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151.
Ecological effects of the North Atlantic Oscillation 总被引:27,自引:3,他引:24
152.
A Synechococcus PglnA::luxAB Fusion for Estimation of Nitrogen Bioavailability to Freshwater Cyanobacteria
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Osnat Gillor Ayelet Harush Ora Hadas Anton F. Post Shimshon Belkin 《Applied microbiology》2003,69(3):1465-1474
In contrast to extensive studies of phosphorus, widely considered the main nutrient limiting phytoplankton biomass in freshwater ecosystems, there have been few studies on the role of nitrogen in controlling phytoplankton populations. This situation may be due partly to the complexity in estimating its utilization and bioavailability. In an attempt to provide a novel tool for this purpose, we fused the promoter of the glutamine synthetase-encoding gene, P glnA, from Synechococcus sp. strain PCC7942 to the luxAB luciferase-encoding genes of the bioluminescent bacterium Vibrio harveyi. The resulting construct was introduced into a neutral site on the Synechococcus chromosome to yield the reporter strain GSL. Light emission by this strain was dependent upon ambient nitrogen concentrations. The linear response range of the emitted luminescence was 1 mM to 1 μM for the inorganic nitrogen species tested (ammonium, nitrate, and nitrite) and 10- to 50-fold lower for glutamine and urea. When water samples collected from along a depth profile in Lake Kinneret (Israel) were exposed to the reporter strain, the bioluminescence of the reporter strain mirrored the total dissolved nitrogen concentrations determined for the same samples and was shown to be a sensitive indicator of the concentration of bioavailable nitrogen. 相似文献
153.
In this article, we discuss molecular mechanisms involved in the evolution of amygdala kindling and the episodic loss of response
to pharmacological treatments during tolerance development. These phenomena allow us to consider how similar principles (in
different neurochemical systems) could account for illness progression, cyclicity, and drug tolerance in affective disorders.
We describe the phenomenon of amygdala-kindled seizures episodically breaking through effective daily pharmacotherapy with
carbamazepine and valproate, suggesting that these observations could reflect the balance of pathological vs compensatory
illness-induced changes in gene expression. Under certain circumstances, amygdala-kindled animals that were initially drug
responsive can develop highly individualized patterns of seizure breakthroughs progressing toward a complete loss of drug
efficacy. This initial drug efficacy may reflect the combination of drug-related exogenous neurochemical mechanisms and illness-induced
endogenous compensatory mechanisms. However, we postulate that when seizures are inhibited, the endogenous illness-induced
adaptations dissipate (the “time-off seizure” effect), leading to the re-emergence of seizures, a re-induction of a new, but
diminished, set of endogenous compensatory mechanisms, and a temporary period of renewed drug efficacy. As this pattern repeats,
an intermittent or cyclic response to the anticonvulsant treatment emerges, leading toward complete drug tolerance.
We also postulate that the cyclic pattern accelerates over time because of both the failure of robust illness-induced endogenous
adaptations to emerge and the progression in pathophysiological mechanisms (mediated by long-lasting changes in gene expression
and their downstream consequences) as a result of repeated occurrences of seizures. In this seizure model, this pattern can
be inhibited and drug responsivity can be temporarily reinstated by several manipulations, including lowering illness drive
(decreasing the stimulation current.), increasing drug dosage, switching to a new drug that does not show crosstolerance to
the original medication, or temporarily discontinuing treatment, allowing the illness to re-emerge in an unmedicated animal.
Each of these variables is discussed in relation to the potential relevance to the emergence, progression, and suppression
of individual patterns of episodic cyclicity in the recurrent affective disorders. A variety of clinical studies are outlined
that specifically test the hypotheses derived from this formulation. Data from animal studies suggest that illness cyclicity
can develop from the relative ratio between primary pathological processes and secondary endogenous adaptations (assisted
by exogenous medications). If this proposition is verified, it further suggests that illness cyclicity is inherent to the
neurobiological processes of episode emergence and amelioration, and one does not need to postulate a separate defect in the
biological clock. The formulation predicts that early and aggressive long-term interventions may be optimal in order to prevent
illness emergence and progression and its associated accumulating neurobiological, vulnerability factors. 相似文献
154.
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156.
Sv. Murbeck 《Plant Systematics and Evolution》1898,48(4):124-127
Ohne Zusammenfassung 相似文献
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159.
The recurrent drying out of small streams in past decades has shown an urgent need to pay attention to the impact of global
climate change. The objectives of this study were to describe the effect of drying out on the composition of the mayfly taxocene
and evaluate the relevance of individual species traits for survival of mayflies to drying out. The mayfly taxocenes of two
model localities, one at an intermittent and one at a permanent brook, were investigated in 2002, 2003 and 2005. Compared
with the permanent stream, the taxocene of the intermittent stream was short of nine species, foremost rheobionts and high
oxygen demand species. To explain further differences between both stream types in survival and recolonisation ability, 15
species traits were evaluated. These included so-called “ecological traits” (e.g., habitat and substrate range, density, distribution,
current velocity adaptation) and “biological traits” connected with life cycle and larval/adult adaptations. Species showing
the highest number of advantageous traits (with only exception of Electrogena sp. cf. ujhelyii — species of taxonomically unclear status) were able to successfully survive under the unfavourable conditions of the intermittent
brook. Biological traits considered more important in many respects seem to be good predictors for assessing sensitivity to
extreme temperature changes, hydrological regime fluctuations and the survival/recolonisation ability of species in exposed
habitats. 相似文献
160.
Robert L. Post Jr. 《Journal of molecular evolution》1990,31(4):257-264
Summary An equation is developed that describes the condition of homeostasis in a general molecular system containing catalysts. In a prebiotic environment, this condition first results from a critical level of catalytic feedback in feedback loops containing differing organic molecular species. This critical level results in temporary exponential growth in concentrations of those catalyst species participating in the feedback loops, leading to homeostasis as the steady-state endpoint. None of the molecules in any feedback loop need be self-replicating for this autocatalysis to occur. Homeostasis is regarded as a definition of life at the lowest possible hierarchical level. A general mathematical boundary condition is derived for the critical level of catalytic feedback mentioned above-in effect, an origin of life condition. The paper argues that any natural prebiotic system of organic molecules in an H2O medium will automatically form many catalytic feedback loops, even if of very low catalytic efficiency. The analysis in this paper indicates that high temperatures strongly increase the efficiency of such catalytic feedback. If the temperature and total concentration of carbon in the system (e.g., in CO2, CH4, etc.) are sufficiently high, the critical condition for initial exponential growth will be attained. High initial temperatures for the earth are predicted by the planetesimal accretion model. 相似文献