This Week in Science

Science  09 May 2003:
Vol. 300, Issue 5621, pp. 861
  1. Not All Added Iron Is Alike

    More than a decade ago, the late John Martin suggested that iron was the major factor limiting oceanic productivity in the high-nutrient, low-chlorophyll (HNLC) marine environments of the subarctic Pacific, the equatorial Pacific, and the Southern Ocean. Tsuda et al. (p. 958) conducted iron fertilization experiments in the subarctic Pacific Ocean, the last of the three major ocean regions to be studied this way. Productivity is also iron-limited, but the observed biological and biogeochemical responses were quite different from the other HNLC waters. A shift of dominant phytoplankton species occurred that the authors believe might have also occurred in HNLC open waters during the past glacial period and that might happen again if longer and larger scale inputs of iron are introduced into the oceans.

  2. Phonons and Phases in Plutonium

    A detailed understanding of the phase diagram of plutonium (Pu) is useful for predicting how the material will behave under a variety of environments. The toxicity and radioactivity of Pu can limit experimental studies, thus increasing our reliance on theoretical predictions of this element's properties. However, modeling of the phase diagram of Pu is challenging in that it exhibits six crystallographic allotropes and calculations that must deal with strong correlations between the f-shell electrons. Dai et al. (p. 953) calculated the phonon (lattice vibration) spectra of Pu at high temperatures by taking into account the electron-electron and electron-phonon interactions. They provide insights into the contraction that occurs during heating as the δ phase transforms in the ϵ phase.

  3. Ant Attacks on the Tree Tops

    Ants are ubiquitous and abundant in the canopy of tropical rain forest, and one hypothesis that accounts for this dominance is that tropical forest ants are herbivores, rather than as omnivores. Davidson et al. (p. 969; see the Perspective by Hunt) used stable nitrogen isotopes to assess the diets of canopy ants. What they find is that many ant species appear to be mainly herbivores, and obtain nitrogen not primarily through predation or scavenging of animals but more likely through bacterial microsymbionts. These latter organisms are closely related to either aphid microsymbionts, known to upgrade nonessential amino acids to essential ones, or related to the nitrogen fixers that nodulate within the roots of plants. These results suggest that rainforest plants experience much greater losses to ant herbivores than previously recognized.

  4. Epoxides Sans Chlorine

    Millions of tons of epoxides are made each year, mainly through a process that relies on chlorine as an oxidant. Kamata et al. (p. 964) report that a tungsten-based polyoxometalate anion allows for the efficient production of a wide variety of epoxides using hydrogen peroxide (H2O2) instead. Unlike many H2O2 routes, this reaction avoids the production of O2 that increases explosion hazards under industrial conditions.

  5. Ancient Asteroid Accident

    A major asteroid collision in the main belt may have produced an increase flux of meteors to Earth in the mid-Ordovician (480 million years ago), but evidence of such an event is scant. Schmitz et al. (p. 961) analyzed relict chromite grains from decomposed meteorites in marine limestone deposited over about 250,000 square kilometers of southern Sweden and found that all of the chromite has an L chondrite composition.

  6. Group Snapshot

    Many fundamental studies of chemical reactions have focused on systems of three atoms in order to avoid the complexities that arise when even one of the products has more than one vibrational mode. Lin et al. (p. 966) introduce a product imaging technique and applied to the reaction of fluorine atoms with deuterated methane to produce DF and the CD3 radicals. This method allowed them to resolve how vibrational energy left in the DF molecule correlated with vibrational energy in different levels of the v2 or “umbrella” mode of CD3.

  7. Fire Escapes for Carbon

    Boreal forests and their underlying soils sequester carbon that can be released by wildfires. Wardle et al. (p. 972) provide quantitative evidence, through the study of an island system in Sweden where different islands have different fire histories, of how wildfire history influences ecosystem carbon input, output, and sequestration in boreal forest. Fire suppression in these forests may increase their role as a net global carbon sink.

  8. How Bacteria Say No to Drugs

    One of the generalized and widespread modes of bacterial resistance to drugs is the development or acquisition of multidrug pumps that actively flush out unrelated species of small molecules, thus reducing cytosolic drug concentrations. Yu et al. (p. 976) describe the structures of four different small molecules bound to AcrB, one of the multidrug efflux pumps in Escherichia coli. They find that a central cavity serves as an all-purpose binding site for small molecules, offering hydrophobic, aromatic, and van der Waals interactions that can be tailored to accommodate diverse ligands.

  9. Tiny but Turbulent

    The small volumes associated with microfluidic channels can make it difficult to observe and take advantage of nonlinear rheological effects. Groisman et al. (p. 955) show that by driving the flow through carefully designed tortuous channels, they can push the fluid from the normal laminar flow regime into an elastic turbulence regime. For small changes in flow rate, they can obtain large changes in pressure drop, and they demonstrate both a flux stabilizer and a bistable flip-flop memory.

  10. The Catalytic Core of the Spliceosome

    One of the central complexes in the spliceosome, which removes introns from pre-messenger RNAs, is SF3b. This complex is involved in the assembly of components at the all-important branch-point nucleotide (the scissors used to make the first cut), which lies within cross-linking distance of SF3b protein p14. Golas et al. (p. 980) report the structure of the SF3b complex from electron cryomicroscopy. They place two structural elements, a ladder of HEAT repeats and several RNA-recognition motifs, within the outer shell and central cavity, respectively.

  11. Proteins Strike a Pose

    Conformation switches can be key events in the regulation of protein activity, but detecting their occurrence can be difficult. Nizak et al. (p. 984) developed a method to select recombinant antibodies rapidly in vitro for use as conformation sensors. Using antibodies selected to recognize guanosine triphosphate (GTP)-bound forms of the small GTPase Rab6, a regulator of intracellular membrane traffic, they determined the localization and behavior of Rab6·GTP in living cells. The method has the potential to create reagents to enable the study of any molecule that can be locked in vitro in a particular conformation.

  12. Simply Doing the Job at Hand

    In vertebrates, the major events in the cell division cycle—synthesis of DNA and mitotic division of the cell—are controlled by complexes of related but distinct cyclin proteins with their partners, the cyclin-dependent kinases, that can have overlapping activities. Moore et al. (p. 987) examined the capacity of cyclin B1, which normally promotes mitosis in combination with cyclin-dependent kinase 1 (Cdk1), to promote DNA synthesis in extracts of frog eggs. Cyclin B1 could in fact promote DNA synthesis if it was targeted to the nucleus and if a moderate stimulation of its activity was attained. Thus, the cyclin B1-Cdk1 complex can phosphorylate critical targets that initiate DNA synthesis. Cells normally prevent such action of the complex through control of its localization and its activation state.

  13. Creating Left-Right Asymmetry in the Hippocampus

    The NR2B subunit of the NMDA receptor is a major prerequisite for learning and memory. A dynamic rearrangement of this subunit occurs during development and after synaptic modifications. Kawakami et al. (p. 990) show that, in the adult mouse hippocampus, the NR2B subunit is asymmetrically distributed in glutamatergic synapses deriving from ipsilateral versus contralateral afferents. There is also an asymmetric NR2B distribution between synapses on the basal and apical dendrites of pyramidal neurons.

  14. Eyes Right, Eyes Left

    Neurons from retinae of the right and left eyes project to different regions of the mature brain's dorsal lateral geniculate nucleus (DLGN). To tease apart the effects of bulk neuronal activity from correlation of neuronal activity, whereby neurons originating from the same eye fire together, Huberman et al. (p. 994) have studied the effects of immunological ablation of amacrine cells in the developing ferret eye. Without the normally interspersed amacrine cells, the ganglion cells of the retina retain activity but lose the tendency to correlate that activity with their near neighbors. Observations of the corresponding DLGN show that the correlated aspect of that activity is not required for eye-specific segregation patterns to develop.