Abdulla, HussainShrestha, Sagar2023-08-282023-08-282022-12https://hdl.handle.net/1969.6/97202A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Coastal and Marine Systems Science.Dissolved organic matter (DOM) in the largest pool of reduced carbon represents one of the ocean's largest exchangeable reservoirs of organic matter. The estuarine system is considered a transitional zone between terrestrial and marine ecosystems where the consumption and transformation of organic matter through microbial and photochemical degradation. Apart from other DON sinks, photochemical degradation is one of the abiotic processes that could enhance the bioavailability of DON. For example, photo-ammonification via DON degradation can occur throughout the euphotic zone of the water column in coastal and marine waters and could link DON to significant NH4+ sources for primary production. However, despite this link between DON and the production of NH4+ fueling primary production, DON and its photoproducts are often overlooked in studies of the nitrogen budget in coastal areas. In this study, I investigate photo-transformation of DON in two semi-arid bays (Baffin Bay and Lavaca Bay) that received different DOM sources. Photo-ammonification rate was quantified in both bays and related to the change in the DOM optical properties and the transformation of DOM chemical composition that analyzed by state-of-the-art Orbitrap Fusion tripled mass spectrometer (OT-FTMS) coupled with both ion chromatography (IC) and ultraperformance liquid chromatography (UPLC) techniques. In Chapter 1, I study the photo-ammonification of surface DOM in seven stations at Baffin Bay, Texas. Baffin Bay is an inverse and hypersaline estuary ecosystem where the evaporation exceeds the freshwater input. DOM and inorganic nitrogen sources are more dominated by phytoplankton/microbes and benthic fluxes than terrestrial sources. Urban and agriculture land use/land cover classes are the most predominant within the surrounding watershed of Baffin Bay, which could be a significant source of nutrients in the bay. I observed a strong significant correlation (P > 0.05; R2=0.94) between NH4+ photoproduction and CDOM(a300) among the stations indicating the high presence of CDOM and the photodegrading responsible for the photoproduction of NH4+ in the bay. The average NH4+ photoproduction rate in Baffin Bay was 0.0472 ± 0.02 µmol. L-1h-1. I also used an electrospray ionization coupled with Orbitrap -Fusion Tribrid Mass Spectrometer (OT-FTMS) to quantify and identify the changes in the chemical composition of surface water from Baffin Bay. The relative abundance of CHON (52%) compounds was found to be higher in the pre-irradiation sample, followed by CHO (23%) and CHONS (9%) compounds. Overall, dissolved organic nitrogen (DON) formulas accounted for 72% of the total formulas detected and represented the largest class of unique formulas detected in pre-irradiated samples. After post-irradiation, molecular composition DOM was dominated by CHON (50%) compounds, followed by CHONS (18%) and CHO (14%) compounds. A higher abundance of nitrogen-containing compounds such as peptides and deaminated peptides in the initial sample was associated with the production of NH4+ by losing terminal anime groups during photooxidation. In Chapter 2, I studied the photochemical transformation of dissolved organic nitrogen (DON) in Lavaca Bay. Lavaca Bay is a secondary bay of Matagorda Bay located North of Port Lavaca, on the Gulf of Mexico. Lavaca River represents the primary freshwater source of Lavaca Bay. The average NH4+ photoproduction rate during 72 hours of irradiation was 0.0204 ± 0.02 µmol. L-1h-1. This photo-ammonification rate is less than half of the rate at Baffin Bay. The optical properties of Lavaca Bay indicated a decrease in molecular weight and aromaticity of DOM after solar irradiation. The transformations and composition of DOM were also investigated at the molecular level using UPLC-OT-FTMS (positive mode) and IC-OT-FTMS (negative mode). The relative abundance of CHON (45%) compounds was found to be higher in the pre-irradiation sample, followed by CHO (25%). After irradiation, molecular composition DOM was dominated by CHON (51%) compounds, followed by CHONS (18%) compounds. My results also suggest that molecular composition after irradiation was more oxygenated and saturated than pre-irradiated samples. This study also showed the increasing/decreasing trend of peptides and deaminated peptides during 72 hours of irradiation which resulted in the production of NH4+ and highlighted the importance of photooxidation as a potential source of NH4+ production in the estuarine system. In Chapter 3, I investigated the effects of photooxidation on sediment porewater dissolved organic matter (DOM) concentrations and molecular composition of Baffin and Lavaca Bays. I also observed a continuous increase in the production of ammonium (NH4+) in both Baffin Bay and Lavaca Bay, but the photoproduction rate was 51% higher at Baffin Bay as compared to Lavaca Bay. Changes in the molecular composition and aromaticity of DOM in both Baffin and Lavaca Bays were also observed by analysis of slope ratio (SR) and specific UV absorbance (SUVA). A closer look at the chemical composition, indicated that Baffin Bay has more labile porewater DOM responsible for photo-ammonification than Lavaca Bay. I found that Baffin Bay higher molecular diversity as represent with higher number of compounds than Lavaca Bay in both ionization modes. Elemental ratio and DBE analysis were also used to conclude that the molecular composition of Baffin Bay was more oxygenated and less aromatic after irradiation. The analysis of molecular composition indicates that 60% of the deamination of peptides was carried out by the oxidative deamination process and was dominated by aliphatic compounds such as glycine, alanine, proline, etc. A detailed analysis of the photo-transformation of peptides and deaminated peptides in Baffin and Lavaca Bays highlights the role of photodegradation of peptides and deaminated peptides as a possible contribution to photo-ammonification in coastal water and the potential degradation to individual amino acids and small organic acids.216 pagesen-USThis material is made available for use in research, teaching, and private study, pursuant to U.S. Copyright law. 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