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91.
Temporal stability of pond zooplankton assemblages 总被引:1,自引:0,他引:1
Christopher F. Steiner 《Freshwater Biology》2005,50(1):105-112
1. A large body of recent theory has recently developed focused on the relationship between the species diversity of competitor assemblages and the temporal stability of total competitor biomass. Many of these models predict that stability can increase with increasing diversity. 2. To explore natural relationships between zooplankton taxonomic diversity and temporal stability of total zooplankton biomass, 18 fishless, permanent ponds located in southern Michigan were surveyed over a 5 month period during a single growing season. 3. Results showed that temporal variability in total zooplankton biomass (measured as the coefficient of variation or CV) decreased with increasing mean zooplankton taxonomic richness. Thus, temporal stability increased with increasing taxonomic richness, consistent with theoretical predictions. 4. Decreases in the CV appeared to be because of portfolio effects (statistical averaging of species’ biomass fluctuations) rather than negative covariances among zooplankton taxa. 5. The CV of zooplankton biomass was also related to several environmental variables, suggesting that taxonomic richness may not be the only mediator of biomass stability. The CV decreased with increasing relative abundance of grazer‐resistant algae (algae >35 μm in size) and the CV increased with increasing pond productivity. 相似文献
92.
Plant Species Composition Effects on Belowground Properties and the Resistance and Resilience of the Soil Microflora to a Drying Disturbance 总被引:3,自引:0,他引:3
We hypothesised that plant species composition and richness would affect soil chemical and microbial community properties,
and that these in turn would affect soil microbial resistance and resilience to an experimentally imposed drying disturbance.
We performed a container experiment that manipulated the composition and species richness of common pasture plant species
(Trifolium repens, Lolium perenne, and Plantago lanceolata) by growing them in monoculture, and in all the possible two and three-way combinations, along with an unplanted control
soil. Experimental units were harvested at four different times over a 16-month period to determine the effect of plant community
development and seasonal changes in temperature and moisture on belowground properties. Results showed that plant species
composition influenced soil chemistry, soil microbial community properties and soil microbial resistance and resilience. Soil
from planted treatments generally showed reduced soil microbial resistance to drying compared to unplanted control soils.
Soils from under T. repens showed a higher resistance and resilience than the soils from under P. lanceolata, and a higher resistance than soils from under L. perenne. We suggest that differences across soils in either resource limitation or soil microbial community structure may be responsible
for these results. Plant species richness rarely affected soil microbial community properties or soil microbial resistance
and resilience, despite having some significant effects on plant community biomass and soil nitrogen contents in some harvests.
The effect that treatments had for most variables differed between harvests, suggesting that results can be altered by the
stage of plant community development or by extrinsic environmental factors that varied with harvest timing. These results
in combination show that soil microbial resistance and resilience was affected by plant community composition, and the time
of measurement, but was largely unrelated to plant species richness. 相似文献
93.
In 1-year experiments, the final population density of nematodes is usually modeled as a function of initial density. Often, estimation of the parameters is precarious because nematode measurements, although laborious and expensive, are imprecise and the range in initial densities may be small. The estimation procedure can be improved by using orthogonal regression with a parameter for initial density on each experimental unit. In multi-year experiments parameters of a dynamic model can be estimated with optimization techniques like simulated annealing or Bayesian methods such as Markov chain Monte Carlo (MCMC). With these algorithms information from different experiments can be combined. In multi-year dynamic models, the stability of the steady states is an important issue. With chaotic dynamics, prediction of densities and associated economic loss will be possible only on a short timescale. In this study, a generic model was developed that describes population dynamics in crop rotations. Mathematical analysis showed stable steady states do exist for this dynamic model. Using the Metropolis algorithm, the model was fitted to data from a multi-year experiment on Pratylenchus penetrans dynamics with treatments that varied between years. For three crops, parameters for a yield loss assessment model were available and gross margin of the six possible rotations comprising these three crops and a fallow year were compared at the steady state of nematode density. Sensitivity of mean gross margin to changes in the parameter estimates was investigated. We discuss the general applicability of the dynamic rotation model and the opportunities arising from combination of the model with Bayesian calibration techniques for more efficient utilization and collection of data relevant for economic evaluation of crop rotations. 相似文献
94.
The characteristics and the possible mechanisms of action of cytotoxic ribonucleases (RNases), promising antitumor drugs, are described. Original experimental data and the results of analysis of recent publications have made it possible to identify the cellular components providing for the selective effects of exogenous RNases on tumor cells, on the one hand, and to estimate the contributions of individual molecular determinants to the enzyme cytotoxicity, on the other hand. The predominant effect of the electric charge of the RNase molecule on the induction of cell death has been demonstrated. The cytotoxic effects of RNases are determined by the catalytic cleavage of accessible RNA, the action of the products of its hydrolysis, and the noncatalytic electrostatic interaction of the exogenous enzyme with cell components. Potential RNase targets in a tumor cell and the role of modulation of calcium-dependent potassium channels and the ras oncogene in RNase-induced cell damage are considered. The effect of cytotoxic RNases on gene expression by affecting RNA interference is discussed.Translated from Molekulyarnaya Biologiya, Vol. 39, No. 1, 2005, pp. 3–13.Original Russian Text Copyright © 2005 by Ilinskaya, Makarov. 相似文献
95.
96.
Annual cycle and inter-annual variability of gross primary production and ecosystem respiration in a floodprone river during a 15-year period 总被引:4,自引:0,他引:4
URS UEHLINGER 《Freshwater Biology》2006,51(5):938-950
1. Temporal variation in ecosystem metabolism over a 15‐year period (1986–2000) was evaluated in a seventh order channelised gravel bed river (mean annual discharge 48.7 m3 s?1) of the Swiss Plateau. The river is subject to frequent disturbance by bed‐moving spates. Daily integrals of gross primary production (GPP) and ecosystem respiration (ER) were calculated based on single‐station diel oxygen curves. 2. Seasonal decomposition of the time series of monthly metabolism rates showed that approximately 50% of the variation of GPP and ER can be attributed to season. Annual GPP averaged 5.0 ± 0.6 g O2 m?2 day?1 and showed no long‐term trend. 3. Ecosystem respiration, averaging 6.2 ± 1.4 g O2 m?2 day?1, declined from 8.8 to 4.1 g O2 m?2 day?1 during the 15‐year period. This significant trend paralleled a decline in nitrate and soluble reactive phosphorus concentrations, and the biochemical oxygen demand discharged by sewage treatment facilities upstream of the study reach. The ratio of GPP to ER (P/R) increased from 0.53 to about 1 as consequence of ER reduction. 4. Bed moving spates reduced GPP by 49% and ER by 19%. Postspate recovery of GPP was rapid between spring and autumn and slow during winter. Recovery of ER lacked any seasonal pattern. Annual patterns of daily GPP and to a minor extent of daily ER can be described as a sequence of recovery periods frequently truncated by spates. 5. The study showed that disturbance by frequent bed‐moving spates resulted in major stochastic variation in GPP and ER but annual patterns were still characterised by a distinct seasonal cycle. It also became evident that stream metabolism is a suitable method to assess effects of gradual changes in water quality. 相似文献
97.
Persistence and stability of macroinvertebrate communities in streams of Denali National Park, Alaska: implications for biological monitoring 总被引:3,自引:0,他引:3
1. Macroinvertebrate communities were studied from 1994 to 2001/2002 (except 1997) in six streams in Denali National Park, interior Alaska. All six streams were potential reference streams with no known impairment. 2. Abundance of individual taxa varied markedly from year to year. Overall, abundance decreased over the study period, particularly with respect to mayflies. Stonefly taxa showed lower persistence and were sometimes absent from a stream in any particular year. 3. Mean community persistence for the six streams, as measured by Jaccard's similarity coefficients between years, varied from 0.48 in the year pair 1999–2000 to 0.78 in 1998–99. Tattler Creek (a small stable stream) supported the most persistent macroinvertebrate community and Highway Pass Creek (a small, unstable creek) the least. Mean community persistence showed a significant relationship with mean winter snowfall (November to March) for the six streams. 4. The highest community compositional stability was found in Tattler Creek and the lowest in Highway Pass Creek, but stability varied markedly over time for the six streams, peaking in 1994–95 and reaching a minimum in 2000–01. Compositional stability was significantly related to the Pfankuch Index of channel stability. 5. The composition metrics % Chironomidae, % dominant taxa, %EPT, % Ephemeroptera and % Plecoptera, employed as part of the Alaska Stream Condition Index, varied over almost their entire range in these pristine streams across the 9 years of the study. 6. This study demonstrates the wide range of natural variation that occurs in benthic macroinvertebrate communities in these pristine central Alaskan streams, potentially limiting the applicability of composition metrics for the biological monitoring of water quality in these systems. 相似文献
98.
Weidner Stanisław Każarnowicz Marta Frączek Ewa Amarowicz Ryszard Karamać Magdalena 《Acta Physiologiae Plantarum》2006,28(6):627-634
Some posttranslational processes that occur in embryos of germinating triticale caryopses treated with different concentrations
of abscisic acid (ABA) were examined. ABA increased the ratio of cytoskeleton-bound polysomes in the total population of polysomes
and depressed the share of free and membrane-bound polysomes. Using exogenous RNase, stability of the total polysomal population
as well as each polysomal fraction was investigated. The total extractable polysomes isolated from embryonic tissues of germinating
triticale caryopses treated with ABA were more stable than the polysomes isolated from the control sample caryopses. The contribution
of the polysomes that were not digested by RNase was increased by higher concentrations of ABA applied during germination.
At high concentrations of ABA (50, 100 μM), the quantitative contribution of polysomes in the total ribosomal fraction was
almost 100% of the amount of polysomes before digestion and the modifications observed consisted mainly of the shift of the
so-called heavy polysomes towards light polysomes, containing a few ribosomes. Within each polysomal population, cytoskeleton-bound
polysomes (CBP and CMBP) were the most stable, which may imply that the bonds between polysomes and these protein filaments,
created in all eukaryotic cells increased their stability. It is assumed that mRNAs are stabilised or destabilised by interaction
of proteins with their various sequences. A plant hormone may depress or elevate the quantities of these proteins, thus regulating
the stability of different mRNAs. The results confirm the multi-faceted mechanism of ABA-induced response, where one of the
constituents is the effect of ABA on the stability of mRNAs molecules. The co-ordinated regulation of mRNAs synthesis and
their stability provide plants with improved adaptability. 相似文献
99.
从土壤中筛选出3株具有抑菌活性的菌株,其中1#菌株对金黄色葡萄球菌、枯草芽孢杆菌、灰霉葡萄孢菌、变异链球菌、大肠杆菌均有较强抑制作用,初步鉴定为地衣芽孢杆菌。对其性质进行考察结果表明,在30℃,pH 7.0,150 r/min的条件下,恒温振荡培养24 h,测其发酵滤液的抑菌活力约为2 000 IU/mL。该发酵滤液具有较强的热稳定性,60℃放置1 h,抑菌活性下降了29%;100℃放置30 min,抑菌活性下降了45%,放置1 h,抑菌活性下降了48%;121℃放置1 h,抑菌活性仍能保持40%。该发酵滤液抑菌作用的最适pH值为7.0,在pH 3.0时能保持71%的抑菌活性,在pH 11.0时,仍能维持67%的抑菌活性,此发酵滤液对蛋白酶K不敏感,并且蛋白酶K对该发酵滤液的抑菌活性也起一定的促进作用。 相似文献
100.
克服位置效应提高外源基因在CHO细胞中稳定高效表达的策略 总被引:4,自引:0,他引:4
能够生产有功用的治疗性蛋白的一个重要前提是获得稳定的重组蛋白高表达细胞株,然而筛选一个能够持续稳定表达外源蛋白的重组细胞株是费时费力的过程。有多篇文献报道了重组蛋白细胞株表达的不稳定性。位置效应是高表达细胞株不稳定性的重要因素,克服或利用位置效应是当前获得稳定高表达重组蛋白细胞株的有效途径。为解决外源基因插入的随机性所带来的不可预知的后果,可以事先在CHO细胞基因组中筛选转录热点区域,再通过位点特异性或同源重组的方式,实现外源基因的定点整合。各种调节位置效应的DNA元件陆续被发现,可以利用它们去调控基因表达及增加细胞株的稳定性。 相似文献