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A model describing the potential amino acid ligands to the four 4Fe-4S centers (P-clusters) within the Azotobacter vinelandii nitrogenase MoFe protein is presented. Based on interspecies and intersubunit amino acid comparisons of the α- and ß-subunits of the MoFe protein, and the FeMoco biosynthetic proteins, NifE and NifN, four conserved residues (Cys62, His83, Cys88, Cys154 all proposed P-cluster ligands) within the α- subunit were targeted for site-directed mutagencsis studies. In order to define a range of acceptable substitutions, 35 specific site-mutants have been constructed, each with a different amino acid replacement at one of the four targeted positions. Previous studies indicated that these residues were important for MoFe activity, and may act as metallocenter ligands. Unusual redox and spectroscopic properties of the Fe-S centers suggest the involvement of ligands other than the four typical cysteines, though extrusion requirements indicate that some thiol ligands are likely. Surprisingly, mutants with an Asp, Gly, Thr, or Ser substituted for Cys88 are still capable of diazotrophic growth (Nif+), though whole cell and crude extract acetylene reduction activity is lowered. Several substitutions (Cys, Asp, Phe, Asn, Met, Tyr, Leu) are tolerated at the His83 position, these Nif+ mutant strains also have varying acetylene reduction rates and growth rates. All mutants with substitutions at positions 62, 154, resulted in complete loss of diazotrophic growth. The results could be interpreted by the following explanations: 1) Our proposed model for the P-cluster ligation within the MoFe protein is incorrect. 2) Some substitutions permit P-cluster rearrangement to a semi-functional state. 3) Either, P-clusters are not absolutely essential for diazotrophic growth, or the enzyme can function with a reduced number of these metal centers. / Master of Science / incomplete_metadata

Young mine soils constructed following surface-mining for coal contain low levels of organic matter and nitrogen. It was hypothesized that nitrogen-fixing plants could be incorporated into a reforestation system in a manner that would meet the short term and long-term nitrogen needs of crop trees while rebuilding the soil and litter nitrogen pools and restoring a stable nitrogen cycle. The nitrogen status of two interplanting studies containing legume ground covers was examined. The first study site was mined prior to 1940 and subsequently abandoned. In 1988, the land was reclaimed, and an interplanting study was established in 1990. Pitch x loblolly pines (Pinus x rigitaeda) and eastern white pines (Pinus strobus L.) were interplanted with four nitrogen-fixing species: black alder (Alnus glutinosa L. IGaertn.l), black locust (Robinia pseudoacacia L.}, bicolor lespedeza (Lespedeza bicolor) and autumn olive (Elaeagnus umbel/ata Thwnb.). Pitch x loblolly pine survival was good; however, eastern white pine survival was only fair, averaging 54%. All of the nurse tree species with the exception of black alder had fair survival rates. Black alder survival was extremely low at 19%. After the third growing season, a ground cover dominated by legumes averaged 79% cover. The black mine spoil on the site was extremely rich in nitrogen, averaging 5,115 mg·kg-1. When the black mine spoil was mixed with the native topsoil, a mine soil that was created had an extremely variable total nitrogen content. However, low mineralizeable nitrogen levels suggested that the nitrogen in the mine spoil was not generally available. Pine responses to the four nitrogen-fixing species were compared. Nurse trees had no effect on pine growth after three years. Pine foliar nitrogen levels were adequate across the study site even in the control treatments and in the microsites where few legumes were present. Microsites surrounding 50 pines of each species were studied. The pitch x loblolly pines grew larger at lower pH levels and higher coarse fragment content reflecting the pines' preference for moderately acid soils, coarse-textured soils. Some competition was present between the crop trees and herbaceous cover, but it was not a controlling factor in pine tree growth. / Master of Science

Master of Science

L'énergie solaire est la source d'énergie la plus abondante au monde et elle peut être convertie en énergie chimique via des processus photocatalytiques. Au cours des dernières décennies, la photocatalyse sous la lumière du soleil est apparue comme une alternative innovante aux combustibles fossiles afin de résoudre et prévenir des problèmes graves liés à la crise environnementale et énergétique. Actuellement, les matériaux à base de semi-conducteurs (tels que TiO₂, C₃N₄, In₂O₃, WO₃) sont intensivement étudiés pour diverses applications photocatalytiques, y compris la réaction d’évolution d'hydrogène (HER) et la réduction de l'azote en ammoniac (NRR). Par conséquent, diverses approches telles que l'ingénierie structurelle, les hétérojonctions nanocomposites ont été étudiées afin de surmonter les problèmes de ces matériaux et ainsi augmenter l'activité catalytique. Dans le cadre de cette thèse, nous avons développé des nouvelles stratégies pour la synthèse des quatre types de photocatalyseurs efficaces pour la production d'hydrogène et la fixation de l'azote sous la lumière du soleil. Nos matériaux présentent une structure unique, qui favorise l'absorption de la lumière visible, la séparation des charges électrons-trous et l’augmentation du nombre de sites actifs.Pour l'application de la génération d'hydrogène photocatalytique, nous avons d'abord synthétisé les sphères de type éponge CdI₂nS₄ monophasées via une méthode solvothermique suivie d'un traitement au gaz contenant H₂S. La formation du complexe Cd/In avec une distribution uniforme de Cd²⁺ et In³⁺ a joué un rôle crucial dans la formation du spinelle monophasé CdIn₂S₄. L'énergie de la bande interdite s'est avérée être significativement réduite, ce qui permet une absorption étendue de la lumière visible jusqu'à 700 nm, ceci est principalement attribué à la dispersion d'espèce sulfure sur la bande de valence du CdIn₂S₄ monophasé. Avec le dépôt de Ni métallique comme cocatalyseur de réduction, le photocatalyseur hybride Ni-CdIn₂S₄ a montré une efficacité améliorée pour la production d'hydrogène sous la lumière solaire, ce qui représente une augmentation de l’activité d’environ, respectivement, 5,5 et 3,6 fois que celle des échantillons Pt-CdIn₂S₄ et Pd-CdIn₂S₄. Le deuxième système photocatalytique développé implique la préparation de nitrure de carbone graphitique dopé au S (Ni-SCN). Ce dernier est chimiquement ancré au nickel par une technique connue sous le nom de processus de photo-dépôt assisté par sulfuration. L'origine de la structure distinctive du Ni-SCN est dû à l'existence de liaisons chimiques NiS-C-N dans le système, ce qui favorisait la séparation des charges photogénérées et améliorait la capacité d’absorption lumineuse du photocatalyseur. Par conséquent, l’échantillon NiSCN synthétisé présente une excellente activité photocatalytique pour la production d'hydrogène sous la lumière du soleil. En effet, ce système présente une activité beaucoup plus élevée que celle des systèmes g-C₃N₄ dopés au S, Ni supporté g-C₃N₄ et Pt supporté g-C₃N₄ dopés au S. Pour une application photo (électro) catalytique de fixation de l'azote, nos travaux sont les premiers à rapporter la synthèse de nanoparticules d'Au chargées de nanoparticules W₁₈O₄₉ dopées au Fe (notées WOF-Au) par une synthèse solvothermique suivie d'un dépôt in situ des nanoparticules d'Au. L'incorporation de dopants Fe peut non seulement guérir les états de défaut de masse dans les réseaux non stœchiométriques W₁₈O₄₉, mais également favoriser la séparation et la migration interfaciale des électrons du photocatalyseur vers les molécules N₂ chimisorbées; tandis que les nanoparticules Au décorées sur la surface dopée au Fe W₁₈O₄₉ ont fourni les électrons à haute énergie pour la réduction de N₂ via l'effet de résonance plasmonique de surface localisé (LSPR). Le système WOF-Au plasmonique résultant montre un rendement amélioré pour la production de NH₃, beaucoup plus élevé que celui du W₁₈O₄₉ pur ainsi qu'une très grande stabilité. L'amélioration des performances photoélectrocatalytiques est principalement due à l'effet synergique des dopants Fe et des nanoparticules Au dans l'hôte W₁₈O₄₉. Enfin, les cacahuètes creuses de In₂O₃ dopées au Ru (dénotées Ru-In₂O₃ HPN) ont été fabriquées par la nouvelle stratégie d'auto-matrice suivie de la calcination des précurseurs synthétisés. Les nanoparticules uniformes In₂O₃ sont étroitement agglomérées ensemble pour former une structure de cacahuète creuse, ce qui facilite la séparation et le transport des l'électrons-trous photoexcités, améliorant l’absorption de la lumière par multi-réflexion. De plus, l'introduction des dopants Ru induit de nombreuses lacunes en oxygène à la surface et réduit l'énergie de la bande interdite du système photocatalytique. Ces lacunes d'oxygène agissent comme des centres de piégeage, facilitant la séparation des électrons trous photoexcités. Par conséquent, le taux de production d'ammoniac des Ru-In₂O₃ HPNs est 5,6 fois plus élevé que celui des In₂O₃ HPNs purs et largement supérieur au matériau en vrac d'In₂O₃, lorsqu’il est soumis à l’irradiation solaire. / Solar energy is the most abundant energy source in the world, and it can be converted into chemical energy via photocatalytic processes. Over the last decades, sunlight-driven photocatalysis has emerged as an innovative alternative to fossil fuels for solving the severe problems related to environmental diseases and the energy crisis. Currently, semiconductorbased materials (such as TiO₂, C₃N₄, In₂O₃, WO₃, BiVO₄) have been intensively studied for diverse photocatalytic applications, including the hydrogen evolution reaction (HER) and the nitrogen reduction reaction (NRR) to produce ammonia. However, the drawbacks of weak visible light absorption, low electron-hole separation with high recombination rate, and lack of surface active-sites have limited the photocatalytic performance of these semiconductorbased photocatalysts. Therefore, various approaches such as structural engineering, nanocomposite heterojunctions have been applied to overcome the limitations of these materials and boosting the catalytic activity. In this thesis, we employed novel strategies to develop four efficient photocatalytic systems for hydrogen production and nitrogen fixation. Our materials possessed a unique structure, which is advantageous to promote the visiblelight absorption, facilitate the separation of charge carrier, and increase the number of surface-active sites. For the photocatalytic hydrogen evolution application, we firstly synthesized the singlephase CdIn₂S₄ sponge-like spheres via solvothermal method followed by H₂S gas treatment. The formation of CdIn-complex with uniform distribution of Cd²⁺ and In³⁺ played a crucial role in achieving the spinel structured-single phase CdIn₂S₄. The bandgap energy was found to be significantly reduced, resulting in the extended visible light absorption up to 700 nm, which was primarily attributed to the sulfide species-mediated modification of the valence band in CdIn₂S₄ single-phase. With the deposition of Ni metal as a reduction cocatalyst, the hybrid Ni-CdIn₂S₄ photocatalyst showed enhanced solar light-driven photocatalytic hydrogen evolution efficiency, which is around 5.5 and 3.6 folds higher than that of Pt-CdIn₂S₄ and Pd-CdIn₂S₄ samples, respectively. The second developed photocatalytic system involved the preparation of chemically bonded nickel anchored S-doped graphitic-carbon nitride (Ni-SCN) through a technique known as sulfidation assisted photo-deposition process. The origin of the distinctive structure of Ni-SCN was due to the existence of Ni-S-C-N chemical bonds in the system, which fundamentally favored the separation of photogenerated electron-hole and improved the light-harvesting capabilities of the photocatalyst. Consequently, the synthesized Ni-SCN exhibited an excellent sunlight-driven photocatalytic activity toward hydrogen evolution, which was several times higher than Sdoped g-C₃N₄, Ni supported g-C₃N₄ and Pt loaded S-doped C₃N₄ systems. For photo(electro)catalytic nitrogen fixation application, our work is the first to report the synthesis of Au nanoparticles loaded Fe doped W₁₈O₄₉ (denoted as WOF-Au) nanorods through a solvothermal synthesis following by in situ deposition of Au nanoparticles. The incorporation of Fe dopants can not only heal the bulk-defect-states in nonstoichiometric W₁₈O₄₉ lattices but also promote the separation and interfacial migration of electrons from photocatalyst to chemisorbed N₂ molecules; while Au nanoparticles decorated on the Fe doped W₁₈O₄₉ surface provided the high energetic electrons for N₂ reduction via the localized surface plasmon resonance effect (LSPR). The obtained plasmonic WOF-Au system shows an enhanced NH₃ yield, which is much higher than that of the bare W₁₈O₄₉, as well as very high stability. The enhancement in photoelectrocatalytic performance is mainly contributed by the synergetic effect of Fe dopants and plasmonic Au nanoparticles on the W₁₈O₄₉ host. Lastly, Ru doped In₂O₃ hollow peanuts (demoted as Ru-In₂O₃ HPNs) were fabricated by the novel self-template strategy followed by the calcination of the as-synthesis precursors. The uniform In₂O₃ nanoparticles were closely packed together to form a hollow peanut structure, which facilitated the separation and transportation of photoinduced electron-hole and favored the light-harvesting ability by the internal multi-reflection process. Furthermore, the introduction of Ru dopants induced numerous surface oxygen vacancies and narrow down the bandgap energy of the photocatalytic system. These oxygen vacancies act as trapping centers, facilitating the separation of photoexcited electrons and holes. Consequently, the ammonia production rate of Ru-In₂O₃ HPNs was 5.6 times and much higher as compared to pure In₂O₃ HPNs and bulk material of In₂O₃ under solar light irradiation.

Made available in DSpace on 2014-06-11T19:22:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-07-27Bitstream added on 2014-06-13T19:48:25Z : No. of bitstreams: 1 ferrarineto_j_me_botfca.pdf: 394206 bytes, checksum: 6b64f3f5824ba5c1c97d98a5556b8a6a (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Áreas que vem sendo cultivadas com pastagens por vários anos podem apresentar deficiências nutricionais e a população de bactérias fixadoras do nitrogênio atmosférico pode estar reduzida. O molibdênio é um micronutriente que faz parte da enzima nitrogenase, a qual é responsável pela fixação simbiótica do nitrogênio nas leguminosas, e está presente nos bacteróides nodulares. Com base nisso, o presente trabalho teve por objetivo avaliar o efeito da aplicação de molibdênio e inoculante, na cultura do amendoim em semeadura direta sobre palhada de forrageiras. Foram conduzidos quatro experimentos, dois no ano agrícola 2008/09 e dois no ano agícola 2009/10, onde a cultura do amendoim foi semeada, respectivamente, sobre as palhadas da Brachiaria brizantha, Brachiaria ruziziensis, Brachiaria brizantha e Brachiaria decumbens no delineamento de blocos casualizados, em esquema de parcelas subdivididas, com quatro repetições. As parcelas dos experimentos foram constituídas por dois tratamentos, com e sem aplicação de inoculante (1- inoculação artificial com estirpe específica de bactéria Bradyrhizobium spp. para a cultura do amendoim e 2- sem inoculação artificial) e as subparcelas dos experimentos do ano agrícola 2008/09 por três doses de molibdênio (27,5, 55, 110g ha-1) aplicadas via tratamento de sementes, na forma de molibdato de amônio e a testemunha. As subparcelas dos experimentos do ano agrícola 2009/10 constituíram-se de três doses de molibdênio (50, 100, 200g ha-1) aplicadas via foliar, na forma de molibdato de amônio e a testemunha. Foram avaliados o estado nutricional das plantas, a nodulação (número e matéria seca de nódulos por planta), a atividade da enzima nitrogenase pelo método da redução do acetileno, a atividade específica da nitrogenase, os componentes da produção... / The areas that have been cultivated with pasture for several years may have nutritional deficiencies and the population of atmospheric nitrogen fixing bacteria may be reduced. Molybdenum, a nutrient that is part of the nitrogenase enzyme, is responsible for symbiotic nitrogen fixation in legumes and is present in bacteria from the nodules. This study aimed at evaluating the effect of molybdenum and inoculant application, in peanut crop sown on a straw of forages. Four experiments were carried out, two in 2008/09 crop year and two in 2009/10 crop year, where peanut crop was directly sown, respectively, on the straws of Brachiaria brizantha, Brachiaria ruziziensis, Brachiaria brizantha and Brachiaria decumbens in the design of randomized blocks in a split-plot with four replications. The plots comprised two treatments, with and without inoculant application (1 - artificial inoculation with specific strain of Bradyrhizobium spp. for peanut crop and 2 - without artificial inoculation) and the subplots of the experiments carried out in the 2008/09 crop year in three molybdenum rates (27,5, 55 e 110 g ha-1) applied in the form of ammonium molybdate by seeds treatment and a control. The subplots of the experiments carried out in the 2009/10 crop year comprised three molybdenum rates (50, 100 e 200 g ha-1) applied as ammonium molybdate by foliar spraying and a control. The nutritional status of plants, nodulation (number and mass of dry matter of the nodules per plant), the activity of nitrogenase enzyme by acetylene reduction method, the specific activity of nitrogenase, yield components and the yield of pods and grains were evaluated. The data of each experiment were subjected to analysis of variance following the split plot model. To compare the average of inoculation, the method of comparison of unprotected average... (Complete abstract click electronic access below)

Nitrogen fixation is no longer considered to be a minor factor of the nitrogen cycle in oceanic ecosystems. Recent geochemical and biological efforts have led to a significant increase in the estimated input of nitrogen to marine ecosystems by biological fixation, while molecular studies have increased our knowledge of the number and diversity of nitrogen fixers known to be active in the ocean. Although Trichodesmium spp. have long been viewed as the primary marine nitrogen fixers, recent efforts have shown that various members of the picoplankton community are also actively involved in nitrogen fixation. The relative abundance of different nitrogen fixers is an important ecosystem parameter since nitrogen fixers may differ significantly in their physiology, life history and ecology. Here we combine rate measurements and stable isotope natural abundance measurements to constrain the impact of N2 fixation in the waters north of Australia. Samples were collected in the Coral, Arafura, and East Timor Seas, thus spanning three distinct hydrographic regions. Our data show that Trichodesmium has a significant influence on the stable nitrogen isotope ratios of particulate and zooplankton biomass and suggest that Trichodesmium is a significant source of nitrogen for the pelagic ecosystem. Based on stable carbon isotope ratios, it is also likely that the pathways are indirect and nitrogen fixed by Trichodesmium enters the higher trophic levels via decomposition as dissolved organic and inorganic nitrogen. Picocyanobacteria showed high diazotrophic activity at some stations, but unlike Trichodesmium, their N2 fixation rate was not reflected in the stable N isotope ratios of particulate and zooplankton biomass. Our results suggest an important N contribution to biomass by diazotrophs in the Coral Sea, Arafura Sea and East Timor Sea.

Nitrogen fixation is an ecologically important microbial process that can contribute bioavailable combined N to habitats low in N. Shipworms, or wood-boring bivalves, host N2-fixing and cellulolytic symbiotic bacteria in gill bacteriocytes, which have been implicated as a necessary adaptation to an N-poor C-rich (wooden) diet. Shipworm symbionts are known to fix N within the gill habitat and newly fixed N is subsequently incorporated into non-symbiont containing host tissue. The presence of N2-fixation in gill bacteriocytes presents a conundrum because N2-fixation is tightly regulated by oxygen in most other diazotrophic microbes. Also, the direct evidence of new N being incorporated into the host tissue indicates that there are potentially complex nutrient cycles in this symbiosis, which have not been investigated. We used the cultivated symbiont Teredinibacter turnerae, which has been isolated from many shipworm species, as a model organism to elucidate controls on N2-fixation and N release in the shipworm symbiosis. Our results indicate that headspace oxygen concentration does not control biomass specific N2-fixation and respiration activity in T. turnerae, but it does influence the magnitude of the growth rate and timing of culture growth. Also, we examined the controls of oxygen on inorganic nutrient uptake rates, and documented a small amount of dissolved inorganic nitrogen release. While the N budget is only partially balanced, we provide indirect evidence for the allocation of fixed N to the excretion of exopolymeric substances and dissolved organic nitrogen; future studies that measure these additional N sinks are necessary to close the N budget. Although there are limitations of using pure cultures to investigate a complex symbiotic system, this study provides direct experimental evidence that T. turnerae has adaptations that are conducive to N2-fixation in gill bacteriocytes.

[Truncated abstract] Renewed interest in the fixation of fractures using plates has been stimulated by an improved understanding of the biology of fracture healing and a drive towards minimally invasive surgery. This has led to a change in the way we use plates nowadays and the way in which we build the bone-plate construct, as well as the development of new implants better suited to these techniques. As a result of this, we have now the potential to safely expand the indications for plate fixation especially in the management of fractures in osteopenic bone. This thesis provides scientific evidence allowing for better formulation of the optimum way to use the modern plating systems in the clinical setting. Biological fracture repair with conventional plates, in terms of a less rigid construct to enhance fracture healing, is becoming increasingly popular. By omitting screws the construct becomes more flexible with a risk of fixation failure. It was the aim of the first paper to investigate in an experimental model the construct strength of different conventional plate lengths and number / position of the screws, and if an oblique screw at the plate end could increase the fixation strength. Our data suggest that the plate length is the most important factor in withstanding forces in cantilever bending. Longer plates with an equal number of screws require greater peak loads to failure than short plates with more screws. Furthermore, an oblique screw at the plate end produces an increased strength of fixation in all different test setups. However, the difference is more significant in shorter plates and in constructs with no screw omission adjacent to the fracture site. ... Following cyclic loading, however, locking plates can better retain fracture reduction compared to compression plates. On the other hand, under torsional load the compression plate appears to be biomechanical superior to the locking system. In supracondylar comminuted femur fractures, combining the two principles results in less plastic deformation, and a higher load to failure compared to their single application. The last two papers examine the behaviour of locking plates in osteopenic bone. In cadaveric intra-articular calcaneal fractures, the locking plate showed a significantly lower irreversible deformation during cyclic loading and a significantly higher load to failure. In dorsal and volar fixed angle distal radius constructs in a cadaveric model, all constructs showed adequate stability with minimal deformation on fatigue testing under physiological conditions in good bone quality. In osteoporotic bone, however, dorsal fixed angle constructs are stiffer and stronger than volar constructs. The addition of a styloid plate to a volar plate does not significantly improve stability.

Thesis equivalent (M.P.H.)--University of Michigan, 1947.