1,721,133 research outputs found
Water column dynamics during the last interglacial anoxic event in the Mediterranean (sapropel S5)
No full textMarine organic-rich sediments are important for long-term carbon sequestration, and as a source of fossil hydrocarbons, but the processes underlying their formation remain elusive. We present a multiproxy analysis of a relatively recent, well-preserved Mediterranean organic-rich deposit known as sapropel S5, which formed 124–119 thousand years ago (ka B.P., where B.P. references year 1950). Combining planktonic foraminiferal abundance records with stable isotope and organic biomarker data, we reconcile the apparently contradicting coexistence of deep-dwelling foraminiferal faunas with green sulphur bacteria that indicate photic zone euxinia. S5 started with a rapid freshwater-induced increase of density stratification that inhibited vertical mixing and deepwater ventilation. Through the first 900 years, anoxic to euxinic deepwater conditions developed and extended up to roughly 200 m water depth. Then followed a 4100-year period of variability in the depth/intensity of winter mixing with a roughly 1400-year periodicity, coincident with fluctuations in water column stability. S5 terminated with a marked decrease in density stratification and consequently renewed convective overturn. <br/
Investigating past methane cycle perturbations through the lens of novel polyfunctionalised hopanoids
No full textEnhanced aerobic methanotrophy has been inferred in wetland environments during past warm climates, notably the Paleocene-Eocene Thermal Maximum (PETM; ∼56 Ma). This is based on a negative carbon isotope excursion in bacterial-derived hopanes (d13Chop). However, these compounds have multiple source organisms and their d13Chop can also reflect changes in the metabolic pathway used to assimilate carbon. In contrast, bacteriohopanepolyols (BHPs) are polyfunctionalised hopanoids that may be a more diagnostic tracer of methane oxidation, but their preservation potential is unclear. In this study, we aim to: (i) determine the preservation of novel BHPs during the PETM; and (ii) explore the potential of BHPs as a proxy to reconstruct past methane cycle dynamics. Overall, our study presents the most diverse bacteriohopanepolyol profile found thus far in sedimentary deposits beyond 50 million years ago, suggesting excellent preservation. Interestingly, a peak in 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol) coincides with the most negative d13Chop value during the PETM. An increase in aminopentol during the PETM was also previously observed, further supporting the utility of aminopentol as a proxy for methane cycling during transient warming events
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
The origin and fate of intact polar lipids in the marine environment
No full textMicroorganisms, such as bacteria, archaea and algae, are the most abundant organisms on Earth and they contain the bulk of the biosphere’s carbon, nitrogen and phosphor.They are also the main drivers of the biogeochemical cycles, and therefore the study of microbes in their environment (microbial ecology) is important for our understanding of Earth system functioning. Unfortunately, the very small size of microbes makes it difficult to study them in situ, and a range of methods is employed to determine their natural diversity, abundance and activity. One such approach is to measure molecular compounds present in a microbial cell. Rather than trying to obtain information about the organism through direct visual or morphological analysis, analysis of its pigments,proteins, DNA or lipids has been shown to contain a wealth of information about their source organism.A promising method for the identification, characterization and enumeration of microbial communities in the natural environment is the measurement of intact polar lipids (IPLs), the basic building blocks of biomembranes. These complex molecules areubiquitous in nature and have several characteristics that make them useful as proxies for living microbial cells. Within the large molecular diversity encountered in IPLs,certain types are uniquely synthesized by particular organisms, and these specific IPLs can consequently be used as biomarkers for those source organisms. Furthermore, IPLs are thought to degrade rapidly upon cell death, meaning they are indicative of livingcells and can thus be used for estimates of viable microbial cell numbers or biomass.Finally, microbes can actively remodel the IPL composition of their biomembranes in adaptation to their environment, meaning that information about environmental conditions can potentially be obtained from IPL measurements.However, the comparatively recent development of suitable analytical instrumentation for direct analysis of IPL molecules (i.e., the coupling of liquid chromatography to mass spectrometry through an electrospray ionization interface) means that only alimited number of studies have been done on IPLs in the marine realm, and in particular the water column. The aim of this thesis was therefore to investigate the origins, dynamics and fate of IPLs in the marine environment. First, the applicability of IPLs as a proxy for living microbial cells was re-assessed by determining IPL degradation rates in decaying diatom cultures, and in targeted study in which IPLs specific to anammoxbacteria were measured in marine sediments in parallel with other proxies for anammox bacterial abundance. Subsequently, a comprehensive analysis of IPLs in surface waters of the North Sea was performed, both on a spatial and a temporal scale. Theseresults were statistically compared with measurements of the environmental conditions and the microbial biogeography and their use as a tool in marine environmental microbiology was evaluatedA laboratory study of IPL degradation in decaying diatom cultures showed that, while degradation rates were initially high, they slowed progressively over time. Furthermore,a substantial fraction of the total IPL pool remained intact for several weeks after the cultures had reached full senescence. Overall, IPL concentrations correlatedwith total cell counts (including both living and dead cells) rather than with living cell counts, implying that they do not exclusively reflect living cells. IPL degradationrates were enhanced when bacteria were introduced into the culture, but also in this scenario the IPL pool did not disappear completely. These results show that IPL degradationis not as uniformly rapid and complete as is currently assumed. While circumstantial evidence suggests that in most marine environments the majority of the IPL pool is degraded fairly rapidly, and most IPLs can be used as proxies for living microbialcells, care should be taking in settings where IPL degradation may be impeded. For example, in anoxic sediments with a high organic matter content the possibility exists that part of the IPL pool escapes degradation and becomes fossilized.In marine sediments of the Gullmar Fjord in southwest Sweden, a specific biomarker IPL (C20-[5]-ladderane monoalkylether phosphatidylcholine) was used to trace abundancesof its source organisms: anaerobic ammonium oxidizing (anammox) bacteria.The good correlations found between ladderane IPL concentrations in the sediment and several other markers for ladderane bacterial abundances and activity show the applicability of this very specific biomarker. However, a discrepancy between concentrationsof the ladderane IPL and ladderane fatty acids was noted, which increased progressively with sediment depth. This shows that these fatty acids are readily fossilized and IPLs provide a better indicator for living microbial cells.To assess the value of IPLs as environmental and chemotaxonomic markers, a comprehensive analysis of the IPL composition, environmental conditions and microbial communitycomposition of marine surface waters was performed in the North Sea. Strong environmental gradients between the North Sea and its adjacent water masses of the eastern North Atlantic Ocean and Skagerrak/Baltic Sea result in a number of distincthydrogeographical regions. These in turn drive the spatial distribution of the diverse microbial community, resulting in biogeographic regions with distinct microbiologicalcompositions during summer.Comprehensive IPL analysis of the North Sea surface waters indicated the presence of a large structural variety, comprising thousands of different IPL species. The IPL poolwas dominated by seven IPL classes, with the sulphur-bearing glycerolipid sulfoquinovosyl- diacylglycerol (SQDG) being the most abundant. The glycerophospholipidsphosphatidylcholine (PC), -glycerol (PG) and -ethanolamine (PE), as well as the betaine lipids diacylglyceryl-trimethylhomoserine (DGTS), -trimethylalanine (DGTA) and -carboxyhydroxymethylcholine(DGCC) were present in smaller and roughly comparableamounts. The overall fatty acid compositions of the different IPL classes were quite distinct, with the SQDGs containing almost exclusively short-chain saturated fatty acids, while the PCs, DGTAs and DGCCs contained many long-chain polyunsaturatedfatty acids (PUFAs).Statistical comparisons were made between the environmental and microbial parameters and the IPL compositions measured throughout the North Sea (spatial distribution) and measured in the Marsdiep tidal inlet over a one-year time series (temporal variation). Little evidence was found for a direct influence of environmentalconditions on IPL composition, and it appears that in this area the microbial community composition is the dominant factor determining the IPL composition. Tentative microbial sources for the predominant IPL classes could be identified, but only withpoor taxonomic resolution (e.g., small algae, cyanobacteria, diatoms), and none of the statistical relationships were particularly strong. This was particularly striking in theMarsdiep time series, where the IPL composition of the surface water remained relatively constant throughout the year, despite major shifts in the phytoplankton community composition. Combined with the observation in other studies that a comparablesuite of IPLs predominates in marine waters at a variety of sites, it thus appears that the most abundant IPLs in the world’s oceans are non-specific. This lack of chemotaxonomic resolution of the majority of the IPL pool makes it difficult to target specific microbial groups by general IPL screening.To bring out the full potential of IPLs as biomarkers in (marine) environmental microbiology, it thus seems necessary to focus on specific biomarker IPLs, such as ladderanes for anammox bacteria. Unfortunately, general full scan IPL screening like thatperformed in the North Sea and at other marine sites, will detect the common, most abundant and least specific IPLs, which obscure the less abundant, but more specific IPLs. This deeper layer of information can be accessed by specifically targeting knownbiomarker IPLs, for example using selective reaction monitoring (SRM). Alternatively, it may be possible to detect these more relevant IPLs by applying techniques that yieldmore structural information, such as MS3, 2D-HPLC-MS or high mass accuracy MS, particularly in combination with bioinformatics approaches to deal with the very large datasets (meta-lipidomics). Regardless of the techniques chosen, more specific biomarker IPLs will need to be obtained from microbial enrichment cultures. If the remaining issues with IPL degradation can be resolved, and their chemotaxonomic specificity can be increased, IPLs will be a useful tool for detecting and quantifying microorganisms in the marine environment
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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