1,721,047 research outputs found
Vivaspin ultrafiltration: A new approach for high resolution measurements of colloidal and soluble iron species
Full text linkVivaspin6® ultrafiltration units with molecular weight “cut-off” membranes of 5, 10, 30, 50, and 100 kDa were used together to examine the size distribution of newly formed iron (Fe) colloids in natural seawater samples and in the presence of several different Fe chelators with varying Fe binding strength. Artificial Fe chelators, such as TAC, and 2 kDG, when added at equimolar levels to Fe, supported the formation of a continuum of Fe-ligand colloids between 5 and 100 kDa. More than 90% of the added 55Fe in these solutions occurred in Fe aggregates/particles larger than 100 kDa. The strong siderophore DFO held the majority of the added 55Fe in the “truly” soluble fraction ≤ 5 kDa, whereas 90% of 55Fe added to UV-irradiated seawater was converted into Fe colloids with a size between 50 to 100 kDa (5–6 nm). Membranes with ≥ 10 kDa showed similar “cut-off” properties on natural seawater samplescollected in the water column off the Peruvian coast. Fe solubility determined with these membranes was approximately six times greater than Fe solubility determined with the 5 kDa membrane and the 0.02 μm syringe filters. This suggests that a seamless size continuum of organic chelators (≤5 kDa–10 kDa) is present in these seawaters and that estimates of ligand production based on 0.02 μm Anotop solubility experiments underestimates the abundance of soluble/colloidal ligands. Regarding these results, we recommend the use of Vivaspin 5 kDa membranes to separate the “truly” soluble from the colloidal Fe fraction
Die Biogeochemie des Mangans in der euphotischen Zone
Full text linkThe trace metal manganese (Mn) plays a significant role in seawater as it is bio-essential for phytoplankton. Mn plays a critical role as a redox center in Photosystem II (PSII) during the conversion of water to oxygen in photosynthesis. It is also essential in other redox related enzymatic processes; in particular Mn is important as the active metal center in superoxide dismutase (SOD) which provides intracellular protection against oxidative stress due to photochemically produced superoxide (O2 ). Mn exists in seawater in three redox states: soluble and prevalent Mn(II), insoluble Mn(III) and Mn(IV)-oxides. In the euphotic zone the biogeochemical cycling of Mn is strongly influenced by reactive oxygen species (ROS). The highly reactive and short-lived superoxide (O2 ) and hydrogen peroxide (H2O2) can both act as oxidants and reductants, and they play a key role in the Mn processes in seawater. For example the dominant Mn sources to the open ocean are the Mn-oxides which are present in atmospheric dust which are reduced to soluble Mn(II) by photochemically produced H2O2. While these processes have been crudely identified, the dominant reactions and mechanisms of Mn and ROS in seawater are poorly understood. This lack of knowledge demands investigations into the in-situ dissolution processes of Mn from dust and into studying the exact reaction mechanisms between Mn and ROS in the euphotic zone.
This thesis comprises four manuscripts. Manuscripts 1 and 2 (Wuttig et al., subm., 2013a; Wuttig et al., subm., 2013b) focus on the cycling and reaction mechanisms of Mn and ROS. Manuscript 3 (Wuttig et al., in prep., 2013) addresses differences in the input and distribution of cadmium (Cd), iron (Fe) and Mn in the Eastern Tropical Atlantic Ocean off Cape Verde, and manuscript 4 (Wuttig et al., 2013) describes Mn cycling after dust additions in a trace metal clean mesocosm experiment in the Mediterranean Sea.
This study has conclusively shown that Mn and organic matter are the dominant sinks for O2 in the Eastern Tropical North Atlantic (manuscripts 1; Wuttig et al., subm., 2013a). Mn dominates this decay especially in the surface waters which are influenced by high atmospheric dust deposition and near the sediment/water interface due to Mn sediment resuspension. This contrasts with current knowledge based on findings from the Mn poor Southern Ocean where copper (Cu) was shown to be the major sink.
In manuscript 2 it is demonstrated that O2 decays by reaction with inorganic Mn(II) in seawater following a first order loss rate which appears to involve a catalytic reaction involving the Mn(II)/MnO2+ couple, in which MnO2+ is a manganous superoxide complex (Wuttig et al., subm., 2013a). Thus in sunlit and oxygenated waters Mn(III) is unlikely to be found in significant concentrations when strong Mn(III) binding ligands are not present. In other studies Mn(III) was found under anoxic conditions in the presence of unknown strong Mn(III) binding ligands. Therefore, in contrast to the Mn(II)/MnO2+ pair, Mn(III) cannot act as a SOD in the oxygenated surface ocean.
In the Eastern Tropical North Atlantic Ocean atmospheric dust is the main source of Mn to surface waters (manuscript 3; Wuttig et al., in prep., 2013). However this study provides clear evidence that equatorial upwelling and sediment resuspension are important Mn sources in this region. In contrast to findings from the Eastern Tropical Pacific, where unexpected high surface concentrations were observed, no secondary Mn(II) maximum was found in the Eastern Tropical North Atlantic Ocean. This could have been introduced by a combination of lateral transport of Mn rich waters from the coastal margins and reduction of Mn-oxides.
While Aeolian sources were predominantly influencing Mn and also Fe cycling in the Eastern Tropical Atlantic, Cd was not controlled by dust deposition (manuscript 3; Wuttig et al., in prep., 2013). These biologically relevant elements exhibited contrasting distribution patterns. For Fe and Mn, atmospheric depositions masked a classical nutrient type profile, while Cd was very depleted at the surface and concentrations steadily increased with depth. Cd was highly correlated to Phosphate (hereafter referred to as P). The Cd/P ratio was mainly controlled by P with elevated concentrations at depth resulting in strongly differing ratios in surface and subsurface layers of 16.6 pmol / µmol and 237 pmol / µmol, respectively.
The complex photochemical processes during the dissolution of Mn dust are also subject of manuscript 4. This paper describes a mesocosm project in the Mediterranean with two consecutive additions of evapocondensed dust conducted. The data also show that the dissolution and loss rates of Mn were comparable during both seedings. The calculated fractional solubilities for the first and the second dust addition were 41 ± 9 % and 27 ± 19 %, respectively.
The results presented in this thesis have significantly improved our understanding of Mn distribution and especially cycling in the euphotic zone. An insight into the mechanisms between Mn and ROS and into the dissolution processes from dust is given.Das Spurenelement Mangan (Mn) ist von zentraler Bedeutung im Meer, da es ein essenzieller Mikronährstoff für Phytoplankton ist. Es spielt eine wichtige Rolle im Photosystem II (PSII) bei der Sauerstoffbildung aus Wasser in der Photosynthese. Des Weiteren ist Mn wichtig für weitere enzymatische Redoxprozesse, insbesondere als aktives Metallzentrum in Superoxiddismutase (SOD), welche als intrazelluläre Schutzmechanismen vor oxidativem Stress durch photochemisch produziertes Superoxid (O2 ) fungieren. Im Meerwasser kommt Mn in drei Oxidationsstufen vor: in erster Linie als lösliches Mn(II), als unlösliches Mn(III) und als Mn(IV)-Oxide. Der biogeochemische Kreislauf von Mn im lichtdurchfluteten Ozean ist signifikant durch reaktive Sauerstoffspezies (ROS) beeinflusst. Hierbei können das stark reaktive und kurzlebige Superoxid (O2 ) und seine Tochterprodukt Wasserstoffperoxid (H2O2) gleichermaßen eine oxidierende als auch eine reduzierende Wirkung haben und spielen somit eine Schlüsselrolle für Mn Prozesse im Meerwasser. Im offenen Ozean beispielsweise ist atmosphärischer Staub die Haupteintragsquelle für Mn und das in oxidierter Form vorliegende Mn im Staub kann durch photochemisch gebildetes H2O2 reduziert und als Mn(II) im Wasser gelöst werden. Obwohl die groben Zusammenhänge dieser Prozesse bekannt sind, sind die Hauptreaktionen und Mechanismen des Zusammenspiels von Mn und ROS im Meerwasser kaum verstanden.
Die Dissertation umfasst vier Manuskripte. Manuskripte 1 (Wuttig et al., subm., 2013a) und 2 (Wuttig et al., subm., 2013b) konzentrieren sich auf Kreisläufe und Reaktionsmechanismen von Mn und ROS. Manuskript 3 beschreibt die Unterschiede zwischen den Eintragsprozessen und der Verteilung von Cd, Fe und Mn im östlichen tropischen Atlantik nahe den Kapverdischen Inseln (Wuttig et al., in prep., 2013). Manuskript 4 (Wuttig et al., 2013) widmet sich der Untersuchung des Mn Kreislaufs in Folge der wiederholten Staubzugabe in das oligotrophe Oberflächenwasser von spurenmetall-sauberen Mesokosmen im Mittelmeer.
Im Gegensatz zu früheren Beobachtungen im Südpolarmeer, konnte in dieser Studie klar gezeigt werden, dass im östlichen tropischen Atlantik Mn und organische Substanzen als vorwiegende Senken für O2 darstellen (Manuskript 1; Wuttig et al., subm., 2013a). Mn dominiert diesen Zerfall besonders im Oberflächenwasser, welches stark durch atmosphärischen Staubfluss beeinflusst ist, und nahe dem Meeresboden durch die Resuspension aus dem Sediment.
Zudem konnte gezeigt werden, dass der Abbau von O2 durch Reaktion mit anorganischem Mn(II) in Seewasser einer Zerfallsrate erster Ordnung folgt (Manuskript 2, Wuttig et al., subm., 2013b). Diese scheint eine katalytische Reaktion des Mn(II)/MnO2+ Paars zu beinhalten, wobei MnO2+ der Superoxidkomplex ist. Des Weiteren ist es in der euphotischen und sauerstoffreichen Zone unwahrscheinlich, nennenswerte Mn(III) Konzentrationen aufzufinden, welche ohne starke Mn(III) bindende Liganden nur unter anoxischen Bedingungen erwartet werden. Somit kann Mn(III) im Gegensatz zum Mn(II)/MnO2+ Paar im sauerstoffreichen Oberflächenwasser nicht als SOD agieren.
Atmosphärischer Staub ist die Haupteintragsquelle von Mn im Oberflächenwasser des östlichen tropischen Atlantiks (Manuskript 3; Wuttig et al., in prep., 2013). Jedoch wurden in dieser Region zusätzlich auch Einträge von Mn durch äquatorialen Auftrieb und Resuspension aus dem Sediment beobachtet. Zusätzlich wurde hier kein sekundäres Mn(II) Maximum gefunden, welches bei gleichen Untersuchungen im östlichen tropischen Pazifik in 200 m Tiefe der Fall war und vermutlich durch eine Kombination aus Einträgen durch atmosphärischen Staub und laterale Transportprozesse verursacht wurde.
Während atmosphärische Quellen dominierend den Mn und Fe Kreislauf beeinflussen, sind Cd Konzentrationen im tropischen Atlantik nicht durch Staubeintrag kontrolliert (Manuskript 3; Wuttig et al., in prep., 2013). Generell zeigten diese Spurenelemente unterschiedliche Verteilungsmuster. Während die für biologisch relevante Elemente zu erwartenden typischen nährstoffähnlichen Tiefenprofiele durch den atmosphärischen Eintrag von Fe und Mn überlagert wurden, waren die Cd Konzentrationen im Oberflächenwasser sehr niedrig und nahmen mit der Tiefe zu. Das Cd/P Verhältnis wurde dabei in erster Linie durch die in der Tiefe erhöhten P Konzentrationen bestimmt und wies dort ein Cd/P Verhältnis von 237 pmol / µmol gegenüber 16.6 pmol / µmol im Oberflächenwasser aus.
Auch im Manuskript 4 (Wuttig et al., 2013), welches sich mit der Löslichkeit und dem Kreislauf von Mn nach zwei Zugaben prozessierten Staubs während eines Mesokosmen Projektes im Mittelmeer beschäftigt, spielen die komplexen photochemischen Experimente eine zentrale Rolle. Hinzukommend zeigen die Daten, dass die Mn Löslichkeits- und Verlustraten nach den beiden Staubzugaben vergleichbar sind. Die anteilige Löslichkeit von Mn aus dem Staubmaterial betrug 41 ± 9 % für die erste Staubzugabe und 27 ± 19 % für die zweite Staubzugabe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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