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Award-winning Theses and Dissertations

Permanent URI for this collection

https://hdl.handle.net/10919/71751
Every year, the Virginia Tech Graduate School honors several outstanding theses and dissertations, and some theses and dissertations have won external awards. Browse these works here. Read about applying for thesis and dissertation awards at graduateschool.vt.edu/academics/awards/.

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Browsing Award-winning Theses and Dissertations by Department "Biological Sciences"

Now showing 1 - 4 of 4
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    Hypolimnetic Oxygenation Mitigates the Effects of Nutrient Loading on Water Quality in a Eutrophic Reservoir
    Gerling, Alexandra Beth (Virginia Tech, 2015-09-03)
    Climate change is predicted to have many diverse effects on freshwater lakes and reservoirs by increasing both hypolimnetic hypoxia and runoff, which will increase nutrient concentrations and degrade water quality. Hypoxic conditions can trigger the release of metals and nutrients from the sediments, i.e., internal loading, while storms can increase external nutrient loading to a waterbody. One potential solution for combating hypoxia is to use side stream supersaturation (SSS), a novel form of hypolimnetic oxygenation. First, in Chapter 1, I tested the efficacy of SSS operation to improve water quality in Falling Creek Reservoir (FCR), a shallow, eutrophic, drinking water reservoir. I found that SSS operation successfully increased hypolimnetic oxygen concentrations in FCR and suppressed internal loading of iron, manganese, and phosphorus. In Chapter 2, I manipulated inflow volumes to FCR and used SSS as a tool to alter hypolimnetic oxygen conditions in whole-ecosystem manipulations of internal and external nutrient loading. I observed that internal nitrogen and phosphorus loading during hypoxic conditions largely controlled the hypolimnetic mass of nutrients in FCR, regardless of inflow volumes, presumably as a result of the accumulated nutrients in its sediment from historical agriculture. Additionally, FCR consistently functioned as net sink of N and P throughout almost all of the treatments and substantially reduced nutrient export to downstream ecosystems. In summary, my research demonstrates the sensitivity of reservoir water quality to global change.
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    Multi-scale Transmission Ecology: How Individual Host Characteristics, Host Population Density, and Community Structure Influence Transmission in a Multi-host Snail Symbiont System
    Hopkins, Skylar R. (Virginia Tech, 2017-05-04)
    We live in an era of global change, where emerging infectious diseases such as Ebola, Zika, bird flu, and white nose syndrome are affecting humans, wildlife, and domesticated species at an increasing rate. To understand and predict the dynamic spread of these infectious agents and other symbionts through host populations and communities, we need dynamic mathematical models that accurately portray host-symbiont transmission. But transmission is an inherently difficult process to measure or study, because it is actually a series of interacting processes influenced by abiotic and biotic factors at multiple scales, and thus empirical tests of the transmission function within epidemiological models are rare. Therefore, in this dissertation, I explore factors at the individual, population, and community-levels that influence host contact rates or symbiont transmission success in a common snail-symbiont system, providing a detailed description of the multi-faceted nature of symbiont transmission. From a review of the ecological literature, I found that most models assume that transmission is a linear function of host population density, whereas most empirical studies describe transmission as a nonlinear function of density. I then quantified the net nonlinear transmission-density relationship in a system where ectosymbiotic oligochaetes are directly transmitted among snail hosts, and I explored the ecological mechanisms underlying the nonlinear transmission-density relationship observed in the field via intraspecific transmission success and contact rate experiments in the laboratory. I found that the field results could be explained by heterogeneity in transmission success among snails with different characteristics and nonlinear contact-density relationships caused by non-instantaneous handling times. After I 'unpacked'population-level transmission dynamics into those individual-level mechanistic processes, I used this same approach to examine higher-level ecological organization by describing the mechanistic underpinnings of interspecific or community-level transmission in the same snail-symbiont system. I found that low interspecific transmission rates in the field were the product of opposing interactions between high population densities, high prevalences of infection, and very low interspecific transmission success caused by strong symbiont preferences for their current host species. Unpacking transmission in this way resulted in one of the most detailed empirical studies of transmission dynamics in a wildlife system, and yielded many surprising new insights in symbiont ecology that would not have been discovered with a purely phenomenological or holistic view of transmission. Though simple, linear, and holistic epidemiological models will always be important tools in disease ecology, 'unpacking'transmission rates and adding heterogeneity and nonlinearity to models, as I have done here, will become increasingly important as we work to maximize model prediction accuracy in this era of increased disease emergence.
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    Responses of Boom-Forming Phytoplankton Populations to Changes in Reservoir Chemistry and Physics
    Hamre, Kathleen Diamond (Virginia Tech, 2016-09-15)
    Phytoplankton populations are integral to the structure and function of aquatic ecosystems, and phytoplankton are an excellent study system for exploring ecological questions. Reservoirs often exhibit high horizontal (inflow to dam) and vertical (surface to sediments) environmental heterogeneity, which plays a large role in determining phytoplankton population dynamics. In this thesis, I explore how three bloom-forming phytoplankton taxa, the dinoflagellates Peridinium and Gymnodinium, and the cyanobacterium Planktothrix, respond to horizontal and vertical environmental gradients, respectively. First, I monitored recruitment, or the process of leaving the sediments and entering the pelagic life stage, of dinoflagellates across a horizontal reservoir ecosystem gradient. Surprisingly, coupling of dinoflagellate biology with reservoir physics and chemistry varied along this continuum; recruiting cells were sensitive to reservoir physics (e.g., flow rate, solar radiation) in the upstream riverine zone, while recruitment was related to reservoir chemistry (e.g., dissolved oxygen, nutrients) in the downstream lacustrine zone. This study indicates that upstream habitats should be monitored when studying reservoir phytoplankton dynamics. Next, I investigated the environmental drivers of the vertical distribution and biomass of a hypolimnetic cyanobacterial bloom over two consecutive summers. I collected high-resolution in situ phytoplankton data, and measured environmental variables throughout the water column. Across both years, the vertical distribution of this population was determined by light availability, while the cyanobacterial biomass was predicted by both light and nutrients. These two studies demonstrate that changing physics and chemistry across environmental gradients can regulate phytoplankton dynamics in reservoirs, and phytoplankton monitoring should include more spatially comprehensive sampling approaches.
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    Understanding PilB, The Type IV Pilus (T4P) Assembly ATPase
    Sukmana, Andreas Binar Aji (Virginia Tech, 2018-06-29)
    The type IV pilus (T4P) is a dynamic long thin fiber found on the surface of many bacterial groups. T4P is a versatile nanomachine; it plays many important roles such as for surface attachment, virulence factor, and surface motility apparatus. This research focuses on understanding the kinetics of PilB, the T4P assembly ATPase. PilB crystal structure exhibits an elongated hexamer with 2-fold symmetry indicating a symmetric rotary mechanism model. Except for its structure, the symmetric rotary mechanism of PilB has not been demonstrated experimentally. Its conformation and relatively low activity constrained previous in vitro studies of PilB. This study identified PilB from thermophilic organism Chloracidobacterium thermophilum (Ct) to be a model for in vitro studies. An active CtPilB was successfully expressed and purified as a hexamer. Malachite green phosphate assay was used to examine CtPilB ATPase activity. The examination indicated that CtPilB is a robust ATPase with a complex kinetics profile. The profile has a stepwise incline in ATPase activity as a function of [ATP] that led to a decline in higher [ATP]. The decline was confirmed to be a substrate inhibition by the enzyme-coupled assay. As for the incline, the detailed mechanism is still less clear to explain the multiphasic profile. The overall incline did not conform with classical Michaelis-Menten kinetic but the first part of the incline was shown to conform with Michaelis-Menten kinetics. The complex kinetics profile of PilB is consistent with the symmetric rotary mechanism of catalysis.