This Week in Science

Science  30 Mar 2001:
Vol. 291, Issue 5513, pp. 639
  1. Bosons Help Cool Fermi Gases

    Evaporative cooling of an atom gas of bosons, whereby the hottest atoms are progressively removed, is an effective and widely used method for forming Bose-Einstein condensates. For Fermi gases, however, collisions between identical fermions are forbidden, and new tricks, such as mixing two Fermi gases with different spin states, have been used to form Fermi condensates. Truscott et al. (p. 2570; see the Perspective by O'Hara and Thomas) now create quantum-degenerate Bose and Fermi condensates simultaneously from a boson-fermion mixture. Heat is removed from the fermions through elastic collisions with the bosons, a process called sympathetic cooling. They show that different statistics associated with the two types of gases can actually be visualized.

  2. Mapping Out Bond Formation

    Several atomic force microscopy (AFM) studies have determined the force necessary to rupture a chemical bond. Lantz et al. (p. 2580; see the Perspective by de Lozanne) have used low-temperature, high-resolution AFM to map out the change in force versus distance for the formation of a chemical bond at a surface. Their atomic-resolution studies of the interaction of a single atom at the end of a silicon (Si) tip with different adsorption sites on the Si(111) 7 × 7 reconstructed surface allowed them to determine the change in short-range chemical force with distance from the surface.

  3. Tharsis Rise and Water Fall

    The Tharsis rise on Mars is a region of thick crust and high topography related to voluminous volcanism and includes the largest volcano identified in our solar system, Olympus Mons. Phillips et al. (p. 2587) have produced a model of the effect of the localized mass load of the Tharsis rise on the global geoid and topography that is consistent with data from Mars Global Surveyor. Their results suggest that the Tharsis rise was formed between 4.0 to 3.6 billion years ago and that about half of the valley networks may be related to hydrologic processes. The release of CO2 and water during this intense period of volcanism may have produced a wetter and warmer martian climate.

  4. Signs of Hydration

    The isotopic composition of oxygen in atmospheric CO2 is determined largely by the interaction of CO2 and liquid water in the ocean, in soils, and in the leaves of plants. Because of the role of leaf water, oxygen isotopes contain important information about the sources and sinks of atmospheric CO2. Gillon and Yakir (p. 2584; see the Perspective by Woodward) show that variations in the activity of the enzyme carbonic anhydrase, which catalyzes the hydration of CO2—and its equilibration with leaf water—in plants cause a wide range of CO2 oxygen isotopic fractionation in different types of vegetation. This information could lead to better estimates of the relative contributions of C3 and C4 plants (defined by their use of the two major photosynthetic pathways) to global productivity.

  5. Magnetic Frustration

    When a current is passed through a metal or semiconductor in the presence of a magnetic field, carriers deflected from their trajectory generate an electric field transverse to the direction of current flow that leads to the voltage drop known as the Hall effect. Measurements of this Hall effect for some magnetic materials—particularly spin-frustrated systems—have revealed an anomalous Hall effect in which the change in transport parameters are opposite to those found for normal materials. Taguchi et al. (p. 2573) combine experimental data from neutron scattering and transport measurements with theoretical work to suggest that geometrical (Berry phase) and spin-chirality effects are responsible for the anomaly.

  6. Alignment Without the Rub

    The application of liquid crystal (LC) materials to display technology is often limited by the ability to design and produce a substrate that can generate the desired surface alignment. Most techniques for generating substrates, such as rubbing, create rough textured surfaces that lead to defects in the orientation of the LC. Lee and Clark (p. 2576) have used smooth neighboring substrates that have different surface treatments to control the LC without any of the angular orientational degeneracies that usually accompany planar isotropic surfaces. Surface treatment was achieved by using self-assembled monolayers that were exposed selectively to ultraviolet light to alter their surface characteristics. More complex devices could be produced through microcontact printing.

  7. First Glimpse from SeaWiFS

    The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has been orbiting Earth for 3 years, gathering data on global fluctuations in oceanic and land photosynthesis (net primary production, or NPP). This period included an El Niño-to-La Niña transition, and Behrenfeld et al. (p. 2594; see the cover) report that substantial increases in oceanic chlorophyll levels were observed that indicate a shift in nutrients available to phytoplankton. No corresponding increase was seen for land plants, but overall NPP from 1997 to 2000 was estimated to have increased by 4.5%.

  8. Disrupted Schedules

    A potential impact of global climate change is a shifting of seasonal changes. Thomas et al. (p. 2598; see the news story by Pennisi) expose the energetic and fitness costs that birds (in this case, blue tits in Corsica) incur when breeding is not perfectly synchronized with food availability in the local environment. Foraging costs increase as breeding becomes mismatched with food supply, which forces the parents to work beyond their sustainable maximum metabolic rate and leads to low survival. These findings provide mechanistic detail for the selective forces acting on the timing of breeding in birds and have implications for the responses of birds to global climate change.

  9. Mutating Mutators

    When bacteria reach the gut of a new host, they must adapt rapidly to these new conditions. Giraud et al. (p. 2606) have analyzed the advantages and disadvantages of the ability of the ubiquitous gut commensal, Escherichia coli, to accelerate mutation rates while colonizing the guts of germ-free mice. Although mutator populations were established in a new mouse and became dominant more quickly than strains that do not have the potential to generate adaptive mutations so rapidly, in the long term, mutators lose their advantage because it is too risky to maintain a high mutation rate. Although only neutral or beneficial mutations will be selected in the gut, any of these may become disadvantageous when the bacteria are flushed out of the gut and are exposed to a different environment before finding a new host.

  10. How Practice Makes Perfect

    During a period of a few months, young songbirds listen to adult birds singing, begin to produce their own unpolished and fragmentary sounds, and gradually hone their skills until they, too, become master singers. How these skills are learned has been difficult to understand. Tchernichovski et al. (p. 2564; see the Perspective by Margoliash) have developed a training regimen in which they can record songs throughout the learning period. They then applied a sophisticated analysis that makes it possible to track from the final, polished song backward to the off-key and monosyllabic precursors. They find, for example, that when the target tone is slightly lower in pitch than the precursor, the young songbird increases the pitch of its note until an abrupt halving of the frequency puts it precisely on-key.

  11. Fat-Burning Mice

    Malonyl-coenzyme A is a key regulator of fatty acid metabolism in mammals and is generated by the action of two acetyl-coenzyme A carboxylases, ACC1 and ACC2. Abu-Elheiga et al. (p. 2613; see the Perspective by Ruderman and Flier) show that mice deficient in ACC2 are fertile and have a normal life-span but also exhibit a persistently elevated rate of fatty acid oxidation. The mice can consume normal amounts of food but store only half the amount of fat as wild-type mice. The identification of ACC2 as a potential therapeutic target for obesity may come as good news to those humans who wish to lose weight without modifying their diet or exercise habits.

  12. Pathfinding Virus

    The decision to begin eating requires the integration of a variety of motivational and metabolic signals emanating from distinct regions of the brain. DeFalco et al. (p. 2608) traced these neural pathways in a rodent model by creating a genetically modified herpes virus (that also encodes a green fluorescent protein) that replicated only in neurons that expressed a gene product of interest and in neurons making synaptic contact with the initially infected cells. They find that hypothalamic neurons expressing the leptin receptor or neuropeptide Y, two proteins known to be involved in the regulation of feeding, receive inputs from a number of different brain areas, including the amygdala, cortex, and other regions of the hypothalamus.

  13. Neighborhood Conflicts

    Development and conservation are often competing goals. Balmford et al. (p. 2616; see the news story by Vogel) have looked at this competition in detail. They show that scattered reports of a link between human settlement and areas of high conservation value are supported for an entire continent (Africa); across 1° grid squares, species richness in four vertebrate groups is positively correlated with human density. This relation is intimately associated with primary productivity. The scope to avoid conservation conflicts is therefore limited because many high-density grid squares contain species that are found nowhere else. Hence, the pervasive conflicts between conservation and development are unlikely to be easily side-stepped.

  14. Freezing Out Nitric Acid

    Stratospheric denitrification—the irreversible removal of nitric acid from the stratosphere—facilitates the destruction of ozone by removing reactive nitrogen species that sequester atomic chlorine, the major catalyst of ozone loss. Reaching a quantitative understanding of how denitrification occurs has been an elusive goal. Tabazadeh et al. (p. 2591) tackle this problem with a cloud model that simulates the formation, growth, and sedimentation of large nitric acid-containing particles, the prime suspects in denitrification. They identify a polar freezing belt that allows denitrification to occur around both the Arctic and the Antarctic. Their results indicate that if global warming leads to stratospheric cooling, recovery from polar ozone loss will be delayed.

  15. Strength in Spider Silk Sequences

    All of the more than 34,000 species of spider utilize silk. The silk, spun from fibroin proteins, can vary dramatically in its properties. Primary dragline silk is extremely tough; silk used in web construction has a high tensile strength; the capture spiral of an orb-web is stretchy and can triple in length before breaking. Whether the properties of spider silk are determined by the fibroin sequences or the spinning process is unclear. Fibroin sequences of only two genera of orb weavers have been characterized until now and represent only a small fraction of spider diversity. To better address the origins of the attributes of silk, Gatesy et al. (p. 2603) have analyzed fibroins from an additional five genera of spiders. Despite intense mutational pressure, specific sequence motifs have been maintained in spider silks since the Cretaceous, which suggests that the structure of the fibroin proteins themselves are important for the mechanical properties of silk. Furthermore, some of these motifs also appear in unrelated insect silks and in oyster and mussel proteins that also display exceptional mechanical properties.

  16. DNA Repair Comes into Bloom

    Bloom Syndrome is an inherited disorder associated with genomic instability, predisposition to cancer, and partial sterility. The causative gene, BLM, encodes a nucleic acid helicase whose precise function in the cell remains mysterious. Kusano et al. (p. 2600) show that the mus309 locus in the fruit fly Drosophila, originally identified in a mutagen sensitivity screen, encodes a homolog of BLM. Flies with mutations in this gene were sterile and showed signs of chromosomal instability. Interestingly, these mutant phenotypes could be partially rescued by an extra copy of the gene encoding Ku70, a protein that binds to double-stranded DNA breaks. These results indicate that BLM may function in DNA repair.

  17. Subatomic Features in Atomic Force Microscopy Images

    Using “a force-detection scheme with superior noise performance and enhanced sensitivity to short-range forces,” Giessibl et al. (Reports, 21 July 2000, p. 422) reported that they could resolve subatomic features on a silicon surface using atomic force microscopy. Hug et al. comment that the frequency shift (Δf) at which the experiments were performed suggests that “more than 95% of Δf is due to long-range …forces,” at variance with the claim of an enhanced sensitivity to short-range forces. Hug et al. conclude that the structures observed by Giessibl et al. were actually due to feedback artifacts. Giessibl et al. respond that “both theoretical and experimental considerations argue against” a feedback explanation, and present additional topography and error signal data that “show clearly that feedback tracking errors are negligible.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/291/5513/2509a

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