1,720,997 research outputs found
Effects of saxitoxin (STX) and veratridine on bacterial Na+ -K+ fluxes: a prokaryote-based STX bioassay
No full textSaxitoxin (STX) is a potent natural sodium channel blocker and represents a significant health concern worldwide. We describe here the antagonistic effects of STX and veratridine (VTD), an Na+ channel activator, on three gram-negative bacteria and their application to an STX bioassay. STX reduced the total cellular levels of both Na+ and K+, as measured by flame photometry, whereas VTD increased the cellular concentrations relative to control ion fluxes in the cyanobacterium Cylindrospermopsis raciborskii AWT205. Endogenous STX production in toxic cyanobacterial strains of C. raciborskii and Anabaena circinalis prevented cell lysis induced by VTD stress. Microscopic cell counts showed that non-STX producing cyanobacteria displayed complete cell lysis and trichome fragmentation 5 to 8 h after addition of VTD and vanadate (VAN), an inhibitor of sodium pumps. The addition of STX, or its analogue neoSTX, prior to treatment with VTD plus VAN prevented complete lysis in non-STX-producing cyanobacteria. VTD also affected cyanobacterial metabolism, and the presence of exogenous STX in the sample also ameliorated this decrease in metabolic activity, as measured by the cellular conversion of tetrazolium into formazan. Reduced primary metabolism was also recorded as a decrease in the light emissions of Vibrio fischeri exposed to VTD. Addition of STX prior to VTD resulted in a rapid and dose-dependent response to the presence of the channel blocker, with samples exhibiting resistance to the VTD effect. Our findings demonstrate that STX and VTD influence bacterial Na+ and K+ fluxes in opposite ways, and these principles can be applied to the development of a prokaryote-based STX bioassay
Fingerprints of Some Chlorinated Hydrocarbons in Plant Foliage from Africa
No full textConcentrations of DDTs, HCHs and HCB in foliage have been used to evaluate the contamination levels of two African areas and Seychelles and Mauritius Islands. Physico-chemical properties in combination with environmental features play the most important role in the global distribution of chlorinated hydrocarbons. However past and present applications appear to be significant in determining the characteristics of the contamination pattern in different areas. Results are discussed in comparison to previous African data. Remarkable differences in contamination levels are evident among the areas considered. Log-Probit and Correspondence Factor Analyses are used for the characterization of the typical distribution pattern of each area. Relative differences in the composition of the pollutant mixture (HCHs dominance, different DDE/DDT ratios etc.) seem to indicate a ''fingerprint'' of the contamination for each geographical-economical homogeneous region
Relationships between chlorinated hydrocarbons in vegetation and socioeconomic indices on a global scale
No full textMany advances have been made in the comprehension of the environmental distribution of persistent organic chemicals. Previous studies on chlorinated hydrocarbons in vegetation have shown that physicochemical properties in combination with environmental characteristics are the most important factors in determining the distribution pattern in remote areas, while present and past use seem to be the most important element near source areas. To better understand how a country's technological level and socioeconomic conditions impact on the environment in terms of organic contamination, several published data on contamination in vegetation by HCHs, HCB, and DDTs along with a new set of results from different countries were considered in relation to socioeconomic indices (Gross National Product per capita and Human Development Index). HCB and secondly γ-HCH seem to be linked to the economic development, while among the DDTs only the DDT/ DDE ratio shows a significant relationship with the considered indices. The proposed relationships between pollution and development highlight new elements that contribute to the understanding of the inputs and distribution of organic contaminants
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
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