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The results of this study suggest that NO^ uptake in many (but not all) species of marine phytoplankton is mediated by a membrane-bound (NO⁻₃, Cl⁻)-activated adenosine triphosphatase. In the presence of NO⁻₃ and Cl⁻, semi-purified membrane preparations exhibit enhanced adenosine triphosphatase activity. The enzyme has characteristics common to other membrane-bound proteins: a break in the Arrhenius plot of 30.9 Kcal/mole at 2.9 C, parallel purification with the (Na⁺ + K⁺)-activated transport adenosine triphosphatase, and activation of catalytic activity by non-ionic and anionic detergents. It is inferred from parallel purification of the (NO⁻₃,Cl⁻)-activated adenosine triphosphatase and the physiological kinetics of NO⁻₃ uptake by intact cells, that the enzyme translocates NO⁻₃ across the cell membrane, into the cytoplasm, against the chemical concentration gradient of the ion. The half-saturation constants for activation of the adenosine triphosphatase by NO⁻₃ are less than 1 µM for most species tested and correlate with half-saturation constants for NO⁻₃ uptake by whole cells. The three dinoflagellates tested did not exhibit any relationship between NO⁻₃ concentrations and ATP hydrolysis, and it is inferred that the (NO⁻₃,Cl⁻)-activated adenosine triphosphatase is probably absent from this group. Results of metabolic inhibitor studies (including KCN, 2,4-dinitrophenol, dichlorodimethylurea, and carbonyl cyanide m-chlorophenylhydrozone) imply that the source of ATP for the NO⁻₃ transport is primarily cyclic photo-phosphorylation in vivo. These results are consistent with observations of selective inhibition of NO⁻₃ uptake in unialgal cultures as well as in natural populations. Field studies with natural phytoplankton communities from Knight Inlet, B. C, suggest a physiological adaptation to external nitrogen concentrations may occur. This adaptation is characterized by increased intracellular chlorophyll a synthesis in response to 10-15% nitrogen enrichment over a 6-8 hr period. During the adaptive period carbon fixation is temporarily suppressed, apparently due to competition between inorganic carbon and inorganic nitrogen for high-energy nucleotides from the light reactions. The results of this study are related to previous proposals for the metabolic pathway of nitrogen in marine phytoplankton. In conclusion, a modified pathway is proposed stressing (1) group differences, in that nitrogen assimilation in dinoflagellates appears different from other groups, and (2) the energetics and biochemical feed-back controls of nitrogen assimilation. / Science, Faculty of / Botany, Department of / Graduate

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It is generally recognized that one of the most important factors limiting crop production is soil nitrogen. Farmers today are using more nitrogen fertilizers than ever before. All indications show that this usage will probably continue to increase. With the expanded use of nitrogen, it has become desirable to increase the efficiency from nitrogen fertilizers. One factor which will affect the efficiency is the position of the nitrogen in the soil in relation to the plant roots.

This thesis details advances made within the field of electroanalytical chemistry through the use of working electrodes that have been activated through application of ultrasound, heat, geometry, chemical modification or composition. Initially the thesis reports the enhanced analytical utility of chemically and compositionally modified working electrodes when directed towards the detection and determination of NO₃¯ and NO₂¯ anions in environmental samples. This has been achieved through the use of electrodes that have been a) modified with a Cu deposit and b) fashioned from a Cu-Ni alloy. Nitrate and nitrite anions have been successfully determined in a variety of passivating matrices, at analytically relevant detection limits of the order of 10^-6 M with a dynamic linear range extending from 10 to 200 μM. The methods presented have been shown to surpass existing electrochemical techniques in terms of nitrate/nitrite speciation through separation of the voltammetric signals, where existing analyses have reported the intereference of both species when present in the same solution. The use of ultrasound as a further enhancement to the sensitivity and versatility of the electrochemical detection of nitrate at a chemically modified electrode is then presented. The influence of ultrasound is shown to remove a portion of the deposited copper, but a significant catalytic layer remains, resulting in greater sensitivity during insonation. The effect of temperature on electrochemical systems involving one- and two-electron redox reactions of K₄Fe(CN)₆, Ru(NH₃)₆Cl₃, Fe(C₅H₅)₂, N,N,N',N'tetramethylphenylenediamine, N,N'dimethylphenylenediamine and tris(4- bromophenyl)amine have been studied under hydrothermal conditions using a novel hydrodynamic method based on a conventional channel flow cell where the working electrode is heated by radio frequency radiation. The diffusion activation parameters obtained with the radio frequency channel cell and computer simulation were compared with independent data from microelectrode high temperature experiments. The application of the heated flow cell as a tool for mechanistic studies is discussed with the investigation of the well characterised ECE reaction of m-iodo-nitrobenzene in acetonitrile, giving a value of 80 ± 5 kJ mol^-1 for the activation energy of the rate constant for the decomposition of the m-iodo-nitrobenzene radical anion. This represents the first observation of an ECE or mechanistically complex reaction at a locally heated electrode. The work presented in the final two chapters of this thesis examines the enhanced activation achieved from modification of the electrode geometry, and in particular the application of microelectrodes to the development of electroanalytical techniques. The electrochemical reduction of the inhalation anaesthetic agent enflurane (2-chloro-1,1,2- trifluoroethyl difluoromethyl ether) is reported at a variety of microelectrode substrates (Au, Ag, Cu, Pt and glassy carbon) with electrode dimensions varying from 5 to 60 μm. The solvents water, dimethylsulfoxide and acetonitrile were investigated along with the supporting electrolytes potassium chloride, tetrabutylammonium hexafluorophosphate and various tetraalkylammonium perchlorates. The use of a gold microelectrode with dimethyl sulfoxide solvent and tetraethylammonium perchlorate as the supporting electrolyte was found to give well-defined voltammetry. Linear calibration curves were obtained between 0 and 2 % v/v (gaseous additions) or up to 135 mM (gravimetric additions), offering scope for the development of a rapid, inexpensive electrochemical gas sensor. The analytical utility of the system has been investigated in the presence of oxygen and nitrous oxide in DMSO solvent. The superoxide anion radical, formed from the electro-reduction of dissolved oxygen, is shown to react with enflurane complicating their simultaneous detection. The kinetics of the enflurane / superoxide reaction are found to be first order with respect to both superoxide and enflurane with a rate constant of 0.25 M^-1 s^-1 determined by three independent methods: steady-state voltammetry, digital simulation of cyclic voltammetric data and UV/Vis spectroscopic analysis.

Master of Science / Department of Geology / Saugata Datta / To understand ambient nitrate dynamics in an unmanaged grassland environment, the stable isotope chemistry of various substrates at the Konza Prairie Biological Station were studied. Sediments consisted of alluvium, loess and bedrock materials are mostly limestone and shale. To gain a better understanding of the biogeochemistry of N cycling in this unmanaged grassland, 34 water samples have been collected along a full stretch of the major onsite creeks (Kings and Shane Creeks). These samples have been analyzed for δD, δ18O, and δ15N to determine the manner by which water allows nitrates to be leached into sediment, and to determine if additional nitrate is contributed from weathering of the bedrock. The anion chemistry of the waters show the presence of bicarbonates, sulfates, chlorides, nitrates, and fluorides. Although an obvious correlation among the anions is not present, data shows high alkalinity content due to the presence of shallow bedrock that is continuously being weathered. Using a hydraulic geoprobe, sediment cores were collected up to 8 ft deep using 2 ft core liners adjacent to the creeks to study δ15N and δ13C spatial variation of the sediments. This research also indicated the presence of C3 and C4 derived soil organic carbon with the latter being dominant. There is also an approximate 1:9 correlation between the total nitrogen and carbon content of these core sediments including NO3 and NH4 that are mostly concentrated in the upper 2 – 4 ft of the sediment profile. The results of this study provide a broader context for ambient nitrate cycling in unmanaged grassland and a baseline comparison for nitrate levels in surrounding agricultural lands.

Thesis (M.S.)--University of Rhode Island, 2006. / Includes bibliographical references (leaves 108-112). Also available online via the University of Rhode Island Watershed Hydrology Laboratory publications home page.

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