Editors' Choice

Science  09 Feb 2007:
Vol. 315, Issue 5813, pp. 739
  1. GEOLOGY

    Leveling the Landscape

    It has been recognized for some time that humans are the most important geomorphic agent modifying our planet's surface, dwarfing the effects of deforestation, desertification, and erosion caused by other processes globally. However, the details of these activities are enlightening, and Wilkinson and McElroy compare the rates of erosion from human activity in different settings with natural processes and with long-term and short-term rates inferred throughout Earth's history. About 5 gigatons (Gt) of sediment per year are thought to have been deposited naturally by rivers during the past 540 million years. There has been high variation about this average, particularly since the Pliocene and during glacial times; the average flux today is about 21 Gt/year. Most of this material (about 80%) comes from mountainous regions where natural erosion rates are highest. For comparison, it is estimated that humans now move about 75 Gt of dirt and rock annually, mostly in low-lying or low-topography areas, and particularly near coasts. This difference implies that large amounts of human-derived detritus are being stored primarily on floodplains and in small stream networks on coastal plains immediately downslope from such areas. This flux greatly exceeds the movement of material by Pleistocene ice sheets. — BH

    Geol. Soc. Am. Bull. 119, 140 (2007).

  2. MOLECULAR BIOLOGY

    Remodeling with a Floral Motif

    The transition from vegetative growth to flowering in the plant Arabidopsis is in part regulated by the gene Flowering Locus C (FLC). The expression of FLC is known to be regulated by various signaling pathways that modify the chromatin in which it is embedded. Deal et al. show that, beyond secondary modifications of existing chromatin components, the addition of a specific histone variant, H2A.Z, adds to the controls on FLC expression. Histone H2A.Z is deposited onto FLC chromatin by the Arabidopsis variant of the SWR1 complex, which has been shown in yeast to participate in the remodeling and individualizing of local chromatin domains. With histone H2A.Z as part of its chromatin packaging, the FLC gene is better poised for active expression. It is the robust expression of FLC that results in maintenance of the vegetative state. — PJH

    Plant Cell 19, 10.1105/tpc.106.048447 (2007).

  3. ECOLOGY/EVOLUTION

    Managing Murrelets

    One challenge in conservation management is estimating what a sustainable population should look like. Geographic and genetic information can readily be obtained from museum specimens, but if, as for many birds, there are morphologically distinguishable age classes, then age-ratio analysis can also provide baseline rates for reproduction and survival. The output of such an analysis can be used to set targets for population recovery.

    Beissinger and Peery have championed the case of the marbled murrelet (Brachyramphus marmoratus), an endangered seabird from the Californian coast which unexpectedly nests high up in old-growth coniferous trees and whose plight has only recently come to attention. The murrelet population is being decimated as a result of attacks from crows, by logging, and especially via the loss of the fish that they eat. Reproduction is expensive for murrelets; they lay a single egg that weighs up to a quarter of the adult bird's weight, and adults will abandon breeding in the face of insufficient food. There are some methodological risks with age-ratio analysis, but for this bird, contemporary data from field studies were compared with the museum data, which showed that the reproductive success of contemporary murrelets is almost an order of magnitude less than it was a century ago. — CA

    Ecology 88, 296 (2007).

  4. CHEMISTRY

    Bromine Scrub

    One drawback of power generation from coal combustion is the atmospheric release of toxic mercury previously trapped in the solid coal. Although current emission filtration systems efficiently remove oxidized mercury compounds, they are less effective at trapping the metal in its water-insoluble elemental state. Combustion of chlorine-rich bituminous coals tends to pass less mercury through the filters, an observation that has been attributed to mercury oxidation by the chlorine. Liu et al. therefore explored the capacity of bromine, chlorine's more polarizable congener, to oxidize residual Hg(0) in coal emission streams. Laboratory-scale tests revealed a greater than hundredfold enhancement in the oxidation rate relative to measured chlorination kinetics. Moreover, the reaction was further accelerated by adsorption of the reagents onto fly-ash particles, with little or no inhibition observed from oxygen, CO, water, or HCl. Sulfur dioxide proved mildly inhibitory, whereas NO had a small promoting effect on the fly-ash-mediated oxidation. By extrapolating their data to the higher temperatures of a power plant emission stream, the authors estimate that the addition of 0.4 part per million of bromine to flue gas could oxidize 60% of residual Hg(0). — JSY

    Environ. Sci. Technol. 41, 10.1021/es061705p (2007).

  5. CHEMISTRY

    All Glow Together

    The appeal of quantum dots stems from their highly tunable optical and electronic properties. Their size-dependent luminescence, for example, is finding use in biological sensing and imaging. However, with the exception of this application, most technologies that could take advantage of quantum dots do not operate in solution. It can be difficult to pack a large concentration of quantum dots into a solid structure, and even if assembly is successful, the luminescent properties can be lost because of the formation of trap states or to quenching interactions between neighboring dots. Arachchige and Brock used a sol-gel process to prepare aerogel and xerogel monoliths with CdSe/ZnS core-shell nanoparticles as the primary building blocks. Throughout the transformation process from sols to wet gels and on to monoliths, little change was observed in the photoluminesence spectra for the three sizes of particles tested. This behavior contrasts strongly with that of nanostructured networks assembled from bare CdSe particles, which lost the favorable absorption and emission properties observed in solution. In both the aerogels and xerogels, transmission electron microscopy images suggested that the ZnS shells may have partially fused together to form a matrix containing CdSe particles, particularly in the 20-fold denser xerogels. — MSL

    J. Am. Chem. Soc. 129, 10.1021/ja066749c (2007).

  6. BIOMEDICINE

    Viruses in the Prostate

    Twenty to 25% of human cancers are caused by viruses. The recognition that a virus plays an etiological role in a specific type cancer can profoundly change the ways in which that particular disease is diagnosed, treated, or prevented. One recent guidepost is cervical cancer, where the identification of human papillomavirus as a causal agent has led to the development of a promising prophylactic vaccine.

    Previous studies have provided evidence in support of a possible viral origin for human prostate cancer. A retrovirus called XMRV (for xenotropic murine leukemia virus-related virus) was detected in 40% of prostate tumors from men who were homozygous for an allelic variant of the RNASEL gene and in only 2% of tumors from men of other genotypes. The RNASEL gene encodes RNase L, a ribonuclease whose activity is required for an innate antiviral response mediated by interferon (IFN); and, intriguingly, the allelic variant associated with XMRV-containing tumors encodes an enzyme with impaired activity.

    Dong et al. show that a molecular viral clone of XMRV is infectious in human prostate cancer cell lines, that replication of the virus in vitro is sensitive to inhibition by IFN, and that suppression of RNase L enhances viral replication. In addition, they localized putative integration sites for the XMRV provirus to several host genes that encode functions with biologically plausible roles in prostate cancer, including a suppressor of androgen receptor transactivation. Still unanswered is the critical question of whether XMRV plays a causal role in prostate cancer, but these provocative observations should stimulate further experiments to sort this out. — PAK

    Proc. Natl. Acad. Sci. U.S.A. 104, 1655 (2007).

  7. GEOPHYSICS

    Rough Core

    Earth's core is split into two zones. A solid inner core lies within a liquid outer core, which drives the geodynamo via heat convection. The smooth and spherical interface comprising the inner core boundary (ICB) is the site of such geochemical changes as solidification and chemical flows. The ICB can be probed by seismic waves that reflect or refract off of it. In the unusual circumstance of repeating earthquakes, sequential rays can travel identical ray paths through Earth. By analyzing one such pair of repeated earthquakes, Cao et al. found changes in the ICB structure over a decade. Two identical ray paths, sampled by earthquakes in the South Sandwich Islands in 1993 and 2003 and recorded in Alaska and Canada, show variations in the shape of their seismic response over this time. Although simple pressure waves were identical, differences were seen between the reflected and refracted waves that intercepted the ICB. These amplitude differences may indicate lumps on the ICB of 10 km in horizontal extent. Features of this size could be created by low convection in the inner core itself, or by rotation of a fixed bump on the inner core by approximately 0.1° per year. — JB

    Proc. Natl. Acad. Sci. U.S.A. 104, 31 (2007).

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