Showing papers by "Aarhus University published in 1985"

Diffusive boundary layers and the oxygen uptake of sediments and detritus1

Abstract: The thickness of diffusive boundary layers and their role for the oxygen uptake of sediments and detritus were studied by the use of microelectrodes. Gradients of oxygen were always detectable within the boundary layer, which varied in thickness from 0.2 mm to > 1 mm. The thickness depended on the flow velocity of the water and on the roughness of the solid surface. Oxygen diffused through the boundary layer with a mean diffusion time of 1.2-9 min. The diffusive boundary layer constituted a transfer resistance for oxygen flux across the solid-water interface which limited the oxygen flux at high uptake rates. Sediments or detritus exposed to aerated water could therefore be almost anoxic at the surface, provided that they had sufficiently high rates of oxygen uptake. This can explain the occurrence of microaerophilic or anaerobic bacteria on exposed sediments where fully oxic conditions would intuitively have been expected.

Anaerobic methane oxidation rates at the sulfate‐methane transition in marine sediments from Kattegat and Skagerrak (Denmark)

Abstract: Concomitant radiotracer measurements were made of in situ rates of sulfate reduction and anaerobic methane oxidation in 2-3-m-long sediment cores. Methane accumulated to high concentrations (> 1 mM CH/sub 4/) only below the sulfate zone, at 1 m or deeper in the sediment. Sulfate reduction showed a broad maximum below the sediment surface and a smaller, narrow maximum at the sulfate-methane transition. Methane oxidation was low (0.002-0.1 nmol CH/sub 4/ cm/sup -3/ d/sup -1/) throughout the sulfate zone and showed a sharp maximum at the sulfate-methane transition, coinciding with the sulfate reduction maximum. Total anaerobic methane oxidation at two stations was 0.83 and 1.16 mmol CH/sub 4/ m/sup -2/ d/sup -1/, of which 96% was confined to the sulfate-methane transition. All the methane that was calculated to diffuse up into the sulfate-methane transition was oxidized in this zone. The methane oxidation was equivalent to 10% of the electron donor requirement for the total measured sulfate reduction. A third station showed high sulfate concentrations at all depths sampled and the total methane oxidation was only 0.013 mmol m/sup -2/ d/sup -1/. From direct measurements of rates, concentration gradients, and diffusion coefficients, simple calculations were made of sulfate and methane fluxes andmore » of methane production rates.« less

Stereological estimation of the volume‐weighted mean volume of arbitrary particles observed on random sections

Abstract: SUMMARY A stereological estimator of the weighted mean volume of particles of arbitrary shape is described. This unbiased estimator is based on simple point-sampling of linear intercept lengths. The complete absence of shape assumptions effectively breaks the long-standing ‘convexity-barrier’: the only requirement here is that individual particles can be unambiguously identified by their profiles on random sections. Practical details of the simple estimation procedure and an example with very irregular particles are reported. Finally, an estimator of the variance of the weighted distribution of particle volume is discussed. This estimator is also valid for particles of arbitrary shape. For any mixture of ellipsoids (spheres, oblates, prolates and triaxial ellipsoids) the estimator is reduced to a simple function of measurements of diameters in the section plane.

Laser Flash Photolysis of HCIO, CIO−, HBrO, and BrO− in Aqueous Solution. Reactions of Cl- and Br-Atoms

Abstract: Spectra and rate constants of reactions were measured of chlorine and bromine atoms formed by photolysis of aqueous solutions of hypochlorous and hypobromous acids and their salts with light of wavelength 248 5 and 308 nm Chlorine and bromine atoms in aqueous solution behave like weak oxygen acids showing the pKa-values 51 and 105, respectively Exchanges of water molecules bound to the halogen atoms are fast processes

A model for the tertiary structure of p21, the product of the ras oncogene

Abstract: A model was developed for the structure of p21, the protein with a molecular weight of 21,000 that is produced by the ras genes. This model predicts that p21 consists of a central core of beta-sheet structure, connected by loops and alpha helices. Four of these loops comprise the guanine nucleotide binding site. The phosphoryl binding region is made up of amino acid sequences from 10 to 16 and from 57 to 63 of p21. The latter sequence may contain a site for magnesium binding. Amino acids defining guanine specificity are Asn-116 and Asp-119, and sequences around amino acid 145 may contribute to guanine binding. The model makes it possible to visualize how oncogenic mutations of p21 affect interaction with guanine nucleotides.

Bacterivory by microheterotrophic flagellates in seawater samples

Abstract: Etude de la consommation bacterienne journaliere des zooflagelles dans un echantillon d'eau de mer. Un nombre de bacteries superieur a environ 10 E ml −1 soutient la croissance des flagelles

Symbiotic photosynthesis in a planktonic foraminiferan, Globigerinoides sacculifer (Brady), studied with microelectrodes1

Abstract: The chemical environment and the symbiotic photosynthesis of the spinose foraminiferan, Globigerinoides sacculifer, from the Gulf of Aqaba, northern Red Sea, were studied with O2 and pH microelectrodes at a resolution of 50-l 00 pm. In the dark, the foraminiferan respiration lowered the 0, concentration at the shell surface to 50% of air saturation, while pH was lowered from the ambient value of 8.23 to 8.15. In the light, the 0, increased to 2.5 times air saturation while pH reached 8.62. These steep chemical gradients were established over a diffusive boundary layer between the spines, which partly separated the animal from the bulk seawater. The 0, pool around the shell was very dynamic, with residence times down to 5 s in the light. Photosynthetic rates were mapped around and within the animal. The compensation light intensity of the symbionthost system was 26-30 PEinst m-2 s-l, while the light saturation intensity, Ik, was 160-170 PEinst m-2 s-l. The action spectrum of symbiont photosynthesis showed peaks at 450 and 670 nm due to chlorophyll a as well as a broad peridinin peak at 500-550 nm. Total respiration of one symbionthost system was 3.0 nmol0, h-l and was not significantly different between dark and light. Gross photosynthesis at light saturation was 18.1 nmol O2 h-l per foraminiferan. The high net photosynthesis showed that the zooxanthellae could theoretically provide all organic carbon required for growth and respiration of the host. Model calculations indicated, however, that the foraminifera were diffusion-limited in the uptake of dissolved nitrogen and phosphorus. The capture of prey was therefore a necessary source of these nutrients, while symbiotic photosynthesis could cover the energy requirements and allow efficient internal recycling of nutrients.

Absolute cross sections for multi-electron processes in low energy Arq+−Ar collisions: Comparison with theory

Abstract: Absolute cross sections have been measured for a variety of multi-electron processes in low-energy collisions of multiply charged argon recoil ions with neutral argon. The cross sections are compared with theoretical estimates based on an extension of the classical barrier model. Comparison is also made with the statistical theory of Muller et al.

The dithizone, Timm's sulphide silver and the selenium methods demonstrate a chelatable pool of zinc in CNS. A proton activation (PIXE) analysis of carbon tetrachloride extracts from rat brains and spinal cords intravitally treated with dithizone.

Abstract: From rats intravitally treated with dithizone (diphenyl-thiocarbazone) brains and spinal cords were removed and freeze-dried. The dithizonates present in the CNS tissue were extracted with carbon tetrachloride and subjected to a multielement analysis (proton activation, PIXE). It was found that the extract contained two metals. Most of the metal was zinc, but small traces of copper were also dectected. Because prior treatment with the chelating agent, dithizone, can block both the Timm and the selenium metal staining methods, it is suggested that the three techniques label predominantly zine in the neuropil (DTS-zine).

Common evolutionary origin of alpha 2-macroglobulin and complement components C3 and C4.

Abstract: A comparison of the sequence of the subunit of human alpha 2-macroglobulin (alpha 2M; 1451 amino acid residues) with that of murine complement component pro-C3 (1639 amino acid residues) reveals eight extended regions of sequence similarity. These regions contain between 19% and 31% identically placed residues and account for 75% and 67%, respectively, of the polypeptide chains of alpha 2M and pro-C3. Published sequence data for complement component C4 show that segments of this protein match well with corresponding stretches in alpha 2M and pro-C3. It is proposed that alpha 2M, C3 and C4, which all contain a unique activatable beta-cysteinyl-gamma-glutamyl thiol ester, have a common evolutionary origin and are homologous proteins. Several larger regions of low sequence similarity indicate the presence of structural domains in each of these proteins that specifically modify an underlying common gross structure. The quartets of basic residues in pro-C3 and pro-C4, at which cleavage takes place to produce the mature subunits of these proteins, and most of the residues forming the anaphylatoxin peptides of C3 and C4 (C3a and C4a) are absent in alpha 2M. In addition, C3 and C4 contain large portions, which extend beyond the COOH terminus of alpha 2M.

Repair in Foreign Language Teaching

Abstract: Repair, defined in this paper as modifications of trouble sources which have manifested themselves in the discourse, is an important activity in FL learning and communication, both in educational and non-educational contexts. It is argued that studies of repair in the FL classroom should include all repair activity rather than focus on one specific repair type, viz., the teacher's correction of learners' errors. In this study the four repair types suggested by Schegloff, Jefferson, and Sacks (1977) are analyzed, and a further distinction is made according to whether the trouble source is produced by the teacher or a learner. It is shown that different preferences for repair patterns vary with the type of classroom activity (language-centered vs. content-centered activities), and it is discussed how these preferences relate to repair in non-educational learner-native speaker discourse.

Mixed reproductive strategy and mate guarding in a semi-colonial passerine, the swallow Hirundo rustica

Abstract: Both male and female swallows Hirundo rustica have a mixed reproductive strategy (parental care for offspring and extra-pair couplations). Mate guarding protects females from male harassment and male swallows from being cuckolded. Females hide their fertile period by copulating successfully with their mates for an extended period during first clutches. Males guard in the pre-fertile period, when many unpaired males are present. Early breeding swallows guard more than late breeders since more sexual chases of females by non-mate males take place in the early period. Solitarily breeding females experience few chases by strange males; copulation frequency, length of copulation period and mate guarding is adjusted to a lower level than among colonial birds. Male guarding activity is more intense in the fertile than in the pre-fertile period. Guarding reduces success of extra-pair copulation attempts.

Forested Arctic: Evidence from North Greenland

Abstract: Shallow-water marine sediments of the Kap Kabenhavn Formation at lat 82°30′ N, Perry Land, North Greenland, contain an abundance of well-preserved remains of terrestrial vegetation and invertebrate fauna indicative of a mosaic of forest tundra environments having similarities to present conditions in Labrador. The presence of foraminifera faunas, marine molluscs, and one mammal species (supported by studies of paleomagnetism and amino acid diagenesis) suggests an age of ∼2 Ma, at the Pliocene-Pleistocene transition. At this time, the arctic tree line was located 2500 km to the north of its present position in the northwest Atlantic region, and forest tundra vegetation existed in lowland areas bordering on an Arctic Ocean without perennial sea-ice cover.

The effect of cattle and sheep grazing on salt-marsh vegetation at Skallingen, Denmark

Abstract: The aggregated effect of cattle and sheep grazing on Puccinellion maritimae and other salt-marsh vegetation has been studied together with changes in species composition, the percentage cover of each species, total cover and the percentage of bare ground, six years after grazing had been prevented by construction of experimental exclosures. The results of these experiments are discussed in relation to the natural development of the vegetation that occurred in a permanent plot in the ungrazed part of the salt marsh. Six years without grazing caused a marked increase in total cover and a dramatic reduction in the amount of bare ground. The species composition of the Puccinellia maritima community did not change during these six years, but all species increased in cover. Whereas Artemisia maritima migrated into the Festuca rubra community and only two species Festuca and Halimione portulacoides, increased in cover, all other species showed reduced cover and Salicornia europaea disappeared from the plot after six years without grazing. During the same period of time, three species, Salicornia europaea, Suaeda maritima, and Glaux maritima, disappeared from the plot in the ungrazed marsh as a result of natural development. During thirty-five years the vegetation originally dominated by Puccinellia maritima and Salicornia europaea has changed into a community dominated by Halimione portulacoides, whereas the grazed salt marsh is still dominated by Puccinellia maritima and Salicornia europaea.

Microprofiles of oxygen in epiphyte communities on submerged macrophytes

Abstract: Microprofiles of oxygen were measured by 2- to 10-μm diameter microelectrodes within 0.2- to 2.0-mm thick epiphyte layers on different marine and freshwater macrophytes during artificial light-dark cycles. The epiphyte community created large fluctuations in oxygen concentrations at the macrophyte surface by increasing the thickness of the layer around the macrophyte in which the transport of solutes occurred by molecular diffusion and by contributing to the combined epiphyte-macrophyte activity. When epiphytes were removed, the diffusive boundary layer was reduced to 100 to 200 μm, and oxygen fluctuations declined. The oxygen concentration at the epiphyte-coverted macrophyte surface varied the most with the photosynthetically very active Scytosiphon lomentaria (Lyngb.), i.e. from 3 times atmospheric saturation of oxygen in the light to 0 in the dark. Low photosynthetic activity and extensive air lacunae in leaves of Littorella uniflora (L.) Aschers reduced oxygen fluctuations and extended the period necessary to reach steady state oxygen profiles. Detailed analysis of the epiphyte community on Potamogeton crispus L. showed that oxygen exchange followed typical light-photosynthesis saturation curves without exhibiting photoinhibition. The light compensation point of the association, (20–37 μE m-2s-1), and the saturation point, Ik (105–135 μE m-2s-1), were both low despite extensive self-shading. With increasing age under nutrient limitation, oxygen release of the epiphyte-P. crispus association under saturating illumination dropped from 10.0 to 5.0 nmol O2 cm-2 min-1, whereas the dark consumption remained about the same (2.4 to 2.9 nmol O2 cm-2 min-1). The contributions of the epiphyte community and the macrophyte to these oxygen exchanges were about equal. Epiphyte communities attenuate light and increase the transfer resistance for fluxes of dissolved substances between the bulk phase and the plant surface. Dense epiphyte communities which develop in eutrophicated waters may therefore be a severe stress to macrophyte metabolism by shading and by generating anoxic conditions at the macrophyte surface in the dark and high O2 and low CO2 concentrations in the light.

Oxygen Responses and Mat Formation by Beggiatoa spp.

Abstract: The behavioral response of single Beggiatoa sp. filaments moving on a gas-permeable membrane was studied by the combined use of microscopy and oxygen microelectrodes during controlled oscillations of oxygen tension. The bacteria reacted to increasing oxygen by reversing the direction of movement. The same step-up phobic response to oxygen was observed when a filament tip or loop glided into a stable microgradient of increasing oxygen. The response was sensitive to a change in oxygen tension of <5% of air saturation min−1. The response time was 20 to 50 s. Frequently, only part of the filament responded, which led to the formation of sharp bends, loops, and coils. This partial response facilitated the positioning of the long filaments within the narrow O2-H2S interface. The structure of whole Beggiatoa mats on sediment surfaces varied from loose to dense in relation to shallow or steep oxygen gradients in the 0.3- to 2-mm-thick, unstirred boundary layer. In an illuminated sediment Beggiatoa spp. lived together with photosynthetic organisms and migrated vertically in accordance with light/dark variations. The combined effect of phobic responses to light and oxygen can explain this migration.