1,720,984 research outputs found
Directing product formation by mixed culture fermentation
No full textOur society is dealing with the shortage of fossil fuels and chemical feedstocks and, on the other hand, with the increase of wastes generated by municipalities, agriculture and industries. Biobased industry, the conversion of renewable resources or wastes to chemicals and fuels, by microbial fermentations or enzymes has been receiving increasingly attention. The aim is to develop new technologies, increase efficiencies and reduce the costs in fermentation, bioconversion, and in downstream processing. A great challenge is to develop fermentation technology that employs the underutilized biomass residues, often considered as wastes. The use of open mixed cultures can be an option to convert these waste streams into bulk chemicals. Based on natural inocula with a high microbial diversity, open mixed cultures are capable of dealing with substrate mixtures of variable composition under non-sterile conditions. Despite the high number of species, selection occurs for a limited number of microorganisms that best adapt to the imposed conditions. The big challenge of mixed culture biotechnology is to design the operational conditions that will select for the desired metabolic conversion. The aim of this project was to study the impact of the operational conditions on mixed culture fermentation: on the product spectrum and on the microbial population established. The variables studied were: pH, carbon source, different influent substrate concentrations and mixtures of substrates. To this end, experiments were performed in a continuous operated bioreactor; and the results were interpreted considering the available knowledge on pure culture fermentation and theoretical predictions, based on thermodynamic calculations.Applied Science
Organics on Mars : Laboratory studies of organic material under simulated martian conditions
Full text linkThe search for organic molecules and traces of life on Mars has been a major topic in planetary science for several decades, and is the future perspective of several missions to Mars. In order to determine where and what those missions should be looking for, laboratory experiments under simulated Mars conditions have been performed. This thesis describes the effects of simulated martian surface conditions on organic material (amino acids) and living organisms (halophilic archaea). Experiments have been performed to study the stability of thin films of glycine and alanine against UV irradiation under different conditions. Thin films of glycine and alanine have a half-life of 22 ± 5 hours and 3 ± 1 hours, respectively, when extrapolated to Mars-like UV flux levels in vacuum. The presence of a 7 mbar CO2 atmosphere does not affect these destruction rates. Cooling the thin films to 210 K (average Mars temperature) lowers the destruction rate by a factor of 7. The intrinsic amino acid composition of two martian soil analogues, JSC Mars-1 and Salten Skov, has been investigated. The results demonstrated that these analogues are inappropriate for a life-science study in their raw state. Besides amino acids, the response of the halophilic archaea Natronorubrum sp. strain HG-1 to Mars-like conditions, such as low pressure, UV radiation and low temperatures, has been studied. From the results we concluded that this strain would not be a good model organism to survive on the surface of Mars.UBL - phd migration 201
Growth and metabolism of Anammox Bacteria
No full textThe anoxic ammonium oxidation (anammox) process is the conversion of nitrite and ammonium under anoxic conditions- to form dinitrogen gas. The process is performed by deep-branching Planctomycetes. The startup of the first full-scale anammox reactor in the world is described in Chapter 2. The described full scale reactor was a granular sludge reactor which was optimized for biomass retention. The reactor was scaled up directly from lab-scale to full-scale without the intermediate step of a pilot plant- and the step from lab-scale to full-scale took three years. In the first phase of the startup, quantification of the number of anammox bacteria, which were present in the reactor by quantitative polymerase chain reaction (Q-PCR) was a reliable indicator of growth of the anammox bacteria. The volumetric conversion of 10 kg N/m3/day is high compared to lab-scale systems. In Chapter 3, anammox bacteria were grown as free suspended (planktonic) cells. Even at a Sludge Retention Time (SRT) of 12 days (doubling time 8.3 days) stable operation was possible. The purity of the biomass was estimated to be 97.6%, which was the highest level of enrichment ever achieved for anammox reactors. The addition of hydroxylamine and the subsequent transient production of hydrazine can be regarded as a benchmark for the anammox process. In Chapter 4, the kinetics of the conversion were studied in detail for "Kuenenia stuttgartiensis". Hydrazine accumulated slightly after addition of hydroxylamine and remained low until near completion of the hydroxylamine. At that moment, the hydrazine level suddenly rose to ca. 100 μM, after which it gradually disappeared. The overall reaction was a disproportionation of hydroxylamine into ammonium and dinitrogen gas. The observed sudden accumulation of hydrazine could only be explained by assuming that hydrazine was an intermediate in this process. Two simple mathematical models, based on the continuous turn over of hydrazine during hydroxylamine conversion, were capable of quantitatively explaining the observed phenomena. The production of nitric oxide, another potential intermediate in the anammox process, was studied in combination with the emission of greenhouse gas nitrous oxide (N2O) in a full scale two reactor nitritation anammox process in Chapter 5. The NO and N2O emissions in the nitritation reactor were 0.2% and 1.7% of the nitrogen load respectively and 0.003% and 0.6% for the anammox reactor. The NO emission in the nitritation reactor was higher at higher aeration flows and NO seemed to be produced mainly in the period when the nitritation reactor was aerated. The N2O emission on the other hand seemed to be mainly produced during anoxic periods. Anammox bacteria have a unique cell plan consisting of several membrane-surrounded compartments. In the main compartment, the anammoxosome, the anammox catabolism is hypothesized to take place. Therefore, also the proton motive force is probably generated between the anammoxosome and the riboplasm by which the anammoxosome is surrounded. 31P nuclear magnetic resonance (NMR) spectroscopy was used to evaluate the pH difference over the anammoxosome membrane of "Kuenenia stuttgartiensis" in vivo in Chapter 6. Two compartments with stable pH values of 6.3 and 7.3 respectively were found in actively converting cells. The pH values were independent of the external pH and were visible already upon exposing the cells to anoxic conditions. The lower pH value was assigned to the anammoxosome, whereas the pH of 7.3 was assigned to the riboplasm. The stability of the pH in both compartments is a strong indication that the anammoxosome is the locus of the catabolism and thus functionally resembles the eukaryotic mitochondrion.Applied Science
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
Exploring and exploiting the mechanism of mycelial pellet formation by Streptomyces
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Streptomyces are multicellular bacteria that grow as branched filaments and are best known for producing the majority of our antibiotics, many immunosuppressant and anticancer compounds. Unfortunately their multicellular life style creates many problems for efficient industrial production. In a bioreactor, depending on the environment and the genetics, it can grow quickly as dispersed mycelia or aggregate in slow growing pellets. Either morphology has advantages and disadvantages, which can be product specific.
For my thesis I studied the mechanism by which these filaments can aggregate into dense pellets. I found a small gene cluster that produces poly-1,6-N-acetylglucosamine, a bacterial glue which binds neighboring cells and required for pellet formation in S. coelicolor. Subsequently we can use these genes to control the morphology of streptomycetes in a liquid environment, tailoring it for production.
My work has given us new insights in the mechanims through which streptomycetes aggregate, but also has the potential to make streptomycetes a more favorable host for industial production.NWO STW VICI grant 10379 to GP van WezelMicrobial Biotechnolog
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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