This Week in Science

Science  27 Apr 2012:
Vol. 336, Issue 6080, pp. 389
  1. A Volcanic Valley on Mars

    CREDIT: NASA/JPL/UNIVERSITY OF ARIZONA

    The formation of the Athabasca Valles, a valley network near the equator of Mars, has been debated for over a decade; both lava and water ice have been implicated. Some studies have attributed the polygonally patterned ground in this region to frost wedging in an ice-rich soil. Using images of the Athabasca region from the High Resolution Imaging Science Experiment onboard Mars Reconnaissance Orbiter, Ryan and Christensen (p. 449) detected numerous spirals on polygonally patterned ground. The spiral shapes are consistent with those of lava coils that form on the surface of Hawaiian lava flows, and cannot be explained by ice-related activities.

  2. Getting Around the Block

    Diblock copolymers provide a rich variety of morphologies that depend on the length of the polymer blocks, the overall fraction of each block, and their chemical dissimilarity. New synthetic methods have made it possible to make copolymers with three or more components and in a range of chemical architectures. However, this growth in design choices can offer too many variables to work with, and rational design is important, especially when trying to transform small-scale products in engineered commodities. Bates et al. (p. 434) review the opportunities and complexities that exist when working in this expanded playground of block copolymers.

  3. Gut Reaction

    The gut needs to keep its trillions of microbial inhabitants contained. The immune system has evolved a multifaceted approach to this problem, which includes the production of large quantities of immunoglobulin A (IgA) in the intestinal mucosa. In a process that is not well understood, plasma cells that produce IgA specific for the gut microflora are selected in Peyer's patches in the gut. Kawamoto et al. (p. 485) used genetically manipulated mice to show that the inhibitory co-receptor, programmed cell death-1 (PD-1), is required for the proper selection IgA-secreting cells in the gut. The effect of PD-1 deletion, however, was not intrinsic to the B cells that produce IgA. Instead, the absence of PD-1 affected the differentiation of T follicular helper cells, which provide important signals to B cells that help guide them as they develop the capacity to produce microflora-specific IgA. Mice deficient in PD-1 exhibited alterations in the composition in their microflora, which suggests that defective selection of IgA can perturb the careful balance that exists between the immune system and resident bacteria.

  4. Synchronize Watches

    Time standards based on the energy-level transitions of atoms and ions provide the most accurate and precise methods of time keeping. Measurements made in one laboratory and in another must be done with clocks that have been synchronized and calibrated to ensure that the same measurement is being made. Such clocks, however, are not particularly mobile and are housed in national metrology labs. Predehl et al. (p. 441; see the Perspective by Warrington) linked two optical clocks separated by over 900 kilometers using optical fiber to show that the clocks can be synchronized, with the clocks showing a frequency stability better than 3.7 × 10−19. Such long-distance synchronization should allow for tests of fundamental physics, such as general relativity and quantum electrodynamics.

  5. Working the Angles on Chemical Bonding

    The forces exerted by chemical bonds depend not only on the distances between atoms but also upon the angles between them. Welker and Giessibl (p. 444; see the cover) probed the angular dependence of the CO molecule adsorbed on top of a copper atom on an atomically flat Cu(111) surface using both atomic force microscopy and scanning tunneling microscopy (STM). Probe tips with three different tip-atom-symmetry environments were used. All three tips delivered similar STM images, but force probes revealed the angular dependence of the CO bond to the surface and provided data for a model of the changes in bond energy.

  6. Space Organics

    The origin of the organic compounds found in meteorites and interplanetary dust particles is a matter of debate. Laboratory experiments suggest that these organics were inherited from the interstellar medium and predate the existence of the solar system. By using particle-tracking models, Ciesla and Sandford (p. 452, published online 29 March; see the Perspective by Nuth and Johnson) explored the possibility that the organics could have been produced within the outer reaches of the protoplanetary disk from which the solar system planets originate. Grains within a protoplanetary disk follow irregular paths; such orbital histories subjected model particles to ultraviolet radiation and temperatures that have been shown in laboratory experiments to generate organic compounds.

  7. Keep It Inside

    Some cyanobacteria form solid-phase calcium carbonate precipitates as a consequence of fixing CO2 during photosynthesis. Usually, such carbonates form extracellularly near the surface of the cells, sometimes generating structures called stromatolites. In a biofilm growing on carbonate deposits in Lake Alchichica, Mexico, Couradeau et al. (p. 459; see the Perspective by Riding) discovered one species of cyanobacteria that also precipitates amorphous carbonate particles internally. Because the structure and chemical composition of these carbonates is distinct from those formed extracellularly, there may be cellular control over the mineralization process. These precipitates may influence physiological processes such as cell buoyancy and the sequestration of excess alkalinity generated during photosynthesis.

    CREDIT: COURADEAU ET AL.
  8. Getting Wetter Faster

    Theoretical projections, based on the relationship between temperature and the amount of water vapor the atmosphere can hold, suggest that global warming should intensify the strength of the atmospheric water cycle by about twice the rate as the thermodynamics and climate models predict. Durack et al. (p. 455) examined 50 years of observations of sea surface salinities and conclude that the patterns of change in the salinity data are consistent with the theoretical projections, rather than with those of the models. Thus, the global water cycle should intensify by 16 to 24% for a future increase of a global average temperature of 2 to 3°C.

  9. Farmer Displaced European Hunters

    Our understanding of prehistoric demography and human evolution has been improved by analysis of ancient DNA. Skoglund et al. (p. 466) describe the retrieval and analysis of genomic DNA from ancient (~5000-year-old) northern European Neolithic individuals within modern Sweden. These include three hunter-gatherers from the Pitted Ware Culture horizon and one farmer ascribed to the Mid-Neolithic North-Central TRB culture. The hunter-gatherers displayed a distinct genetic signature, similar to that of extant northern Europeans, whereas the farmer's genetic signature more closely resembled southern Europeans, suggesting migration and admixture during the spread of farming.

  10. Acetylation and Autophagy

    CREDIT: LIN ET AL.

    Autophagy allows cells to digest their own components when necessary to survive stressful conditions. Lin et al. (p. 477) and Yi et al. (p. 474) describe signaling mechanisms in mammalian cells and yeast, respectively, by which autophagy is regulated by protein acetylation. In mammalian cells deprived of serum, the acetyltransferase TIP60 was activated by phosphorylation by the protein kinase GSK3 (glycogen synthase kinase 3). TIP60's target appeared to be a protein kinase central to autophagy regulation, ULK1. This activating pathway was required for autophagy in the absence of serum, but was not needed for autophagy in cells deprived of glucose. In the yeast Saccharomyces cerevisiae starved of nitrogen, another acetylation mechanism was uncovered. Starvation led to activation of the histone acetyltransferase Esa1, which acetylated the protein Atg3, a key component of the autophagy machinery, thus increasing its interaction with another autophagy protein, Atg8.

  11. Microbes: Early and Often

    Epidemiological studies have suggested that the increase in the incidence of asthma and other inflammatory diseases seen in many parts of the world may be due to a reduced exposure to microbes during early childhood. Olszak et al. (p. 489, published online 22 March) now show that commensal microflora help to regulate the numbers and functions of natural killer T (NKT) cells in the colon and lung in mice. Germ-free mice had elevated numbers of NKT cells in these tissues and were more susceptible to chemically induced colitis and allergic asthma. Neonatal recolonization of germ-free mice with microflora prevented enhanced colitis and asthma sensitivity; however, exposure of adult mice to these conditions was not effective. Thus, early exposure to microbes has important, lasting effects on the immune system's sensitivity to inflammation.

  12. Overthinking Religion?

    Many theories of human cognition make a distinction between System I, which tends to be rapid and to rely on heuristics or rules of thumb, and System II, which tends to be more deliberative and analytic. This dual-process framework, within which both processes may operate simultaneously and competitively, has been used to explain a variety of situational influences upon decision-making. Gervais and Norenzayan (p. 493) studied the application of a dual-process framework to religious disbelief and found that triggering analytic thinking processes through a variety of experimental manipulations resulted in a tendency for subjects to report lower levels of religious belief.

  13. Acid Rate

    The low species richness of the acid mine drainage in the Richmond Mine (Iron Mountain, California, USA) provides an excellent opportunity to study microbial process on a genome scale in real time. Denef and Banfield (p. 462; see the Perspective by DeLong) report the further analysis of metagenomic data from 24 microbial biofilm samples collected over 8 years, at different sites. High-throughput, shotgun sequencing of the DNA was used to look for high-frequency, single-nucleotide polymorphisms in six genotypic variants of a chemolithoautotroph, Leptospirillum group II, the dominant primary producer and iron-oxidizer. Thirteen samples collected at one site over 5 years were analyzed to infer the accumulation of single-nucleotide substitutions for free-living populations over time. A series of very recent branching points in the phylogenetic tree that arose via homologous recombination were revealed.

  14. Handling Nitrosative Stress

    S-nitrosylation of proteins is a principal mechanism of cellular signaling in eukaryotes but has not been reported in microbes. The transcription factor OxyR serves to protect bacterial cells from reactive oxygen species produced by cell metabolism in the presence of oxygen. Seth et al. (p. 470) found that when cells grew in the presence of nitrate, OxyR was modified by S-nitrosylation of the same cysteine residue that gets oxidized in cells grown aerobically. However, the nitrosylated OxyR activated a different set of genes—some of which appeared to protect the cell from excessive S-nitrosylation.

  15. Generating Inflammasomes

    Inflammasomes are large, multiprotein complexes that assemble in response to infection that are also involved in the pathogenesis of a variety of other diseases, including type 2 diabetes and atherosclerosis. The assembly of the inflammasome triggers an inflammatory cascade that results in the activation of caspase-1 and production of the cytokines interleukin-1 and -18. Very little, however, is known about the specific signals that trigger inflammasome assembly. Shenoy et al. (p. 481, published online 29 March; see the Perspective by Caffrey and Fitzgerald) now show that guanylate binding protein 5 (GBP5) promotes the assembly of the NLRP3-containing inflammasome in response to certain activation signals, such as pathogenic bacteria and adenosine triphosphate, but not others, like crystalline stimuli. Mice deficient in GBP5 exhibited impaired caspase-1 activation and production of cytokines. NLRP3 inflammasome–dependent responses to pathogenic bacteria and inflammatory stimuli were also impaired in mice lacking GBP5.

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