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1.
The phytoplankton communities of eleven shallow lakes from Buenos Aires Province, Argentina, were studied seasonally from 1987 to 1989. Several physical and chemical properties were measured in each lake (pH, temperature, dissolved oxygen, transparency, nutrients), in order to interpret the structural and dynamic traits of the phytoplankton community.Important differences between the lakes studied were put in evidence by means of multivariate techniques (Cluster Analysis and PCA). The shallow lakes densely populated by macrophytes hosted lowest phytoplanktonic densities, with average values ranging from 690 to 16500 algae ml–1. High species diversities were observed in these lakes (4.0–4.8). Lakes less colonized by macrophytes had higher phytoplankton densities. In some of them important blooms of Cyanophyceae were recorded, with between 60 000 and 179 000 algae ml–1, and concomitant low diversities.The results of this investigation support the hypothesis that the phytoplankton community is strongly influenced by the macrophytes, by direct competition and/or by competition from periphytic algae associated with higher plants. 相似文献
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The changes in germination, peroxidase activity and isoperoxidase spectrum have been studied in apple embryos at 5°C (stratification) and at 20°C in the presence or absence of seed coats. The embryo dormancy is progressively released at 5°C, but not at 20°C. The peroxidase activity in embryos covered with seed coats is very low at 5°C as well as at 20°C which corresponds to a restricted number of isoenzymes. In isolated embryos the peroxidase activity increases significantly. This is due to an increase in both the number and the activity of the isoperoxidases and it is more pronounced at 20°C than at 5°C. The obtained results suggest that the soluble peroxidases are not involved in the process of the release of embryo dormancy. The variations observed are attributed to the growth process following germination, which can occur even at low temperature. 相似文献
4.
Paul A. del Giorgio Alicia L. Vinocur Ruben J. Lombardo Hector G. Tell 《Hydrobiologia》1991,224(3):129-154
The phytoplankton of the River Lujan (Buenos Aires, Argentina) was studied for a period of 18 months, together with physical and chemical variables, in relation to a pollution gradient. 167 taxa were recorded within a seasonal succession characterized by dominance of diatoms with a brief summer green algae facies. A combination of several biotic indices and multivariate analysis was employed to assess the impact of pollution on the phytoplankton community. The biotic indices used were species diversity and richness, algal quotients (green algae/diatom ratio, Centrales/Pennales ratio) and the SD succession rate index. Multivariate procedures included cluster analysis and ordination by PCA of both species and samples, stepwise discriminant analysis and multiple discriminant analysis of variance (MANOVA). Results indicate that community dynamism is attenuated at the more polluted sites, concomitant with an increased predominance of a broad-tolerance algal assemblage, co-dominated by Cyclotella meneghiniana and Nitzschia stagnorum. The changes in the community structure and dynamics described herein involved alterations in the distribution and relative proportions of the algae, rather than modifications in the basic species composition. These changes may not be readily detectable by methods which over-simplify the ecological information, such as systems of indicator species and biotic indices, designed to assess the degree of pollution. The suitability of multivariate analysis and biotic indices in river phytoplankton studies is further discussed. 相似文献
5.
AB Kane RP Stanton EG Raymond ME Dobson ME Knafelc JL Farber 《The Journal of cell biology》1980,87(3):643-651
The relationship between intracellular lysosomal rupture and cell death caused by silica was studied in P388d(1) macrophages. After 3 h of exposure to 150 μg silica in medium containing 1.8 mM Ca(2+), 60 percent of the cells were unable to exclude trypan blue. In the absence of extracellular Ca(2+), however, all of the cells remained viable. Phagocytosis of silica particles occurred to the same extent in the presence or absence of Ca(2+). The percentage of P388D(1) cells killed by silica depended on the dose and the concentration of Ca(2+) in the medium. Intracellular lyosomal rupture after exposure to silica was measured by acridine orange fluorescence or histochemical assay of horseradish peroxidase. With either assay, 60 percent of the cells exposed to 150 μg silica for 3 h in the presence of Ca(2+) showed intracellular lysosomal rupture, was not associated with measureable degradation of total DNA, RNA, protein, or phospholipids or accelerated turnover of exogenous horseradish peroxidase. Pretreatment with promethazine (20 μg/ml) protected 80 percent of P388D(1) macrophages against silica toxicity although lysosomal rupture occurred in 60-70 percent of the cells. Intracellular lysosomal rupture was prevented in 80 percent of the cells by pretreatment with indomethacin (5 x 10(-5)M), yet 40-50 percent of the cells died after 3 h of exposure to 150 μg silica in 1.8 mM extracellular Ca(2+). The calcium ionophore A23187 also caused intracellular lysosomal rupture in 90-98 percent of the cells treated for 1 h in either the presence or absence of extracellular Ca(2+). With the addition of 1.8 mM Ca(2+), 80 percent of the cells was killed after 3 h, whereas all of the cells remained viable in the absence of Ca(2+). These experiments suggest that intracellular lysosomal rupture is not causally related to the cell death cause by silica or . Cell death is dependent on extracellular Ca(2+) and may be mediated by an influx of these ions across the plasma membrane permeability barrier damaged directly by exposure to these toxins. A23187相似文献
6.
A covalently bound photoisomerizable agonist. Comparison with reversibly bound agonists at electrophorus electroplaques 总被引:2,自引:2,他引:0
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HA Lester ME Krouse MM Nass NH Wassermann BF Erlanger 《The Journal of general physiology》1980,75(2):207-232
After disulphide bonds are reduced with dithiothreitol, trans-3- (α-bromomethyl)-3’-[α- (trimethylammonium)methyl]azobenzene (trans-QBr) alkylates a sulfhydryl group on receptors. The membrane conductance induced by this “tethered agonist” shares many properties with that induced by reversible agonists. Equilibrium conductance increases as the membrane potential is made more negative; the voltage sensitivity resembles that seen with 50 [mu]M carbachol. Voltage- jump relaxations follow an exponential time-course; the rate constants are about twice as large as those seen with 50 μM carbachol and have the same voltage and temperature sensitivity. With reversible agonists, the rate of channel opening increases with the frequency of agonist-receptor collisions: with tethered trans-Qbr, this rate depends only on intramolecular events. In comparison to the conductance induced by reversible agonists, the QBr-induced conductance is at least 10-fold less sensitive to competitive blockade by tubocurarine and roughly as sensitive to “open-channel blockade” bu QX-222. Light-flash experiments with tethered QBr resemble those with the reversible photoisomerizable agonist, 3,3’,bis-[α-(trimethylammonium)methyl]azobenzene (Bis-Q): the conductance is increased by cis {arrow} trans photoisomerizations and decreased by trans {arrow} cis photoisomerizations. As with Bis-Q, ligh-flash relaxations have the same rate constant as voltage-jump relaxations. Receptors with tethered trans isomer. By comparing the agonist-induced conductance with the cis/tans ratio, we conclude that each channel’s activation is determined by the configuration of a single tethered QBr molecule. The QBr-induced conductance shows slow decreases (time constant, several hundred milliseconds), which can be partially reversed by flashes. The similarities suggest that the same rate-limiting step governs the opening and closing of channels for both reversible and tethered agonists. Therefore, this step is probably not the initial encounter between agonist and receptor molecules. 相似文献
7.
JÉRÔME ORIVEL JULIEN GRANGIER JULIEN FOUCAUD JULIEN LE BRETON FRANÇOIS‐XAVIER ANDRÈS HERVÉ JOURDAN JACQUES H. C. DELABIE DENIS FOURNIER PHILIPPE CERDAN BENOIT FACON ARNAUD ESTOUP ALAIN DEJEAN 《Ecological Entomology》2009,34(4):504-512
Abstract 1. The biology of most invasive species in their native geographical areas remains largely unknown. Such studies are, however, crucial in shedding light on the ecological and evolutionary processes underlying biological invasions. 2. The present study focuses on the little fire ant Wasmannia auropunctata, a species native to Central and South America that has been widely introduced and which has become invasive throughout the tropics. We characterise and compare several ecological traits of native populations in French Guiana with those in one of its introduced ranges, New Caledonia. 3. We found ecologically heterogeneous populations of W. auropunctata coexisting in the species’ native geographical area. First, we found populations restricted to naturally perturbed areas (particularly floodplains) within the primary forest, and absent from the surrounding forest areas. These populations were characterised by low nest and worker densities. Second, we found dominant populations in recent anthropogenic areas (e.g. secondary forest or forest edge along road) characterised by high nest and worker densities, and associated with low ant species richness. The local dominance of W. auropunctata in such areas can be due to the displacement of other species (cause) or the filling‐up of empty habitats unsuitable to other ants (effect). With respect to their demographic features and ant species richness, the populations of native anthropogenic habitats were to a large extent similar to the invasive populations introduced into remote areas. 4. The results point to the need for greater research efforts to better understand the ecological and demographic features of invasive species within their native ranges. 相似文献
8.
FLUXNET and modelling the global carbon cycle 总被引:3,自引:0,他引:3
ANDREW D. FRIEND ALMUT ARNETH† NANCY Y. KIANG‡ MARK LOMAS§ JÉRÔME OGÉE¶ CHRISTIAN RÖDENBECK STEVEN W. RUNNING JEAN-DIEGO SANTAREN STEPHEN SITCH†† NICOLAS VIOVY F. IAN WOODWARD§ SÖNKE ZAEHLE†† 《Global Change Biology》2007,13(3):610-633
Measurements of the net CO2 flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO2 source–sink patterns, CO2 flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO2 (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model‐data comparison. Flux measurements made over four vegetation types were used to calibrate the land‐surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land‐surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO2 uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite‐measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO2 measurements. Combining the PEM, DGVM, and inversion results suggests that CO2 fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO2 uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of −1.2 Pg [C] yr−1 for 1982–1995 is compatible with the PEM‐ and DGVM‐predicted trends in NPP. There is, thus, a convergence in understanding derived from process‐based models, remote‐sensing‐based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night‐time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land‐atmosphere CO2 fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO2 uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO2 variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO2 exchange. 相似文献
9.
Periphyton on artificial substrata from three lakes of different trophic status at Hope Bay (Antarctica) 总被引:5,自引:0,他引:5
The nutrient characteristics of aquatic environments at Hope Bay (Antarctica) depend on different factors, penguin enrichment being one of the most important. In this study we surveyed three water bodies of different morphometry and trophic status: Esperanza Lake (oligotrophic), Boeckella Lake (meso-eutrophic), and Pingüi Pond (hypereutrophic). This research provides a preliminary characterization of the periphyton colonization on artificial substrata (glass slides) of these water bodies from the end of January to mid-March 2000. Qualitative and quantitative analyses of the algae, chlorophyll a, dry weight, ash-free dry weight, and physical and chemical variables of the lakes were assessed. The Autotrophic Index and the Lakatos system of classification were also applied. All three lakes differed clearly in all the variables considered. The periphyton from Esperanza Lake was first dominated by Bacillariophyceae and subsequently by the chrysophyce Phaeogloea mucosa. P. mucosa was the dominant species at Boeckella Lake and Chlamydomonas subcaudata was dominant at Pingüi Pond. Highest mean maximum density values were reached at Pingüi Pond, followed by Boeckella and Esperanza Lakes. Highest biomass figures were registered with increasing trophic status. According to the Lakatos index, the periphyton of Esperanza Lake was the least developed and it was mainly organic and heterotrophic; Boeckella Lake showed intermediate development in periphyton mass, which was autotrophic and mainly inorganic whereas Pingüi Pond showed the highest development in periphytic mass, and it was mainly autotrophic. 相似文献
10.
Owing to its lipophilic property, carbon tetrachloride (CCl4) is rapidly absorbed by both the liver and brain. We investigated the protective effects of crocin against brain damage caused by CCl4. Fifty rats were divided into five groups of ten: control, corn oil, crocin, CCl4 and CCl4 + crocin. CCl4 administration decreased glutathione (GSH) and total antioxidant status (TAS) levels, and catalase (CAT) activity, while significant increases were observed in malondialdehyde (MDA) and total oxidant status (TOS) levels and superoxide dismutase (SOD) activity. The cerebral cortex nuclear lamina developed a spongy appearance, neuronal degeneration was observed in the hippocampus, and heterochromatic and pyknotic neurons with increased cytoplasmic eosinophilia were observed in the hippocampus after CCl4 treatment. Because crocin exhibits strong antioxidant properties, crocin treatment increased GSH and TAS levels and CAT activities, and decreased MDA and TOS levels and SOD activity; significant improvements also were observed in histologic architecture. We found that crocin administration nearly eliminated CCl4 induced brain damage by preventing oxidative stress. 相似文献