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961.
Willerslev E Cappellini E Boomsma W Nielsen R Hebsgaard MB Brand TB Hofreiter M Bunce M Poinar HN Dahl-Jensen D Johnsen S Steffensen JP Bennike O Schwenninger JL Nathan R Armitage S de Hoog CJ Alfimov V Christl M Beer J Muscheler R Barker J Sharp M Penkman KE Haile J Taberlet P Gilbert MT Casoli A Campani E Collins MJ 《Science (New York, N.Y.)》2007,317(5834):111-114
It is difficult to obtain fossil data from the 10% of Earth's terrestrial surface that is covered by thick glaciers and ice sheets, and hence, knowledge of the paleoenvironments of these regions has remained limited. We show that DNA and amino acids from buried organisms can be recovered from the basal sections of deep ice cores, enabling reconstructions of past flora and fauna. We show that high-altitude southern Greenland, currently lying below more than 2 kilometers of ice, was inhabited by a diverse array of conifer trees and insects within the past million years. The results provide direct evidence in support of a forested southern Greenland and suggest that many deep ice cores may contain genetic records of paleoenvironments in their basal sections. 相似文献
962.
Stephens BB Gurney KR Tans PP Sweeney C Peters W Bruhwiler L Ciais P Ramonet M Bousquet P Nakazawa T Aoki S Machida T Inoue G Vinnichenko N Lloyd J Jordan A Heimann M Shibistova O Langenfelds RL Steele LP Francey RJ Denning AS 《Science (New York, N.Y.)》2007,316(5832):1732-1735
Measurements of midday vertical atmospheric CO2 distributions reveal annual-mean vertical CO2 gradients that are inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes. The three models that most closely reproduce the observed annual-mean vertical CO2 gradients estimate weaker northern uptake of -1.5 petagrams of carbon per year (Pg C year(-1)) and weaker tropical emission of +0.1 Pg C year(-1) compared with previous consensus estimates of -2.4 and +1.8 Pg C year(-1), respectively. This suggests that northern terrestrial uptake of industrial CO2 emissions plays a smaller role than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for CO2. 相似文献
963.
The structure of an RNA polymerase II-transcribing complex has been determined in the posttranslocation state, with a vacancy at the growing end of the RNA-DNA hybrid helix. At the opposite end of the hybrid helix, the RNA separates from the template DNA. This separation of nucleic acid strands is brought about by interaction with a set of proteins loops in a strand/loop network. Formation of the network must occur in the transition from abortive initiation to promoter escape. 相似文献
964.
Hulme M Mahony M Beck S Görg C Hansjürgens B Hauck J Nesshöver C Paulsch A Vandewalle M Wittmer H Böschen S Bridgewater P Diaw MC Fabre P Figueroa A Heong KL Korn H Leemans R Lövbrand E Hamid MN Monfreda C Pielke R Settele J Winter M Vadrot AB van den Hove S van der Sluijs JP 《Science (New York, N.Y.)》2011,333(6043):697-698
965.
Scanning electron microscope techniques show that Eocene opaline claystones (fuller's earth and buhrstone) of the Atlantic and Gulf Coastal Plain, deposits long considered volcanic in origin, are actually highly altered diatomites formed as transgressive facies in normal marine continental shelf environmnents. These findings are in agreement with a biogenic origin for time-equivalent horizon A and A deep-sea cherts of the North Atlantic and Caribbean. 相似文献
966.
Jeandet P Douillet-Breuil AC Bessis R Debord S Sbaghi M Adrian M 《Journal of agricultural and food chemistry》2002,50(10):2731-2741
Resistance of plants to infection by phytopathogenic microorganisms is the result of multiple defense reactions comprising both constitutive and inducible barriers. In grapevine, the most frequently observed and best characterized defense mechanisms are the accumulation of phytoalexins and the synthesis of PR-proteins. Particular attention has been given here to stilbene phytoalexins produced by Vitaceae, specifically, their pathway of biosynthesis (including stilbene phytoalexin gene transfer experiments to other plants) and their biological activity together with fungal metabolism. 相似文献
967.
Olivier?GimenezEmail author Rémi?Choquet Laurent?Lamor Paul?Scofield David?Fletcher Jean-Dominique?Lebreton Roger?Pradel 《Journal of Agricultural, Biological & Environmental Statistics》2005,10(2):184-196
In a capture-recapture analysis, uncertainty in the parameter estimates is usually expressed by presenting classical Wald-type
confidence intervals. This approach involves (1) the assumption that the maximum likelihood estimates are asymptotically normal
and (2) numerical computation of the variance-covariance matrix of these estimates. When the sample size is small or when
the estimates are on the boundary of their domain, a Wald confidence interval often performs badly. A natural alternative
is to use profile-likelihood confidence intervals. In general, these intervals require a greater amount of computation. We
propose a new implementation of this approach that is efficient, both in reducing the amount of computation and in coping
with boundary estimates. We also show how profile-likelihood confidence intervals can be adjusted for overdispersion. Simulations
were used to check whether nominal coverage levels were attained, and allowed us to compare this approach with the classical
Wald procedure. We illustrate this work by considering a multi-state model for a sooty shearwater (Puffinus griseus) population. 相似文献
968.
P. A. Roger S. Santiago-Ardales P. M. Reddy I. Watanabe 《Biology and Fertility of Soils》1987,5(2):98-105
Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 102 to 8.0 × 106 CFU/cm2 (median 6.4 × 104) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 104 to 2.8 × 107 CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N2-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.Maître de Recherches ORSTOM (France), Visiting Scientist at IRRI 相似文献
969.
Land usage is a strong determinant of soil microbial community composition and activity, which in turn determine organic matter decomposition rates and decomposition products in soils. Microbial communities in permanently flooded wetlands, such as those created by wetland restoration on Sacramento-San Joaquin Delta islands in California, function under restricted aeration conditions that result in increasing anaerobiosis with depth. It was hypothesized that the change from agricultural management to permanently flooded wetland would alter microbial community composition, increase the amount and reactivity of dissolved organic carbon (DOC) compounds in Delta waters; and have a predominant impact on microbial communities as compared with the effects of other environmental factors including soil type and agricultural management. Based on phospholipid fatty acid (PLFA) analysis, active microbial communities of the restored wetlands were changed significantly from those of the agricultural fields, and wetland microbial communities varied widely with soil depth. The relative abundance of monounsaturated fatty acids decreased with increasing soil depth in both wetland and agricultural profiles, whereas branched fatty acids were relatively more abundant at all soil depths in wetlands as compared to agricultural fields. Decomposition conditions were linked to DOC quantity and quality using fatty acid functional groups to conclude that restricted aeration conditions found in the wetlands were strongly related to production of reactive carbon compounds. But current vegetation may have had an equally important role in determining DOC quality in restored wetlands. In a larger scale analysis, that included data from wetland and agricultural sites on Delta islands and data from two previous studies from the Sacramento Valley, an aeration gradient was defined as the predominant determinant of active microbial communities across soil types and land usage. 相似文献
970.
Wandering spiders limit densities of a major microbi-detritivore in the forest-floor food web 总被引:3,自引:2,他引:3
In a long-term field experiment, densities of wandering spiders (i.e. species that do not build webs to capture prey) were reduced in order to determine whether or not a major group of microbi-detritivores, the Collembola (springtails), would increase in response to lowered spider predation. Thirty 4-m2 fenced plots (15 spider-removal, 15 control plots) and 15 unfenced, undisturbed reference areas were established in a deciduous forest. Spiders were taken from the removal-treatment after being collected by periodic trapping with pitfall traps that contained no preservative, and by searching the litter surface. Judging from a comparison of the numbers captured during periodic censuses in spider-removal and control plots (one census period was two consecutive days of pitfall trapping), wandering spiders were rapidly reduced by ca. 50% and were maintained at levels lower than control plots for 1.3 years. The reduction in numbers was greatest for wolf spiders (Lycosidae; ≈90%). Of six families of Collembola abundant in the leaf litter during the experiment, one family, the Tomoceridae, exhibited a significant release from spider predation. Densities of tomocerids, which are the largest and most active springtails on the research site, gradually increased until they were consistently 2× higher in the spider-removal treatment than in either fenced control plots or unfenced, open reference areas. 相似文献