全文获取类型
收费全文 | 762篇 |
免费 | 114篇 |
国内免费 | 2篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 20篇 |
2020年 | 9篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 7篇 |
2016年 | 15篇 |
2015年 | 21篇 |
2014年 | 32篇 |
2013年 | 39篇 |
2012年 | 56篇 |
2011年 | 53篇 |
2010年 | 33篇 |
2009年 | 30篇 |
2008年 | 37篇 |
2007年 | 38篇 |
2006年 | 34篇 |
2005年 | 40篇 |
2004年 | 40篇 |
2003年 | 26篇 |
2002年 | 36篇 |
2001年 | 17篇 |
2000年 | 15篇 |
1999年 | 30篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 11篇 |
1995年 | 12篇 |
1994年 | 9篇 |
1993年 | 11篇 |
1992年 | 10篇 |
1991年 | 17篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1973年 | 2篇 |
1969年 | 5篇 |
1967年 | 2篇 |
1966年 | 3篇 |
排序方式: 共有878条查询结果,搜索用时 15 毫秒
151.
Accumulation of Trehalose and Sucrose in Cyanobacteria Exposed to Matric Water Stress 总被引:5,自引:1,他引:4 下载免费PDF全文
The drought-resistant cyanobacteria Phormidium autumnale, strain LPP4, and a Chroococcidiopsis sp. accumulated trehalose, sucrose, and both trehalose and sucrose, respectively, in response to matric water stress. Accumulated sugar concentrations reached values of up to 6.2 μg of trehalose per μg of chlorophyll in P. autumnale, 6.9 μg of sucrose per μg of chlorophyll in LPP4, and 4.1 μg of sucrose and 3.2 μg of trehalose per μg of chlorophyll in the Chroococcidiopsis sp. The same sugars were accumulated by these cyanobacteria in similar concentrations under osmotic water stress. Cyanobacteria that did not show drought resistance (Plectonema boryanum and Synechococcus strain PCC 7942) did not accumulate significant amounts of sugars when matric water stress was applied. 相似文献
152.
Reduction of nitrosubstituted aromatic compounds by the halophilic anaerobic eubacteria Haloanaerobium praevalens and Sporohalobacter marismortui. 总被引:4,自引:3,他引:1 下载免费PDF全文
The moderately halophilic, obligately anaerobic eubacteria Haloanaerobium praevalens DSM 2228 and Sporohalobacter marismortui ATCC 35420 are able to reduce a variety of nitrosubstituted aromatic compounds at a high rate to the corresponding amines. Compounds degraded included nitrobenzene, o-nitrophenol, m-nitrophenol, p-nitrophenol, nitroanilines, 2,4-dinitrophenol, and 2,4-dinitroaniline. Most of these compounds, when added at concentrations of 50 to 100 mg/liter, were completely transformed within 24 h, but at the highest concentrations growth rates were somewhat lowered. Growth of H. praevalens in the presence of 14C-labeled p-nitrophenol showed that the compound was not incorporated by the cells or degraded to acid-volatile compounds. 相似文献
153.
Recent decades have seen profound changes in species abundance and community composition. In the marine environment, the major anthropogenic drivers of change comprise exploitation, invasion by nonindigenous species, and climate change. However, the magnitude of these stressors has been widely debated and we lack empirical estimates of their relative importance. In this study, we focused on Eastern Mediterranean, a region exposed to an invasion of species of Red Sea origin, extreme climate change, and high fishing pressure. We estimated changes in fish abundance using two fish trawl surveys spanning a 20‐year period, and correlated these changes with estimated sensitivity of species to the different stressors. We estimated sensitivity to invasion using the trait similarity between indigenous and nonindigenous species; sensitivity to fishing using a published composite index based on the species’ life‐history; and sensitivity to climate change using species climatic affinity based on occurrence data. Using both a meta‐analytical method and random forest analysis, we found that for shallow‐water species the most important driver of population size changes is sensitivity to climate change. Species with an affinity to warm climates increased in relative abundance and species with an affinity to cold climates decreased suggesting a strong response to warming local sea temperatures over recent decades. This decrease in the abundance of cold‐water‐associated species at the trailing “warm” end of their distribution has been rarely documented. Despite the immense biomass of nonindigenous species and the presumed high fishing pressure, these two latter factors seem to have only a minor role in explaining abundance changes. The decline in abundance of indigenous species of cold‐water origin indicates a future major restructuring of fish communities in the Mediterranean in response to the ongoing warming, with unknown impacts on ecosystem function. 相似文献
154.
155.
156.
After a period of more than ten years in which bacterial and algal community sizes were extremely small, a dense bloom of halophilic archaea developed in the upper 5–10 m of the Dead Sea water column in the summer of 1992. The development of this bloom followed a dilution of the upper water layer by winter rainfloods, which enabled the development of a short-lived dense bloom of the unicellular green alga Dunaliella parva. The dense archaeal community (up to 3.5 × 107 cells m1–1 in June 1992) imparted a red coloration to the Dead Sea, due to its high content of bacterioruberin. Bacteriorhodopsin was not detected. High levels of potential heterotrophic activity were associated with the bloom, as measured by the incorporation of labeled organic substrates. After the decline of the algal bloom, archaeal numbers in the lake decreased only little, and most of the community was still present at the end of 1993. The amount of carotenoid pigment per cell, however, decreased 2–3-fold between June 1992 and August 1993. No new algal and archaeal blooms developed after the winter floods of 1992–1993, in spite of the fact that salinity values in the surface layer were sufficiently low to support a new algal bloom. A remnant of the 1992 Dunaliella bloom maintained itself at the lower end of the pycnocline at depths between 7 and 13 m (September 1992–August 1993). Its photosynthetic activity was small, and very little stimulation of archaeal growth and activity was associated with this algal community. 相似文献
157.
158.
Classical burster models are based on a fast
system that either oscillates or is quiescent, depending on temporarily
fixed values of slow variables. In a study of the lobster heart
ganglion, we found a new type of burster for which the
fast system is globally stable for all relevant fixed values of
the slow variables. We describe how this burster works and
speculate on its biological significance.
Received: 14 July 1994 / Accepted in revised form: 25
October 1994 相似文献
159.
David S. Ellsworth Ram Oren Ce Huang Nathan Phillips George R. Hendrey 《Oecologia》1995,104(2):139-146
Physiological responses to elevated CO2 at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO2 using a free-air CO2 enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO2 over an 8-day exposure period was similar to that of trees exposed to current ambient CO2 under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO2-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO2 conditions. Short-term (minutes) direct stomatal responses to elevated CO2 were also relatively weak (5% reduction in stomatal aperture in response to high CO2 concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO2 for nearly 80 days compared to foliage grown under current ambient CO2, so intrinsic leaf water-use efficiency at elevated CO2 was enhanced primarily by direct responses of photosynthesis to CO2. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO2 (A
net-C
i curves) for Pinus taeda foliage grown under elevated CO2 (550 mol mol–1) for 50–80 days compared to those for foliage grown under current ambient CO2 from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol–1 CO2 was enhanced by approximately 65% compared to the rate at ambient CO2 (350 mol mol–1). A similar level of enhancement under elevated CO2 was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO2 during the study period appears to be primarily attributable to direct photosynthetic responses to CO2 in the pine forest, longer-term CO2 responses and feedbacks remain to be evaluated. 相似文献
160.
Uptake and turnover of acetate in hypersaline environments 总被引:2,自引:0,他引:2
Aharon Oren 《FEMS microbiology ecology》1995,18(1):75-84
Abstract: Acetate uptake and turnover rates were determined for the heterotrophic community in hypersaline environments (saltern crystallizer ponds, the Dead Sea) dominated by halpphilic Archaea. Acetate was formed from glycerol, which is potentially the major available carbon source for natural communities of halophilic Archaea. Values of [ K t + S n ] (the sum of the substrate affinity and the substrate concentration present in situ) for acetate measured in saltern crystallizer ponds were around 4.5–11.5 μM, while in the Dead Sea during a Dunaliella bloom values up to 12.8 μM were found. Maximal theoretical rates ( V max ) of acetate uptake in saltern crystallizer ponds were 12–56 nmol l−1 h−1 , with estimated turnover times for acetate ( T t ) between 127–730 h at 35°C. V max values measured in the Dead Sea were between 0.8 and 12.8 nmol l−1 h−1 , with turnover times in the range of 320–2190 h. V max values for acetate were much lower than those for glycerol. Comparisons with pure cultures of halophilic Archaea grown under different conditions showed that the natural communities were not adapted for preferential use of acetate. Both in natural brines and in pure cultures of halophilic Archaea, acetate incorporation rates rapidly decreased above the optimum pH value, probably since acetate enters the cell only in its unionized form. The low affinity for acetate, together with low potential utilization rates result in the long acetate turnover times, which explains the accumulation of acetate observed when low concentrations of glycerol are supplied as a nutrient to natural communities of halophilic Archaea. 相似文献