Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Optical Properties of Marine and Picocyanobacteria-derived Dissolved Organic Matter in the Atlantic, Pacific and during Long-term Incubation Experiments(2022) Lahm, Madeline Amelia; Gonsior, Michael; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Marine dissolved organic matter (DOM) is a large, dynamic, and complex pool of carbon, comparable in size to the carbon dioxide pool in the atmosphere, yet it is arguably the least understood component of the global carbon cycle. DOM deriving from picocyanobacterial cells via situationally unique mechanisms, such as viral lysis and metazoan grazing, complicate the picture as the resident pool present reflects sequestration processes that occur at time scales ranging from days to hundreds of thousands of years. Recently virus induced cell lysis released from the globally distributed picocyanobacteria, such as Synechococcus and Prochlorococcus, have been shown to release optically active DOM known as Chromophoric DOM (CDOM) that closely matches the “humic-like” appearance of marine CDOM raising questions about our understanding of this carbon pool given the reliance on spectral measures to assess its composition. Hence, this thesis is seeking to understand CDOM released by lysed picocyanobacteria and to investigate the molecular chemical composition of picocyanobacteria-derived DOM in general. A special focus will be to confirm the refractory nature of chromophores released by lysed picocyanobacteria (Synechococcus) given the reliance on optical properties of recalcitrant DOM being used in the investigation of timescales of carbon storage and biological processing of carbon. As we consider the outcomes of the current global carbon inventory with a sizable error in flux, linking products of microbial processes to chromophore structures and spectrometry is a capstone in understanding the global carbon cycle for decades of research. This study offers a direct comparison of fluorescence signatures from the Bermuda Atlantic Time-Series (BATS) and the Hawai'i Ocean Time-series (HOT), observes optical and nutrient profiles tracking long-term incubation experiments of oligotrophic microbial communities amended with Synechococcus-derived DOM, and explores new techniques in DOM solid-phase extraction (SPE). This work is part of a National Science Foundation project - The Fate of Lysis Products of Picocyanobacteria Contributes to Marine Humic-like Chromophoric Dissolved Organic Matter – linking the accumulating evidence of picocyanobacterial-derived DOM to our understanding of marine organic carbon. Furthermore, we seek to understand how picocyanobacteria-derived DOM is degraded and what role changing heterotrophic microbial communities plays. This research is important to the concept of a microbial carbon pump that supplies a constrained and constant source of DOM which has important implications for the marine carbon cycle and its role in global climate.Item Investigating the mechanism of the hydrogen peroxide photoproduction from chromophoric dissolved organic matter(2014) Zhang, Yi; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The photochemical pathways for H2O2 production from chromophore dissolved organic matter (CDOM) were investigated extensively, employing in part a selective sodium borohydride reduction method to examine the structural basis of the H2O2 formation. Estimates of the lifetime of possible H2O2 precursor(s) have been acquired by examining the dependence of H2O2 production rates on dioxygen concentration. The results indicate that H2O2 arises from intramolecular electron transfer from an excited singlet donor to a ground-state acceptor. Possible donors include substituted phenols, while possible acceptors included quinones, which unlike ketones/aldehydes are not irreversibly reduced by borohydride. The relationship between the rate of H2O2 formation and the rate of reducing intermediate(s) formation upon irradiation of CDOM was thoroughly investigated employing radical molecular probe. The results obtained from the dependence of dioxygen concentrations and wavelengths indicate that approximate 90% of produced one-electron reducing intermediates is converted to superoxide. The stoichiometric ratio between the H2O2 and total reducing radicals suggests that 67% of the photochemically produced superoxide decays through oxidant sinks other than dismutation to H2O2. The effect of adding external phenol electron donors on the H2O2 production rate was also examined. Substantially enhanced rates of H2O2 production are observed, which is substantially inhibited by borohydride reduction (40-50%), similar to the loss of TMP previously reported. In addition, H2O2 production rate increases as the dioxygen concentration decreases, consistent with reaction with a triplet state intermediate. The results all indicate that an additional pathway of H2O2 formation is introduced in the presence of sufficient high concentration electron donors. Reaction between the phenol electron donors and the excited triplet state of aromatic ketones/aldehydes yields a ketyl radical that subsequently react with O2 to form O2- and then H2O2. The enhancement generally follows the reduction potential of the added phenols, as DMOP> MOP> PHE, with the exception of TMP. In the case of TMP, secondary radical reactions could be the cause of this difference.Item Investigating the Mechanism of Phenol Photooxidation by Humic Substances(2014) Sikorski, Kelli Ann; Blough, Neil V; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)It is well established that organic pollutants such as phenols are degraded in the presence of chromophoric dissolved organic matter (CDOM) and sunlight in natural waters. Early work attributed the photochemical loss of phenols to the involvement of photoproduced reactive oxygen species (ROS) such as singlet oxygen (1O2), hydroxyl radical (*OH) or peroxy radicals (RO2*). However, evidence for the involvement of triplet excited states of aromatic ketones/aldehydes within CDOM has accumulated in the literature. To probe the mechanism of the photosensitized loss of phenols by humic substances (HS), the dependence of the initial rate of 2,4,6-trimethylphenol (TMP) loss (RTMP) on dioxygen concentration and irradiation wavelength was examined both for a variety of untreated as well as borohydride-reduced HS and C18 extracts from the Delaware Bay and Mid-Atlantic Bight. The effect of [O2] and borohydride-reduction of SRFA was also examined for a series of substituted phenols of varying one-electron reduction potentials. We find that RTMP was inversely proportional to dioxygen concentration at [O2] > 50 μM, a dependence consistent with reaction with triplet excited states, but not with 1O2 or RO2. Modeling the dependence of RTMP on [O2] provided rate constants for TMP reaction, O2 quenching and lifetimes compatible with a triplet intermediate. Borohydride reduction significantly reduced TMP loss, supporting the role of aromatic ketone triplets in this process. However, for most samples, the incomplete loss of sensitization following borohydride reduction, as well as the inverse dependence of RTMP on [O2] for these reduced samples, suggests that there remains another class of oxidizing triplet sensitizer, perhaps quinones. However, the results of the wavelength dependence reveal that the sensitization is driven primarily by shorter wavelength UV-B and UV-A absorbing moieties, consistent with the involvement of aromatic ketones and aldehydes but appearing to exclude the longer wavelength (visible) absorbing quinones as sensitizers. An inverse dependence of Φ on one-electron reduction potential was observed where DMOP ≈ TMP > 4-MOP > 4-MP > phenol. Similar dependencies were observed for TMP and 4-MOP in the dependence of Rprobe on [O2] whereas DMOP did not exhibit a substantially lower Rprobe at high [O2] as would be expected for a triplet sensitization mechanism. Moreover, that a significant amount of sensitization is observed following borohydride reduction of SRFA for DMOP under high [O2], as well as the very low sensitization observed at low [O2] indicates that a separate pathway, unrelated to triplets, may be important for the mechanism of DMOP photooxidation by chromophoric dissolved organic matter.