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991.
Jackson WE de Leon JM Brown GE Waychunas GA Conradson SD Combes JM 《Science (New York, N.Y.)》1993,262(5131):229-233
X-ray absorption spectroscopy (XAS) of Fe(2+) in Fe(2)SiO(4) liquid at 1575 kelvin and 10(-4) gigapascal (1 bar) shows that the Fe(2+) -O bond length is 1.98 +/- 0.02 angstroms compared with approximately 2.22 angstroms in crystalline Fe(2)SiO(4) (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe(2+) coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe(2+) in olivine-composition melts, which helps explain some of the properties of Fe(2)SiO(4) liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed. 相似文献
992.
The signaling pathways by which beta-adrenergic agonists modulate voltage-dependent cardiac sodium currents are unknown, although it is likely that adenosine 3'5'-monophosphate (cAMP) is involved. Single-channel and whole-cell sodium currents were measured in cardiac myocytes and the signal transducing G protein Gs was found to couple beta-adrenergic receptors to sodium channels by both cytoplasmic (indirect) and membrane-delimited (direct) pathways. Hence, Gs can act on at least three effectors in the heart: sodium channels, calcium channels, and adenylyl cyclase. The effect on sodium currents was inhibitory and was enhanced by membrane depolarization. During myocardial ischemia the sodium currents of depolarized cells may be further inhibited by the accompanying increase in catecholamine levels. 相似文献
993.
The spatial variation of changes in intracellular calcium ions were studied with a one-dimensional scanning microphotometer. Changes in intracellular calcium were measured with a metallochromic dye, arsenazo III. Both the magnitude and the kinetics of changes in calcium were dramatically different in different regions of a cell. In Limulus ventral photoreceptors the maximum change was probably restricted to the rhabdomeric lobe. 相似文献
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998.
Productivity is a poor predictor of plant species richness 总被引:5,自引:0,他引:5
Adler PB Seabloom EW Borer ET Hillebrand H Hautier Y Hector A Harpole WS O'Halloran LR Grace JB Anderson TM Bakker JD Biederman LA Brown CS Buckley YM Calabrese LB Chu CJ Cleland EE Collins SL Cottingham KL Crawley MJ Damschen EI Davies KF DeCrappeo NM Fay PA Firn J Frater P Gasarch EI Gruner DS Hagenah N Hille Ris Lambers J Humphries H Jin VL Kay AD Kirkman KP Klein JA Knops JM La Pierre KJ Lambrinos JG Li W MacDougall AS McCulley RL Melbourne BA Mitchell CE Moore JL Morgan JW Mortensen B 《Science (New York, N.Y.)》2011,333(6050):1750-1753
For more than 30 years, the relationship between net primary productivity and species richness has generated intense debate in ecology about the processes regulating local diversity. The original view, which is still widely accepted, holds that the relationship is hump-shaped, with richness first rising and then declining with increasing productivity. Although recent meta-analyses questioned the generality of hump-shaped patterns, these syntheses have been criticized for failing to account for methodological differences among studies. We addressed such concerns by conducting standardized sampling in 48 herbaceous-dominated plant communities on five continents. We found no clear relationship between productivity and fine-scale (meters(-2)) richness within sites, within regions, or across the globe. Ecologists should focus on fresh, mechanistic approaches to understanding the multivariate links between productivity and richness. 相似文献
999.
Smith AD Brown CJ Bulman CM Fulton EA Johnson P Kaplan IC Lozano-Montes H Mackinson S Marzloff M Shannon LJ Shin YJ Tam J 《Science (New York, N.Y.)》2011,333(6046):1147-1150
Low-trophic level species account for more than 30% of global fisheries production and contribute substantially to global food security. We used a range of ecosystem models to explore the effects of fishing low-trophic level species on marine ecosystems, including marine mammals and seabirds, and on other commercially important species. In five well-studied ecosystems, we found that fishing these species at conventional maximum sustainable yield (MSY) levels can have large impacts on other parts of the ecosystem, particularly when they constitute a high proportion of the biomass in the ecosystem or are highly connected in the food web. Halving exploitation rates would result in much lower impacts on marine ecosystems while still achieving 80% of MSY. 相似文献
1000.
Brown K 《Science (New York, N.Y.)》2002,295(5560):1634-1635