Browsing by Subject "Oxygen"

Now showing 1 - 4 of 4
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    High Frequency Oxygen Data from Eight Shallow Prairie Pothole Lakes, 2009-2013
    (2020-10-15) Rabaey, Joseph S; Cotner, James B; Zimmer, Kyle D; Domine, Leah M; Rabae005@umn.edu; Rabaey, Joseph S; University of Minnesota Cotner Lab
    Dissolved oxygen controls important processes in lakes, from chemical reactions to organism community structure and metabolism. In shallow lakes, small volumes allow for large fluctuations in dissolved oxygen concentrations, and the oxygen regime can greatly affect ecosystem-scale processes. This data includes high frequency dissolved oxygen measurements that we used to examine differences in oxygen regimes between two alternative stable states that occur in shallow lakes. We compared annual oxygen regimes in four macrophyte-dominated, clear state lakes to four phytoplankton-dominated, turbid state lakes by quantifying oxygen concentrations, anoxia frequency, and measures of whole-lake metabolism. Oxygen regimes were not significantly different between lake states throughout the year except for during the winter under-ice period. During winter, clear lakes had less oxygen, higher frequency of anoxic periods, and higher oxygen depletion rates. Oxygen depletion rates correlated positively with peak summer macrophyte biomass. Due to lower levels of oxygen, clear shallow lakes may experience anoxia more often and for longer duration during the winter, increasing the likelihood of experiencing fish winterkill. These observations have important implications for shallow lake management, which typically focuses efforts on maintaining the clearwater state.
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    Improving Electrochemical Techniques for Studying Dissimilatory Metal Reducing Bacteria
    (2016-04) Joshi, Komal
    Advancements in the field of biotechnology over the past few decades have provided solutions to many research problems and have paved the way to new scientific discoveries. From development of different applications of microbial fuel cells and discovery and characterization of new classes of microorganisms to progress in whole-genome sequencing and annotation methods, the world of biotechnology is expanding every day. Currently, the world is dependent on fossil fuels to meet its energy demand but for future, we have to invent new strategies to fulfill global energy needs. One strategy is the use of biofuels produced by genetically engineered microbes. The dissimilatory metal reducing bacteria like Shewanella oneidensis utilize metals including Fe(III), Mn(IV), and U(VI) as terminal electron acceptors for anaerobic respiration and possess the ability of extracellular electron transfer. This ability of extracellular electron transfer can be useful to many biotechnological applications, however there are three major technological challenges that must be addressed: scalability, control over operation process, and defined biological pathways. The dissimilatory metal reducing bacterium (DMRB) Shewanella oneidensis MR-1 is used as a model organism for studies in microbial fuel cells and bioelectrochemical reactors. In this study that follow, aspects related to increasing control of bioelectrochemical reactors were addressed. Work was done to improve the coulombic efficiency of S. oneidensis in reactors by improving design of anoxic bioreactors. The new reactors were found to have < 3 ppm oxygen and showed no planktonic growth. The current density was also higher in new reactors (23 µA/cm2) compared to 10 µA/cm2 in old bioreactors. The coulombic efficiency of S. oneidensis in new reactors was measured to be 86.4 ± 10.37%, a vast improvement over alternative electrochemical systems. Hydrogen metabolism in S. oneidensis biofilms was also studied in these new better controlled reactors, and the role of hydrogenases in electron transfer from S. oneidensis to electrodes in a bioreactor was studied. It was found that deletion of the hydrogenases in S. oneidensis bioreactors funneled the electron transfer to electrodes and improved the coulombic efficiency by 30% indicating their role in extracellular electron transfer. In summary, the improved bioelectrochemical system described herein will be useful in studying phenotypes of various mutants of S. oneidensis and other metal reducing bacteria under anaerobic conditions on electrodes of defined redox potential. This well-defined and robust bioelectrochemical system will aid the study of extracellular electron transfer and may provide a platform for the design of microbial fuel cells and the production of alternative fuels.
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    Kinetic and spectroscopic characterization of intermediates in the soluble methane monooxygenase catalytic cycle: the old and the new
    (2013-02) Banerjee, Rahul
    Soluble methane monooxygenase (sMMO) catalyzes the difficult reaction of oxidative hydroxylation of methane to methanol, thereby allowing the host methanotroph to grow on methane as the sole source of carbon and energy. As methane possesses the strongest aliphatic C-H bond, the chemical mechanism of this enzyme is the epitome of oxygen activation in metalloenzymes. Exhaustive research of the catalytic mechanism of sMMO in recent years has elucidated many aspects of the chemistry occurring at the dinuclear iron active site center. These studies include the three dimensional structures of the protein components of sMMO and fairly detailed outlines of both the chemical mechanism and the method of regulatory control. The discovery of a diferric peroxo intermediate P, and more crucially, a high valent bis-μ-oxo diiron(IV) intermediate Q have provided the greatest advances in the understanding of oxygen activation in diiron oxygenases. The research described here has extended this understanding in several aspects. A long standing issue of low accumulation of enzyme reaction intermediates for spectroscopic studies has been addressed and in part solved. A new intermediate P* that had been proposed to exist based on kinetic studies has been trapped and characterized through spectroscopic techniques. P* appears to be a diferrous cluster intermediate that binds O2 weakly with little transfer of charge density onto the oxygen atoms. This result suggests a general theme for heme and non-heme oxygen activating enzymes in which ferrous centers initially form a weak complex with O2, which is strengthened in following steps by interactions such as stabilizing hydrogen bonds and charge donation from trans ligands. The long sought after goal of characterization of the vibrational spectrum of compound Q has been achieved through a time resolved resonance Raman technique. The preliminary results corroborate the diamond core structure that has been proposed for Q. Another putative high valent intermediate Q' has been discovered to arise from compound Q in the catalytic cycle. This is potentially a very significant finding as it seem likely that Q' rather than Q reacts directly with substrate. Following from a precedent from synthetic diiron model compound studies, it is possible that Q' is an open core form of the Q intermediate in which a reactive, terminal Fe(IV)=O moiety replaces the more stable bis-μ-oxo bridging structure.
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    Modulation of neurovascular coupling in the retina:effects of oxygen and diabetic retinopathy.
    (2011-07) Mishra, Anusha
    Neurovascular coupling is a process by which neuronal activity leads to localized increases in blood flow in the central nervous system. When neurovascular coupling results in hyperperfusion of the neural tissue, the response is termed functional hyperemia and serves to satisfy the increased energy demand of active neurons. In brain slices, high [O2] alters neurovascular coupling, decreasing activity-dependent vasodilations and increasing vasoconstrictions. However, in vivo, hyperoxia has no effect on neurovascular coupling. In order to resolve these conflicting reports of O2 modulation, I examined neurovascular coupling in both ex vivo and in vivo rat retina preparations. In the ex vivo retina, 100% O2 reduced the amplitude of light-evoked arteriole vasodilations by 3.9-fold and increased the amplitude of vasoconstrictions by 2.6-fold when compared to responses in atmospheric [O2] (21%), consistent with slice data. Oxygen exerted its effect by decreasing vasodilatory prostaglandin signaling and increasing vasoconstrictory 20-hydroxyeicosatetraenoic acid signaling. However, in vivo, hyperoxia (breathing 100% O2) had no effect on light-evoked arteriole vasodilations or on blood flow. We found that the differing effects of O2 arise because retinal pO2 increases to a much greater extent in the ex vivo preparation (to 548 mmHg) than in vivo (to 53 mmHg; Yu et al. Am J Physiol 267:H2498-H2507). When retinal pO2 was raised to 53 mmHg in the ex vivo retina, no change in neurovascular coupling was observed. These results demonstrate that although O2 can modulate neurovascular signaling pathways when pO2 is raised high enough, such levels are not attained in vivo, even when an animal breaths 100% O2. Functional hyperemia can also be modulated by pathological conditions. It is diminished in the retinas of diabetic patients, possibly contributing to the development of diabetic retinopathy. I investigated the mechanism responsible for this loss in a streptozotocin-induced rat model of type 1 diabetes. Here I show that light-evoked arteriole dilation was reduced by 58% in these diabetic rats at 7 month survival time. The diabetic retinas showed neither a decrease in the thickness of the retinal layers nor an increase in neuronal loss, although signs of early glial reactivity were observed. Functional hyperemia is believed to be mediated, at least in part, by glial cells and we found that glial-evoked vasodilation was reduced by 60% in diabetic animals. An upregulation of inducible nitric oxide synthase (iNOS) was detected by immunohistochemistry, and inhibition of iNOS restored both light- and glial-evoked dilations to control levels. These findings suggest that high NO levels resulting from iNOS upregulation alters glial control of vessel diameter and may underlie the loss of functional hyperemia observed in diabetic retinopathy. I further tested whether inhibiting iNOS reverses the loss of flicker-induced vasodilation in diabetic rat retinas in vivo. Flicker-evoked arteriolar dilations were diminished by 61% in diabetic animals, compared to non-diabetic controls. Treating diabetic animals with aminoguanidine (an iNOS inhibitor), either acutely via IV injection or long-term in drinking water, restored flicker-induced arteriole dilations in diabetic rats to control levels. The amplitude of the electroretinogram b-wave was similar in control and diabetic animals, suggesting that the deficit in functional hyperemia was not due to a reduction in neuronal activity. These findings demonstrate that inhibiting iNOS with AG is effective in preventing the loss of, and restoring, normal flicker-induced vasodilation in the diabetic rat retina. Treatment with iNOS inhibitors early in the course of diabetes has the potential to slow the progression of retinopathy by maintaining normal neurovascular coupling.