1,721,025 research outputs found
Interactions between Cd and Cu, and Zn influence particulate phytochelatin concentrations in marine phytoplankton: Laboratory results and preliminary field data
No full textThe effect of metal interactions on phytochelatin production by marine phytoplankton has received little attention but yet is critical to understanding the biochemical production of this potentially important metal-binding ligand in the field. Cd, Cu, and Zn additions were made singly and in combination to three species of laboratory cultures and to a natural algal assemblage from pristine coastal seawater. In the laboratory cultures intracellular phytochelatin varied with metal exposure and demonstrated metal- and concentration-dependent synergisms and antagonisms. Most notably, the addition of all three metals together greatly suppressed phytochelatin production in all cultures. Particulate phytochelatin was also measured at two field sites. In the field, phytochelatin production is related to ambient Cd, Cu, and Zn levels, and the deviations from the dose-response relationship are potentially explained by metal interactions similar to those observed in the laboratory cultures. Though particulate glutathione concentrations were very low in some field samples, it did not appear to limit phytochelatin production. Particulate phytochelatin concentrations in samples from both field sites were very similar to those measured in the laboratory cultures when exposed to all three metals together, and thus phytochelatin levels in the field may be regulated by the interaction of Cd, Cu, and Zn
Origin of iron and aluminium in large particles (> 53 µm) in the Crozet region, Southern Ocean
Full text linkNatural iron fertilization processes are occurring around the Crozet Islands (46°26?S–52°18?E), thus relieving the water masses from the normally encountered High Nutrients Low Chlorophyll (HNLC) conditions of the Southern Ocean. During austral summers 2004/2005 and 2005/2006, iron and aluminium concentrations were investigated in large particles (> 53 µm) collected from just below the mixed layer at stations under the influence of island inputs, and also in adjacent HNLC waters. These large particles are anticipated to sink out of the mixed layer, and to reflect the net effects of input and cycling of these elements in the overlying mixed layer. Labile and refractory fractions were determined by a two-stage leaching technique. Data showed that water masses downstream of the islands were enriched in total iron and aluminium (0.25–2.68 nmol L? 1 and 0.34–3.28 nmol L? 1 respectively), relative to the southern HNLC control sites (0.15–0.29 nmol L? 1 for Fe and 0.12–0.29 nmol L? 1 for Al), with only a small fraction (typically < 1%) being acid leachable in both environments. Particulate iron predominantly derived from the island system represents a significant fraction of the total water column iron inventory and may complement dissolved Fe inputs that help support the high summer productivity around the Crozet islands
Simultaneous determination of in situ vertical transitions of color, pore-water metals, and visualization of infaunal activity in marine sediments
No full textThe vertical color transition from brown to gray-green in marine sediments is linked to the Fe redox boundary and is commonly used as a proxy for biogeochemical state. We combine time-lapse sediment profile imaging with diffusive gradient thin (DGT) gels to obtain simultaneous in situ measurements of sediment color profiles, pore-water Fe and Mn profiles, and qualitative estimates of faunal activity at the Oyster Ground and North Dogger (North Sea). Analysis of Fe and Mn profiles using generalized additive modeling reveals that high variability between profiles within the sites makes it difficult to determine any intersite differences in trace metal behavior. At the Oyster Ground, the depth of sediment color transition (4.78 +/- 0.76 cm) was not significantly different from the Fe redox boundary (7.67 +/- 4.04 cm). At the North Dogger, there was a significant discrepancy between the depth of the sediment color transition (2.86 +/- 0.78 cm) and the Fe redox boundary (10.17 +/- 1.04 cm), which most likely results from high sulfate reduction rates at the North Dogger, leading to complexation of reduced iron to a form not available to the DGT technique. The differences in the coupling of sediment color and the Fe redox boundary between stations is likely to be related to variations in recent infaunal bioturbation activity, rather than variations in sediment source or fundamental differences in bulk sediment chemistry. Our results highlight the importance of the infaunal community in mediating Fe and Mn cycles, which are key pathways in the degradation of organic matter, and suggest that descriptions of bulk chemistry alone may be insufficient to understand the dynamics of biogeochemical cycling
Fluxes of particulate iron from the upper ocean around the Crozet Islands: A naturally iron-fertilized environment in the Southern Ocean
No full textDespite a large macronutrient reservoir, the Southern Ocean has low levels of chlorophyll, primarily due to low iron availability. Exceptions to this situation are island systems where natural terrestrial iron inputs allow the development of large blooms. Particulate organic carbon (POC) and particulate (labile and refractory) iron analyses were performed on large (>53 ?m) particles collected at the base of the mixed layer within such a system (the Crozet Islands) and in adjacent high-nutrient, low-chlorophyll (HNLC) waters. Biogenic iron was obtained by removal of estimated lithogenic Fe from the total Fe present. We combine these data with 234Th measurements to determine downward particulate Fe fluxes. Fluxes of Fe ranged from 4 to 301 nmol m?2 d?1 (labile), not detectable to 50 ?mol m?2 d?1 (biogenic), and from 3 to 145 ?mol m?2 d?1 (total) and, on average, were approximately four times larger below the highly productive, naturally iron-fertilized region than below the adjacent HNLC area. Downward labile iron fluxes are close to the sum of dissolved terrestrial, atmospheric, and upwelled iron calculated from the Planquette et al. (2007), model. Refractory iron fluxes are ?2 orders of magnitude larger, and these can only have come from particles advected from the plateau itself. The “biogenic Fe,” is a substantial fraction (0–76, mean 23%) of the total particulate Fe to the north of the islands. The origin of this Fe pool must be dominantly biological conversion from the lithogenic fraction, as other supply terms including aeolian, deep mixing, and lateral advection of dissolved Fe are inadequate to account for the magnitude of this Fe. Inclusion of the offshore biologically available fraction of the lithogenic iron flux is therefore required to calculate fully the yield of carbon exported per unit iron injected
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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