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51.
Fire is an important natural disturbance process in arid grasslands but current fire regimes are largely the result of both human and natural processes and their interactions. The collapse of the Soviet Union in 1991 spurred substantial socioeconomic changes and was ultimately followed by a rapid increase in burned area in southern Russia. What is unclear is whether this increase in burned area was caused by decreasing livestock numbers, vegetation changes, climate change, or interactions of these factors. Our research goal was to identify the driving forces behind the increase in burned area in the arid grasslands of southern Russia. Our study area encompassed 19,000 km2 in the Republic of Kalmykia in southern Russia. We analyzed annual burned area from 1986 to 2006 as a function of livestock population, NDVI, precipitation, temperature, and broad-scale oscillation indices using best subset regressions and structural equation modeling. Our results supported the hypothesis that vegetation recovered within 5–6 years after the livestock declined in the beginning of the 1990s, to a point at which large fires could be sustained. Climate was an important explanatory factor for burning, but mainly after 1996 when lower livestock numbers allowed fuels to accumulate. Ultimately, our results highlight the complexity of coupled human-natural systems, and provide an example of how abrupt socioeconomic change may affect fire regimes. 相似文献
52.
53.
Effects of disturbance intensity and frequency on bacterial community composition and function 总被引:2,自引:0,他引:2
Disturbances influence community structure and ecosystem functioning. Bacteria are key players in ecosystems and it is therefore crucial to understand the effect of disturbances on bacterial communities and how they respond to them, both compositionally and functionally. The main aim of this study was to test the effect of differences in disturbance strength on bacterial communities. For this, we implemented two independent short-term experiments with dialysis bags containing natural bacterial communities, which were transplanted between ambient and 'disturbed' incubation tanks, manipulating either the intensity or the frequency of a salinity disturbance. We followed changes in community composition by terminal restriction fragment analysis (T-RFLP) and measured various community functions (bacterial production, carbon substrate utilization profiles and rates) directly after and after a short period of recovery under ambient conditions. Increases in disturbance strength resulted in gradually stronger changes in bacterial community composition and functions. In the disturbance intensity experiment, the sensitivity to the disturbance and the ability of recovery differed between different functions. In the disturbance frequency experiment, effects on the different functions were more consistent and recovery was not observed. Moreover, in case of the intensity experiment, there was also a time lag in the responses of community composition and functions, with functional responses being faster than compositional ones. To summarize, our study shows that disturbance strength has the potential to change the functional performance and composition of bacterial communities. It further highlights that the overall effects, rates of recovery and the degree of congruence in the response patterns of community composition and functioning along disturbance gradients depend on the type of function and the character of the disturbance. 相似文献
54.
Somatic embryogenesis (SE) is expected to play an important role in increasing productivity, sustainability, and uniformity
of future US forests. For commercial use, SE technology must work with a variety of genetically diverse trees. Initiation
in loblolly pine (Pinus taeda L.), the main commercial US forest species, is often recalcitrant for desirable genotypes. Liquid initiation medium with
no or low gelling agent or placement of the explant on gelled medium followed later by a liquid medium overlay during the
initiation process increased initiation for loblolly pine and Norway spruce (Picea abies). Loblolly pine liquid medium required reduction of NAA from 2 mg/l in gelled medium to 0.3 mg/l in liquid medium. Once the
NAA concentration was adjusted, loblolly pine initiation occurred in liquid medium with fully immersed megagametophytes, explants
supported at the liquid medium surface, or on gelled medium overlaid with liquid medium. Liquid overlays (0.25 ml) consisting
of medium with NAA reduced to 0.3 mg/l, 9 mg/l ABA and no gelling agent applied to explants on 2 ml of gelled medium provided
excellent initiation results. Greatest initiation percentages occurred when the liquid overlay was applied 14 days after placement
of the megagametophyte on gelled medium. Initiation increases ranged from +8.5% with high-value cross-pollinated seed sources
to +6.5 to +9.9% with open-pollinated and often recalcitrant seed sources. Liquid medium addition allows rapid replenishment
of nutrients and adjustment or change of pH, hormones, or other parameters without disturbing the tissue. 相似文献
55.
Tom Grygar Anna Blhov David Hradil Petr Bezdi
ka Jaroslav Kadlec Petr Schnabl George Swann Hedi Oberhnsli 《Palaeogeography, Palaeoclimatology, Palaeoecology》2007,250(1-4):50-67
The environmental record from Lake Baikal, Russia, from 310 to 50 ky BP (MIS 9a to MIS 3) was interpreted using rock magnetic, UV–Vis spectral, mineralogical, and diatom analyses. The age model was based on a correlation of the diatom and chemical weathering records and the summer insolation curve at 55°N and checked against an age model based on the proxy of relative palaeointensity of the Earth's magnetic field. Peaks in chemical weathering within the watershed, inferred from maximum concentration of magnetic and coloured minerals and mica, the lowest mean Fe oxidation state in silicates and highs in expandable clay minerals correlated with the Northern Hemisphere summer insolation minima at 55°N. Reconstructed changes in weathering intensity are better correlated to insolation patterns than to global ice volume records. We propose a scheme of yet missing palaeoenvironmental interpretation of the diatom assemblage, including also some extinct species. Aulacoseira baicalensis and Aulacoseira skvortzowii were abundant in the early stages of lake flora recovery immediately after deglaciation and during MIS 7e and MIS 5e; periods of more pronounced continental climate and peak chemical weathering. Stephanodiscus formosus var. minor, Cyclotella minuta and Cyclotella ornata dominated in intervals of decreased seasonality and decreased humidity at the end of most interglacial/interstadial diatom zones. Stephanodiscus grandis, Stephanodiscus carconeiformis and Stephanodiscus formosus were ubiquitous between MIS 8 and MIS 5, an interval marked by high seasonality, i.e., large differences between winter and summer insolation, and low humidity revealed by a low hydrolysis of expandable clay minerals in the watershed. Diatom concentrations peaked in the climatic optima of MIS 7e and MIS 5e and in the short periods marked by shifts to warmer conditions in the upper sections of MIS 5: MIS 5c (103–99 ky BP), MIS 5b (90–88 ky BP), and MIS 5a (84–79 ky BP) in which increased humidity resulted in enhanced hydrolysis of clay minerals. No such short similar climatic optimums were found from MIS 9a to MIS 6. Sharp climate deteriorations recorded in the diatom and clay mineral records at 107, 94, and 87 ky BP, however, occurred within 1–2 ky of cold extremes in North Atlantic sea surface temperature emphasizing the strong teleconnections between the two localities. 相似文献
56.
57.
Curium(III) complexation with pyoverdins secreted by a groundwater strain of Pseudomonas fluorescens
Henry Moll Anna Johnsson Mathias Schäfer Karsten Pedersen Herbert Budzikiewicz Gert Bernhard 《Biometals》2008,21(2):219-228
Pyoverdins, bacterial siderophores produced by ubiquitous fluorescent Pseudomonas species, have great potential to bind and thus transport actinides in the environment. Therefore, the influence of pyoverdins secreted by microbes on the migration processes of actinides must be taken into account in strategies for the risk assessment of potential nuclear waste disposal sites. The unknown interaction between curium(III) and the pyoverdins released by Pseudomonas fluorescens (CCUG 32456) isolated from the granitic rock aquifers at the Äspö Hard Rock Laboratory (Äspö HRL), Sweden, is the subject of this paper. The interaction between soluble species of curium(III) and pyoverdins was studied at trace curium(III) concentrations (3 × 10?7 M) using time-resolved laser-induced fluorescence spectroscopy (TRLFS). Three Cm3+—P. fluorescens (CCUG 32456) pyoverdin species, MpHqLr, could be identified from the fluorescence emission spectra, CmH2L+, CmHL, and CmL?, having peak maxima at 601, 607, and 611 nm, respectively. The large formation constants, log β121 = 32.50 ± 0.06, log β111 = 27.40 ± 0.11, and log β101 = 19.30 ± 0.17, compared to those of other chelating agents illustrate the unique complexation properties of pyoverdin-type siderophores. An indirect excitation mechanism for the curium(III) fluorescence was observed in the presence of the pyoverdin molecules. 相似文献
58.
Anna Lynn Suer David W. Phillips 《Journal of experimental marine biology and ecology》1983,67(3):243-259
Larvae of Urechis caupo Fisher & MacGinitie, reared in the laboratory, were exposed to potential settlement stimuli, including natural sediment from adult burrows, and “scent” obtained from the skin of adult animals. Competent larvae settled rapidly and specifically in response to adult burrow sediment when compared with their responses to other natural and abiotic sediments. Larvae also responded specifically to chemical “scent” from adult animals when the “scent” of another echiuran worm, Listriolobus pelodes Fisher served as a control. Larval responses to chemical “scent” were as great as their responses to natural burrow sediment. Hence, it is likely that larvae settle gregariously in nature in response to “scent” on sediment grains of adult burrows. The chemical “scent” had a molecular weight between ≈3500 and 14000 daltons, as determined by dialysis. It quickly lost its effectiveness in promoting settlement after it was heated to 80 °C, but was relatively stable at ambient ocean temperatures, retaining its effectiveness for several days. It was soluble in sea water. However, larvae did not respond to the chemical “scent”, unless it was adsorbed onto a surface. Purely tactile stimuli, such as the shape, texture, and size-distribution of particles, were not important settlement cues during these experiments. 相似文献
59.
Anna Corcione Elisa Ferretti Maria Bertolotto Franco Fais Lizzia Raffaghello Andrea Gregorio Claudya Tenca Luciano Ottonello Claudio Gambini Glaucia Furtado Sergio Lira Vito Pistoia 《PloS one》2009,4(12)
Background
Fractalkine/CX3CL1, a surface chemokine, binds to CX3CR1 expressed by different lymphocyte subsets. Since CX3CL1 has been detected in the germinal centres of secondary lymphoid tissue, in this study we have investigated CX3CR1 expression and function in human naïve, germinal centre and memory B cells isolated from tonsil or peripheral blood.Methodology/Principal Findings
We demonstrate unambiguously that highly purified human B cells from tonsil and peripheral blood expressed CX3CR1 at mRNA and protein levels as assessed by quantitative PCR, flow cytometry and competition binding assays. In particular, naïve, germinal centre and memory B cells expressed CX3CR1 but only germinal centre B cells were attracted by soluble CX3CL1 in a transwell assay. CX3CL1 signalling in germinal centre B cells involved PI3K, Erk1/2, p38, and Src phosphorylation, as assessed by Western blot experiments. CX3CR1+ germinal centre B cells were devoid of centroblasts and enriched for centrocytes that migrated to soluble CX3CL1. ELISA assay showed that soluble CX3CL1 was secreted constitutively by follicular dendritic cells and T follicular helper cells, two cell populations homing in the germinal centre light zone as centrocytes. At variance with that observed in humans, soluble CX3CL1 did not attract spleen B cells from wild type mice. OVA immunized CX3CR1−/− or CX3CL1−/− mice showed significantly decreased specific IgG production compared to wild type mice.Conclusion/Significance
We propose a model whereby human follicular dendritic cells and T follicular helper cells release in the light zone of germinal centre soluble CX3CL1 that attracts centrocytes. The functional implications of these results warrant further investigation. 相似文献60.
PIAS proteins as repressors of Oct4 function 总被引:1,自引:0,他引:1
Tolkunova E Malashicheva A Parfenov VN Sustmann C Grosschedl R Tomilin A 《Journal of molecular biology》2007,374(5):1200-1212