Editors' Choice

Science  04 Sep 2009:
Vol. 325, Issue 5945, pp. 1182
  1. Ecology

    Fishing Down a Halo

    1. Sherman J. Suter

    Breeding seabirds are among the many animals that return to a central location after obtaining food. Such central-place foragers are expected to favor prey captured nearby. This preference could deplete prey in the area around the colony (forming a “Storer-Ashmole's halo”) through the removal of benthic species or the departure of mobile pelagic species. For marine predators, tests of this hypothesis must also consider water depth, which might have a greater impact than distance on foraging efficiency. Breeding thick-billed murres (Uria lomvia) usually return to their chick with a single easily identifiable item of food, and the birds are large enough to carry time-depth recorders that have little effect on their behavior. Thus, by monitoring prey deliveries of chick-rearing murres equipped with recorders, Elliott et al. were able to attribute individual prey items to specific dives. Drawing on data gathered over many years at a colony numbering roughly 100,000 individuals on Coats Island in northern Hudson Bay, they restricted their analyses to the interval of constant energy-delivery rates to chicks. Prey mass, and hence energy content, increased with distance from the colony, and dive depths became shallower as distance increased. As the breeding season progressed, murres flew farther for a given prey. Within each season, birds fished down the food web: taking larger fish first, then smaller fish, and even smaller invertebrate prey last. The prey depletion demonstrated by these four foraging patterns can lead to density-dependent population regulation and the life-history strategies of delayed maturity, low fecundity, and high adult survival found in many seabirds.

    Auk 126, 613 (2009).

  2. Astronomy

    Clouds in the Tropics

    1. Maria Cruz

    Previous observations of Saturn's largest moon showed methane clouds at high southern latitudes, where summer prevailed, and ethane clouds in the winter north polar region. The tropics, by contrast, tended to appear virtually cloudless. Observations from Cassini and ground-based telescopes have now revealed a greater recent incidence of clouds in Titan's tropical atmosphere. Using Cassini observations taken between 3 July 2004 and 28 May 2008, Griffith et al. detected the presence of five tropical clouds with characteristics similar to those of the summer polar methane clouds. This increasing cloud presence provides evidence for the beginning of an overturn in Titan's atmospheric circulation; Titan's pole-to-pole circulation is expected to reverse near the equinox, which occurred this August, inducing the polar clouds to swap hemispheres.

    High-latitude clouds can reach altitudes of 45 km, but the Cassini observations show that tropical clouds are confined to altitudes below 26 km. Although this finding points to a dry and stable tropical climate, another recent study implicates rather different weather patterns. Using images from two ground-based telescopes, Schaller et al. report large cloud outbursts starting at southern mid-latitudes in April 2008, triggering cloud activity near the south pole and the equator of Titan over the next 3 weeks. The cause of this outburst is unknown, though it is thought to be associated with stormy weather conditions and substantial amounts of methane rain.

    Astrophys. J. 702, L105 (2009); Nature 460, 873 (2009).

  3. Biophysics

    Swimming Through Mucus

    1. Gilbert Chin

    The stomach is an acidic environment of pH ∼ 2 as anyone who has suffered a bout of emesis can testify. So how does Helicobacter pylori, which is not an acidophilic bacterium, survive? First, it secretes the enzyme urease, which hydrolyzes urea to produce ammonia. This helps to neutralize the ambient hydrochloric acid, at least locally. Another strategem is to burrow beneath the layer of protective mucus that lies between the digestive juices and the epithelial cell lining of the stomach; once there, it is protected in part by locally secreted bicarbonate. The problem for H. pylori is how to penetrate the mucus, which has the remarkable property of transitioning from a viscous solution at neutral pH to a gel-like substance at low pH.

    Celli et al. demonstrate that the capacity of this bacterium to neutralize its immediate surroundings, via urease activity, also contributes to reducing the viscoelastic moduli of gastric mucin. Measurements of the time-dependent rotation of the bacterial cell body and its flagellum, when trapped within mucin in its gel-like state, reveal that it possesses a high-torque motor operating at roughly 1 × 10−17 N m, a value that is 3- to 10-fold larger than what has been calculated for other bacteria. Adding urea raises the pH, which reduces the moduli by two orders of magnitude and allows H. pylori to swim through the mucus at 30 µm/s.

    Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0903438106 (2009).

  4. Plant Science

    Sending Out an ROS

    1. Nancy R. Gough

    Local stresses, such as the damage caused by an insect on a leaf, can produce signals that are transmitted systemically to parts of the plant that are not directly injured or stressed. These signals help the plant acclimate to environmental stress or defend against pathogens. Miller et al. show that the gene RbohD, which encodes a plant NADPH oxidase that generates reactive oxygen species (ROS), is critical for rapid systemic signaling in response to wounding by a scalpel, heating to 42°C, cooling in ice water, high-intensity light, or increased salinity.

    Sci. Signal. 2, ra45 (2009).

  5. Biomedicine

    Deadly Collaboration

    1. Paula A. Kiberstis

    Goblet cells (blue) in gastric mucosa.

    CREDIT: LIU ET AL., GASTROENTEROLOGY 137, 10.1053/J.GASTRO.2009.07.041 (2009)

    By some estimates, more than 50% of the world's population is infected by the bacterium Helicobacter pylori, a gastrointestinal pathogen most famous for its role in the development of gastric ulcers. H. pylori infection is also a risk factor for gastric cancer, but because only a small percentage of infected individuals develop the disease, it would be of great interest to identify controllable lifestyle factors that might contribute to an enhanced risk of cancer. Correlative data from epidemiological studies have suggested a potential interaction between H. pylori infection and diet, but long-term human studies that would establish a cause-effect relationship are not feasible.

    In a carefully controlled 5-year study of Rhesus monkeys that were monitored at frequent intervals by gastroscopy and biopsy, Liu et al. found that gastric neoplasia (precancerous and cancerous lesions) developed in H. pylori–infected animals that had also consumed a carcinogen similar to one found in pickled vegetables and smoked meats, but not in animals exposed to either the bacterium or the carcinogen alone. In terms of cancer prevention strategies, these findings underscore the importance of dietary awareness in individuals known to be infected with H. pylori.

    Gastroenterology 137, 10.1053/j.gastro.2009.07.041 (2009).

  6. Applied Physics

    Directing Optical Traffic

    1. Ian S. Osborne

    Optical applications such as quantum cryptography and communication are reliant on the ability to direct single photons between sender and receiver stations. Though there are several implementations capable of generating single photons, the direction of photon emission tends to be random, and the coupling efficiency into the carrier medium (an optical fiber) tends to be rather low. Toishi et al. place a quantum dot in a specially prepared photonic crystal cavity which controls the lifetime of the quantum dot and the direction of the emitted photons. Through careful design of the photonic crystal to match the output modes of the source with the mode of the carrier fiber, they show that they can increase the coupling efficiency into the fiber. The designer optical components should find direct application in opto-electronic circuitry.

    Opt. Express 17, 14618 (2009).

  7. Chemistry

    Edging In with Iron

    1. Jake Yeston

    Hydroboration is widely used in organic chemistry to append a borane and a hydrogen atom to respective carbons at either end of a double bond; the borane group can then be efficiently replaced to form a range of compounds such as alcohols. Wu et al. now show that an iron catalyst facilitates a variation on this reaction in 1,3-dienes: hydrocarbon derivatives with two double bonds separated by a single bond (C=C–C=C). Instead of adding boron and hydrogen to adjacent carbons, the reaction proceeds to add them at opposite edges of the four-carbon sequence, concomitantly creating a double bond between the central carbons. The catalyst forms in situ through reduction of an iminopyridine iron(II) complex by magnesium. In substrates substituted at one of the central carbons in the diene framework, the reaction is highly selective for the E product geometry about the newly formed double bond, complementing a previously developed palladium-catalyzed variant of this reaction class that favors the opposite stereochemistry. Varying the imine substituent on the ligand also allows the authors to modulate which end carbon receives the boron.

    J. Am. Chem. Soc. 131, 10.1021/ja9048493 (2009).