全文获取类型
收费全文 | 4430篇 |
免费 | 432篇 |
专业分类
4862篇 |
出版年
2023年 | 13篇 |
2022年 | 43篇 |
2021年 | 98篇 |
2020年 | 42篇 |
2019年 | 46篇 |
2018年 | 69篇 |
2017年 | 46篇 |
2016年 | 99篇 |
2015年 | 181篇 |
2014年 | 218篇 |
2013年 | 244篇 |
2012年 | 319篇 |
2011年 | 354篇 |
2010年 | 223篇 |
2009年 | 201篇 |
2008年 | 276篇 |
2007年 | 283篇 |
2006年 | 219篇 |
2005年 | 255篇 |
2004年 | 253篇 |
2003年 | 237篇 |
2002年 | 251篇 |
2001年 | 42篇 |
2000年 | 24篇 |
1999年 | 58篇 |
1998年 | 79篇 |
1997年 | 44篇 |
1996年 | 39篇 |
1995年 | 31篇 |
1994年 | 42篇 |
1993年 | 43篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 42篇 |
1989年 | 19篇 |
1988年 | 23篇 |
1987年 | 14篇 |
1986年 | 28篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 19篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 20篇 |
1978年 | 18篇 |
1977年 | 18篇 |
1976年 | 21篇 |
1975年 | 14篇 |
1973年 | 21篇 |
1970年 | 12篇 |
排序方式: 共有4862条查询结果,搜索用时 15 毫秒
81.
82.
Sara Kleindienst Sharon Grim Mitchell Sogin Annalisa Bracco Melitza Crespo-Medina Samantha B Joye 《The ISME journal》2016,10(2):400-415
The Deepwater Horizon (DWH) oil well blowout generated an enormous plume of dispersed hydrocarbons that substantially altered the Gulf of Mexico''s deep-sea microbial community. A significant enrichment of distinct microbial populations was observed, yet, little is known about the abundance and richness of specific microbial ecotypes involved in gas, oil and dispersant biodegradation in the wake of oil spills. Here, we document a previously unrecognized diversity of closely related taxa affiliating with Cycloclasticus, Colwellia and Oceanospirillaceae and describe their spatio-temporal distribution in the Gulf''s deepwater, in close proximity to the discharge site and at increasing distance from it, before, during and after the discharge. A highly sensitive, computational method (oligotyping) applied to a data set generated from 454-tag pyrosequencing of bacterial 16S ribosomal RNA gene V4–V6 regions, enabled the detection of population dynamics at the sub-operational taxonomic unit level (0.2% sequence similarity). The biogeochemical signature of the deep-sea samples was assessed via total cell counts, concentrations of short-chain alkanes (C1–C5), nutrients, (colored) dissolved organic and inorganic carbon, as well as methane oxidation rates. Statistical analysis elucidated environmental factors that shaped ecologically relevant dynamics of oligotypes, which likely represent distinct ecotypes. Major hydrocarbon degraders, adapted to the slow-diffusive natural hydrocarbon seepage in the Gulf of Mexico, appeared unable to cope with the conditions encountered during the DWH spill or were outcompeted. In contrast, diverse, rare taxa increased rapidly in abundance, underscoring the importance of specialized sub-populations and potential ecotypes during massive deep-sea oil discharges and perhaps other large-scale perturbations. 相似文献
83.
Jonathan J. Smart Andrew Chin Leontine Baje Madeline E. Green Sharon A. Appleyard Andrew J. Tobin Colin A. Simpfendorfer William T. White 《PloS one》2016,11(4)
Fisheries observer programs are used around the world to collect crucial information and samples that inform fisheries management. However, observer error may misidentify similar-looking shark species. This raises questions about the level of error that species misidentifications could introduce to estimates of species’ life history parameters. This study addressed these questions using the Grey Reef Shark Carcharhinus amblyrhynchos as a case study. Observer misidentification rates were quantified by validating species identifications using diagnostic photographs taken on board supplemented with DNA barcoding. Length-at-age and maturity ogive analyses were then estimated and compared with and without the misidentified individuals. Vertebrae were retained from a total of 155 sharks identified by observers as C. amblyrhynchos. However, 22 (14%) of these were sharks were misidentified by the observers and were subsequently re-identified based on photographs and/or DNA barcoding. Of the 22 individuals misidentified as C. amblyrhynchos, 16 (73%) were detected using photographs and a further 6 via genetic validation. If misidentified individuals had been included, substantial error would have been introduced to both the length-at-age and the maturity estimates. Thus validating the species identification, increased the accuracy of estimated life history parameters for C. amblyrhynchos. From the corrected sample a multi-model inference approach was used to estimate growth for C. amblyrhynchos using three candidate models. The model averaged length-at-age parameters for C. amblyrhynchos with the sexes combined were = 159 cm TL and = 72 cm TL. Females mature at a greater length (l50 = 136 cm TL) and older age (A50 = 9.1 years) than males (l50 = 123 cm TL; A50 = 5.9 years). The inclusion of techniques to reduce misidentification in observer programs will improve the results of life history studies and ultimately improve management through the use of more accurate data for assessments. 相似文献
84.
Sharon L. McDonnell Carole Baggerly Christine B. French Leo L. Baggerly Cedric F. Garland Edward D. Gorham Joan M. Lappe Robert P. Heaney 《PloS one》2016,11(4)
BackgroundHigher serum 25-hydroxyvitamin D [25(OH)D] concentrations have been associated with a lower risk of multiple cancer types across a range of 25(OH)D concentrations.ObjectivesTo investigate whether the previously reported inverse association between 25(OH)D and cancer risk could be replicated, and if a 25(OH)D response region could be identified among women aged 55 years and older across a broad range of 25(OH)D concentrations.MethodsData from two cohorts representing different median 25(OH)D concentrations were pooled to afford a broader range of 25(OH)D concentrations than either cohort alone: the Lappe cohort (N = 1,169), a randomized clinical trial cohort (median 25(OH)D = 30 ng/ml) and the GrassrootsHealth cohort (N = 1,135), a prospective cohort (median 25(OH)D = 48 ng/ml). Cancer incidence over a multi-year period (median: 3.9 years) was compared according to 25(OH)D concentration. Kaplan-Meier plots were developed and the association between 25(OH)D and cancer risk was examined with multivariate Cox regression using multiple 25(OH)D measurements and spline functions. The study included all invasive cancers excluding skin cancer.ResultsAge-adjusted cancer incidence across the combined cohort (N = 2,304) was 840 cases per 100,000 person-years (1,020 per 100,000 person-years in the Lappe cohort and 722 per 100,000 person-years in the GrassrootsHealth cohort). Incidence was lower at higher concentrations of 25(OH)D. Women with 25(OH)D concentrations ≥40 ng/ml had a 67% lower risk of cancer than women with concentrations <20 ng/ml (HR = 0.33, 95% CI = 0.12–0.90).Conclusions25(OH)D concentrations ≥40 ng/ml were associated with substantial reduction in risk of all invasive cancers combined. 相似文献
85.
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
86.
The rate at which catalytic capacity of microbial exo-enzymes degrades post-exudation will influence the time during which return on microbes’ investment in exo-enzyme production can be realized. Further, if exo-enzyme degradation rates vary across exo-enzymes, microbial investment returns may vary by element across time. We quantify how aging of two soil organic matter (SOM)-decaying enzymes (β-D-cellobioside, BGase; and N-acetyl-β-D-glucosaminide, NAGase) influences enzyme-substrate V max at multiple temperatures (5, 15, 25 °C), and compute how enzyme age influences relative availabilities of C and N. Both BGase and NAGase exhibited similar, exponential declines in catalytic rate with age at 25 °C (0.22 ± 0.02 and 0.36 ± 0.14 d?1, respectively). At 15 °C, NAGase exhibited exponential declines in catalytic rates with age (0.79 ± 0.31 d?1), but BGase exhibited no decline. Neither enzyme exhibited a decline in catalytic rate over 72 h at 5 °C. At 15 °C, the amount of C liberated from cellulose and chitin analogues relative to N increased, on average, by more than one order of magnitude. The ratio of C:N liberated from the two substrates remained constant across enzyme age at 25 and 5 °C, but for different reasons: no differences in decay rate across enzymes at 25 °C, and no observed decay at 5 °C. Thus, temperature-dependent decreases of catalytic activity over time may influence microbial resource allocation strategies and rates of SOM decomposition. Because the enzyme decay rates we observed differ considerably from values assumed in most models, such assumptions should be revisited when parameterizing microbial process models. 相似文献
87.
Christopher A Lamb Stefanie Nühlen Delphine Judith David Frith Ambrosius P Snijders Christian Behrends Sharon A Tooze 《The EMBO journal》2016,35(3):281-301
Macroautophagy requires membrane trafficking and remodelling to form the autophagosome and deliver its contents to lysosomes for degradation. We have previously identified the TBC domain‐containing protein, TBC1D14, as a negative regulator of autophagy that controls delivery of membranes from RAB11‐positive recycling endosomes to forming autophagosomes. In this study, we identify the TRAPP complex, a multi‐subunit tethering complex and GEF for RAB1, as an interactor of TBC1D14. TBC1D14 binds to the TRAPP complex via an N‐terminal 103 amino acid region, and overexpression of this region inhibits both autophagy and secretory traffic. TRAPPC8, the mammalian orthologue of a yeast autophagy‐specific TRAPP subunit, forms part of a mammalian TRAPPIII‐like complex and both this complex and TBC1D14 are needed for RAB1 activation. TRAPPC8 modulates autophagy and secretory trafficking and is required for TBC1D14 to bind TRAPPIII. Importantly, TBC1D14 and TRAPPIII regulate ATG9 trafficking independently of ULK1. We propose a model whereby TBC1D14 and TRAPPIII regulate a constitutive trafficking step from peripheral recycling endosomes to the early Golgi, maintaining the cycling pool of ATG9 required for initiation of autophagy. 相似文献
88.
Sabra Ahmed Stanford Sophia N. Storton Sharon Lawrence Matthew D’Silva Lindsay Morris Roger H. K. Evans Vanessa Wani Mushtaq Potter John F. Evans Phillip A. 《BMC neurology》2016,16(1):1-8
The Qatari law, as in many other countries, uses brain death as the main criteria for organ donation and cessation of medical support. By contrast, most of the public in Qatar do not agree with the limitation or withdrawal of medical care until the time of cardiac death. The current study aims to examine the duration of somatic survival after brain death, organ donation rate in brain-dead patients as well as review the underlying etiologies and level of support provided in the state of Qatar. This is a retrospective study of all patients diagnosed with brain death over a 10-year period conducted at the largest tertiary center in Qatar (Hamad General Hospital). Among the 53 patients who were diagnosed with brain death during the study period, the median and mean somatic survivals of brain-dead patients in the current study were 3 and 4.5 days respectively. The most common etiology was intracranial hemorrhage (45.3%) followed by ischemic stroke (17%). Ischemic stroke patients had a median survival of 11 days. Organ donation was accepted by only two families (6.6%) of the 30 brain dead patients deemed suitable for organ donation. The average somatic survival of brain-dead patients is less than one week irrespective of supportive measures provided. Organ donation rate was extremely low among brain-dead patients in Qatar. Improved public education may lead to significant improvement in resource utilization as well as organ transplant donors and should be a major target area of future health care policies. 相似文献
89.
Anandarajan Thiyagarajan Masaaki Toyama Masanori Baba Ashoke Sharon 《Nucleosides, nucleotides & nucleic acids》2016,35(6):305-314
The present study includes the exploration of new possible nucleoside mimetics based on 4-methoxy-7H-pyrrolo[2,3-d]pyrimidine carbocyclic nucleosides (4a–g), which were synthesized by 10–15 synthetic steps and characterized adequately. We report the anti-HCV activities and cytotoxicities of 4a–g. Compound 4a was analyzed by single crystal X-ray diffraction which showed some puckering in the cyclopentene ring with a 2′-endo conformation and anti-base disposition (χ = ?125.7°). 相似文献
90.