Editors' Choice

Science  19 Oct 2007:
Vol. 318, Issue 5849, pp. 361
  1. ECOLOGY/EVOLUTION

    Ins and Outs of Extinction

    1. Andrew M. Sugden

    The global extinction of a species is the end point of a series of smaller-scale local extinctions of populations. Hence, the causes of extinction can be understood by studying patterns of extinction at the local scale. Species vary in their intrinsic vulnerability to extinction, and there is a range of extrinsic factors that can influence a population's survival; the probability of extinction might depend on the interplay of these two broad considerations. To study these questions, Fréville et al. took advantage of the Park Grass experiment, in which the fate of populations of herbaceous plants subjected to different fertilizer treatments have been followed for 60 years at a site in southeast England. The interactions of 11 intrinsic factors (life-history traits relating, for example, to reproduction and growth) with four extrinsic factors (such as nitrogen enrichment and acidification) were investigated. It transpired that population extinction could in most cases be related to the interaction of just one life-history trait with one extrinsic factor, but that the pairs of factors differed in different species. These findings point the way to a more accurate and predictive science of extinction, which will in turn provide a new tool for conservation managers attempting to reduce the rate of local extinctions caused by human activity. — AMS

    Ecology 88, 2662 (2007).

  2. CHEMISTRY

    Pulling Copper Along

    1. Jake Yeston

    Copper is a common choice for constructing pipes that carry drinking water because of its relatively strong resistance to corrosion, but over time oxidative chemistry can introduce metal ions into the streams emerging from the faucet. A complex series of factors contributes to the ion concentration, ranging from the water's pH to the precipitation equilibria of various hydroxide, oxide, and carbonate salts, as well as biochemical processes that accompany the formation of bacterial biofilms on the pipes' inner surfaces. In general though, a simplifying assumption has been that the aqueous copper ion concentration is limited by diffusion during stagnant periods between flow, when water rests in the pipe. Calle et al. have now found that the influence of flow dynamics cannot in general be neglected. Through a series of measurements on a pipe system connecting a well to a household in Chile, they uncovered concentration patterns suggesting that a significant quantity of ions is dislodged from surface biofilms by virtue of interfacial forces arising during flow. Thus, the interplay of hydrodynamics with sorption equilibria in these systems merits further study. — JSY

    Environ. Sci. Technol. 41 10.1021/es071079b (2007).

  3. NEUROSCIENCE

    Too Quick to Glimpse?

    1. Katrina L. Kelner

    An optical illusion can help define which parts of the brain are responsible for human consciousness. People cannot consciously perceive a number flashed on a screen for 16 ms if it is quickly followed by another stimulus in the same area. As the time between the two stimuli increases, the first stimulus becomes visible; that is, it is accessible to the person's consciousness. Del Cul et al. recorded electrical brain waves from people's scalps as they were shown these stimuli and reported to the investigators whether they were visible or invisible. One brain wave in particular, P3, occurring 270 to 400 ms after the beginning of the trial, correlated with conscious perception of the stimulus. This wave seems to arise from sudden simultaneous activity in several parts of the brain, specifically the frontal, parietal, and temporal cortices of both hemispheres. These data are inconsistent with several proposed correlates of consciousness, including the rapid induced activity in the visual areas of the brain and the later more distributed, but still local, neural reverberations. Rather, they suggest that conscious perception is associated with a sudden global reverberation of neural activity, about 300 ms after the stimulus, encompassing several cortical areas bilaterally. — KK

    PLoS Biol. 5, 10.1371/journal.pbio.0050260 (2007).

  4. GEOLOGY

    Heat Bursts in the Highlands

    1. Brooks Hanson

    Because rocks are good insulators, it is generally thought that temperatures deep in the crust evolve slowly, rising and falling over millions to tens of millions of years. Rapid pulses of fluid or the intrusion of hot magmas can heat or cool rocks more quickly, as can rapid uplift along a fault (which juxtaposes hot and cold rocks at a rate faster than heat conduction). Thus metamorphic processes are also thought to act over these time scales. Ague and Baxter challenge some of these notions in well-studied metamorphic rocks in Scotland, known as the Barrovian metamorphic belt and thought to represent burial and heating of rocks during continental collision. They show that concentrations of a trace element, strontium, across the mineral apatite are surprisingly variable. Laboratory data imply that if the minerals were at the temperatures inferred for the host rocks for even 1 million years, diffusion should have homogenized any gradients. Thus the authors infer that the rocks were heated and cooled in less time. This would seem to require rapid heat input by fluids and rapid exhumation, but at scales and rates that start to challenge what have been thought to be geologic limits. Stay tuned. — BH

    Earth Planet. Sci. Lett. 261, 500 (2007).

  5. CHEMISTRY

    A Light Switch in SWNTs

    1. Phil Szuromi

    Although most realizations of molecular electronics make use of metallic leads, single-walled carbon nanotubes (SWNTs) can also serve as contacts. Oxidative cutting leaves carboxylate-decorated ends that can be covalently linked to diamine molecules so that the SWNT is reconnected through the molecule via amide linkages. Whalley et al. now use this approach to study ethene-bridged dithiophene and dipyrrole derivatives that photoconvert from ring-opened to ring-closed forms. Ultraviolet irradiation of the ring-opened thiophene derivative created a conjugated ring-closed form that was 25 times more conductive. Unlike the case for molecules bridging gold break-junction electrodes, neither visible light exposure nor heating recovered the open isomer, which the authors attribute to the greater energy dissipation from the excited state in this system. The pyrrole derivative could be thermally switched back, and showed a staircase rise in conductance with ultraviolet light exposure that was attributed to the presence of several molecules bridging the SWNT gap. — PDS

    J. Am. Chem. Soc. 129, 10.1021/ja073127y (2007).

  6. ECOLOGY

    Kelp in the Depths

    1. Caroline Ash

    Kelp forests are exceptionally productive marine ecosystems, iconic of high-latitude, shallow, cold waters. There are a few rare records of tropical deep-water species, but these are thought to be relicts of glacial-era populations. Graham et al. suggest that kelp may not be as restricted in distribution as once thought. By modeling the coincidence of the water temperature, bottom depth, and light penetration with nutrient circulation, they derive a map of potential tropical kelp beds, a rough contour of 25 to 236 m. A quick look offshore of the Galapagos Islands indeed revealed kelp at around 60 m depth. The authors also predict extensive kelp forests off Brazil, West Africa, and the Malay Archipelago. Essentially, wherever clear tropical water allows light to penetrate into cooler depths and bathymetries allow nutrient upwelling, kelp should survive in the tropics. Hence, even in strong El Niño years, tropical kelp can escape surface warming. — CA

    Proc. Natl. Acad. Sci. U.S.A. 104, 10.1073/pnas.0704778104 (2007).

  7. STKE

    How Clocks See the Light

    1. L. Bryan Ray

    The circadian clocks that regulate daily rhythms in various processes in living organisms are entrained to a 24-hour cycle by mechanisms that detect daily changes in the amount of light in the environment. Hirayama et al. show that hydrogen peroxide (H2O2) can function as a required signaling molecule to transmit the sensation of light to changes in timing of the biochemical clock. In zebrafish, oscillators present in peripheral tissues and organs are sensitive to exposure to light. The authors used Z3 cells to show that exposure of the cells to light caused increased production of H2O2. Exposure of the cells to H2O2 increased expression of zebrafish Cryptochrome and Period genes (which encode components of the core clock machinery) with a time course similar to that observed when cells were exposed to light. Catalase is an antioxidant enzyme that can degrade H2O2, and the authors confirmed that light exposure stimulated expression of the zCat gene, but did so with a delayed time course consistent with its possible function in a negative feedback loop to cyclically suppress expression of the clock genes that initially resulted from light-induced generation of H2O2. In mammalian cells, H2O2 did not influence the expression of the clock genes, but mammalian peripheral tissues are not responsive to light. The identity of the phototransducer in the zebrafish system remains unknown. — LBR

    Proc. Natl. Acad. Sci. U.S.A. 104, 15747 (2007).

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