Catálogo de publicaciones - revistas

<jats:title>The Third Way</jats:title> <jats:p> Because organic carbon contains a larger fraction of the light isotope <jats:sup>12</jats:sup> C than inorganic carbonate, variations in the carbon isotopic record of sedimentary rocks are thought to represent changes in the amount of organic carbon buried as sediments (and thus removed from the rest of the carbon cycle). <jats:bold> Schrag <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="540" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1229578">540</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="6119" page="533" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1231981">Canfield and Kump</jats:related-article> </jats:bold> ) suggest that historically a third component was important: authigenic carbonate. Authigenic carbonate is not produced in any appreciable quantity today, but was much more abundant when the level of atmospheric oxygen was low. </jats:p>

<jats:title>Dissecting Diversity</jats:title> <jats:p> Solid tumors are composed of functionally diverse tumor cells. The prevailing view is that this "intratumoral heterogeneity" arises from the accumulation of mutations during tumor growth, resulting in multiple genetically defined subclones of cells that respond in different ways to selective pressures such as chemotherapy. <jats:bold> Kreso <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="543" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1227670">543</jats:related-article> , published online 13 December; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6119" page="528" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1234415">Marusyk and Polyak</jats:related-article> </jats:bold> ) simultaneously monitored the genetic profiles and growth behavior of human colorectal cancer cells that were serially passaged in mice. Individual tumor cells within a uniform genetic lineage displayed extensive variation in survival, growth dynamics, and response to a chemotherapeutic drug. Thus, additional diversity-generating mechanisms such as epigenetic regulation or microenvironmental variability appear to operate within a genetic clone, endowing a subset of tumor cells with robust survival potential, especially during stress. </jats:p>

<jats:title>Not Just Wasting</jats:title> <jats:p> Malnutrition is well known in Malawi, including a severe form—kwashiorkor—in which children do not simply waste away, they also suffer edema, liver damage, skin ulceration, and anorexia. <jats:bold> Smith <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="548" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1229000">548</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="6119" page="530" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1234723">Relman</jats:related-article> </jats:bold> ) investigated the microbiota of pairs of twins in Malawian villages and found notable differences in the composition of the gut microbiota in children with kwashiorkor. In these children, a bacterial species related to <jats:italic>Desulfovibrio</jats:italic> , which has been associated with bowel disease and inflammation, was noticeable. When the fecal flora from either the healthy or the sick twin was transplanted into groups of germ-free mice, the mice that received the kwashiorkor sample started to lose weight, like their human counterpart. </jats:p>

<jats:title>Linking Mass and Time</jats:title> <jats:p> The precision of atomic clocks is based on the transitions between two well-defined energy levels—the frequency of oscillation. We know from relativity that mass and energy are equivalent and from quantum mechanics that energy relates to frequency. Therefore, the ticking of a clock can be related, in principle, to the mass of a particle. The oscillation frequency of a particle is known as its Compton frequency and, because of the high frequency involved and stability of atoms, it has been argued that a clock linking mass and time would offer very high precision. Ordinarily, the Compton frequency is extremely high and not accessible to direct excitation. <jats:bold> Lan <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="554" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1230767">554</jats:related-article> , published online 10 January; see the Perspective by Debs et al.) demonstrate the operation of a Compton clock exploiting a related parameter, the phase accumulation rate of cold cesium atoms. Using an atom interferometer and an optical frequency comb to bring the Compton frequency into an experimentally accessible regime, mass and time could be directly linked. </jats:p>

<jats:title>Nanoscale NMR with Diamond Defects</jats:title> <jats:p> Although nuclear magnetic resonance (NMR) methods can be used for spatial imaging, the low sensitivity of detectors limits the minimum sample size. Two reports now describe the use of near-surface nitrogen-vacancy (NV) defects in diamond for detecting nanotesla magnetic fields from very small volumes of material (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6119" page="529" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1233222">Hemmer</jats:related-article> </jats:bold> ). The spin of the defect can be detected by changes in its fluorescence, which allows proton NMR of organic samples only a few nanometers thick on the diamond surface. <jats:bold> Mamin <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="557" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1231540">557</jats:related-article> ) used a combination of electron spin echoes and pulsed NMR manipulation of the proton spins to detect the very weak fields. <jats:bold> Staudacher <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="561" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1231675">561</jats:related-article> ) measured statistical polarization of a population of about 10 <jats:sup>4</jats:sup> spins near the NV center with a dynamical decoupling method. </jats:p>

<jats:title>Nanoscale NMR with Diamond Defects</jats:title> <jats:p> Although nuclear magnetic resonance (NMR) methods can be used for spatial imaging, the low sensitivity of detectors limits the minimum sample size. Two reports now describe the use of near-surface nitrogen-vacancy (NV) defects in diamond for detecting nanotesla magnetic fields from very small volumes of material (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6119" page="529" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1233222">Hemmer</jats:related-article> </jats:bold> ). The spin of the defect can be detected by changes in its fluorescence, which allows proton NMR of organic samples only a few nanometers thick on the diamond surface. <jats:bold> Mamin <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="557" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1231540">557</jats:related-article> ) used a combination of electron spin echoes and pulsed NMR manipulation of the proton spins to detect the very weak fields. <jats:bold> Staudacher <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="561" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1231675">561</jats:related-article> ) measured statistical polarization of a population of about 10 <jats:sup>4</jats:sup> spins near the NV center with a dynamical decoupling method. </jats:p>

<jats:title>The Change of Winds</jats:title> <jats:p> As the combined effects of Antarctic stratospheric ozone depletion and climate warming have forced the westerly surface winds in the Southern Hemisphere to shift toward the pole, mixing between the upper ocean and deeper waters has also changed. <jats:bold> Waugh <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="568" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225411">568</jats:related-article> ) now show that water originating at the surface at subtropical latitudes is mixing into the deeper ocean at a higher rate than 20 years ago, while the reverse is true for those originating at higher latitudes. The summer westerly winds that blow in the Southern Hemisphere have shifted toward the South Pole over the past several decades, but why? <jats:bold>Lee and Feldstein</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="563" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225154">563</jats:related-article> ) show that greenhouse gas forcing and ozone depletion impart different signatures to wind patterns and conclude that ozone depletion has been responsible for more than half of the observed shift. </jats:p>

<jats:title>The Change of Winds</jats:title> <jats:p> As the combined effects of Antarctic stratospheric ozone depletion and climate warming have forced the westerly surface winds in the Southern Hemisphere to shift toward the pole, mixing between the upper ocean and deeper waters has also changed. <jats:bold> Waugh <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="568" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225411">568</jats:related-article> ) now show that water originating at the surface at subtropical latitudes is mixing into the deeper ocean at a higher rate than 20 years ago, while the reverse is true for those originating at higher latitudes. The summer westerly winds that blow in the Southern Hemisphere have shifted toward the South Pole over the past several decades, but why? <jats:bold>Lee and Feldstein</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="563" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225154">563</jats:related-article> ) show that greenhouse gas forcing and ozone depletion impart different signatures to wind patterns and conclude that ozone depletion has been responsible for more than half of the observed shift. </jats:p>

<jats:title>Chloroplast Translocon Revealed</jats:title> <jats:p> Protein translocation across biological membranes requires supramolecular complexes, called translocons. Chloroplasts require translocons in their double-envelope membranes to import thousands of nucleus-encoded proteins synthesized in the cytosol. However, the identity of the translocon at the inner envelope of the chloroplast (TIC) has long been a matter of debate; two proteins, Tic20 and Tic110, have been proposed to be central to protein translocation across the inner envelope membrane. Using transgenic <jats:italic>Arabidopsis</jats:italic> plants expressing a tagged form of Tic20, <jats:bold> Kikuchi <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="571" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1229262">571</jats:related-article> ) report the isolation of a 1-megadalton complex composed of Tic56, Tic100, and Tic214 involved in protein translocation across the inner envelope. Thorough in vitro biochemical and in vivo genetic experimentation suggest that the isolated translocon contains both nuclear- and organellar-encoded components. Tic110 was not part of the isolated translocon. </jats:p>

<jats:title>The Power of the Collective</jats:title> <jats:p> Sensing the environment is generally considered to require significant cognitive sampling and comparison, not to mention time. However, species that do not necessarily have the cognitive ability, or the time, have also proven to be quite adept at sensing and evaluating their environment. <jats:bold> Berdahl <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="574" related-article-type="in-this-issue" vol="339" xlink:href="10.1126/science.1225883">574</jats:related-article> ) show that in shiners, a species of schooling fish, mere attraction to, and movement toward, neighboring individuals allows the group to track preferred darkness in a variable-light environment. </jats:p>