Catálogo de publicaciones - revistas

<jats:title>Early Ore Formation</jats:title> <jats:p> Ore deposits contain most of the world's metal resources, from commonly used metals such as iron, to precious and expensive metals such as platinum. Understanding how these ancient deposits form may lead to more efficient metal extraction and give clues about early Earth. <jats:bold> Bekker <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1086" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177742">1086</jats:related-article> ) studied sulfur and iron isotopes in 2.7-billion-year old Fe-Ni sulfide deposits from Canada and Australia and found that most of the metal-scavenging sulfur was originally atmospheric in origin. Photochemical reactions in the ancient oxygen-free atmosphere produced sulfide that eventually circulated to the sea floor and mixed with newly erupted komatite magmas. Thus, global surface processes in the oceans, atmosphere, and on continents are geochemically linked to ore-forming processes within Earth. </jats:p>

<jats:title>Building Early Continents</jats:title> <jats:p> Cratons, the roots of Earth's continents, have survived billions of years of accretion, volcanism, and plate motion. Due to this tumultuous history, existing evidence for how and when they formed is hard to find. <jats:bold> C.-W. Chen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1089" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178477">1089</jats:related-article> ) use geophysical data collected below the Slave craton in Canada to show that subduction of lithospheric plates in the Archean may have been a major process that controlled its assembly. Spatially aligned seismic and conductive discontinuities over 100 kilometers below the surface are caused by minerals that formed from hot fluids generated as ancient crust melted at a subduction zone. Other old cratons on Earth show similar features, suggesting plate tectonics was operating at least 3.5 billion years ago. </jats:p>

<jats:title>Plasmonic Probing of Catalysis</jats:title> <jats:p> An understanding of catalytic reactions on surfaces, such as those used in industrial processes, often requires some measure of reactant concentration on the surface. Often this is expressed as the surface coverage of metal particles that are dispersed on oxide supports. Although optical probes of surface coverage would be convenient, they usually lack sufficient sensitivity to detect the small number of molecules on the surface. <jats:bold> Larsson <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1091" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1176593">1091</jats:related-article> , published online 22 October) have used shifts in plasmon resonances to measure surface coverages. They grow oxide coatings, decorated with metal catalyst particles, on a nanoscale gold disk, and find that these model catalysts are within the region of plasmon sensitivity. Reactions such as CO oxidation on platinum can be followed for different ratios of reactant gases with a sensitivity of 0.1 monolayer of surface coverage. </jats:p>

<jats:title>Methane's Path to Captivity</jats:title> <jats:p> The mutual repulsion of oil and water is well known. It is thus somewhat baffling that in arctic regions and in marine sediments, enormous quantities of methane lie trapped under pressure in surrounding cages of ice. <jats:bold> Walsh <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1095" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1174010">1095</jats:related-article> , published online 8 October; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5956" page="1070" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1183027">Debenedetti and Sarupria</jats:related-article> </jats:bold> ) undertook extended simulations to probe the steps that guide these two normally incompatible molecules along convergent, rather than divergent, paths. Computed 2- and 5-microsecond trajectories trace the process of methane capture as ice crystals nucleate and ultimately assemble into a cage network. </jats:p>

<jats:title>Acidic Ocean</jats:title> <jats:p> One consequence of the historically unprecedented level of CO <jats:sup>2</jats:sup> in the atmosphere that fossil fuel burning has caused, in addition to a warmer climate, is higher concentrations of dissolved CO <jats:sup>2</jats:sup> in the oceans. This dissolved CO <jats:sup>2</jats:sup> makes the oceans more acidic, and thus less saturated with respect to calcium carbonate. This has important ramifications for organisms that have calcium carbonate skeletons, which depend for their survival on the saturation state of calcium carbonate in the waters where they live. <jats:bold> Yamamoto-Kawai <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1098" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1174190">1098</jats:related-article> ) report that in 2008, surface waters of the Canada Basin became undersaturated with respect to aragonite, a relatively soluble form of calcium carbonate incorporated into the shells or skeletons of many types of marine plankton and invertebrate. This undersaturation occurred much sooner than had been anticipated and has important implications for the composition of the Arctic ecosystem. </jats:p>

<jats:title>Demise of the Megafauna</jats:title> <jats:p> Approximately 10,000 years ago, the Pleistocene-Holocene deglaciation in North America produced widespread biotic and environmental change, including extinctions of megafauna, reorganization of plant communities, and increased wildfire. The causal links and sequences of these changes remain unclear. <jats:bold> Gill <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1100" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1179504">1100</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5956" page="1072" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1182770">Johnson</jats:related-article> </jats:bold> ) unravel these connections in an analysis of pollen, charcoal, and the dung fungus <jats:italic>Sporormiella</jats:italic> from the sediments of Appleman Lake, Indiana. The decline in Pleistocene megafaunal population densities (inferred from fungal spore abundances) preceded both the formation of the lateglacial plant communities and a shift to an enhanced fire regime, thus contradicting hypotheses that invoke habitat change or extraterrestrial impact to explain the megafaunal extinction. The data suggest that population collapse and functional extinction of the megafauna preceded their final extinction by several thousand years. </jats:p>

<jats:title>Gardening for Ants and Termites</jats:title> <jats:p> Among the social insects, ants and termites are the most diverse and ecologically dominant. Termites are known to engage in a mutualism with nitrogen-fixing bacteria, and <jats:bold> Pinto-Tomás <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1120" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1173036">1120</jats:related-article> ) have identified similar relationships occurring among leaf-cutter ants, which maintain specialized nitrogen-fixing bacteria in their fungus gardens. Together, these mutualisms are a major source of nitrogen in terrestrial ecosystems. How is the evolutionary stability of such mutualistic cooperation maintained? <jats:bold> Aanen <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1103" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1173462">1103</jats:related-article> ) show that the <jats:italic>Termitomyces</jats:italic> fungus cultured by termites remains highly related because mycelia of the same clone fuse together and grow more efficiently to out-compete rare clones. </jats:p>

<jats:title>Extinction Distinctions</jats:title> <jats:p> The distribution of affected organisms can help reveal the mechanisms of mass or background extinctions. This has often been studied by comparing effects on land versus in the ocean, or across latitudes, for example. <jats:bold>Miller and Foote</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1106" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1180061">1106</jats:related-article> ) now analyze extinction patterns in shallow seas around continents, versus open-ocean coastal regions from the Permian through the Cretaceous, a period spanning three major mass extinctions. The mass extinctions all affected organisms in the open coastal environment greater than those inhabiting shallow seas, but at other times the pattern was opposite. Thus, organisms in these environments may have very different evolutionary histories. </jats:p>

<jats:title>Periplasmic Redox Regulation</jats:title> <jats:p> The oxidation state of intracellular and extracellular proteins are carefully managed by cellular redox machineries. <jats:bold> Depuydt <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1109" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1179557">1109</jats:related-article> ) discovered a reducing system that protects single cysteine residues from oxidation in the bacterial periplasm. DsbG, a thioredoxin-related protein, appears to be a key player in that system and is the first reductase identified in the periplasm of <jats:italic>Escherichia coli</jats:italic> . Together with DsbC, DsbG controls the global sulfenic acid content of this compartment. Sulfenic acid formation is a major posttranslational modification in the periplasm, and three homologous L,D-transpeptidases are substrates of DsbG. Sulfenic acid formation is not restricted to <jats:italic>E. coli</jats:italic> , but is ubiquitous. Because proteins from the thioredoxin superfamily are widespread, similar thioredoxin-related proteins may control cellular sulfenic acid more widely. </jats:p>

<jats:title>A-Maize-ing</jats:title> <jats:p> Maize is one of our oldest and most important crops, having been domesticated approximately 9000 years ago in central Mexico. <jats:bold> Schnable <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1112" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178534">1112</jats:related-article> ; see the cover) present the results of sequencing the B73 inbred maize line. The findings elucidate how maize became diploid after an ancestral doubling of its chromosomes and reveals transposable element movement and activity and recombination. <jats:bold> Vielle-Calzada <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1078" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178437">1078</jats:related-article> ) have sequenced the <jats:italic>Palomero Toluqueño</jats:italic> ( <jats:italic>Palomero</jats:italic> ) landrace, a highland popcorn from Mexico, which, when compared to the B73 line, reveals multiple loci impacted by domestication. <jats:bold> Swanson-Wagner <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1118" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1178294">1118</jats:related-article> ) exploit possession of the genome to analyze expression differences occurring between lines. The identification of single nucleotide polymorphisms and copy number variations among lines was used by <jats:bold> Gore <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1115" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1177837">1115</jats:related-article> ) to generate a Haplotype map of maize. While chromosomal diversity in maize is high, it is likely that recombination is the major force affecting the levels of heterozygosity in maize. The availability of the maize genome will help to guide future agricultural and biofuel applications (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5956" page="1071" related-article-type="in-this-issue" vol="326" xlink:href="10.1126/science.1183463">Feuillet and Eversole</jats:related-article> </jats:bold> ). </jats:p>