1,720,966 research outputs found
Mass dependent stable isotope fractionation of mercury during its microbial transformations
No full textMercury (Hg) is often cited in fish consumption advisories across the world due to the extreme neurotoxicity of its methylated forms. Given the complex biogeochemical cycling of Hg, a differentiation between local vs. global and natural vs. anthropogenic sources of Hg(0) and determination of transformations that are dominant in a given ecosystem is critical. Mercury has seven stable isotopes and Hg isotope ratios can become a novel biogeochemical tool to track sources and transformations of Hg in the environment. However, development of a stable isotope based tool requires the determination of the extent of fractionation during individual biotic and abiotic transformations that can occur in the environment. This thesis reports the extent of fractionation of Hg isotopes during two biological transformations: 1) degradation of monomethyl-Hg (MMHg) via the mercury resistance (mer) pathway in Escherichia coli JM109/pPB117 and 2) Hg(II) reduction by four Hg(II) reducing strains, including three Hg(II) resistant strains (E. coli JM109/pPB117, Bacillus cereus Strain 5 and Anoxybacillus spp. FB9) and a Hg(II) sensitive strain (Shewanella oneidensis MR-1). Using a multi-collector inductively coupled plasma mass spectrometer, it was found that MMHg and Hg(II) that remained in the reactors became progressively heavier (increasing delta202Hg) with time and underwent mass dependent Rayleigh-type fractionation with average fractionation factors (alpha 202/198 for reactant/instantaneous product) of 1.0004 and 1.0016, respectively. Mass independent fractionation (MIF) was not observed and based on the nature of microbe-Hg interactions, it is suggested that the nuclear spin dependent MIF is unlikely to occur during biological processes. A multi-step framework for understanding the extent of fractionation seen during the mer mediated MMHg degradation and Hg(II) reduction experiments is provided, and based on the biochemistry and kinetics of the steps involved in the two pathways, the steps in the process that could contribute to the observed extent of fractionation are suggested in the thesis. A clear effect of Hg(II) bioavailability on the extent of fractionation of Hg was observed and is also discussed. The framework discussed here can guide future experiments on Hg isotope fractionation during other transformations in its biogeochemical cycle, and ultimately facilitate a more rigorous development of a Hg isotope based geochemical tool.Ph.D.Includes bibliographical references (p. 140-151)by Krite
MeHg concentrations and stable isotope ratios for individual samples from mercury reduction experiments conducted on marine phytoplankton from 2012-2013
No full textDataset: ISOMeHg SamplesMeHg concentrations and stable isotope ratios for individual samples from mercury reduction experiments conducted on marine phytoplankton from 2012-2013.
For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/826724NSF Division of Ocean Sciences (NSF OCE) OCE-163415
MeHg photoreduction rates and stable isotope enrichment from mercury reduction experiments conducted on marine phytoplankton from 2012-2013.
No full textDataset: ISOMeHg RatesMeHg photoreduction rates and stable isotope enrichment from mercury reduction experiments conducted on marine phytoplankton from 2012-2013.
For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/826633NSF Division of Ocean Sciences (NSF OCE) OCE-163415
Hg(II) concentrations and stable isotope ratios for individual samples from mercury reduction experiments conducted on marine phytoplankton from 2011-2013
No full textDataset: ISOHgII SamplesHg(II) concentrations and stable isotope ratios for individual samples from mercury reduction experiments conducted on marine phytoplankton from 2011-2013.
For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/826662NSF Division of Ocean Sciences (NSF OCE) OCE-163415
Mercury stable isotope ratios for standard reference material UM-Almaden during 2011-2013
No full textDataset: Mercury SRMMercury stable isotope ratios for standard reference material UM-Almaden during 2011-2013.
For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/826746NSF Division of Ocean Sciences (NSF OCE) OCE-163415
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Hg(II) photoreduction rates and stable isotope enrichment from mercury reduction experiments conducted on marine phytoplankton from 2011-2013
No full textDataset: ISOHgII RatesHg(II) photoreduction rates and stable isotope enrichment from mercury reduction experiments conducted on marine phytoplankton from 2011-2013.
For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/826598NSF Division of Ocean Sciences (NSF OCE) OCE-163415
Variations on the Author
No full text“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
No full textWe 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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