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排序方式: 共有519条查询结果,搜索用时 78 毫秒
511.
Hao Ye Hossein Lotfi Lu Li Robert T. Hinkey Rui Q. Yang Lin Lei Joel C. Keay Matthew B. Johnson Tetsuya D. Mıshıma Michael B. Santos 《科学通报(英文版)》2014,59(10):950-955
Interband cascade (IC) photovoltaic (PV)device structures, consisting of multiple discrete InAs/GaSb superlattice absorbers sandwiched between electronand hole barriers, were grown by molecular beam epitaxy.Details of the molecular beam epitaxy growth and materialcharacterization of the structures are presented. The dis-crete absorber architecture enables certain advantages, suchas high open-circuit voltage, high collection efficiency,high operating temperature, and smooth integration ofcascade stages with different bandgaps. The two- andthree-stage ICPV devices presented in this article operate atroom temperature with substantial open-circuit voltages ata cutoff wavelength of 5.3 μm (corresponding to a bandgapof 0.23 eV), the longest ever reported for room temperaturePV devices. The device characteristics indicate a high levelof current matching and demonstrate the advantages of theinterband cascade approach in thermophotovoltaic celldesign. 相似文献
512.
Allee effects and pulsed invasion by the gypsy moth 总被引:1,自引:0,他引:1
Biological invasions pose considerable threats to the world's ecosystems and cause substantial economic losses. A prime example is the invasion of the gypsy moth in the United States, for which more than $194 million was spent on management and monitoring between 1985 and 2004 alone. The spread of the gypsy moth across eastern North America is, perhaps, the most thoroughly studied biological invasion in the world, providing a unique opportunity to explore spatiotemporal variability in rates of spread. Here we describe evidence for periodic pulsed invasions, defined as regularly punctuated range expansions interspersed among periods of range stasis. We use a theoretical model with parameter values estimated from long-term monitoring data to show how an interaction between strong Allee effects (negative population growth at low densities) and stratified diffusion (most individuals disperse locally, but a few seed new colonies by long-range movement) can explain the invasion pulses. Our results indicate that suppressing population peaks along range borders might greatly slow invasion. 相似文献
513.
Chung Y Klimanskaya I Becker S Marh J Lu SJ Johnson J Meisner L Lanza R 《Nature》2006,439(7073):216-219
The most basic objection to human embryonic stem (ES) cell research is rooted in the fact that ES cell derivation deprives embryos of any further potential to develop into a complete human being. ES cell lines are conventionally isolated from the inner cell mass of blastocysts and, in a few instances, from cleavage stage embryos. So far, there have been no reports in the literature of stem cell lines derived using an approach that does not require embryo destruction. Here we report an alternative method of establishing ES cell lines-using a technique of single-cell embryo biopsy similar to that used in pre-implantation genetic diagnosis of genetic defects-that does not interfere with the developmental potential of embryos. Five putative ES and seven trophoblast stem (TS) cell lines were produced from single blastomeres, which maintained normal karyotype and markers of pluripotency or TS cells for up to more than 50 passages. The ES cells differentiated into derivatives of all three germ layers in vitro and in teratomas, and showed germ line transmission. Single-blastomere-biopsied embryos developed to term without a reduction in their developmental capacity. The ability to generate human ES cells without the destruction of ex utero embryos would reduce or eliminate the ethical concerns of many. 相似文献
514.
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516.
James TY Kauff F Schoch CL Matheny PB Hofstetter V Cox CJ Celio G Gueidan C Fraker E Miadlikowska J Lumbsch HT Rauhut A Reeb V Arnold AE Amtoft A Stajich JE Hosaka K Sung GH Johnson D O'Rourke B Crockett M Binder M Curtis JM Slot JC Wang Z Wilson AW Schüssler A Longcore JE O'Donnell K Mozley-Standridge S Porter D Letcher PM Powell MJ Taylor JW White MM Griffith GW Davies DR Humber RA Morton JB Sugiyama J Rossman AY Rogers JD Pfister DH Hewitt D Hansen K Hambleton S Shoemaker RA Kohlmeyer J 《Nature》2006,443(7113):818-822
The ancestors of fungi are believed to be simple aquatic forms with flagellated spores, similar to members of the extant phylum Chytridiomycota (chytrids). Current classifications assume that chytrids form an early-diverging clade within the kingdom Fungi and imply a single loss of the spore flagellum, leading to the diversification of terrestrial fungi. Here we develop phylogenetic hypotheses for Fungi using data from six gene regions and nearly 200 species. Our results indicate that there may have been at least four independent losses of the flagellum in the kingdom Fungi. These losses of swimming spores coincided with the evolution of new mechanisms of spore dispersal, such as aerial dispersal in mycelial groups and polar tube eversion in the microsporidia (unicellular forms that lack mitochondria). The enigmatic microsporidia seem to be derived from an endoparasitic chytrid ancestor similar to Rozella allomycis, on the earliest diverging branch of the fungal phylogenetic tree. 相似文献
517.
Surface ultraviolet-B radiation and atmospheric CO2 concentrations have increased as a result of ozone depletion and burning of fossil fuels. The effects are likely to be most apparent in polar regions where ozone holes have developed and ecosystems are particularly sensitive to disturbance. Polar plant communities are dependent on nutrient cycling by soil microorganisms, which represent a significant and highly labile portion of soil carbon (C) and nitrogen (N). It was thought that the soil microbial biomass was unlikely to be affected by exposure of their associated plant communities to increased UV-B. In contrast, increasing atmospheric CO2 concentrations were thought to have a strong effect as a result of greater below-ground C allocation. In addition, there is a growing belief that ozone depletion is of only minor environmental concern because the impacts of UV-B radiation on plant communities are often very subtle. Here we show that 5 years of exposure of a subarctic heath to enhanced UV-B radiation both alone and in combination with elevated CO2 resulted in significant changes in the C:N ratio and in the bacterial community structure of the soil microbial biomass. 相似文献
518.
Stream denitrification across biomes and its response to anthropogenic nitrate loading 总被引:11,自引:0,他引:11
Mulholland PJ Helton AM Poole GC Hall RO Hamilton SK Peterson BJ Tank JL Ashkenas LR Cooper LW Dahm CN Dodds WK Findlay SE Gregory SV Grimm NB Johnson SL McDowell WH Meyer JL Valett HM Webster JR Arango CP Beaulieu JJ Bernot MJ Burgin AJ Crenshaw CL Johnson LT Niederlehner BR O'Brien JM Potter JD Sheibley RW Sobota DJ Thomas SM 《Nature》2008,452(7184):202-205
Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks. 相似文献
519.
Regeneration of the photoprotein aequorin. 总被引:1,自引:0,他引:1