1,721,033 research outputs found
The influence of solar ultraviolet radiation on the photochemical production of H2O2 in the equatorial Atlantic Ocean
Get PDFHydrogen peroxide (H2O2) was measured in marine surface waters of the eastern Atlantic Ocean between 25degreesN and 25degreesS. H2O2 concentrations decreased from 80 nM in the north to 20 nM in the south, in agreement with earlier observations. A diel cycle of H2O2 production as a function of sunlight in surface waters was followed twice whilst the ship steamed southward. Around 23degreesN a distinct diel cycle could be measured which correlated well with irradiance conditions.
The wavelength dependency of H2O2 formation was studied near the equator. For 16 hours, water samples were incubated with wavelength hands of the solar spectrum, i.e. visible (VIS: 400-700 nm), VIS and ultraviolet A radiation (UVAR: 320-400 nm) and VIS, UVAR and ultraviolet B radiation (UVBR: 280-320 rim). A significant relationship was found between wavelength band and the production of H2O2. In addition, a clear positive relationship between intensity and production was found. UVAR was 6.5 times more efficient than VIS in producing 1 nM of H2O2, whereas UVBR was 228 times more efficient than VIS. When these data were weighted with respect to the energy of the solar spectrum at zenith hour, 28% of the H2O2 was formed by VIS, 23% was formed by UVAR and 48% was formed by UVBR. Considering the strong attenuation of UVBR in marine waters as compared with UVAR and VIS radiation, the role of UVAR deeper in the water column is recognised. Furthermore results of this research emphasise the importance of VIS radiation in the formation of H2O2
The influence of UV irradiation on the photoreduction of iron in the Southern Ocean
No full textAn iron enrichment experiment, EisenEx, was performed in the Atlantic sector of the Southern Ocean during the Antarctic spring of 2000. Deck incubations of open ocean water were performed to investigate the influence of ultraviolet B (UVB: 280-315 nm) and ultraviolet A (UVA: 315-400 nm) on the speciation of iron in seawater, using an addition of the radioisotopes Fe-59(III) (1.25 nM) or Fe-55(III) (0.5 nM). Seawater was sampled inside and outside the iron-enriched region. The radioisotopic Fe(II) concentration was monitored during daylight under three different light conditions: the full solar spectrum (total), total minus UVB, and total minus UVB+UVA. A distinct diel cycle was observed with a clear distinction between the three different light regimes. A clear linear relationship was found for the concentration of radioisotopic Fe(II) versus irradiance. UVB produced most of the Fe(II) followed by UVA and visible light (VIS: 400-700 nm), respectively. UVB produced 4.89 and 0.69 pM m(2) W-1 radioisotopic Fe(II) followed by UVA with 0.33 and 0.10 pM M-2 W-1 radioisotopic Fe(II) and VIS with 0.04 and 0.03 pM m(2) W-1 radioisotopic Fe(II)
Photochemistry and organic complexation of iron: Interactions in the Southern Ocean
Get PDFHet spoor-metaal ijzer (Fe) speelt in grote delen van de oceaan een belangrijke rol in de limitatie van algengroei. De chemie van Fe is daarom ondermeer van belang voor het koolstofdioxide budget in de Zuidelijke IJszee. De fotoreductie van ijzer is essentieel in de transformatie van colloidaal en organisch gebonden ijzer naar een reactievere Fe vorm die beter beschikbaar is voor opname door algen. Iedere lente ontstaat boven Antarctica een gat in de ozonlaag resulterend in een verhoogde intensiteit ultaviolet-B licht (UVB: 280-315 nm) die het aard- en zeeoppervlak bereikt. Onderzocht werd of deze hogere intensiteit aan UVB de fotoreductie van Fe(III) bevordert, resulterend in hogere concentraties biologisch beschikbaar Fe. Daarnaast werd onderzocht hoe de aanwezigheid van Fe-bindende organische moleculen (liganden) en twee typische Antarctische algen van invloed zijn op de fotoreductie van Fe. Natuurlijke organische liganden, zoals die voorkomen in de Westerschelde en het Marsdiep, werden onderzocht op hun gevoeligheid voor afbraak door ultraviolet licht. Zie: Samenvattin
De natuurlijke fluorescentie en de chloriniteit van het interstitiële water in de wadbodem van het westelijk deel van de Waddenzee in het voorjaar (april-juni 1978)
No full textA little investigation is made concerning the natural fluorescence and the chlorinity of interstitial water in the bottom of the western part of the Dutch Wadden Sea in the spring- summer situation. The results are compared with the results of the study of the summer situation by van Geldermalsen (1977-12) and of the winter situation by Zondervan (1978-1)
Interactions of algal ligands, metal complexation and availability, and cell responses of the diatom Ditylum brightwellii with a gradual increase in copper
No full textA continuous culture experiment was conducted to study interactions between copper-binding ligands released by light- limited Ditylum brightwellii, and toxic effects of Cu. on this diatom. Over 6 months, the Cu concentration in the medium has been increased in seven steps (3-173 nM). At each Cu. addition, Cu. speciation, characteristics of Cu sorption to cellular binding sites, and cell characteristics were determined. Physiological effects of Cu. were studied, using indicators for metal detoxification (thiols) and lipid peroxidation (malondialdehyde). Minor amounts of Cu (< 1.4%) were chelated by a minimum amount of EDTA (57 nM), required to maintain a stable long-term continuous culture. The responses of D. brightwellii to Cu were monitored. (1) From 3 to 47 nM added Cu, decreasing pools of glutathione, increasing malondialdehyde contents, an increased release of lipophilic ligands, and cell lysis indicated the enhancement of lipid peroxidation. (2) From 47 to 94 nM Cu, a 16-fold increase in high-affinity (strong) hydrophilic ligands was measured (conditional stability constants K' approximate to 10(12)) that complexed most Cu (maximum 97%); sexual reproduction was stimulated and cell volumes increased. (3) From 126 nM Cu, glutathione pools increased again, whereas cell division rates decreased slightly. (4) At 142 nM Cu, the number of lysed cells reached a maximum, as did the production of lipophilic compounds that complexed similar to2% Cu. As the binding sites of the strong ligands became Cu-saturated above 142 nM Cu, larger amounts of Cu were bound to low-affinity (weak) dissolved ligands (3-30%) and cellular binding sites (0.2-2.5%). Probably due to saturation of organic complexes at 142 nM Cu, the MINEQL- calculated Cu2+ concentrations increased markedly; pCu values decreased from > 11 to similar to 10; division rates were further inhibited; gamma-glutamylcysteine (phytochelatin precursor) was produced. (5) At 157 nM Cu, phytochelatin synthesis started, and Cu-sorption capacities (cell walls and internal binding sites) increased. (6) At 173 nM Cu, the phytochelatin pool sizes and the number of cellular Cu-binding sites increased further. These results suggest that ligands released by a dense bloom of D. brightwellii, either by active excretion or lysis, would have lower affinities for Cu (K' approximate to 10(9)-10(12)) and moderate the availability of Cu less effectively than ligands in natural environments (10(13)-10(14)). In this diatom, the concurring release of ligands, enhanced malondialdehyde production, increasing numbers of presexual cells and cell enlargement may serve as early-warning signals for Cu toxicity, rather than metal- specific phytochelatins that appeared at a stage when cell division was already clearly inhibited. [KEYWORDS: binding site; cell volume; copper; diatom; malondialdehyde; speciation Oxidative stress; nitzschia-closterium; silene-cucubalus; maize seedlings; cadmium; phytoplankton; damage; accumulation;speciation; defense]
Organic Iron-binding ligands in the Arctic, Antarctic and subtropical Regions
No full textOrganic ligands determine the threshold of dissolved-Fe (DFe) concentrations in seawater. Understanding the processes controlling the dynamics of natural organic ligands is of fundamental importance towards a better understanding of the impact of global warming on the Fe cycle. However, there is a paucity of Fe speciation data to comprehensively assess the effects of global climate change on the biogeochemical cycle of Fe. Therefore, this thesis reports organic ligand properties in the oceanic regions that are vulnerable to global climate change and in an area subject to temporal variability. This thesis used an electrochemical approach, competitive ligand exchange - adsorptive cathodic stripping voltammetry (CLE-AdCSV). This method is an indirect method to determine organic ligand properties, the concentrations ([Lt] in nM equivalents of Fe (nM eq. Fe) and the conditional stability constant ("K" _"Fe'L" ^"cond" in M-1; it is shown in logarithmic value, log "K" _"Fe'L" ^"cond" ).
The study of dissolved organic Fe-binding ligands in Fram strait reveals that the binding strength of organic ligands regulates the export of DFe from the largest glacier of northeast Greenland. A high [Lt] (up to ~3 nmol eq. Fe), yet with relatively weak binding strength, is found in the vicinity of the glacier terminus. These relatively weak ligands compete less efficient against scavenging and precipitation, relative to the stronger ligands.
In the waters adjacent to the western Antarctic Peninsula, Fe-binding ligand concentrations and characteristics are investigated in different hydrographic regions. Under sea ice cover near the coast, organic ligand production is linked to ice-associated algae. Sediment-water interaction and resuspension result in high concentrations of DFe and ligands in upwelling deep waters over the continental slope. Towards the open ocean, phytoplankton blooms deplete nutrients and iron, while actively or passively producing organic ligands. The presence of fronts affects the distribution of the ligands, marking the border between shelf derived ligands and bloom-associated ligands offshore.
The Hauraki Gulf in New Zealand, serves as a natural laboratory to study DFe speciation in a highly variable system, where changes of the microbial ecosystem can have a profound effect on DFe speciation. In the shelf regions, [Lt] was relatively high (~2.5 to 7.5 nM eq. Fe). The high [Lt] enabled relatively high DFe to persist on the shelf, and consequently, increased the potential export of DFe from the shelf to the open ocean, where Fe can be an important driver of primary productivity.
This thesis reports the concentration, distribution, and characteristics of Fe-binding ligands in different ocean regions with distinct environmental circumstances, adding a valuable data set to expand our current knowledge about global Fe speciation. The study areas in Fram strait and the Northeast Greenland shelf, the Western Antarctic Peninsula, as well as the Hauraki Gulf, all have distinct hydrographic settings. The study areas in the high latitude oceans are particularly susceptible to changes in climate, and our results help provide the fundamental overview that is needed to predict the consequences of climate change on the Fe biogeochemical cycle in these globally important high latitude oceans
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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