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排序方式: 共有140条查询结果,搜索用时 15 毫秒
101.
Nathan F. Putman Katherine L. Mansfield Ruoying He Donna J. Shaver Philippe Verley 《Biology letters》2013,9(5)
The inaccessibility of open ocean habitat and the cryptic nature of small animals are fundamental problems when assessing the distribution of oceanic-stage sea turtles and other marine animals sharing similar life-history traits. Most methods that estimate patterns of abundance cannot be applied in situations that are extremely data limited. Here, we use a movement ecology framework to generate the first predicted distributions for the oceanic stage of the Kemp''s ridley sea turtle (Lepidochelys kempii). Our simulations of particle dispersal within ocean circulation models reveal substantial annual variation in distribution and survival among simulated cohorts. Such techniques can help prioritize areas for conservation, and supply inputs for more realistic demographic models attempting to characterize population trends. 相似文献
102.
Sammy R. Shaver William Pendergast Suhaib M. Siddiqi Benjamin R. Yerxa Dallas K. Croom Robert W. Dougherty 《Nucleosides, nucleotides & nucleic acids》2013,32(7-9):1099-1102
Abstract Uridine 5′-O-triphosphate (UTP) is a potent agonist of the purinergic receptor designated P2Y2. UTP is rapidly metabolized in human tissue. To find a compound with similar activity that may be more slowly metabolized, a series of 4-substituted uridine 5′-triphosphates were prepared and evaluated in a P2Y2 receptor second messenger assay. 相似文献
103.
Michael M. Loranty Scott J. Goetz Edward B. Rastetter Adrian V. Rocha Gaius R. Shaver Elyn R. Humphreys Peter M. Lafleur 《Ecosystems》2011,14(1):76-93
We scale a model of net ecosystem CO2 exchange (NEE) for tundra ecosystems and assess model performance using eddy covariance measurements at three tundra sites.
The model, initially developed using instantaneous (seconds–minutes) chamber flux (~m2) observations, independently represents ecosystem respiration (ER) and gross primary production (GPP), and requires only
temperature (T), photosynthetic photon flux density (I
0), and leaf area index (L) as inputs. We used a synthetic data set to parameterize the model so that available in situ observations could be used to
assess the model. The model was then scaled temporally to daily resolution and spatially to about 1 km2 resolution, and predicted values of NEE, and associated input variables, were compared to observations obtained from eddy
covariance measurements at three flux tower sites over several growing seasons. We compared observations to modeled NEE calculated
using T and I
0 measured at the towers, and L derived from MODIS data. Cumulative NEE estimates were within 17 and 11% of instrumentation period and growing season observations,
respectively. Predictions improved when one site-year experiencing anomalously dry conditions was excluded, indicating the
potential importance of stomatal control on GPP and/or soil moisture on ER. Notable differences in model performance resulted
from ER model formulations and differences in how L was estimated. Additional work is needed to gain better predictive ability
in terms of ER and L. However, our results demonstrate the potential of this model to permit landscape scale estimates of
NEE using relatively few and simple driving variables that are easily obtained via satellite remote sensing. 相似文献
104.
M. Syndonia Bret-Harte Michelle C. Mack Gaius R. Shaver Diane C. Huebner Miriam Johnston Camilo A. Mojica Camila Pizano Julia A. Reiskind 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2013,368(1624)
Fire causes dramatic short-term changes in vegetation and ecosystem function, and may promote rapid vegetation change by creating recruitment opportunities. Climate warming likely will increase the frequency of wildfire in the Arctic, where it is not common now. In 2007, the unusually severe Anaktuvuk River fire burned 1039 km2 of tundra on Alaska''s North Slope. Four years later, we harvested plant biomass and soils across a gradient of burn severity, to assess recovery. In burned areas, above-ground net primary productivity of vascular plants equalled that in unburned areas, though total live biomass was less. Graminoid biomass had recovered to unburned levels, but shrubs had not. Virtually all vascular plant biomass had resprouted from surviving underground parts; no non-native species were seen. However, bryophytes were mostly disturbance-adapted species, and non-vascular biomass had recovered less than vascular plant biomass. Soil nitrogen availability did not differ between burned and unburned sites. Graminoids showed allocation changes consistent with nitrogen stress. These patterns are similar to those seen following other, smaller tundra fires. Soil nitrogen limitation and the persistence of resprouters will likely lead to recovery of mixed shrub–sedge tussock tundra, unless permafrost thaws, as climate warms, more extensively than has yet occurred. 相似文献
105.
L. C. Johnson G. R. Shaver A. E. Giblin K. J. Nadelhoffer E. R. Rastetter J. A. Laundre G. L. Murray 《Oecologia》1996,108(4):737-748
We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO2 exchanges, methane (CH4) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO2 respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO2 storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO2 losses from increased ecosystem respiration at elevated temperature were compensated by increased CO2 uptake (gross photosynthesis). Although CO2 losses from ecosystem respiration were primarily limited by drainage, CH4 emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra. 相似文献
106.
Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes 总被引:3,自引:0,他引:3
Cornelissen JH van Bodegom PM Aerts R Callaghan TV van Logtestijn RS Alatalo J Chapin FS Gerdol R Gudmundsson J Gwynn-Jones D Hartley AE Hik DS Hofgaard A Jónsdóttir IS Karlsson S Klein JA Laundre J Magnusson B Michelsen A Molau U Onipchenko VG Quested HM Sandvik SM Schmidt IK Shaver GR Solheim B Soudzilovskaia NA Stenström A Tolvanen A Totland Ø Wada N Welker JM Zhao X;M.O.L. Team 《Ecology letters》2007,10(7):619-627
Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition. 相似文献
107.
Effects of suspended sediment and desiccation on the benthic tailwater community in the Colorado River, USA 总被引:2,自引:2,他引:0
Michael L. Shaver Joseph P. Shannon Kevin P. Wilson Peggy L. Benenati Dean W. Blinn 《Hydrobiologia》1997,357(1-3):63-72
We demonstrated that differences in habitatrequirements by C. glomerata and Oscillatoria havea profound bottom-up influence on the foodweb in thetailwaters below Glen Canyon Dam in the Colorado Riverthrough Grand Canyon National Park, USA. We examinedthe effects of suspended sediment and desiccation onthe colonization sequence of Cladophora glomerata andOscillatoria spp. and the consequent effects onmacroinvertebrates in each algal community in a seriesof reciprocal transplants in the regulated ColoradoRiver, AZ. Our experiments showed that C. glomeratagrows best in continuously submerged, clear-water,stable habitats, whereas Oscillatoria forms densemat-like matrices of trichomes and sand in varialzones and habitats with high suspended sedimentstypical of many southwestern USA streams. Varial zoneconditions have a stronger influence on communitystructure than habitats with high suspended sediments. Recruitment by chironomid larvae was less dependent onC. glomerata and less affected by suspended sedimentand periodic desiccation than Gammarus lacustris. Weestimated the energy from macroinvertebrate biomassassociated with tufts of C. glomerata to be an orderof magnitude higher than that in Oscillatoriamatrices. Therefore, loss of C. glomerata andreplacement of habitat more suitable for Oscillatoriaas a result of regulated flows indirectly reducespotential energy flow in the Colorado Riverfoodweb. 相似文献
108.
Alan Shaver Jesse B. Ng David A. Hall Barry I. Posner 《Molecular and cellular biochemistry》1995,153(1-2):5-15
The inorganic coordination chemistry of peroxovanadium compounds relevant to insulin mimesis is reviewed. The structure and kinetic reactivity of solutions of vanadate anion, vanadyl complexes and peroxovanadate complexes are briefly compared. Peroxovanadium compounds contain an oxo group, one or two peroxo ligands (O2
2–) and an ancillary ligand which is usually bidentate. These compounds approximate a trigonal bipyramidal structure which can be divided conceptually into a polar oxo half and a relatively non-polar organic half. This presents a number of interesting design variations which are discussed with respect to the development of a rudimentary structure-activity correlation of insulin mimetic ability.Abbreviations phen
1,10-phenanthroline
- 4,7-Me2phen
4,7-dimethyl-1,10-phenanthroline
- 3,4,7,8-Me4phen
3,4,7,8-tetramethyl-1,10-phenanthroline
- 5-CH3phen
5-methyl-1,10-phenanthroline
- 5-NO2phen
5-nitro-1,10-phenanthroline
- 5-NH2phen
5-amino-1,10-phenanthroline
- bipy
2,2-bipyridine
- bipyH
2,2-bipyridinium
- 4,4-Me2bipy
4,4-dimethyl-2,2-bipyridine
- bipy-4,4-(COO)2
2,2-bipyridine-4,4-dicarboxylato
- pic
pyridine-2-carboxylato
- 3-OHpic
3-hydroxypyridine-2-carboxylato
- 3-acetpic
3-acetatoxypyridine-2-carboxylato
- m,n-pdc
pyridine-m,n-dicarboxylato
- ox
oxalato
- pzc
pyrazine-2-carboxylato
- 3-NH2pzc
3-aminopyrazine-2-carboxylato
- 4OH-2,6pdc
4-hydroxy-2,6-pyridinedicarboxylato
- IDA
iminodiacetato
- cit
citrato
- EDTA
ethylenediaminetetraacetato
- HEDTA
ethylene-diaminetetraacetic acid
- isoquin
isoquinoline-2-carboxylato
- quin
quinolato
- NTA
nitrilotriacetato
- glyH
glycine
- cystH
cysteine
- nicH
nicotinic acid
- Hheida
N-(2-hydroxyethyl)iminodiacetato 相似文献
109.
Yevette J. Shaver Madan L. Nagpal Karen F. Fox Rivka Rudner Alvin Fox 《Molecular microbiology》2001,42(1):101-109
The genome of the Bacillus subtilis 168-type strain contains 10 ribosomal RNA (rRNA) operons. In the intergenic spacer region (ISR) between the 16S and 23S rRNA genes, five rRNA operons, rrnI-H-G and rrnJ-W, lack a trinucleotide signature region. Precise determination of molecular weight (MW), using electrospray mass spectrometry (MS), of the polymerase chain reaction (PCR) products from a segment of the ISR from the 168-type strain and B. subtilis 168-like strain 23071 demonstrated 114 and 111 basepair (bp) PCR products (due to the presence or absence of the insert in the operons) as predicted from sequence. However, PCR of the ISR segment for five other B. subtilis 168 isolates generated only a 114 bp PCR product, suggesting the presence of the trinucleotide signature region in all rRNA operons for these strains. Additional genetic variability between the seven B. subtilis 168 isolates was demonstrated by restriction fragment length polymorphism (RFLP) of the rRNA operons, with three distinct patterns found upon Southern blot analysis. The 168-type strain and three others (23066, 23067, and 23071) exhibited the same Southern pattern. Thus, operon deletion is not responsible for the absence of a 111 bp product on MS analysis for strains 23066 and 23067. Restriction analysis confirmed the presence of the trinucleotide signature region in the ISR of all rRNA operons for five B. subtilis 168 isolates; sequencing of rrnW/H from a representative strain also upheld this finding. These results help provide a better understanding of variations in sequence, operon number and chromosomal organization, both within a genome and among isolates of B. subtilis subgroup 168. It is also hypothesized that the presence of the trinucleotide insert in certain rRNA operons may play a role in rRNA maturation and protein synthesis. 相似文献
110.
Christopher M. Yates Edward P. Garvey Sammy R. Shaver Robert J. Schotzinger William J. Hoekstra 《Bioorganic & medicinal chemistry letters》2017,27(15):3243-3248
While the orally-active azoles such as fluconazole and posaconazole are effective antifungal agents, they potently inhibit a broad range of off-target human cytochrome P450 enzymes (CYPs) leading to various safety issues (e.g., drug-drug interactions, liver, and reproductive toxicities). Recently we described the rationally-designed, antifungal agent VT-1161 that is more selective for fungal CYP51 than related human CYP enzymes such as CYP3A4. Herein, we describe the use of a homology model of Aspergillus fumigatus to design and optimize a novel series of highly selective, broad spectrum fungal CYP51 inhibitors. This series includes the oral antifungal VT-1598 that exhibits excellent potency against yeast, dermatophyte, and mold fungal pathogens. 相似文献