Browsing by Subject "nitrogen fixation"
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Item Evolution of energy storage traits in symbiotic rhizobia(2018-01) Muller, KatherineThe mutually-beneficial symbiosis between legume plants and nitrogen-fixing rhizobia involves an inherent conflict-of-interest over how rhizobia allocate the resources they receive from the host plant. In theory, rhizobia could enhance their future fitness by diverting resources from nitrogen-fixation into storage compounds like polyhydroxybutyrate (PHB). Although the conflict-of-interest between PHB accumulation and nitrogen-fixation has been discussed as a driving factor in the evolution of legume-rhizobia interactions, its role in natural populations is unclear. Therefore, this dissertation fills in key empirical gaps between what we know about the functional role of PHB and hypotheses about how natural selection might act on continuous variation in the amount of PHB that rhizobia acquire during symbiosis. The first chapter assesses the extent of heritable phenotypic variation within natural rhizobia populations interacting with soybean (Glycine max) and partridge pea (Chamaecrista fasciculata) and evaluates implications for fitness in the free-living stage after rhizobia are released from nodules. The results from my first chapter show that 1) natural populations of rhizobia contain heritable, quantitative variation in the amount of PHB they accumulate during symbiosis (a prerequisite for evolution by natural selection) and, 2) natural selection on PHB accumulation may be mediated by how rhizobia allocate PHB over time and among life functions, which could vary independently from traits underlying the amount of PHB acquired during symbiosis. The second chapter assesses phenotypic response to selection based on resident rhizobia populations from long-term agricultural plots varying in host (soybean) or non-host (maize) frequency over years. The mean PHB per cell (measured in nodules) was two times higher in rhizobia populations from plots with 5 or 30 years of continuous maize than from plots where soybean was grown in the previous year. An apparent decrease in mean PHB per cell after the first year of soybean following five years of maize supports the hypothesis that low-PHB rhizobia have higher reproduction in nodules, perhaps due to host sanctions against rhizobia that divert more resources to PHB. A model used to interpret the results suggests that, 1) PHB acquired during symbiosis may contribute to fitness variation for several years after the last host crop, and 2) host sanctions against less-beneficial rhizobia may be stronger in the first soybean crop due to a combination of lower initial rhizobia population size and negative frequency-dependent selection during symbiosis. Collectively, these findings provide empirical support for previously unsubstantiated hypotheses about how conflicts-of-interest over resource allocation contribute to the evolution of the legume-rhizobia mutualism and develop a more nuanced framework for future research.Item Nitrogen Fixation Provides a Significant Source of Biologically Available Nitrogen in the Great Lakes(2020-08) Natwora, KaelaNitrogen fixation (NFix) is an important yet understudied microbial process in aquatic ecosystems and especially in the Laurentian Great Lakes (LGL). Nitrogen fixation (NFix) or diazotrophy is a lynchpin in the nitrogen cycle that converts dinitrogen (N2) gas to ammonia (NH3) using the nitrogenase protein complex (Nif). Nitrogen fixation re-mobilizes atmospheric nitrogen and N lost to denitrification processes. Early work suggested the contribution of NFix in the LGL is minimal to the nitrogen budget. However, recent work has shown during bloom events, NFix can help alleviate nitrogen limitations. Evidence of nitrogen accumulation in the LGL, like in Superior, suggest that we do not have a firm grasp on the nitrogen cycle in large lakes. Furthermore, for most of the Great Lakes, there is a dearth of nitrogen fixation rates from near-shore and surface water environments. Thus, we sought to revisit NFix in the LGL and comprehensively sample from near and offshore stations and with depth to understand the spatial variability of NFix. In our quest to survey the Great Lakes, we carried out a series of preliminary studies to determine our methodology and understanding of nitrogen fixation is aqueous samples. Preliminary studies exploring seasonality and size fractionation showed detectable nitrogen fixation rates in Lake Superior, counter to previous expectations as a result of readily available dissolved inorganic nitrogen. Likewise, preliminary studies on nitrogen speciation and concentrations were found to inhibit nitrogen fixation rates in pure culture experiments. Inorganic nitrogen exemplified a significant threshold where nitrogen fixation rates decreased. In our Great Lakes survey, we found that each lake is significantly different in NFix rates from one another and that rates are depth dependent. Linear regression models show that chlorophyll-a and oxidized nitrogen species are correlated, directly and inversely, with NFix but with the caveat that lakes like Erie and Huron do not fully adhere to the model. Together this suggest that traditional controls on diazotrophy may not always adhere and that NFix is a significant source of N.Item Optimization of Ammonium and Biohydrogen Production from Mutant Strains of Azotobacter vinelandii Deregulated for Nitrogen Fixation(2018-05) Plunkett, MaryThe increase in demand for food and fuel as a result of an increasing population must be sustainable and renewable in the face of global climate change. Azotobacter vinelandii, an aerobic nitrogen fixing bacterium, has the potential to supplement or replace a major consumer of global energy, which is the production of ammonium (NH4+) for use in fertilizers. A requisite by-product of nitrogen fixation includes the production of hydrogen gas (H2), which can be used for many applications, including renewable hydrogen fuel cells. A. vinelandii produces both of these in a biochemical process which takes place at ambient temperatures and pressures using renewable carbon sources for energy. Within this research, improvement of the conditions needed for higher NH4+ and H2 production from a strain deregulated for the production of nitrogenase was explored and H2 output was characterized as a result of multiple genetic modifications and changes to culture conditions.Item Optimizing soybean water use and nitrogen fixation to improve productivity under climate change(2021-07) Monnens, DanielClimate change effects are driving crop yield decreases globally, and yield penalties are often associated with increases in frequency and intensity of heat waves and soil moisture deficits, particularly during reproductive development. Additionally, climate change is driving increases in vapor pressure deficit (VPD) in recent decades, suggesting global atmospheric drying. In a U.S. study, VPD during reproductive stages was identified as the most important predictor of soybean yield variation. Investigations into the effect of VPD on soybean yields often consider its effect on crop transpiration rate (TR) and gas exchange, and such efforts have led to development of drought-tolerant soybean cultivars, which take advantage of particular TR responses to VPD. Despite such progress, no studies have yet examined the effect of VPD on nitrogen fixation, likely due to difficulties of taking non-invasive, real-time measurements.A common technique for monitoring nitrogen fixation rates non-destructively is the acetylene reduction assay (ARA), which has received critiques due to hazards posed to researchers and potential effects on root nodules. The first goal of this investigation was to develop an alternative method for non-invasively measuring nitrogen fixation that minimizes the above difficulties. A system was designed that enabled us to quantify N2 fixation as a function of the rate of by-product hydrogen gas (H2) production by soybean nodules. A second goal was to examine rates of nitrogen fixation in relation to concurrent changes in environmental variables, namely VPD and temperature. The system was successfully tested on two soybean genotypes in both field and controlled environment conditions. A key result is the confirmation of H2 production being associated with root nodules, seen by a lack of H2 signal from plants without nodules. Additional key observations include increases in H2 production rate from the morning to afternoon in the field, driven at least partly by increases in VPD, consistent with H2 production rate increases observed when shifting from low to high VPD in a controlled environment. Consistent genotypic differences were observed in both settings, signifying potential for genotypic diversity, which could be exploited in breeding efforts. This study invites further research into the effect of VPD and temperature on N fixation, as soybean growing environments continue to undergo increases in atmospheric drying. A second focus of this research on soybean is centered on plant water use, which is largely driven by evaporative demand as VPD increases drive increases in TR. Increased TR drives processes beneficial to carbon fixation, so in an environment where available water is not limiting, such behavior should lead to higher yield, or at least be neutral. However, in a water-deficit prone environment, genotypes exhibiting lower TR at high VPD will outperform those exhibiting a ‘profligate’ TR, by reducing the amount of water lost during the time of day when evaporative demand is the highest. Despite progress in phenotyping soybean TR responses to VPD, so far no QTL have been detected for traits directly reflecting such response curves, due to methodological and logistical challenges. The QTL mapping performed here resulted in the detection, for the first time, of two QTL for the response of TR to both low and high VPD. If confirmed, these QTL could be leveraged to design soybean varieties that are optimized for specific water availability regimes in order to maximize productivity.