1,720,956 research outputs found
Integrated bioprocess for conversion of gaseous substrates to liquids
No full textIn the quest for inexpensive feedstocks for the cost-effective production of liquid fuels, we have examined gaseous substrates that could be made available at low cost and sufficiently large scale for industrial fuel production. Here we introduce a new bioconversion scheme that effectively converts syngas, generated from gasification of coal, natural gas, or biomass, into lipids that can be used for biodiesel production. We present an integrated conversion method comprising a two-stage system. In the first stage, an anaerobic bioreactor converts mixtures of gases of CO₂ and CO or H₂ to acetic acid, using the anaerobic acetogen Moorella thermoacetica. The acetic acid product is fed as a substrate to a second bioreactor, where it is converted aerobically into lipids by an engineered oleaginous yeast, Yarrowia lipolytica. We first describe the process carried out in each reactor and then present an integrated system that produces microbial oil, using synthesis gas as input. The integrated continuous bench-scale reactor system produced 18 g/L of C16-C18 triacylglycerides directly from synthesis gas, with an overall productivity of 0.19 g⋅L⁻¹⋅h⁻¹ and a lipid content of 36%. Although suboptimal relative to the performance of the individual reactor components, the presented integrated system demonstrates the feasibility of substantial net fixation of carbon dioxide and conversion of gaseous feedstocks to lipids for biodiesel production. The system can be further optimized to approach the performance of its individual units so that it can be used for the economical conversion of waste gases from steel mills to valuable liquid fuels for transportation.United States. Advanced Research Projects Agency-Energy (DE-AR0000059)United States. Department of Energy (DE-SC0008744
Improving formaldehyde consumption drives methanol assimilation in engineered E. coli
Get PDFDue to volatile sugar prices, the food vs fuel debate, and recent increases in the supply of natural gas, methanol has emerged as a promising feedstock for the bio-based economy. However, attempts to engineer Escherichia coli to metabolize methanol have achieved limited success. Here, we provide a rigorous systematic analysis of several potential pathway bottlenecks. We show that regeneration of ribulose 5-phosphate in E. coli is insufficient to sustain methanol assimilation, and overcome this by activating the sedoheptulose bisphosphatase variant of the ribulose monophosphate pathway. By leveraging the kinetic isotope effect associated with deuterated methanol as a chemical probe, we further demonstrate that under these conditions overall pathway flux is kinetically limited by methanol dehydrogenase. Finally, we identify NADH as a potent kinetic inhibitor of this enzyme. These results provide direction for future engineering strategies to improve methanol utilization, and underscore the value of chemical biology methodologies in metabolic engineering
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
Enabling Cl-Based bioconversion with metabolic engineering
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, May, 2017Cataloged from the official PDF of thesis.Includes bibliographical references (pages 235-261).Single-carbon (C) substrates, such as synthesis gas and methanol, are attractive feedstocks for biochemical processes, as they are widely available, can be produced renewably, and do not compete with food supply. However, their use in industrial bioprocessing remains limited, primarily because microbes that utilize these substrates are poorly characterized biochemically, and limited tools exist for their genetic modification. This leaves the metabolic engineer with a choice: to develop genetic tools to enable engineering in the desired host, or to import the relevant catabolic pathway into a more tractable organism, such as Escherichia coli. This thesis explores both options within the context of developing strains for the conversion of C1 substrates into value-added chemicals and fuels.Clostridium ljungdahlii is an acetogen that grows autotrophically on synthesis gas (CO, H₂ , and CO₂) using the Wood-Ljungdahl pathway, and is a promising candidate for non-photosynthetic CO₂ fixation. In the first section, we extended its primitive genetic tools by developing a CRISPRi system for the targeted knockdown of specific genes. Constitutive downregulation of several genes with putative roles in energy conservation and carbon flux by up to 30-fold was demonstrated, and the associated phenotypes analyzed. Optimization of the promoter controlling dCas9 expression allowed for inducible knockdown, paving the way for dynamic metabolic control strategies to redirect carbon flux in engineered strains. To demonstrate this concept, several variants of a heterologous pathway for the biosynthesis of 3-hydroxybutyrate (3HB) were constructed, to probe 3HB production in the wild-type background and with various CRISPRi plasmids.The CRISPRi system represents a valuable contribution to the metabolic engineering field for its ability to redirect carbon flux, and is also useful to the microbiology community to probe gene function to answer open questions in the biochemistry underlying the Wood-Ljungdahl pathway. Efforts to develop genetic tools for the related acetogen Moorella thermoacetica are also described. To explore the alternative approach of importing a single-carbon catabolic pathway into a tractable host, in the second section E. coli was engineered to metabolize methanol. Screening various candidates of the three heterologous pathway enzymes enabled robust incorporation of 3 C-labeled methanol into central carbon metabolism. To further improve methanol assimilation, a kinetic-thermodynamic modeling framework was developed and combined with novel isotopic tracing experiments to probe potential pathway limitations.Flux leakage from the cyclical ribulose monophosphate (RuMP) pathway was identified as the primary bottleneck, as this led to the build-up of the toxic intermediate formaldehyde and ablation of the thermodynamic driving force for methanol oxidation. Strategies were developed to re-wire central metabolism accordingly, which restored the driving force and led to the identification of the kinetics of the first enzyme of the pathway - methanol dehydrogenase (MDH) - as the next limitation. These results represent the first systematic analysis of flux limitations in E. coli engineered for methanol metabolism, and provide clear targets for further metabolic engineering to enable synthetic methylotrophy. Finally, the development of a formaldehyde biosensor in support of evolutionary approaches to enhance MDH activity and partition carbon flux between formaldehyde assimilation and growth is described.by Benjamin Michael Woolston.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Chemical Engineerin
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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