全文获取类型
收费全文 | 1008篇 |
免费 | 81篇 |
出版年
2021年 | 17篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 17篇 |
2016年 | 14篇 |
2015年 | 31篇 |
2014年 | 30篇 |
2013年 | 29篇 |
2012年 | 60篇 |
2011年 | 50篇 |
2010年 | 34篇 |
2009年 | 30篇 |
2008年 | 37篇 |
2007年 | 50篇 |
2006年 | 55篇 |
2005年 | 32篇 |
2004年 | 47篇 |
2003年 | 34篇 |
2002年 | 20篇 |
2001年 | 23篇 |
2000年 | 39篇 |
1999年 | 24篇 |
1998年 | 22篇 |
1997年 | 10篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 8篇 |
1993年 | 8篇 |
1992年 | 27篇 |
1991年 | 20篇 |
1990年 | 17篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1979年 | 14篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1975年 | 11篇 |
1974年 | 9篇 |
1972年 | 11篇 |
1970年 | 5篇 |
1969年 | 6篇 |
1967年 | 6篇 |
1877年 | 4篇 |
排序方式: 共有1089条查询结果,搜索用时 189 毫秒
51.
David Fowler Mhairi Coyle Ute Skiba Mark A. Sutton J. Neil Cape Stefan Reis Lucy J. Sheppard Alan Jenkins Bruna Grizzetti James N. Galloway Peter Vitousek Allison Leach Alexander F. Bouwman Klaus Butterbach-Bahl Frank Dentener David Stevenson Marcus Amann Maren Voss 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2013,368(1621)
Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr−1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40–70 Tg N yr−1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr−1) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 102–103 years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced. 相似文献
52.
53.
DNA sequencing reveals unexpected Recent diversity and an ancient dichotomy in the American marsupial genus Marmosops (Didelphidae: Thylamyini) 下载免费PDF全文
Juan F. Díaz‐Nieto Sharon A. Jansa Robert S. Voss 《Zoological Journal of the Linnean Society》2016,176(4):914-940
To assess species‐level diversity in the didelphid marsupial genus Marmosops, we obtained sequence data from the mitochondrial cytochrome b (CYTB) gene from > 200 specimens, including exemplars of every currently recognized species together with multiple specimens of all geographically widespread forms. Analyses of these data using the general mixed Yule coalescent (GMYC) model suggest that the genus could be twice as speciose as currently recognized, but putative species identified by the GMYC criterion require careful evaluation using other data. To assess phylogenetic relationships within Marmosops, we additionally sequenced a large fragment of the breast cancer activating 1 (BRCA1) gene from one specimen each of the putative species identified by the GMYC analyses of CYTB. Maximum likelihood and Bayesian analyses of a concatenated gene (CYTB + BRCA1) matrix revealed a basal dichotomy between two ancient, morphologically diagnosable clades with apparently distinct distributions and adaptive phenotypes. We describe those clades as subgenera and assign 12 nominal taxa to Sciophanes subgen. nov. (with type species Marmosops parvidens) and 27 nominal taxa to the nominotypical subgenus (with type species Marmosops incanus). 相似文献
54.
55.
We tested hypotheses concerning the origin of bird and mammal sex chromosomes by mapping the location of amniote sex-chromosome loci in a salamander amphibian (Ambystoma). We found that ambystomatid orthologs of human X and chicken Z sex chromosomes map to neighboring regions of a common Ambystoma linkage group 2 (ALG2). We show statistically that the proportion of human X and chicken Z orthologs observed on ALG2 is significantly different from the proportion that would be expected by chance. We further show that conserved syntenies between ALG2 and amniote chromosomes are identified as overlapping conserved syntenies when all available chicken (N = 3120) and human (N = 14,922) RefSeq orthologs are reciprocally compared. In particular, the data suggest that chromosomal regions from chicken chromosomes (GGA) Z and 4 and from human chromosomes (HSA) 9, 4, X, 5, and 8 were linked ancestrally. A more distant outgroup comparison with the pufferfish Tetraodon nigroviridis reveals ALG2/GGAZ/HSAX syntenies among three pairs of ancestral chromosome duplicates. Overall, our results suggest that sex chromosomal regions of birds and mammals were recruited from a common ancestral chromosome, and thus our findings conflict with the currently accepted hypothesis of separate autosomal origins. We note that our results were obtained using the most immediate outgroup to the amniote clade (mammals, birds, and other reptiles) while the currently accepted hypothesis is primarily based upon conserved syntenies between in-group taxa (birds and mammals). Our study illustrates the importance of an amphibian outgroup perspective in identifying ancestral amniote gene orders and in reconstructing patterns of vertebrate sex-chromosome evolution. 相似文献
56.
The effect of tyrosine nitration on mammalian GS activity and stability was studied in vitro. Peroxynitrite at a concentration of 5 micro mol/l produced tyrosine nitration and inactivation of GS, whereas 50 micro mol/l peroxynitrite additionally increased S-nitrosylation and carbonylation and degradation of GS by the 20S proteasome. (-)Epicatechin completely prevented both, tyrosine nitration and inactivation of GS by peroxynitrite (5 micro mol/l). Further, a putative "denitrase" activity restored the activity of peroxynitrite (5 micro mol/l)-treated GS. The data point to a potential regulation of GS activity by a reversible tyrosine nitration. High levels of oxidative stress may irreversibly damage and predispose the enzyme to proteasomal degradation. 相似文献
57.
Jasuja R Ge G Voss NG Lyman-Gingerich J Branam AM Pelegri FJ Greenspan DS 《The Journal of biological chemistry》2007,282(12):9053-9062
Highly purified fractions of bone extracts capable of inducing ectopic bone formation have been reported to contain peptides corresponding to the mature active regions of the TGF-beta-like bone morphogenetic proteins (BMPs) 2-7, and to the prodomain region of the metalloproteinase BMP1. Co-purification of BMPs 2-7 with BMP1 prodomain sequences through the multiple biochemical steps used in these previous reports has suggested the possibility of interactions between the BMP1 prodomain and BMPs 2-7. Here we demonstrate that the BMP1 prodomain binds BMPs 2 and 4 with high specificity and with a KD of approximately 11 nM, in the physiological range. It is further demonstrated that the BMP1 prodomain is capable of modulating signaling by BMPs 2 and 4 in vitro and in vivo, and that endogenous BMP1 prodomain-BMP4 complexes exist in cell culture media and in tissues. 相似文献
58.
It is well known that simultaneous presentation of incongruent audio and visual stimuli can lead to illusory percepts. Recent data suggest that distinct processes underlie non-specific intersensory speech as opposed to non-speech perception. However, the development of both speech and non-speech intersensory perception across childhood and adolescence remains poorly defined. Thirty-eight observers aged 5 to 19 were tested on the McGurk effect (an audio-visual illusion involving speech), the Illusory Flash effect and the Fusion effect (two audio-visual illusions not involving speech) to investigate the development of audio-visual interactions and contrast speech vs. non-speech developmental patterns. Whereas the strength of audio-visual speech illusions varied as a direct function of maturational level, performance on non-speech illusory tasks appeared to be homogeneous across all ages. These data support the existence of independent maturational processes underlying speech and non-speech audio-visual illusory effects. 相似文献
59.
Falk Eigemann Eyal Rahav Hans-Peter Grossart Dikla Aharonovich Daniel Sher Angela Vogts Maren Voss 《Environmental microbiology》2022,24(5):2467-2483
Marine bacteria rely on phytoplankton exudates as carbon sources (DOCp). Yet, it is unclear to what extent phytoplankton exudates also provide nutrients such as phytoplankton-derived N and P (DONp, DOPp). We address these questions by mesocosm exudate addition experiments with spent media from the ubiquitous pico-cyanobacterium Prochlorococcus to bacterial communities in contrasting ecosystems in the Eastern Mediterranean – a coastal and an open-ocean, oligotrophic station with and without on-top additions of inorganic nutrients. Inorganic nutrient addition did not lower the incorporation of exudate DONp, nor did it reduce alkaline phosphatase activity, suggesting that bacterial communities are able to exclusively cover their nitrogen and phosphorus demands with organic forms provided by phytoplankton exudates. Approximately half of the cells in each ecosystem took up detectable amounts of Prochlorococcus-derived C and N, yet based on 16S rRNA sequencing different bacterial genera were responsible for the observed exudate utilization patterns. In the coastal community, several phylotypes of Aureimarina, Psychrosphaera and Glaciecola responded positively to the addition of phytoplankton exudates, whereas phylotypes of Pseudoalteromonas increased and dominated the open-ocean communities. Together, our results strongly indicate that phytoplankton exudates provide coastal and open-ocean bacterial communities with organic carbon, nitrogen and phosphorus, and that phytoplankton exudate serve a full-fledged meal for the accompanying bacterial community in the nutrient-poor eastern Mediterranean. 相似文献
60.