Berlin, Humboldt-Univ., Diss., 2003. / Computerdatei im Fernzugriff.

Berlin, Humboldt-Univ., Diss., 2003. / Computerdatei im Fernzugriff.

Berlin, Humboldt-Universiẗat, Diss., 2003.

Regensburg, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Regensburg, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Regensburg, Universiẗat, Diss., 2001.

The majority of power generated worldwide is from combustion of fossil fuels. The sustainability and environmental impacts of this non renewable process are severe. Alternative fuels and power generation systems are needed, however, to cope with increasing energy demands. Ammonia shows promise for use in power generation, however it is costly to produce and very few methods of using it as a fuel are developed. To address the need for alternative methods of ammonia synthesis, this research designed and tested a bioelectrochemical device that generates NH3 through electrode induced enzyme catalysis. The ammonia generating device consists of an electrode modified with a polymer that contains whole cell Anabaena variabilis, a photosynthetic cyanobacterium. A. variabilis contains nitrogenase and nitrate/nitrite reductase, catalysts for the production of ammonia. In this system, the electrode supplies driving force and generates a reductive microenvironment near cells to facilitate enzymatic production of NH3 at ambient temperatures and pressures. Farm animal wastes contain significant amounts of NO2- and NO3-, which can leech into groundwater sources and contaminate them. The system described here recycles NO2- and NO3- to NH4sup+ by the nitrate/nitrite reductase enzyme. Unlike nitrogen fixation by the nitrogenase enzyme whose substrate is atmospheric N2, the substrates for nitrate/nitrite reductase are NO2- and NO3-. The ammonia produced by this system shows great potential as a crop fertilizer. While the substrates and enzymatic basis for ammonia production by nitrogenase and nitrate/nitrite reductase are very different, there is utility in the comparison of commercially produced ammonia by the Haber Bosch synthesis and by the bioelectrocatalytic device described here. In one day, the Haber Bosch process produces 1800 tons of NH3 at an energetic cost of $500/ton. Per ton of ammonia, the Haber Bosch process consumes 28 GJ of energy. The bioelectrocatalytic device produces 1 ton of NH3 for $10/ton, consuming only 0.04 GJ energy, which can be obtained by sunlight via installation of a photovoltaic device. Thus, the system presented here demonstrates ammonia production with significant impact to the economy. NH3 production by the bioelectrocatalytic is dependent upon A. var. cell density and electrode polarization. The faradaic current response from cyclic voltammetry is linearly related to cell density and ammonia production. Without electrode polarization, immobilized A. var. do not produce ammonia above the basal level of 2.8 ± 0.4 ΜM. Ten minutes after cycled potential is applied across the electrode, average ammonia output increases to 22 ± 8 ΜM depending on the mediator and substrate chemicals present. Ammonia is produced by this system at 25 °℃ and 1 atm. The electrochemical basis for enhanced NH3 by immobilized cyanobacteria is complex with multiple levels of feedback.

Ammonia is an important compound to many industries around the world. Most of the fertilizers used by crop growers have ammonia as an essential ingredient. It can also be useful as a fuel source, offering greater energy density per unit than hydrogen and greater safety. Currently, the predominant method for producing ammonia on an industrial scale is by the Haber-Bosch process. This process uses steam evolution of methane to provide H2 gas, which is then combined with N2 gas over an iron catalyst to form NH3. This process requires large amounts of energy as well as high temperatures and pressures. Here, an alternative method for ammonia production is explored. With Anabaena Variabilis, a photosynthetic cyanobacteria, on a carbon electrode, ammonia can be generated at ambient temperatures and pressures at little energy cost, a few tenths of a volt. A bioelectrocatalytic device has been constructed by immobilizing whole cell a. variabilis in a Nafion film modified with a trimethyl octadecyl ammonium bromide (TMODA) salt at an electrode surface [3]. The polymer modified electrode provides the driving force and reductive microenvironment to facilitate production of NH3 by nitrogenase and nitrate/nitrite reductase enzymes present in a. variabilis. Ammonia production by cyanobacteria were increased from basal levels of 2.8 ± 0.4 µM produced over a two week period, to 22 ± 8 µM produced in 20 minutes under mild voltage perturbation, roughly 104% increase in rate. Control of ammonia producing structures (nitrogenase in heterocystic cells or nitrate/nitrite reductase in vegetative cells) can be accomplished by growing the algae with and without fixed sources of nitrogen in the growth media. With the addition of various nitrogen-containing gases to the electrolyte solution during cyclic voltammetry, there is evidence that biofilms containing a mixture of cell types increases ammonia production above controls when the nitrogen is present as NO2-, NO, or N2O. Chronoamperometric perturbation studies show increased ammonia production at near +600 mV and -300 mV vs SCE. In cyclic voltammetric studies, nitrate/nitrite reductase in vegetative-only biofilms responds favorably to positive voltage ranges, while isolated heterocyst biofilms containing nitrogenase can be effectively targeted with the application of a negative voltage profile. References: [1] Johna Leddy and Timothy M. Pashkewitz, Ammonia Production Using Bioelectrocatalytic Devices, US Patent Application 20140011252 [2] Timothy M. Paschkewitz, Ammonia Production at Ambient Temperature and Pressure: An Electrochemical and Biological Approach, Ph.D., University of Iowa, 2012.

Univ., Diss.--Regensburg, 1992.

L’azote est un des éléments les plus importants dans la nature. Sa disponibilité limite la productivité d’un grand nombre d’écosystèmes naturels, et influencera sans doute de manière importante leurs réponses aux changements climatiques globaux. La première source d’azote dans les écosystèmes non anthropisés est la fixation biologique de l’azote. Ce processus repose sur un groupe de métallo-enzymes spécifiques, les nitrogénases, dont le cofacteur métallique contient soit du fer et un atome de molybdène, soit du fer et un atome de vanadium, soit uniquement du fer. A ce jour, seule la nitrogénase au molybdène est prise en considération dans la dynamique de l’azote dans les écosystèmes, et ce malgré de nombreux indices indiquant que la nitrogénase au vanadium pourrait avoir un rôle important. Est-ce que la nitrogénase au vanadium est utilisée dans les écosystèmes naturels et quelles sont les conditions favorisant son utilisation ? Nous avons cherché à répondre à ces questions à l’aide d’un modèle symbiotique tripartite, un lichen, association entre une algue, un champignon et une cyanobactérie fixatrice d’azote. Nous avons tout d’abord développé une méthode d’étude des contenus en métaux des différents symbiontes, puis nous avons étudié la répartition et la régulation du vanadium au sein des différents symbiontes dans différentes conditions environnementales. Nous avons pu démontrer que dans ce modèle, le vanadium possède toutes les caractéristiques d’un micronutriment essentiel à la fixation d’azote. Nous avons également démontré que la disponibilité du molybdène ainsi que les températures, telles que rencontrées en milieux boréaux, seraient deux facteurs importants contrôlant l’utilisation de la V-Nase. Les résultats présentés dans cette étude apportent une meilleure compréhension de la gestion des métaux cofacteurs de la nitrogénase au sein de la symbiose lichénique. Mais ils permettent surtout de remettre en question le paradigme de l’hégémonie du molybdène sur la fixation biologique de l’azote. Ainsi, la fixation d’azote en milieu continental repose sur un ensemble hétérogène d’enzymes, ce qui autorise aux organismes fixateurs d’azote une grande flexibilité vis-à-vis des paramètres environnementaux comme les basses températures. Cela leurs permet également une meilleure adaptation au stress métallique résultant de carences en micronutriments, notamment celle en molybdène. Ces résultats invitent également à réévaluer les modèles biogéochimiques liant les cycles des micronutriments aux cycles des macronutriments, particulièrement celui de l’azote.