全文获取类型
收费全文 | 13188篇 |
免费 | 979篇 |
国内免费 | 8篇 |
出版年
2023年 | 50篇 |
2022年 | 117篇 |
2021年 | 272篇 |
2020年 | 177篇 |
2019年 | 187篇 |
2018年 | 245篇 |
2017年 | 224篇 |
2016年 | 413篇 |
2015年 | 618篇 |
2014年 | 682篇 |
2013年 | 869篇 |
2012年 | 1080篇 |
2011年 | 1092篇 |
2010年 | 698篇 |
2009年 | 607篇 |
2008年 | 794篇 |
2007年 | 839篇 |
2006年 | 806篇 |
2005年 | 669篇 |
2004年 | 663篇 |
2003年 | 548篇 |
2002年 | 553篇 |
2001年 | 133篇 |
2000年 | 94篇 |
1999年 | 119篇 |
1998年 | 162篇 |
1997年 | 82篇 |
1996年 | 91篇 |
1995年 | 101篇 |
1994年 | 90篇 |
1993年 | 87篇 |
1992年 | 71篇 |
1991年 | 69篇 |
1990年 | 62篇 |
1989年 | 57篇 |
1988年 | 46篇 |
1987年 | 59篇 |
1986年 | 43篇 |
1985年 | 60篇 |
1984年 | 47篇 |
1983年 | 51篇 |
1982年 | 43篇 |
1981年 | 46篇 |
1980年 | 30篇 |
1979年 | 29篇 |
1978年 | 24篇 |
1977年 | 36篇 |
1976年 | 21篇 |
1975年 | 18篇 |
1974年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Göran Dave Eva Nilsson 《Journal of Aquatic Ecosystem Stress and Recovery (Formerly Journal of Aquatic Ecosystem Health)》1994,3(3):193-206
Sediments were sampled from 62 sites in the Kattegat and Skagerrak, which are located between the Baltic and the North Sea in the Western Atlantic, during autumn 1989 and spring 1990. From each site 5 to 6 samples were taken wit ha box-corer. After mixing to composite samples on board, transport and storage (at 4 °C for 2 to 4 weeks), the samples were tested for toxicity to Daphnia magna and Nitocra spinipes. Immobility in Daphnia after exposure to 16 percent sediment (wet wt) in reconstituted standardized water (ISO, 1982) ranged from 0 to 88 percent after 24 h and from 3 to 95 percent after 48 h. For Nitocra the toxicity, determined as the 96-h LC50 (% wet wt) at 7 salinity, ranged from > > 32 percent (nontoxic) to 1.8 percent (most toxic). All exposures were made in duplicates and the effects obtained in the duplicates with the same sediment were correlated to each other. However, sediment toxicity to Daphnia and Nitocra was not. The test with Nitocra, which was made at several concentrations of sediment, was considered to give the most reliable picture of sediment toxicity in the Kattegat and Skagerrak. This ambient toxicity assessment identified three areas with toxic sediment, (1) the Göta älv estuary (outside the city of Göteborg) and its surroundings, (2) the Bay of Laholm in southern Kattegat, which is an area with periodic oxygen depletion and where repeated mussel kills have occurred during the last decade, and (3) an area in the open Skagerrak northwest of Skagen (the tip of the Jutland peninsula). Sediments, which had been stored at 4 °C, were tested again after 6 to 13 mos with the Nitocra test. Stored sediment toxicity was poorly correlated with fresh sediment toxicity. The average detoxification during storage was 5 times, but the range was 3 orders of magnitude, from 17 times more toxic to 73 times less toxic. The reasons for the observed areal and storage differences in sediment toxicity are so far not understood. 相似文献
132.
What is Microthrix parvicella ? 总被引:1,自引:0,他引:1
Microthrix parvicella is one of the filamentous bacteria that is known to create problems in the operation of activated sludge plants. Its physiology has already been investigated, although primarily in the context of its being a bulking species. It is now recognized that it is one of the major foam-forming organisms and, as such, needs further study. The initial isolation of M. parvicella did not prove to be as easy as would be expected from the earlier work and, eventually, micromanipulation was required. Growth studies showed that it exhibited several morphological forms, only one of which was that described previously. This has led to doubts about the classification of this species which does not yet have a clearly defined bacteriological designation. 相似文献
133.
Butler Michael J. Aphale Jayant S. DiZonno Michele A. Krygsman Phyllis Walczyk Eva Malek Lawrence T. 《Journal of industrial microbiology & biotechnology》1994,13(1):24-29
Summary We have investigated the aminopeptidase activities present inStreptomyces lividans strains. The majority of these activities proved to be intracellular with multiple active species. Two aminopeptidase P genes were identified to be responsible for the ability to hydrolyze amino terminal peptide bonds adjacent to proline residues. Two other broad spectrum aminopeptidases were found to display homology at both the DNA and protein levels. One showed significant homology to PepN proteins, particularly around the putative zinc-binding residues which are important for catalysis. The second broad spectrum activity was not analyzed in detail but showed a different spectrum of substrate specificity to that of PepN. 相似文献
134.
135.
Rosemary S. Gray David P. Muehleisen Eva J. Katahira Walter E. Bollenbacher 《Cellular and molecular neurobiology》1993,13(1):39-58
1. | A 28-kDa peptide from the brain of the tobacco hornworm,Manduca sexta, was purifiedvia HPLC. The peptide copurified with the insect neurohormone, prothoracicotropic hormone (PTTH), through two HPLC columns. |
2. | Immunocyctochemistry using polyclonal antibodies against the 28-kDa peptide revealed that the peptide was produced in the same protocerebral neurons that produce PTTH. Western blot analysis demonstrated that the 28-kDa peptide and big PTTH are different molecules. |
3. | A PTTHin vitro bioassay indicated that despite having chromatographic properties similar to those of big PTTH and being produced by the same neurons, the 28-kDa peptide did not have PTTH activity. |
4. | Amino acid sequence analysis yielded a 27 N-terminal amino acid sequence that had no similarity with known peptides. |
5. | Immunocytochemical studies revealed that the 28-kDa peptide is present as early as 30% embryonic development and is absent by adult eclosion. This is in contrast to big PTTH, which is expressed throughout theManduca life cycle. |
6. | These data suggest that the 28-kDa peptide is another secretory phenotype of the lateral neurosecretory cell group III (L-NSC III) which may have functions distinct from those for big PTTH or may act synergistically with big PTTH. |
136.
Because of its low solubility it is unlikely that elemental sulfur serves as the direct substrate for sulfur-reducing bacteria. To test the hypothesis that polysulfide may represent a soluble intermediate of sulfur reduction, the maximal polysulfide concentrations formed from elemental sulfur in aqueous sulfide solutions were measured at near neutral pH and at temperatures up to 90°C. The saturation concentrations decreased by two orders of magnitude when the pH was lowered from 7 to 6 at a given temperature, and increased about tenfold when the temperature was raised from 37°C to 90°C at a given pH. The dissolution of 0.1 mM zerovalent sulfur in 1 mM sulfide (H2S+HS–) required a pH of 7.5 at 20°C and of only 6.1 at 100°C. A comparison with the growth optima of sulfur-reducers suggests that polysulfide is present at sufficient concentration at the growth conditions of the Bacteria and the moderately acidophilic Archaea. Polysulfide is apparently not available at the growth conditions of the extremely acidophilic Archaea. Alternative mechanisms for the sulfur utilization under these conditions are discussed.Abbreviations MOPS
Morpholinopropanesulfonate
- PIPES
1,4 piperazine-N,N-bis(2-ethanesulfonate)
- HEPES
N-2-hydroxy-ethylpiperazine-N-ethanesulfonate 相似文献
137.
Analysis of polymerase chain reaction amplified products from the sixth intron of the human interleukin-1 gene reveals a high polymorphism (polymorphism information content = 0.51) in a Caucasian population. Altogether, seven alleles have been defined ranging from 620 to 1220bp. This polymorphism is probably attributable to a variable number of 46-bp tandem repeats, each containing potential regulatory sequences. 相似文献
138.
Marjolijn J. A. Christianen Fee O. H. Smulders Jan Arie Vonk Leontine E. Becking Tjeerd J. Bouma Sabine M. Engel Rebecca K. James Mabel I. Nava Jaco C. de Smit Jurjan P. van der Zee Per J. Palsbøll Elisabeth S. Bakker 《Global Change Biology》2023,29(1):215-230
Large grazers (megaherbivores) have a profound impact on ecosystem functioning. However, how ecosystem multifunctionality is affected by changes in megaherbivore populations remains poorly understood. Understanding the total impact on ecosystem multifunctionality requires an integrative ecosystem approach, which is especially challenging to obtain in marine systems. We assessed the effects of experimentally simulated grazing intensity scenarios on ecosystem functions and multifunctionality in a tropical Caribbean seagrass ecosystem. As a model, we selected a key marine megaherbivore, the green turtle, whose ecological role is rapidly unfolding in numerous foraging areas where populations are recovering through conservation after centuries of decline, with an increase in recorded overgrazing episodes. To quantify the effects, we employed a novel integrated index of seagrass ecosystem multifunctionality based upon multiple, well-recognized measures of seagrass ecosystem functions that reflect ecosystem services. Experiments revealed that intermediate turtle grazing resulted in the highest rates of nutrient cycling and carbon storage, while sediment stabilization, decomposition rates, epifauna richness, and fish biomass are highest in the absence of turtle grazing. In contrast, intense grazing resulted in disproportionally large effects on ecosystem functions and a collapse of multifunctionality. These results imply that (i) the return of a megaherbivore can exert strong effects on coastal ecosystem functions and multifunctionality, (ii) conservation efforts that are skewed toward megaherbivores, but ignore their key drivers like predators or habitat, will likely result in overgrazing-induced loss of multifunctionality, and (iii) the multifunctionality index shows great potential as a quantitative tool to assess ecosystem performance. Considerable and rapid alterations in megaherbivore abundance (both through extinction and conservation) cause an imbalance in ecosystem functioning and substantially alter or even compromise ecosystem services that help to negate global change effects. An integrative ecosystem approach in environmental management is urgently required to protect and enhance ecosystem multifunctionality. 相似文献
139.
Lasse T. Keetz Eva Lieungh Kaveh Karimi-Asli Sonya R. Geange Emiliano Gelati Hui Tang Yeliz A. Yilmaz Kjetil S. Aas Inge H. J. Althuizen Anders Bryn Stefanie Falk Rosie Fisher Anne Fouilloux Peter Horvath Sunniva Indrehus Hanna Lee Danica Lombardozzi Frans-Jan W. Parmentier Norbert Pirk Vigdis Vandvik Ane V. Vollsnes Olav Skarpaas Frode Stordal Lena M. Tallaksen 《Global Change Biology》2023,29(15):4440-4452
Dynamic Global Vegetation Models (DGVMs) provide a state-of-the-art process-based approach to study the complex interplay between vegetation and its physical environment. For example, they help to predict how terrestrial plants interact with climate, soils, disturbance and competition for resources. We argue that there is untapped potential for the use of DGVMs in ecological and ecophysiological research. One fundamental barrier to realize this potential is that many researchers with relevant expertize (ecology, plant physiology, soil science, etc.) lack access to the technical resources or awareness of the research potential of DGVMs. Here we present the Land Sites Platform (LSP): new software that facilitates single-site simulations with the Functionally Assembled Terrestrial Ecosystem Simulator, an advanced DGVM coupled with the Community Land Model. The LSP includes a Graphical User Interface and an Application Programming Interface, which improve the user experience and lower the technical thresholds for installing these model architectures and setting up model experiments. The software is distributed via version-controlled containers; researchers and students can run simulations directly on their personal computers or servers, with relatively low hardware requirements, and on different operating systems. Version 1.0 of the LSP supports site-level simulations. We provide input data for 20 established geo-ecological observation sites in Norway and workflows to add generic sites from public global datasets. The LSP makes standard model experiments with default data easily achievable (e.g., for educational or introductory purposes) while retaining flexibility for more advanced scientific uses. We further provide tools to visualize the model input and output, including simple examples to relate predictions to local observations. The LSP improves access to land surface and DGVM modelling as a building block of community cyberinfrastructure that may inspire new avenues for mechanistic ecosystem research across disciplines. 相似文献
140.
Terje Kalland Mikael Dohlsten Lars Abrahmsén Gunnar Hedlund Per Björk Peter A. Lando Anette Sundstedt Eva Åkerblom Peter Lind 《Cell biochemistry and biophysics》1993,22(1-3):147-164
The bacterial superantigen staphylococcal enterotoxin A (SEA) is an extremely potent activator of T lymphocytes when presented on MHC class II antigens. In order to induce T lymphocytes to reject a tumor, we substituted the specificity of SEA for MHC class II molecules with specificity for tumor cells by combining SEA with a MAb recognizing colon carcinomas. Chemical conjugates or recombinant fusion proteins of the MAb C215 and SEA retained excellent antigen binding properties whereas the binding to MHC class II was markedly reduced. The hybrid proteins directed SEA responsive T cells to tumors with specificity determined by the specificity of the MAb. Significant tumor cell killing was obtained at picomolar concentrations of the hybrid proteins and was the result of direct cell mediated by cytotoxicity as well as production of tumoricidal cytokines by T cells. Targeting of superantigens represents a novel approach to specific immunomodulation and deserves further study as a potential therapy for malignant disease. 相似文献