This Week in Science

Science  24 Mar 2006:
Vol. 311, Issue 5768, pp. 1669
  1. Ice Sheet Stability

    CREDIT: OTTO-BLIESNER ET AL.

    The world is warming, and higher temperatures can cause melting of polar ice sheets. How fast will the ice sheets of Greenland and Antarctica disappear, and how fast and far will sea level rise in the coming century? These issues are addressed in a news story by Kerr (see the cover), the Editorial by Hanson and Kennedy, Perspectives by Bindschadler and Joughin, and four Reports. Otto-Bliesner et al. (p. 1751) integrate climate model simulations, an ice sheet model, and paleoclimate data to show that the northern latitudes, and particularly the Arctic, were significantly warmer during the Last Interglaciation, when sea level was several meters higher than at present. They also estimate that the Greenland Ice Sheet contributed between 2.2 and 3.4 meters of sea level rise in the penultimate deglaciation. Overpeck et al. (p. 1747) compare the model's predictions of warming during the next 130 years to this reconstruction, and conclude that surface temperatures will be as high by the end of this century as they were 130,000 years ago. These conditions would melt enough of the Greenland Ice Sheet to raise sea level by several meters. Determining how quickly Antarctic ice may be disappearing has been difficult to assess. The Gravity Recovery and Climate Experiment (GRACE) satellites were designed to make the needed measurements, and Velicogna and Wahr (p. 1754, published online 23 February) show that the mass of the ice sheet has been decreasing by 152 ± 80 cubic kilometers per year from 2002 to 2005, mostly from losses of the West Antarctic Ice Sheet. Contrary to some projections, ice loss around the margins is proceeding faster than the center of the ice sheet is growing. Glacial earthquakes are triggered by the large and sudden sliding of glaciers and can be observed by global seismic networks. Ekström et al. (p. 1756; see the Perspective by Joughin) recorded glacial earthquakes on Greenland and found that these events were more common in summer and that their annual number has doubled since 2002. Both of these findings are consistent with the observed accelerating motion of outlet glaciers from the Greenland Ice Sheet and correlate with its more widespread melting in recent years.

    CREDIT: VELICOGNA AND WAHR
  2. Beating Entropy

    It is typically difficult to mix two polymers together or to mix particles into polymers unless there is a strong attraction between the dissimilar materials because entropic effects favor phase separation. Mackay et al. (p. 1740) show that when the size of the particles is smaller than the radius of gyration of the polymer, the mixed state may be thermodynamically favored because of an increase in surface contacts between the particles and the polymer. However, they also show that processing strategies must be taken into consideration for this favored state to be reached for certain mixtures.

  3. Making Oxygen Glow in the Dark

    Aqueous mixtures of organic matter in the environment contain many molecules that, when irradiated by sunlight, can excite dissolved oxygen to its singlet state (1O2). Highly reactive 1O2 can play a significant role in both the direct degradation of pollutants and the internal chemistry of local bacteria. However, the short lifetime of 1O2 hinders accurate measurements of its concentration. Latch and McNeill (p. 1743, published online 23 February; see the Perspective by Hassett) use a hydrophobic probe molecule to trap 1O2 from deep within the suspended pockets of organic matter and then quantify concentrations with induced chemiluminescence. They measure values more than 100 times greater than those found with traditional probes that fail to penetrate the organic phase. A kinetic model based on competing quenching and diffusion rates accounts well for the partitioning.

  4. Bloated and Not-So-Bloated Genomes

    Eukaryotic genomes are bloated with so-called “junk” DNA including introns, mobile elements, and large intergenic regions. Curiously, animal mitochondrial genomes are tiny, essentially junk-free, and conserved in gene structure, whereas plant mitochondrial genomes are relatively large, full of junk, and do not show a rigid conservation of gene structure. What underlies these very different patterns of genome size and complexity? Lynch et al. (p. 1727) review how mutation rates correlate with organelle genome complexity, being for the most part much higher in animal mitochondria than in plant mitochondria, which suggests that nonadaptive evolutionary forces play a critical role in shaping the structure of organelle genomes and possibly nuclear genomes. A stumbling block in annotating bacterial genomes is the presence of pseudogenes. Ochman and Davalos (p. 1730) review systematic methods for identifying pseudogenes in particular genomes, using the well-studied Escherichia coli as an example.

  5. Plankton Biogeography

    Prochlorococcus is the most common oxyphototroph in the open ocean and plays a key role in ocean-based fixation of CO2, oceanic primary production, and the composition of the marine ecosystem. Johnson et al. (p. 1737) show that closely related strains (>97% similarity in 16S ribosomal RNA) have dramatically different distribution patterns in the water column, and indeed over the entire Atlantic Ocean. These closely related microbes appear to have ecologically distinct roles related to temperature, light, and competitors. Coleman et al. (p. 1768) analyzed two closely related Prochlorococcus strains and found that diversity was concentrated in genomic islands, putatively acquired via lateral gene transfer mediated by phage. Genomic islands may be a fundamental mechanism for niche differentiation across microbial systems (see the news story by Pennisi).

  6. A Foe Motif

    Pattern recognition receptors recognize conserved components found in pathogens, but not in the host, are central to the innate immune response. Chang et al. (p. 1761) describe the crystal structure at 2.1 angstrom resolution of tracheal cytotoxin (TCT), a fragment of a peptidoglycan specific to Gram-negative bacteria, bound to the ectodomains of the peptidoglycan recognition proteins LCa and LCx. The structure shows how a specificity determinant of Gram-negative bacteria is recognized in the complex and how TCT induces heterodimerization of LCa and LCx to activate downstream signaling.

  7. Adding Oxygen to the Evolutionary Mix

    What was the effect of developing the ability to use oxygen safely in metabolic reactions? Raymond and Segrè (p. 1764; see the Perspective by Falkowski) modeled how metabolic networks would have evolved from the Late Archean to Late Proterozoic periods of Earth's history. The complexity of networks that could use oxygen increased to levels far beyond those seen before the presence of oxygen. Comparisons between enzyme distributions and phylogenies suggest that adaptation to oxygen occurred after the major phylum-level divergences.

  8. Rethinking β-Islet Cell Replacement

    CREDIT: CHONG ET AL.

    Type 1 diabetes mellitus (T1DM) occurs when the insulin producing β-islet cells of the pancreas become depleted through autoimmune attack. As well as finding means of limiting this destructive immune response, a great deal of research effort is being placed in finding ways of regenerating β-islet cells. It had been reported that spleen cells could reverse T1DM by replacing lost β-islet cells through transdifferentiation when injected together with an immune adjuvant into diabetic mice [Science302, 1223 (2003)]. Three groups (Chong et al., p. 1774; Nishio et al., p. 1775; and Suri et al., p. 1778) now report that the same protocol does result in some reversal of established T1DM in the same mouse model, but not via spleen cell transdifferentiation (see the news story by Couzin). Simple injection of the immune adjuvant alone promoted recovery. Presumably, the immune-modifying activity of the adjuvant provides a window of opportunity for the few remaining β-islet cells to proliferate to the extent that they become a sufficient source of insulin. Although these studies do not support the contribution of spleen cell transdifferentiation to the reversal of T1DM, they do provide hope for future development of immune-based therapies for the condition.

  9. A Trojan Horse to Battle Cancer

    One of the major hurdles in cancer therapy is delivering drugs efficiently to the tumor cell target. Thorne et al. (p. 1780) addressed this problem by designing a “Trojan horse” therapy in which immune effector cells that naturally migrate to tumors (cytokine-induced killer, or CIK cells) were used to deliver a potent oncolytic virus (vaccinia) to tumors growing in mice. The CIK cells transported the virus deep within the tumors to provide a uniform distribution of infection. The viral infection in turn enhanced tumor cell killing by the CIK cells and significantly inhibited tumor growth. Although each component of the therapy had been shown previously to have antitumor activity, the combination proved to be much more effective.

  10. Ordering Up Ice Phases

    Ice forms numerous phases at high pressures that are denser than liquid water. Exploring the phase diagram has been an ongoing process, and success has often been obtained by finding a method or trick to accelerate the transformation from one phase, in which it may be trapped, into another. Salzmann et al. (p. 1758) show that the addition of hydrochloric acid to two disordered phases of ice, ice V and ice VII, could unlock their geometrical frustration to form two previously uncharacterized phases, ice XIII and ice XIV.

  11. Shedding Light on Tuberculosis Susceptibility

    Since the late 1800s, sunlight and other forms of light therapy have been considered potentially beneficial for tuberculosis, most likely because of the antimicrobial effects of sunlight-induced synthesis of vitamin D. Liu et al. (p. 1770) reveal that vitamin D signaling contributes to the Toll-like receptor (TLR) pathway of microbicidal defense to Mycobacterium tuberculosis (MTB) in monocytes and macrophages. Activation of TLR2/1 by a bacterial lipoprotein led to vitamin D receptor expression and processing of the provitamin D precursor, which stimulated vitamin D-induced up-regulation of an antimicrobial peptide and killing of MTB bacilli. The low levels of circulating provitamin D3 hormone and limited ability to induce the microbicidal peptide observed in sera of African-American individuals may contribute to their increased susceptibility to tuberculosis.

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