全文获取类型
收费全文 | 1509篇 |
免费 | 158篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 23篇 |
2021年 | 51篇 |
2020年 | 39篇 |
2019年 | 33篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 63篇 |
2015年 | 85篇 |
2014年 | 102篇 |
2013年 | 111篇 |
2012年 | 143篇 |
2011年 | 126篇 |
2010年 | 74篇 |
2009年 | 61篇 |
2008年 | 105篇 |
2007年 | 92篇 |
2006年 | 84篇 |
2005年 | 87篇 |
2004年 | 69篇 |
2003年 | 56篇 |
2002年 | 54篇 |
2001年 | 21篇 |
2000年 | 4篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 8篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1981年 | 3篇 |
1978年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1963年 | 1篇 |
1960年 | 1篇 |
1950年 | 1篇 |
1949年 | 1篇 |
排序方式: 共有1667条查询结果,搜索用时 15 毫秒
941.
942.
Phillip J. Brumm Scott Monsma Brendan Keough Svetlana Jasinovica Erin Ferguson Thomas Schoenfeld Michael Lodes David A. Mead 《PloS one》2015,10(10)
Thermus aquaticus Y51MC23 was isolated from a boiling spring in the Lower Geyser Basin of Yellowstone National Park. Remarkably, this T. aquaticus strain is able to grow anaerobically and produces multiple morphological forms. Y51MC23 is a Gram-negative, rod-shaped organism that grows well between 50°C and 80°C with maximum growth rate at 65°C to 70°C. Growth studies suggest that Y51MC23 primarily scavenges protein from the environment, supported by the high number of secreted and intracellular proteases and peptidases as well as transporter systems for amino acids and peptides. The genome was assembled de novo using a 350 bp fragment library (paired end sequencing) and an 8 kb long span mate pair library. A closed and finished genome was obtained consisting of a single chromosome of 2.15 Mb and four plasmids of 11, 14, 70, and 79 kb. Unlike other Thermus species, functions usually found on megaplasmids were identified on the chromosome. The Y51MC23 genome contains two full and two partial prophage as well as numerous CRISPR loci. The high identity and synteny between Y51MC23 prophage 2 and that of Thermus sp. 2.9 is interesting, given the 8,800 km separation of the two hot springs from which they were isolated. The anaerobic lifestyle of Y51MC23 is complex, with multiple morphologies present in cultures. The use of fluorescence microscopy reveals new details about these unusual morphological features, including the presence of multiple types of large and small spheres, often forming a confluent layer of spheres. Many of the spheres appear to be formed not from cell envelope or outer membrane components as previously believed, but from a remodeled peptidoglycan cell wall. These complex morphological forms may serve multiple functions in the survival of the organism, including food and nucleic acid storage as well as colony attachment and organization. 相似文献
943.
944.
Jürg Brendan Logue Silke Langenheder Anders F Andersson Stefan Bertilsson Stina Drakare Anders Lanzén Eva S Lindstr?m 《The ISME journal》2012,6(6):1127-1136
A central goal in ecology is to grasp the mechanisms that underlie and maintain biodiversity and patterns in its spatial distribution can provide clues about those mechanisms. Here, we investigated what might determine the bacterioplankton richness (BR) in lakes by means of 454 pyrosequencing of the 16S rRNA gene. We further provide a BR estimate based upon a sampling depth and accuracy, which, to our knowledge, are unsurpassed for freshwater bacterioplankton communities. Our examination of 22 669 sequences per lake showed that freshwater BR in fourteen nutrient-poor lakes was positively influenced by nutrient availability. Our study is, thus, consistent with the finding that the supply of available nutrients is a major driver of species richness; a pattern that may well be universally valid to the world of both micro- and macro-organisms. We, furthermore, observed that BR increased with elevated landscape position, most likely as a consequence of differences in nutrient availability. Finally, BR decreased with increasing lake and catchment area that is negative species–area relationships (SARs) were recorded; a finding that re-opens the debate about whether positive SARs can indeed be found in the microbial world and whether positive SARs can in fact be pronounced as one of the few ‘laws'' in ecology. 相似文献
945.
Nan Shang Ammar Saleem Lina Musallam Brendan Walshe-Roussel Alaa Badawi Alain Cuerrier John T. Arnason Pierre S. Haddad 《PloS one》2015,10(8)
We evaluated and compared the antidiabetic potential and molecular mechanisms of 17 Cree plants’ ethanol extracts (EE) and hot water extracts (HWE) on glucose homeostasis in vitro and used metabolomics to seek links with the content of specific phytochemicals. Several EE of medical plants stimulated muscle glucose uptake and inhibited hepatic G6Pase activity. Some HWE partially or completely lost these antidiabetic activities in comparison to EE. Only R. groenlandicum retained similar potential between EE and HWE in both assays. In C2C12 muscle cells, EE of R. groenlandicum, A. incana and S. purpurea stimulated glucose uptake by activating AMP-activated protein kinase (AMPK) pathway and increasing glucose transporter type 4 (GLUT4) expression. In comparison to EE, HWE of R. groenlandicum exhibited similar activities; HWE of A. incana completely lost its effect on all parameters; interestingly, HWE of S. purpurea activated insulin pathway instead of AMPK pathway to increase glucose uptake. In the liver, for a subset of 5 plants, HWE and EE activated AMPK pathway whereas the EE and HWE of S. purpurea and K. angustifolia also activated insulin pathways. Quercetin-3-O-galactoside and quercetin 3-O-α-L-arabinopyranoside, were successfully identified by discriminant analysis as biomarkers of HWE plant extracts that stimulate glucose uptake in vitro. More importantly, the latter compound was not identified by previous bioassay-guided fractionation. 相似文献
946.
Premise
Continental-scale leaf trait studies can help explain how plants survive in different environments, but large data sets are costly to assemble at this scale. Automating the measurement of digitized herbarium collections could rapidly expand the data available to such studies. We used machine learning to identify and measure leaves from existing, digitized herbarium specimens. The process was developed, validated, and applied to analyses of relationships between leaf size and climate within and among species for two genera: Syzygium (Myrtaceae) and Ficus (Moraceae).Methods
Convolutional neural network (CNN) models were used to detect and measure complete leaves in images. Predictions of a model trained with a set of 35 randomly selected images and a second model trained with 35 user-selected images were compared using a set of 50 labeled validation images. The validated models were then applied to 1227 Syzygium and 2595 Ficus specimens digitized by the National Herbarium of New South Wales, Australia. Leaf area measurements were made for each genus and used to examine links between leaf size and climate.Results
The user-selected training method for Syzygium found more leaves (9347 vs. 8423) using fewer training masks (218 vs. 225), and found leaves with a greater range of sizes than the random image training method. Within each genus, leaf size was positively associated with temperature and rainfall, consistent with previous observations. However, within species, the associations between leaf size and environmental variables were weaker.Conclusions
CNNs detected and measured leaves with levels of accuracy useful for trait extraction and analysis and illustrate the potential for machine learning of herbarium specimens to massively increase global leaf trait data sets. Within-species relationships were weak, suggesting that population history and gene flow have a strong effect at this level. Herbarium specimens and machine learning could expand sampling of trait data within many species, offering new insights into trait evolution. 相似文献947.
Vivian X. Y. Sim Katherine A. Dafforn Stuart L. Simpson Brendan P. Kelaher Emma L. Johnston 《PloS one》2015,10(6)
Multi-use marine parks achieve conservation through spatial management of activities. Zoning of marine parks in New South Wales, Australia, includes high conservation areas and special purpose zones (SPZ) where maritime activities are concentrated. Although such measures geographically constrain anthropogenic impacts, we have limited understanding of potential ecological effects. We assessed sediment communities and contaminants adjacent to boating infrastructure (boat ramps, jetties and a marina) in a SPZ from the Clyde Estuary in Batemans Marine Park. Metal concentrations and fines content were elevated at boating structures compared to reference sites. Species richness was higher at sites with boating structures, where capitellid polychaetes and nematodes dominated the communities. Changes associated with boating structures were localised and did not extend beyond breakwalls or to reference sites outside the SPZ. The study highlights the benefits of appropriate zoning in a multi-use marine park and the potential to minimise stress on pristine areas through the application of spatial management. 相似文献
948.
Dickson Oh Kate Porter Brendan Russ David Burns Mike Dyall-Smith 《Extremophiles : life under extreme conditions》2010,14(2):161-169
Haloquadratum walsbyi is frequently a dominant member of the microbial communities in hypersaline waters. 16S rRNA gene sequences indicate that
divergence within this species is very low but relatively few sites have been examined, particularly in the southern hemisphere.
The diversity of Haloquadratum was examined in three coastal, but geographically distant saltern crystallizer ponds in Australia, using both culture-independent
and culture-dependent methods. Two 97%-OTU, comprising Haloquadratum- and Halorubrum-related sequences, were shared by all three sites, with the former OTU representing about 40% of the sequences recovered at
each site. Sequences 99.5% identical to that of Hqr. walsbyi C23T were present at all three sites and, overall, 98% of the Haloquadratum-related sequences displayed ≤2% divergence from that of the type strain. While haloarchaeal diversity at each site was relatively low (9–16 OTUs), seven phylogroups (clones and/or isolates) and
4 different clones showed ≤90% sequence identity to classified taxa, and appear to represent novel genera. Six of these branched
together in phylogenetic tree reconstructions, forming a clade (MSP8-clade) whose members were only distantly related to classified
taxa. Such sequences have only rarely been previously detected but were found at all three Australian crystallizers. 相似文献
949.
950.
A series of potential UDP-sugar mimics were readily synthesised by copper(I) catalysed modified Huisgen cycloaddition of the corresponding α-propargyl glycosides with 5-azido uridine in aqueous solution. None of the compounds accessed displayed significant inhibitory activity at concentrations of up to 4.5 mM in an assay against bovine milk β-1,4-galactosyltransferase. 相似文献