering media such as whole blood.

  5. How High Was It?

    Oxygen isotope fractionation in rain generally decreases with elevation and temperature, and this effect can be used to determine changes in elevation of a region over geological time. However, changes in the path of storms or the season of precipitation add great uncertainty. Ghosh et al. (p. 511; see the Perspective by Poage and Chamberlain) have developed a thermometer based on the binding of the temperature-dependent binding of rate 13C and 18O isotopes in carbonate minerals. This independent estimate of temperature can be related to lapse rate and other data used to infer elevation of minerals that form in soils. An analysis of soil carbonates in Bolivia shows that the high plateau there rose between 6 and 10 million years ago.

  6. Marine Microbial Gene Ecology

    CREDIT: DELONG ET AL.

    Depth stratification occurs in the open ocean not only for large planktonic creatures but also for microbial plankton. DeLong et al. (p. 496) sampled and sequenced the microorganisms in the water column in the North Pacific Subtropical Gyre with the aim of identifying sequences that tracked major environmental features. Above 200 meters, distinct photic zone sequences were found characteristic of photosynthetic and mobile microorganisms requiring iron, mostly Prochlorococcus (itself dividing into high- and low-light-tolerant clades) and Peligabacter, accompanied by Euryarchaea. Strikingly, photic zone microbes showed evidence of high rates of viral infection. Below 200 meters, Chloroflexi, SAR202, Planctomycetales, and Crenarchaea were found, with sequences suggesting a predominance of “adhesive” microbes that produce pili and synthesize polysaccharides and antibiotics.

  7. Dispersal Patterns of Marine Population

    The scale of dispersal among marine populations, or “population connectivity,” has been a notoriously intractable problem. Cowen et al. (p. 522, published online 15 December 2005; see the cover and the Perspective by Steneck) analyzed larval dispersal patterns for a suite of coastal fish species in the Caribbean Sea, a large region with complex ocean currents. Typical dispersal distances were on the scale of only 10 to 100 kilometers, and larval movement was a key factor in their dispersal potential. These robust estimates of population connectivity levels have broad relevance for the spatial management of marine resources and for understanding the spread of invasive species and disease in the marine environment.

  8. The Not-So-Quiet Cricket

    Our own behavior often generates intense sensory feedback, for example, during loud shouting. How do we prevent self-induced desensitization of our auditory pathway and distinguish between self-generated and external sounds? Inhibitory neural signals, called corollary discharges, are sent from motor to sensory areas in the brain that suppress responses at the precise time that we generate sensory information. Using singing crickets as a model system, Poulet and Hedwig (p. 518) identified the cellular basis for a corollary discharge that is indispensable in order to distinguish self-generated sensory feedback from external information. The corollary discharge interneuron in the cricket is driven by the song pattern generator and monosynaptically inhibits crucial elements of the auditory pathway.

  9. Working an Active Site into an Existing Scaffold

    CREDIT: PARK ET AL.

    Designing enzymes that catalyze industrial reactions is one goal of protein engineering. Although there has been progress in rational design, it is hindered by a limited understanding of structure-function relations. Park et al. (p. 535; see the Perspective by Tawfik) have used a strategy that mimics natural evolution to change the function of an existing protein scaffold. By insertion, deletion, and substitution of several active-site loops, followed by point mutations, they introduced β-lactamase activity into the αβ/βα metallohydrolase scaffold of glyoxalase II. Extending the process to other scaffolds may allow creation of new enzyme activities with practical applications.

  10. Maintaining Different Trees in the Forest

    Frequency-dependent models for the maintenance of high species diversity of trees in tropical forests predict that locally rare species survive preferentially when compared with common species. Wills et al (p. 527; see the news story by Pennisi) present a longitudinal survey of species frequencies from a network of large plots (50 hectares) in seven tropical forest sites in the Old and New Worlds. In all of the sites, the diversity of recruits into large size classes did increase as the forests aged. Forests suffering from limited, temporary disturbance should have the ability to recover former levels of diversity, and selection processes should favor increasing differences between species.

  11. Viruses Reveal the Secrets of the Cougar

    Conservationists and research scientists have discussed the idea that pathogens could be used as genetic tags to record changes in the demography of the host population, but until now have failed to get to grips with any specific system. Biek et al. (p. 538) have characterized the spatial and temporal distribution of nonpathogenic feline immunodeficiency virus (FIV) and its natural host, the cougar, as the cats' populations recovered from heavy hunting pressure in the first half of the 20th century. Fast-evolving RNA viruses such as FIV provide insights into what the host population has been doing on an ecological time scale, despite the slow pace of chance of the host population.

  12. Building the Postsynaptic Density

    The postsynaptic density (PSD) is a large and dynamic protein complex on the postsynaptic side of neural synapses that helps communicate incoming signals to cytoplasmic targets. Shank proteins are located deep in the PSD and may be involved in organizing the structure of the complex. Baron et al. (p. 531) have determined the structure of the sterile alpha motif domain of a Shank protein and show that it forms large sheets composed of helical fibers stacked side-by-side. Sheets of the Shank protein may form a platform for the construction of the PSD complex.

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