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Extracellular electron transfer (EET) is a bioelectrochemical pathway in which microorganisms reduce electron acceptors in their environment. Through EET, microorganisms can enhance energy conservation and balance intracellular redox homeostasis while simultaneously reducing their extracellular environment. Outside of these direct impacts EET-capable microbial metabolism, the broader effects of EET in conducive environments remain underexplored outside of applications for microbial fuel cells. Chapter 1 of this dissertation focuses on the EET capabilities of bacteria found in diverse plant- and animal-associated niches, including the plant rhizosphere, animal mucosal surfaces, and during the fermentation of plant and animal foods. Ecological impacts of EET metabolism are explored, including the modulation of metal uptake by plants, the growth and proliferation of pathogenic bacteria in mucosal niches, and the increasing environmental acidification and reduction potential during food fermentations. Food fermentations are of particular interest regarding EET due to the impact of lactic acid bacteria (LAB), a diverse group of Gram-positive organisms characterized by their production of lactic acid during fermentation. LAB are found in diverse environmental, plant, mammal, and insect-associated microbiomes, and used in the fermentation of hundreds of foods and beverages. In Chapter 2, the EET is identified and explored in Lactiplantibacillus plantarum, a model organism for studying LAB. We characterized L. plantarum EET through the reduction of extracellular, insoluble ferrihydrite (iron(III) oxyhydroxide) and the generation of current with a graphite anode. The genetic conservation of the L. plantarum flavin-mediated EET (FLEET) locus was identified in other LAB and led to the discovery that LAB EET required Ndh2 (a type-II NADH dehydrogenase) and conditionally required PplA (a membrane-associated flavin-binding reductase) for iron reduction or current generation. The metabolic impacts of EET were also quantified and these data showed that L. plantarum had a shorter lag phase, greater cell abundance and viability, greater intracellular ATP and NAD+/NADH under EET-conducive conditions. Furthermore, L. plantarum EET was demonstrated during the fermentation of kale juice and increased environmental acidification. With the physiological impacts of EET on L. plantarum characterized, we turned towards exploring the role of quinones, an indispensable class of electron shuttles used by all bacteria for EET. In Chapter 3, the specificity of quinones and their physiological and ecological impacts on L. plantarum EET was explored. L. plantarum growth with DHNA led to membrane-associated menaquinone production but was not sufficient to stimulate EET in the absence of an exogenous DHNA. Instead, co-exposure of L. plantarum to DHNA during growth and EET significantly increased iron reduction at both high and low, environmentally relevant concentrations. DHNA and other quinones inhibited L. plantarum growth via hydrogen peroxide production and induced expression of oxidative stress response genes and redox-associated amino acid metabolism and transport. However, co-exposure of DHNA and ferric iron, a terminal electron acceptor, reversed these effects and partially restored L. plantarum growth. Other purified, naphthoquinone-based compounds, as well as spent cell-free growth medium from the LAB Lactococcus lactis and Leuconostoc mesenteroides was also sufficient in stimulated L. plantarum EET. Furthermore, we confirmed quinone cross-feeding in situ by showing that L. plantarum in co-culture with these LAB results in enhanced iron reduction and accelerated environmental acidification. Taken together the results of this dissertation highlight a novel metabolic pathway present in L. plantarum and other LAB and explores both the physiological and ecological impacts of EET on niches inhabited by EET-capable LAB. These findings inform the use of LAB during the fermentation of foods and beverages, during which successful outcomes as well as modulating flavor profiles may be achievable using EET.

Thermometric titrimetry has been used by many scientists as a means of determining the end-point of a reaction in acid-base and oxidation-reduction chemistry. Acetaminophen is an effective method for deterring pain in an individual, therefore, we sought to develop a unique relationship between the weight of sodium hypochlorite added to acetaminophen and the temperature from the heat that evolves from this reaction. One gram of acetaminophen was reacted with varying weights of sodium hypochlorite. As the weight increased, so did the average temperature change; yet, the average change of temperature per gram of bleach decreased, indicating that the temperature evolved from the reaction is dependent on the combined weights of the two reactants. Thus, we have effectively demonstrated an inexpensive and useful means of further determining the quality of acetaminophen via an inexpensive reaction combined with an inexpensive apparatus.

Carotenoids are yellow, orange and red pigments found in fruits and vegetables. Some carotenoids can act as dietary precursors of vitamin A. Humans and other animals generate retinal (vitamin A aldehyde) from provitamin A carotenoids by oxidative cleavage of the central 15-15′ double bond by the enzyme β-carotene 15-15′-oxygenase (BCO1). Another carotenoid oxygenase, β-carotene 9′-10′-oxygenase (BCO2), catalyzes the oxidative cleavage of the 9′-10′ double bond of various carotenoids to yield apo-10′-carotenals and ionones.In this dissertation, we elucidate the substrate specificity of these two enzymes. Recombinant His-tagged human BCO1 was expressed in Escherichia coli strain BL21-Gold (DE3) and purified by cobalt ion affinity chromatography. The enzyme was incubated with various dietary carotenoids and β-apocarotenals, and the reaction products were analyzed by reverse-phase high-performance liquid chromatography (HPLC). We found that BCO1 catalyzes the oxidative cleavage of only provitamin A dietary carotenoids and β-apocarotenals specifically at the 15-15′ double bond to yield retinal. A notable exception is lycopene, which is cleaved by BCO1 to yield two molecules of acycloretinal. Previous studies have found lycopene to be unreactive with BCO1. Our results warrant a fresh look at acycloretinal and its alcohol and acid forms as possible metabolites of lycopene. We also found that BCO1 does not react with 9-cis-β-carotene. It has been previously suggested the 9-cis-retinoic acid, a ligand of retinoid X receptors (RXR's), is generated by BCO1 cleavage of 9-cis-β-carotene to 9-cis-retinal and subsequently oxidized to the acid. However, our results strongly argue against this.Similarly, the substrate specificity of purified recombinant chicken BCO2 was also tested by incubating purified enzyme with the test substrates and analysis of the products by HPLC. Unlike BCO1, BCO2 reacts with full length provitamin A carotenoids as well as non-provitamin A carotenoids that contain 3-hydroxy ionone rings. However, it does not react with lycopene and β-apocarotenals.Carotenoid cleavage oxygenases (CCO's) oxidatively cleave carotenoids to yield aldehydes and/or ketones. Aldehydes readily exchange their carbonyl oxygen with water, making oxygen labeling experiments challenging. BCO1 has been thought to be a monooxygenase based on a study that used conditions that favored oxygen exchange with water. We elucidated the reaction mechanism of BCO1 using the same principles of oxygen labeling experiments, but minimized the reaction and processing times to minimize oxygen exchange between retinal and water. We incubated purified recombinant human BCO1 and β-carotene in an 16O2-H218O medium for 15 minutes at 37°C, and the relative amounts of 18O-retinal and 16O-retinal were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The 18O-retinal makes up only 3-10% of the total retinal. Incubation of 16O-retinal under the same conditions yields 5-13% 18O-retinal. We also incubated BCO1 and β-carotene in an 18O2-H216O medium. Under these conditions, 18O-retinal makes up 79-85% of the total retinal product. Similarly, incubation of 91% 18O-retinal under the same conditions yields 67-84% 18O-retinal. Our results show that BCO1 incorporates only oxygen from O2 into retinal, and BCO1 is therefore a dioxygenase.

Previous literature regarding students' understandings about oxidation-reduction reactions has focused primarily on students' understandings at the symbolic domain, while literature regarding students' understandings about electrochemical cells has focused primarily on the particulate domain. This study attempts to explore the gap in the literature between students' symbolic oxidation-reduction understandings and particulate electrochemistry understandings by investigating students' understandings of multiple representations of oxidation-reduction reactions using sequential exploratory mixed-methods study. In the first phase of this study, students' misconceptions about oxidation-reduction at the symbolic, macroscopic, and particulate domains were elicited through qualitative research methods, and the results of this phase were used to create a concept inventory to measure students' understandings on a large scale in a quick and efficient manner. Six major misconceptions themes emerged during the students' interviews including 1) oxidation numbers, 2) surface features of the chemical representations, 3) electron transfer processes, 4) the role of the spectator ion, 5) the particulate and dynamic reaction process, and 6) charges & bonding. Using these themes, the Redox Concept Inventory (ROXCI) was developed and each item response choice was created based upon students' responses from the interviews. Therefore, the ROXCI is inherently designed to measure students' understandings. The ROXCI went through several rounds of revisions and evidence for the content validity, response process validity, test-criterion validity, internal consistency, and test-retest reliability are presented. The ROXCI was answered by more than 2000 students throughout the course of this study, and in the final round of implementations, the ROXCI was administered to over 1000 students in a national study. While previous studies have identified students' oxidation-reduction misconceptions in qualitative studies, the ROXCI study is the first to present students' misconceptions on a national scale. The students' results provide evidence that the ROXCI can be used as a formative assessment of students' understandings about oxidation-reduction misconceptions. Future studies should develop pedagogy to target students' misconceptions in the classroom.

As species diversity declines worldwide, there has been a push to better understand the relationship between biodiversity and ecosystem function. The majority of studies which explore how function relates to plant diversity and community composition have focused on aboveground processes in terrestrial systems. In this study, we investigated the relationship of plant diversity and community composition to the degree of soil reduction—measured by oxidation-reduction potential (Eh) and ferrous (reduced) iron (Fe2+) levels in experimental wetlands. A previous experiment which used the same plant functional groups found that lower methane emissions and higher root biomass were correlated with greater diversity. A possible mechanism for the lower methane emissions could be that roots were facilitating methanotrophy over methanogenesis (which necessitates very low redox potentials) by oxygenating the soil. Based on these findings, we hypothesized that greater belowground biomass in the shallower depths of the soil in higher diversity communities and in communities with highly productive functional groups (reeds, tussocks) would lead to increased oxygenation of the rhizospheres surrounding the plant roots; this enhanced oxygenation would manifest itself in more positive Eh measurements and lower levels of Fe2+. To test our hypotheses, 80 experimental wetland mesocosms were installed at an outdoor research site on Ohio State University’s campus. The mesocosms were planted with varying levels of diversity, using 0, 1, 2, 3, or 4 wetland plant functional groups with all possible functional group combinations represented with five replicates. Three platinum-tipped redox electrodes were permanently installed in each of the 80 mesocosms at depths of 5, 15, and 25 cm in May 2008. The mesocosms were partially drained several times, but were kept flooded for most of the sampling season. Redox readings were taken eight times from June to August 2008. Reduced iron levels were measured in porewater samples at the same three depths in August 2008. Temperature, pH, and above and belowground biomass were considered as explanatory variables. We found that redox potentials did not differ among diversity treatments (P = 0.10) and community composition (P = 0.37), but showed an increase with depth (P = 0.01). Fe2+ did not vary by diversity treatment (P = 0.88) or community composition (P=0.33); however, Fe2+ levels increased significantly with depth (P2 = 0.09) and belowground (P = 0.02, r2 = 0.05) biomass. The data may suggest that the redox potentials are driven more by organic matter production (which results in electrons being donated to redox reactions) than oxygenation.

The product of interaction between Al and TiO2 at elevated temperature has a wide range of applications in refractory, structural and electronics industries (refractory tiles, tank armor, fuel cells, and microelectronic devices). This research attempts to understand the extent of interaction between Al and TiO2 when the reactant surfaces are in contact at elevated temperature and normal atmospheric pressure. The interfacial region between the reactant compounds is examined using analytical techniques; and the formation of TiAl as the interfacial compound is described. The thermodynamics of the Al – Ti – O system is explained as it relates to the particular conditions for the Al – TiO2 reaction research. Thermodynamic principles have been used to demonstrate that the formation of TiAl is favored instead of other TixAly compounds for the set of conditions outlined in this thesis. A study of the mechanism of interactions in the interfacial region can help towards being able to determine the reaction kinetics that lead to the control of microstructure and thus an improvement in the material performance. An appropriate model that describes the formation of TiAl at the interface is described in this study. The formation of TiAl at the interface is a result of the reduction reaction between TiO2 and Al. The O released during the reduction of TiO2 has been investigated and demonstrated to partly remain dissolved in TiAl at the interfacial region. Some O reacts with Al as well to form crystalline Al2O3 in the interfacial layer.