全文获取类型
收费全文 | 101550篇 |
免费 | 909篇 |
国内免费 | 885篇 |
专业分类
103344篇 |
出版年
2024年 | 13篇 |
2023年 | 42篇 |
2022年 | 120篇 |
2021年 | 170篇 |
2020年 | 133篇 |
2019年 | 181篇 |
2018年 | 12029篇 |
2017年 | 10812篇 |
2016年 | 7744篇 |
2015年 | 1064篇 |
2014年 | 875篇 |
2013年 | 947篇 |
2012年 | 5036篇 |
2011年 | 13493篇 |
2010年 | 12437篇 |
2009年 | 8625篇 |
2008年 | 10295篇 |
2007年 | 11845篇 |
2006年 | 665篇 |
2005年 | 896篇 |
2004年 | 1343篇 |
2003年 | 1351篇 |
2002年 | 1044篇 |
2001年 | 431篇 |
2000年 | 347篇 |
1999年 | 148篇 |
1998年 | 51篇 |
1997年 | 68篇 |
1996年 | 43篇 |
1995年 | 30篇 |
1994年 | 27篇 |
1993年 | 49篇 |
1992年 | 52篇 |
1991年 | 76篇 |
1990年 | 41篇 |
1989年 | 31篇 |
1988年 | 37篇 |
1987年 | 22篇 |
1986年 | 9篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 25篇 |
1982年 | 9篇 |
1975年 | 10篇 |
1972年 | 251篇 |
1971年 | 280篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Jian Liang Jun Xie Jing Gao Chao-Qun Xu Yi Yan Gan-Chu Jia Liang Xiang Li-Ping Xie Rong-Qing Zhang 《Marine biotechnology (New York, N.Y.)》2016,18(6):645-658
Mantle can secret matrix proteins playing key roles in regulating the process of shell formation. The genes encoding lysine-rich matrix proteins (KRMPs) are one of the most highly expressed matrix genes in pearl oysters. However, the expression pattern of KRMPs is limited and the functions of them still remain unknown. In this study, we isolated and identified six new members of lysine-rich matrix proteins, rich in lysine, glycine and tyrosine, and all of them are basic matrix proteins. Combined with four members of the KRMPs previously reported, all these proteins can be divided into three subclasses according to the results of phylogenetic analyses: KRMP1–3 belong to subclass KPI, KRMP4–5 belong to KPII, and KRMP6–10 belong to KPIII. Three subcategories of lysine-rich matrix proteins are highly expressed in the D-phase, the larvae and adult mantle. Lysine-rich matrix proteins are involved in the shell repairing process and associated with the formation of the shell and pearl. What’s more, they can cause abnormal shell growth after RNA interference. In detail, KPI subgroup was critical for the beginning formation of the prismatic layer; both KPII and KPIII subgroups participated in the formation of prismatic layer and nacreous layer. Compared with different temperatures and salinity stimulation treatments, the influence of changes in pH on KRMPs gene expression was the greatest. Recombinant KRMP7 significantly inhibited CaCO3 precipitation, changed the morphology of calcite, and inhibited the growth of aragonite in vitro. Our results are beneficial to understand the functions of the KRMP genes during shell formation. 相似文献
992.
Biodegradability of Vegetation-Derived Dissolved Organic Carbon in a Cool Temperate Ombrotrophic Bog
Andrew J. Pinsonneault Tim R. Moore Nigel T. Roulet Jean-François Lapierre 《Ecosystems》2016,19(6):1023-1036
Dissolved organic carbon (DOC) plays a key role in the peatland carbon balance and serves numerous ecological and chemical functions including acting as a microbial substrate. In this study, we quantify the concentration, biodegradability, and intrinsic properties of DOC obtained from peat, fresh material, and litter from nine species of ombrotrophic bog vegetation. Potential biodegradability was assessed by incubating vegetation extracts for 28 days in the dark and measuring percent DOC loss as the fraction of biodegradable DOC (%BDOC) while DOC properties were characterized using UV–Vis absorbance and fluorescence measurements. The mean initial DOC concentration extracted differed significantly among species (P < 0.05) and was significantly higher in fresh material, 217 ± 259 mg DOC l?1, than either litter or peat extracts with mean concentrations of 82.1 ± 117 mg DOC l?1 and 12.7 ± 1.0 mg DOC l?1, respectively (P < 0.05). %BDOC also differed significantly among species (P < 0.05) and ranged from 52 to 73% in fresh cuttings with the greatest fraction observed in S. magellanicum; 22–46% in litter; and 24% in peat. The majority of variability (82.5%) in BDOC was explained by initial absorbance at 254 nm and total dissolved nitrogen concentration which was further resolved into significant non-linear relationships between %BDOC and both humic-like and protein-like DOC fractions (P < 0.05). Our results highlight the extremely heterogeneous nature of the surface vegetation-derived DOC input in peatlands and stress the importance of vegetation species in peatland ecosystem function. 相似文献
993.
In species-poor communities, genetic diversity potentially plays an important role for ecosystem functioning, though this is still largely unexplored in marine and estuarine ecosystems. We studied how genetic diversity (sensu genotypic diversity and/or allelic richness) affects ecosystem functioning in marine habitat-forming plant communities. First, we conducted a 15-month field experiment in the highly seasonal Baltic Sea and established mono- and polycultures of different genotypes and genotype combinations of Zostera marina. Second, we reviewed existing literature and performed a meta-analysis of 12 studies including this study. We found no evidence of positive genetic diversity effects on shoot production in the field experiment, but diversity enhanced community stability over time. The literature review revealed that a majority of the included studies observed positive effects of genetic diversity on ecosystem functions such as primary production and nutrient uptake. The results from the meta-analysis support the hypothesis that genetic diversity effects on productivity are stronger during or after periods of stress. These diversity effects were also more positive in the field compared to mesocosm studies. Our results indicate that genetic diversity has positive effects on ecosystem functioning, particularly during increased environmental stress. Thus, local genetic diversity should be preserved especially in species-poor ecosystems, where it potentially provides insurance against environmental change. 相似文献
994.
Wouter Suykerbuyk Tjeerd J. Bouma Laura L. Govers Kris Giesen Dick J. de Jong Peter Herman Jan Hendriks Marieke M. van Katwijk 《Ecosystems》2016,19(2):296-310
Changes in the seascape often result in altered hydrodynamics that lead to coinciding changes in sediment dynamics. Little is known on how altered sediment dynamics affect long-term seagrass persistence. We studied the thresholds of sediment dynamics in relation to seagrass presence by comparing sediment characteristics and seagrass presence data of seven separate seagrass meadows. All meadows had a long-term (>20 years) presence. Within these meadows, we distinguish so-called “hotspots” (areas within a meadow where seagrass was found during all mapping campaigns) and “coldspots” (with infrequent seagrass presence). We monitored static sediment characteristics (median grain size, bulk density, silt content) and sediment dynamics (that is, bed level change and maximum sediment disturbance depth), bioturbation (that is, lugworm densities and induced fecal pit and mound relief), and seagrass cover. We statistically analyzed which sediment characteristic best explains seagrass cover. Densely vegetated hotspots were shown to have lower sediment dynamics than sparsely vegetated hotspots and coldspots, whereas static sediment characteristics were similar (grain size, bulk density). The vegetation cover was either low (2–15%) or high (>30%) and sediment dynamics showed a threshold for vegetation cover. From this correlative finding, we postulate a self-sustaining feedback of relatively dense seagrass via sediment stabilization and accordingly a runaway feedback once the seagrass cover becomes too sparse. The sensitivity for sediment dynamics shown in our study implies that future existence of seagrass meadows may be at risk as ongoing climate change might directly (increased environmental extremes) or indirectly (changing seascapes) negatively affect seagrass beds. 相似文献
995.
John M. Heydinger 《Ecosystems》2016,19(4):661-673
Founded upon sustainability science, the ecosystem services concept is increasingly defined by an economic valuation approach to natural capital. This latter-day addition risks subsuming the central message of the ecosystem services concept: that humanity is reliant upon the natural world. Three arenas of inappropriate application of the economic valuation approach to ecosystem services are detailed, these are defined as ecological, social-natural, and socioeconomic problems. Each problematic arena suggests the primary shortcoming of the economic valuation approach: it lacks an incorporation of the temporal component. More clearly incorporating the natural conditions and processes which compose and maintain human benefits into the ecosystem services concept will more fully reflect contemporary economics and sustainability science. The framework of Social-ecological Systems (SES) theory provides a broad foundation for the economic valuation of ecosystem services. Emphasizing the importance of human and environmental change, SES theory encapsulates a needed awareness of the dynamic interactions which compose ecosystem services. 相似文献
996.
Jianfeng Ren Zhanhui Hou Haiyan Wang Ming-an Sun Xiao Liu Bin Liu Ximing Guo 《Marine biotechnology (New York, N.Y.)》2016,18(2):242-254
A large number of Crassostrea oysters are found in Asia-Pacific. While analyses of interspecific variation have helped to establish historical relationships among these species, studies on intraspecific variation are necessary to understand their recent evolutionary history and current forces driving population biology. We resequenced 18 and analyzed 31 mitogenomes of five Crassostrea species from China: Crassostrea gigas, Crassostrea angulata, Crassostrea sikamea, Crassostrea ariakensis, and Crassostrea hongkongensis. Our analysis finds abundant insertions, deletions, and single-nucleotide polymorphisms in all species. Intraspecific variation varies greatly among species with polymorphic sites ranging from 54 to 293 and nucleotide diversity ranging from 0.00106 to 0.00683. In all measurements, C. hongkongensis that has the narrowest geographic distribution exhibits the least sequence diversity; C. ariakensis that has the widest distribution shows the highest diversity, and species with intermediate distribution show intermediate levels of diversity. Low sequence diversity in C. hongkongensis may reflect recent bottlenecks that are probably exacerbated by human transplantation. High diversity in C. ariakensis is likely due to divergence of northern and southern China populations that have been separated without gene flow. The significant differences in mitogenome diversity suggest that the five sister species of Crassostrea have experienced different evolutionary forces since their divergence. The recent divergence of two C. ariakensis populations and the C. gigas/angulata species complex provides evidence for continued diversification and speciation of Crassostrea species along China’s coast, which are shaped by unknown mechanisms in a north–south divide. 相似文献
997.
Yin Chen Wen-Jun Mao Meng-Xia Yan Xue Liu Shu-Yao Wang Zheng Xia Bo Xiao Su-Jian Cao Bao-Qin Yang Jie Li 《Marine biotechnology (New York, N.Y.)》2016,18(3):301-313
Marine sponges are ancient and simple multicellular filter-feeding invertebrates attached to solid substrates in benthic habitats and host a variety of fungi both inside and on their surface because of its unique ingestion and digest system. Investigation on marine sponge-associated fungi mainly focused on the small molecular metabolites, yet little attention had been paid to the extracellular polysaccharides. In this study, a homogeneous extracellular polysaccharide AS2-1 was obtained from the fermented broth of the marine sponge endogenous fungus Alternaria sp. SP-32 using ethanol precipitation, anion-exchange, and size-exclusion chromatography. Results of chemical and spectroscopic analyses showed that AS2-1 was composed of mannose, glucose, and galactose with a molar ratio of 1.00:0.67:0.35, and its molecular weight was 27.4 kDa. AS2-1 consists of a mannan core and a galactoglucan chain. The mannan core is composed of (1→6)-α-Manp substituted at C-2 by (1→2)-α-Manp with different degrees of polymerization. The galactoglucan chain consists of (1→6)-α-Glcp residues with (1→6)-β-Galf residues attached to the last glucopyranose residue at C-6. (1→6)-β-Galf residues have additional branches at C-2 consisting of disaccharide units of (1→2)-β-Galf and (1→2)-α-Glcp residues. The glucopyranose residue of the galactoglucan chain is linked to the mannan core. AS2-1 possessed a high antioxidant activity as evaluated by scavenging of 1,1-diphenyl-2-picrylhydrazyl and hydroxyl radicals in vitro. AS2-1 was also evaluated for cytotoxic activity on Hela, HL-60, and K562 cell lines by the MTT and SRB methods. The investigation demonstrated that AS2-1 was a novel extracellular polysaccharide with different characterization from extracellular polysaccharides produced by other marine microorganisms. 相似文献
998.
999.
1000.
Thomas Schmitt Dirk Louy Edineia Zimmermann Jan Christian Habel 《Organisms Diversity & Evolution》2016,16(4):791-808
The distributions of European high mountain species are often characterised by small and geographically isolated populations and, in many cases, have highly complex biogeographic histories. The butterfly genus Erebia represents one of the best examples for small-scale diversification in the European high mountain systems and therefore to understand speciation processes and associated range dynamics of high mountain species. In this study, we analysed 17 polymorphic allozyme loci of 1731 individuals from 49 populations representing four species, one of which has three subspecies: Erebia nivalis; Erebia tyndarus; Erebia ottomana; and Erebia cassioides cassioides, Erebia cassioides arvernensis, and Erebia cassioides neleus. Samples were collected in the high mountain systems of Europe (i.e. Pyrenees, Massif Central, Alps, Apennines, Carpathians, Balkan high mountains). Genetic analyses supported all previously accepted species. However, the genetic differentiation within E. cassioides sensu lato into three geographically delimited groups is justifying species rank: E. arvernensis distributed in the Pyrenees, Massif Central and western Alps; E. cassioides sensu stricto in the eastern Alps and Apennines; and E. neleus in the Balkan mountains and the south-western Carpathians. While the differentiation between western Alps and Massif Central as well as eastern Alps and Apennines was low, the Pyrenees as well as the south-western Carpathians were significantly differentiated from the other regions within the respective taxon. In general, the differentiation among the populations of E. neleus was stronger than between populations of the other taxa. Within E. cassioides, we found a west-east gradient of genetic similarity over the eastern Alps. Based on the obtained genetic structures, we are able to delineate glacial refugia and interglacial range modifications. Based on the genetic structures and genetic diversity patterns, we conclude that, triggered by the glacial-interglacial cycles, repeated range modifications have taken place with subsequent differentiation and speciation in the region of the Alps and Balkans. Colonisations to Pyrenees (E. arvernensis pseudomurina, E. arvernensis pseudocarmenta), Massif Central (E. ottomana tardenota, E. a. arvernensis) and Apennines (E. cassioides majellana) appear to be recent and most probably not older than the last interglacial period. 相似文献