4 results on '"Johnson, Thomas M."'
Search Results
2. Mass-dependent selenium isotopic fractionation during microbial reduction of seleno-oxyanions by phylogenetically diverse bacteria
- Author
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Schilling, Kathrin, Basu, Anirban, Wanner, Christoph, Sanford, Robert A., Pallud, Celine, Johnson, Thomas M., Mason, Paul R.D., Petrology, and Petrology
- Subjects
010504 meteorology & atmospheric sciences ,Isotope ,biology ,chemistry.chemical_element ,Fractionation ,Microbial reduction ,010502 geochemistry & geophysics ,biology.organism_classification ,01 natural sciences ,Selenate ,Redox ,Cell-specific reduction rate ,Selenium ,chemistry.chemical_compound ,Isotope fractionation ,chemistry ,Geochemistry and Petrology ,Environmental chemistry ,550 Earth sciences & geology ,Geobacter sulfurreducens ,0105 earth and related environmental sciences ,Anaeromyxobacter dehalogenans - Abstract
Selenium (Se) isotope fractionation has been widely used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of the controls on fractionation during microbial reduction of Se-oxyanions is based on a limited number of studies. Here we complement and expand the currently available pure culture data for Se isotope fractionation by investigating for the first time six phylogenetically diverse, mesophilic, and non-respiring bacterial strains that reduce Se-oxyanions to elemental Se [Se(0)]. Experiments were performed with either selenate [Se(VI)] or selenite [Se(IV)] at lower, more environmentally-relevant Se (9–47 μM) and carbon (500 μM) concentrations than previously investigated. Enterobacter cloacae SLD1a-1, Desulfitobacterium chlororespirans Co23 and Desulfitobacterium sp. Viet-1 were incubated with Se(VI) and Se(IV). Geobacter sulfurreducens PCA, Anaeromyxobacter dehalogenans FRC-W and Shewanella sp. (NR) were examined for their ability reducing Se(IV) to Se(0). Our data confirm that microbial reduction of both Se-oxyanions is accompanied by large kinetic isotopic fractionation (reported as 82/76e = 1000×(82/76α-1) in ‰). Under our experimental conditions, microbial reduction of Se(VI) shows consistently greater isotope fractionation (e = −9.2‰ to −11.8‰) than reduction of Se(IV) (e = −6.2 to −7.8‰) confirming the difference in metabolic pathways for the reduction of the two Se-oxyanions. For Se(VI), an inverse relationship between normalized cell specific reduction rate (cSRR) and Se isotope fractionation suggests that the kinetic isotope effect for Se(VI) reduction is governed by an enzymatically-specific pathway related to the bacterial strain-specific physiology. In contrast, the lack of correlation between normalized cSRR and isotope fractionation for Se(IV) reduction indicates a non-enzyme specific pathway which is dominantly extracellular. Our study highlights the importance to understand microbially-mediated Se isotope fractionation depending on Se species, and cell-specific reduction rates before Se isotope ratios can become a fully applicable tool to interpret Se isotopic changes in modern and ancient environments.
- Published
- 2020
3. Field Application of 238U/235U Measurements To Detect Reoxidation and Mobilization of U(IV)
- Author
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Jemison, Noah E, Shiel, Alyssa E, Johnson, Thomas M, Lundstrom, Craig C, Long, Philip E, and Williams, Kenneth H
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Nitrates ,Biodegradation ,Uranium ,Water Pollutants ,Radioactive ,Oxidation-Reduction ,Groundwater ,Environmental Sciences ,Environmental - Abstract
Biostimulation to induce reduction of soluble U(VI) to relatively immobile U(IV) is an effective strategy for decreasing aqueous U(VI) concentrations in contaminated groundwater systems. If oxidation of U(IV) occurs following the biostimulation phase, U(VI) concentrations increase, challenging the long-term effectiveness of this technique. However, detecting U(IV) oxidation through dissolved U concentrations alone can prove difficult in locations with few groundwater wells to track the addition of U to a mass of groundwater. We propose the 238U/235U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces 238U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a high 238U/235U ratio into the aqueous U(VI) pool produces an increase in 238U/235U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, 238U/235U consistently increased, suggesting 238U/235U is broadly applicable for detecting mobilization of U(IV).
- Published
- 2018
4. Open architecture as an enabler for FORCEnet Cruise Missile Defense
- Author
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Camacho, Juan G., Guest, Lawrence F., Hernandez, Belen M., Johnson, Thomas M., Kang, Alan H., Le, Giang T., MacGillivray, Brian J., Ngo, Tu K., Norman, Kyle B., Tomei, Franklin, Green, John M., and Systems Engineering (SE)
- Subjects
ComputerApplications_COMPUTERSINOTHERSYSTEMS - Abstract
Advancements in missile technology have made cruise missile capability available worldwide. Current US naval weapon systems lack full interoperability across multiple platforms and full integration of detection, control, and engagement processes against incoming targets. The key to defeating future threats to our military assets is in gaining additional reaction time. This can be accomplished by leveraging collective sensor detection data throughout the battlespace, utilizing the FORCEnet data resources to evaluate the threat, and engaging the threat with a tiered defense. The objective of this capstone project is to address the above issues through the use of Open Architecture (OA) within a FORCEnet environment. This report focuses on the development of a conceptual architecture for Cruise Missile Defense (CMD) that combines FORCEnet architecture requirements with Program Executive Office of Integrated Warfare Systems (PEO IWS)'s OA functional architecture technical requirements. Further, this conceptual architecture is compared with PEO IWS's functional architecture via a series of systems engineering diagrams. These diagrams culminate in a simulation model that analyzes and determines the validity of the conceptual architecture. Results from the simulation model show that the conceptual architecture performed significantly better than PEO IWS's. These results are attributed to the addition of a re-engagement loop called Observe-Orient-Decide-Act (OODA). http://archive.org/details/openrchitectures109456930
- Published
- 2007
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