Energy allocation in the cladoceran Daphnia magna Straus,under starvation and refeeding

Summary Models incorporating the energetics of individual daphnids (Cladocera) have been developed to predict the effect of environmental variables, particularly food availability, on population dynamics. One of them, that of Kooijman (1986), assumes that all assimilated energy enters a storage compartment prior to use in production and metabolism, and that under starvation the stores are used to support maintenance, reproduction and somatic growth, in that order of priority. This predicts that, under starvation, reproduction and growth will continue for a time, and that after they cease death will be immediate. Another model, that of McCauley et al. (1990), assumes that assimilated energy is used directly for maintenance and production, and that stores are accumulated to support maintenance metabolism under starvation. This predicts that growth and reproduction should cease immediately upon starvation and that death will not be immediate. We have carried out laboratory experiments, manipulating starvation time, on Daphnia magna to distinguish between these two models. The results support features of both models in that reproduction, but not growth, ceases upon starvation. We therefore developed a third model in which both maintenance and growth are supported from stores under starvation, with maintenance taking priority over growth under these conditions.     

Morphological and physiological responses to increased salinity in marsh and dune ecotypes ofSporobolus virginicus (L.) Kunth

Summary Sporobolus virginicus (L.) Kunth is a halophytic grass native to tropical and warm temperate coasts throughout the world. A rhizomatous perennial with erect culms,S. virginicus occurs as two genetically distinct growth forms, which are designated by their characteristic habitats as marsh and dune. What accounts for the specific distribution and maintenance of two separate ecotypes ofS. virginicus is not known. The present study examined the effects of seawater salinity on several morphological and physiological responses of hydroponically cultivated marsh and dune plants to determine whether differential tolerance to substrate salinity might contribute to the observed pattern of habitation. Both marsh and dune form plants survived prolonged exposure to full-strength seawater and reproduced vegetatively via culms and rhizomes. Salinity-induced reductions in culm height, internode length, and leaf size led to a miniaturization of marsh and dune plants. Sodium ion levels were low (<1.0 mmol/g dry weight) in various organs of salinized plants irrespective of ecotype, and potassium ion content increased in all salt-challenged plants, as did quarternary ammonium compounds and proline. Significant differences, however, between marsh and dune plants with respect to the effects of salinity on resource allocation, flowering phenology, and protein composition suggested that external salt concentration has a role in determining ecotype distribution.     

Nutrient transfer between the root zones of soybean and maize plants connected by a common mycorrhizal mycelium

The objective of the study was to determine whether nutrient fluxes mediated by hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi between the root zones of grass and legume plants differ with the legume's mode of N nutrition. The plants, nodulating or nonnodulating isolines of soybean [ Glycine max (L.) Merr.], were grown in association with a dwarf maize ( Zea mays L.) cultivar in containers which interposed a 6-cm-wide root-free soil bridge between legume and grass container compartments. The bridge was delimited by screens (44 μm) which permitted the passage of hyphae, but not of roots and minimized non VAM interactions between the plants. All plants were colonized by the VAM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe. The effects of N input to N-sufficient soybean plants through N₂-fixation or N-fertilization on associated maize-plant growth and nutrition were compared to those of an N-deficient (nonnodulating, unfertilized) soybean control. Maize, when associated with the N-fertilized soybean, increased 19% in biomass, 67% in N content and 77% in leaf N concentration relative to the maize plants of the N-deficient association. When maize was grown with nodulated soybean, maize N content increased by 22%, biomass did not change, but P content declined by 16%. Spore production by the VAM fungus was greatest in the soils of both plants of the N-fertilized treatment. The patterns of N and P distribution, as well as those of the other essential elements, indicated that association with the N-fertilized soybean plants was more advantageous to maize than was association with the N₂-fixing ones.     

Inhibition of the transthylakoid gradient of electrochemical proton potential by the local anesthetic dibucaine

The effects of the local anesthetic dibucaine on coupling between electron transport and ATP synthesis-hydrolysis by the coupling-factor complex (CF₀CF₁ ATPase) were investigated in thylakoid membranes from Spinacia oleracea L. cv. Monatol. Evidence is presented that inhibition of ATP synthesis was produced by a specific uncoupling mechanism which was based on dibucaine-membrane surface interactions rather than on the interaction of dibucaine with the ATPase complex. Dibucaine reduced the osmotic space of thylakoid vesicles. At low pH of the medium it stimulated ATP hydrolysis beyond the rates obtained with optimum concentrations of ‘classical’ uncouplers. After addition of dibucaine, there was displacement of membrane-bound Mg²⁺ and strong thylakoid stacking in the presence of only low Mg²⁺ concentrations. Inhibition of ATP synthesis and transmembrane pH gradient increased with medium pH. Hydrolysis of ATP by isolated CF₁ and the CF₀CF₁ complex was only slightly affected by dibucaine. The data are discussed assuming the involvement of localized proton channels on the membrane surface in protonic coupling of electron transport and ATP synthesis. A hypothesis for the mechanisms of action of local anesthetics at the thylakoid membrane is presented.     

Calcium in the synergid cells and other regions of pearl millet ovaries

Summary The synergids and other cells of mature, unpollinated pearl millet ovaries were investigated using: (1) freeze-substitution fixation in conjunction with scanning electron microscope observations and energy-dispersive X-ray microanalysis to localize total calcium (Ca) and other elements, and (2) antimonate precipitation to selectively localize loosely sequestered, exchangeable calcium (Ca⁺⁺). In freeze-fixed ovaries, the synergid cells, ovary wall, nucellus, and other regions of the ovary displayed, respectively and relatively, extremely high, high, moderate, and low levels of Ca. In antimonate-fixed ovaries, Ca-containing antimonate precipitates exhibited similar distribution patterns. In ovaries fixed using the conventional 2% (w/v) antimonate in fixatives, the synergids were disrupted due to precipitate overload. In the ovary wall, precipitates were mainly located in the intercellular spaces. Some precipitates were observed at the micropyle and along the outer ovule integument, associated with diffuse extracellular material, and in the cell walls of nucellar cells proximal to the micropyle. Examination of precipitate distribution inside the synergids was possible in ovaries fixed using 0.5% (w/v) antimonate in the fixatives. Cytoplasmic organelles of all synergids examined exhibited variable states of disintegration. The amount of precipitates associated with the degenerated organelles appeared to be proportional to the degree of their degeneration. Distinct precipitates were localized in contiguous regions of the nucellar cells fused with the embryo sac, the micropylar half of the embryo sac wall, and the filiform apparatus. The results are discussed in relation to the involvement of Ca⁺⁺ in mediating the functions of synergid cells during fertilization in angiosperms.On Specific Cooperative Agreement 58-43YK-8-0026 with the Department of Biochemistry, University of Georgia, Athens, GA 30602, USA     

主养青鱼池塘生态系统能量转换率的研究

1985—1987年对苏州市郊区主养青鱼池塘生态系统的能量转换率进行了分析。结果表明,主养青鱼净产7.5、11.25、15t/ha 3个产量级型池塘青鲤团头鲂产出能占养鱼总产出能的比例分别为82.49、78.03、79.34％;总投入能(太阳辐射能+辅助能)转移到鱼的总产出能转换率分别为0.19、0.24、0.31％;太阳辐射能转移到毛和净初级生产力的能量转换率分别为0.76、0.90、0.96％和0.61、0.72、0.77％;净初级生产力转移到滤食性鱼净产量的能量转换率分别为4.02、4.63、5.27％;辅助能转移到鱼净产量的能量转换率分别为12.20、11.33。11.74％。在3个产量级型池塘中,以15t/ha产量级的能量转换率为最佳型。     

Thermogenic effect of adrenaline: interaction with insulin

The contribution of insulin (3.6 pmol.kg body mass-1.min-1) to adrenaline-induced (0.164 nmol.kg fat free mass-1.min-1) thermogenesis was studied in ten postabsorptive healthy volunteers using two sequential protocols. Variables considered were oxygen consumption as well as carbon dioxide production, heart rate, blood pressure, plasma concentrations of glucose, insulin, glycerol, free fatty acids, beta-HO-butyrate and lactate. Adrenaline increased plasma concentrations of glucose, glycerol, free fatty acids, and beta-HO-butyrate, and heart rate and metabolic rate during normo-insulinaemia [61.3 (SEM 6.6) pmol.l-1]. Similar effects were observed during hyperinsulinaemia [167.9 (SEM 18.7) pmol.l-1], but the effect of adrenaline on oxygen consumption was reduced. On average, metabolic rate increased by 12.9% during normo-insulinaemia and by 8.9% during hyperinsulinaemia. We concluded that relative hyperinsulinaemia resulted in decreased adrenaline-induced thermogenesis and therefore increased whole body anabolism.     

Regulation of nuclear membrane assembly and maintenance during in vitro maturation of mouse oocytes: role of pyruvate and protein synthesis

Summary In the absence of a suitable energy source, mouse oocytes cultured in vitro resume, but fail to complete, meiotic maturation. However, little is known about the underlying mechanisms leading to this meiotic failure. We utilized pyruvate-deficient medium to test for the role of pyruvate throughout the meiotic maturation process. Germinal vesicle-stage (GV) oocytes underwent germinal vesicle breakdown (GVBD), but failed to form a polar body when cultured continuously in pyruvate-free medium. However, when GV oocytes were preincubated for 4 h in pyruvate-free medium containing dibutyryl cyclic adenosine monophosphate (dbcAMP) and then cultured in pyruvate-free medium, GVBD was markedly inhibited. Preincubation of GV oocytes in dbcAMP and cycloheximide, followed by culture in cycloheximide only, also inhibited GVBD. A longer preincubation period was required in the cycloheximide-dbcAMP case (12 h) than in pyruvate-free-dbcAMP medium situation (4 h). Strikingly, reassembly of the nuclear membrane without polar body formation was observed following GVBD in oocytes continuously cultured in pyruvate-free medium. The reassembled nuclear membrane increased in size with continued culture, and it surrounded partially-decondensed chromatin. Nuclear membrane reassembly also occurred in oocytes which had undergone GVBD during continuous culture in medium containing only cycloheximide. Reformation of nuclear membranes after GVBD was confirmed by electron-microscopic analyses of oocytes cultured in pyruvate-free medium or in the presence of cycloheximide. We conclude that both pyruvate and protein synthesis are required for nuclear membrane disassembly, whereas lack of pyruvate or protein synthesis is associated with interruption of the metaphase state and reassembly of the nuclear membrane. The evidence suggests that assembly and maintenance of an intact nucleus and its disintegration are all amenable to regulation by pyruvate, possibly via mechanism(s) involving protein synthesis.     

A re-assessment of bacterial growth efficiency: the heat production and membrane potential of Streptococcus bovis in batch and continuous culture

Glucose-limited, continuous cultures (dilution rate 0.1 h^-1) of Streptococcus bovis JB1 fermented glucose at a rate of 3.9 mol mg protein^-1 h^-1 and produced acctate, formate and ethanol. Based on a maximum ATP yield of 32 cells/mol ATP (Stouthamer 1973) and 3 ATP/glucose, the theoretical glucose consumption for growth would have been 2.1 mol mg protein^-1 h^-1. Because the maintenance energy requirement was 1.7 mol/mg protein/h (Russell and Baldwin 1979), virtually all of the glucose consumption could be explained by growth and maintenance and the Y_ATP was 30. Glucose-limited, continuous cultures produced heat at a rate of 0.29 mW/mg protein, and this value was similar to the enthalpy change of the fermentation (0.32 mW/mg protein). Batch cultures (specific growth rate 2.0 h^-1) fermented glucose at a rate of 81 mol mg protein^-1 h^-1, and produced only lactate. The heat production was in close agreement with the theoretical enthalpy change (1.72 versus 1.70 mW/mg protein), but only 80% of the glucose consumption could be accounted by growth and maintenance. The Y_ATP of the batch cultures was 25. Nitrogen-limited, glucose-excess, non-growing cultures fermented glucose at a rate of 6.9 mol mg protein^-1 h^-1, and virtually all of the enthalpy for this homolactic fermentation could be accounted as heat (0.17 mW/mg protein). The nitrogenlimited cultures had a membrane potential of 150 mV, and nearly all of the heat production could be explained by a futile cycle of protons through the cell membrane (watts = amperes x voltage where H⁺/ATP was 3). The membrane voltage of the nitrogen-limited cells was higher than the glucose-limited continuous cultures (150 versus 80 mV), and this difference in voltage explained why nitrogen-limited cultures consumed glucose faster than the maintenance rate. Batch cultures had a membrane potential of 100 mV, and this voltage could not account for increased glucose consumption (more than growth plus maintenance). It appears that another mechanism causes the increased heat production and lower growth efficiency of batch cultures.     

The tertiary structure of Aspergillus saitoi minor ribonuclease (Ms) predicted from the structure of RNase T1

Ribonuclease Ms from Aspergillus saitoi is a small acidic protein (11 714 Da) containing 106 amino acids of known sequence. Unlike other enzymes belonging to the RNase T₁ family this ribonuclease is base-unspecific. Using interactive computer graphics and energy minimisation we predicted the structure of RNase Ms on the basis of sequence homology to RNase T₁ of known structure. In this report the predicted structure of this protein is presented and characterised.