1,721,002 research outputs found
Novel methanotrophic community assemblages in a terrestrial methane seep in Svalbard
Full text linkRecent studies have concluded that groundwater driven methane escape through open system pingos is an important greenhouse gas source in the vulnerable high Arctic. In wetlands and marine sediments, large quantities of methane are consumed by methane oxidizing bacteria (MOB) and ANaerobic MEthanotrophic (ANME) archaea, preventing its release to the atmosphere. The methanotrophic capacity of open system pingos is understudied, and microbial community profiling is of key interest, to infer the magnitude of the methane filter and for high resolution prediction of methane evasion. In this thesis, we mapped the microbial community assemblages across hydrological transitions at the open system pingo Lagoon pingo (N78°14’22 E015°45’16). In summer, methane-saturated and oxygen limited groundwater discharges continuously through a main source, forming crater-like ponds, providing potential habitats for methanotrophy. We sampled sediments in August 2019 and coordinated these with methane flux measurements and oxidation rate assays. Environmental parameters and 16S rRNA gene diversity revealed a radial mosaic of habitat patches, made by the seep water. We found distinct and unusual microbial communities inhabiting these habitat patches, suggesting high levels of specialization and adaptation to an unusual terrestrial system with marine influences. Phylogenetic analyses of 16S amplicons unveiled MOB and ANME communities in the crater pond. The MOB sequences were dominated by the type I genus Methylobacter, which was >97% affiliated to the arctic wetland strain Methylobacter tundripaludum SV97. The highest relative abundances of Methylobacter coincided with methane oxidation rates in waterlogged habitats. The distribution of the MOB could not be explained by any environmental parameters measured in this study alone, however it is likely that the MOB are largely controlled by water and/or the dissolved methane, oxygen and nutrients. Surprisingly, Methylobacter was prevalent in sediments continuously flushed with anoxic groundwater, indicating adaptations to oxygen limitation. The archaeal 16S library was dominated by ANME-3 and 2a/b, with lesser representation of ANME-1a. ANME OTU affiliation to sulfate dependent marine clades coincided with sulphur cycling taxa in the source sediments, suggesting a potential for anaerobic oxidation of methane coupled with sulfate reduction, making this an intriguing terrestrial equivalent of cold marine methane seeps. The microbial community we present in this thesis depicts a system contrasting fundamentally from previously reported ecosystems in the Arctic and elsewhere. We confirm that methane is one of the primary energy sources at the seep site and that the atmospheric transfer of methane is mitigated by MOB and possibly ANME
Linking methane fluxes and oxidation rates to methane oxidizing bacteria in an Arctic terrestrial methane seep, Svalbard
Full text linkGlobal warming has especially detrimental effects on Arctic regions. One major issue is permafrost thaw and sub-permafrost methane escape via surface seeps. While the mitigation potential of methane-consuming bacteria on marine pingo-like methane seeps is well described, terrestrial methane seeps are still understudied. Recently, open system pingos have gained attention as terrestrial methane seeps. Lagoon Pingo (N 78°14'22'', E15°45'16''), a near-shore open system pingo in the Adventdalen valley, Svalbard, was chosen as a study site to investigate the potential impact of methane-oxidizing bacteria on methane evasion from open system pingos. During a fieldwork campaign in August 2019, methane fluxes were measured across the entire site and compared to the distribution of the site’s methane oxidation potentials. The centre of evasion was found at the groundwater-discharging source. The discharged waters were carried away by an associated stream, which gradually emitted methane to a distance of up to 80 meters from the source. While waters from the site were not shown to possess the ability to oxidize methane aerobically, an abundance of methane oxidizing bacteria was found in sediments that were covered with methane emitting waters, creating the potential to oxidize methane. Furthermore, using Lagoon Pingo sediments as inoculum, enrichments have brought a methane oxidizing bacterial strain in culture. This novel Methylobacter sp. seems to produce exospores, a feature not previously described for cultured Methylobacter species. The new knowledge provided by this thesis is a fundament for evaluating the bacterial impact on methane evasion from open system pingos. This in turn could be used to improve predictions of the contribution of open system pingos to the global methane budget
Dynamics and role of microorganisms in the deep-sea sunken wood ecosystem
No full textLorsqu’un morceau de bois atteint le fond de l’océan, il provoque la mise en place d’un écosystème capable de se développer en absence de lumière. Cet écosystème est qualifié de chimiosynthétique du fait de la présence d’une faune pouvant fixer le carbone inorganique présent dans l’eau de mer. De plus, ce système attire une faune ultra-specialisée qui utilise des symbiontes bactériens pour digèrer le bois. Avant ces travaux, la plupart des études s’interressaient principalement à la macrofaune et le rôle des microorganismes libres demeurait inconnu. Nous avons pu démontrer dans cette thèse le rôle essentiel que jouent les microorganismes libres dans la mise en place de cet écosystème. Nous avons prouvé que des communautés de microorganismes se succédaient au cours de la première année de colonisation et que cette succession était influencée par le type de bois et l’environment dans lequel il se trouve. La première phase de cette succession aboutit au développement après un mois, d’une population de bactéries sulfato-réductrices produisant de l’hydrogène sulfuré et ce, même en l’absence d’organismes foreurs. Cette production d’hydrogène sulfuré est à la base (1) du développement rapide d’un biofilm chimiolithoautotrophe et (2) du recrutement d’espèces possédants des symbiontes chimiosynthétiques. Nos résultats ont permis d’aboutir à la proposition d’une succession d’étapes clés liées permettant la transformation d’un substrat térrigène en un écosysteme qui, il y a plusieurs millions d’années, aurait permis à la faune chimiosynthétique de coloniser les grands fonds.When wood sinks to the deep-sea floor it creates a new ecosystem that does not depend directly on energy from sunlight. This ecosystem is called chemosynthetic because of the presence of a fauna associated with symbiotic bacteria that can assimilate inorganic carbon from seawater. Furthermore this system is colonized by specialized fauna that use symbiotic bacteria to digest the wood matrix. Previous studies mostly focused on these symbiotic macroorganisms and the role played by non-symbiotic microorganisms in the sunken wood ecosystem remains unknown. We demonstrate in this thesis the important role played by non symbiotic microorganisms during the sunken wood ecosystem establishment. We reveal the ecological succession of microorganisms driven by time and wood structure. The first step of this succession is characterized by a microbial population able to produce hydrogen sulfide after one month of immersion. This hydrogen sulfide production is the basis for (1) a chemolithoautotroph biofilm development on the wood surface and (2) a recruitment of species associated with chemoautotrophic bacteria. Our results suggest a succession of different phases that transform a terrigeneous substrate into an environment that may have helped, million years ago, the colonization of the deep sea by chemosynthetic species
Dynamique et rôle des microorganismes dans l'écosystème bois coulé en milieu profond
Full text linkWhen wood sinks to the deep-sea floor it creates a new ecosystem that does not depend directly on energy from sunlight. This ecosystem is called chemosynthetic because of the presence of a fauna associated with symbiotic bacteria that can assimilate inorganic carbon from seawater. Furthermore this system is colonized by specialized fauna that use symbiotic bacteria to digest the wood matrix. Previous studies mostly focused on these symbiotic macroorganisms and the role played by non-symbiotic microorganisms in the sunken wood ecosystem remains unknown. We demonstrate in this thesis the important role played by non symbiotic microorganisms during the sunken wood ecosystem establishment. We reveal the ecological succession of microorganisms driven by time and wood structure. The first step of this succession is characterized by a microbial population able to produce hydrogen sulfide after one month of immersion. This hydrogen sulfide production is the basis for (1) a chemolithoautotroph biofilm development on the wood surface and (2) a recruitment of species associated with chemoautotrophic bacteria. Our results suggest a succession of different phases that transform a terrigeneous substrate into an environment that may have helped, million years ago, the colonization of the deep sea by chemosynthetic species.Lorsqu’un morceau de bois atteint le fond de l’océan, il provoque la mise en place d’un écosystème capable de se développer en absence de lumière. Cet écosystème est qualifié de chimiosynthétique du fait de la présence d’une faune pouvant fixer le carbone inorganique présent dans l’eau de mer. De plus, ce système attire une faune ultra-specialisée qui utilise des symbiontes bactériens pour digèrer le bois. Avant ces travaux, la plupart des études s’interressaient principalement à la macrofaune et le rôle des microorganismes libres demeurait inconnu. Nous avons pu démontrer dans cette thèse le rôle essentiel que jouent les microorganismes libres dans la mise en place de cet écosystème. Nous avons prouvé que des communautés de microorganismes se succédaient au cours de la première année de colonisation et que cette succession était influencée par le type de bois et l’environment dans lequel il se trouve. La première phase de cette succession aboutit au développement après un mois, d’une population de bactéries sulfato-réductrices produisant de l’hydrogène sulfuré et ce, même en l’absence d’organismes foreurs. Cette production d’hydrogène sulfuré est à la base (1) du développement rapide d’un biofilm chimiolithoautotrophe et (2) du recrutement d’espèces possédants des symbiontes chimiosynthétiques. Nos résultats ont permis d’aboutir à la proposition d’une succession d’étapes clés liées permettant la transformation d’un substrat térrigène en un écosysteme qui, il y a plusieurs millions d’années, aurait permis à la faune chimiosynthétique de coloniser les grands fonds
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
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