Showing 3951 - 3960 of 5240 Items
Diatom blooms in Harpswell Sound: seasonality, succession, and origin
Date: 2023-01-01
Creator: Charlie Francis O'Brien
Access: Open access
- Harpswell Sound (HS) is an inlet in northeastern Casco Bay that exerts control on Gulf of Maine ecosystem health, yet its complex phytoplankton community dynamics have not been characterized with sufficiently detailed analyses. In this research, high-resolution automated microscopy and current velocity observations were used to test the seasonality, ecological succession, bloom origin location, and potential toxicity of populations in HS between 2020 and 2022. Winter months could exhibit slow accumulation of diatom biovolume. Cold, salty surface water has net outflow in winter as nutrients from depth are replenished during net upwelling conditions, and populations could be exported from the inlet at the surface. Extreme current velocity variability in spring due to the Kennebec River plume in HS destabilizes spatially uniform populations. Warm, low-salinity surface water with net inflow in summer (net downwelling which retains populations at the head of the sound) corresponds with temporally separate dinoflagellate and diatom blooms. Large, multi-peaked spring and fall diatom blooms are recurrent, contrasting small, short-lived blooms in summer. A successional pattern from diatoms to dinoflagellates is confirmed for summer but refuted for other seasons. The hypothesis that diatom succession during all blooms in HS is characterized by large centric cells preceding small cells or pennate cells was explored but no clear pattern in decreasing cell size was observed. Observed tidal effects on biovolume concentration could mask that blooms develop at coherent times but spatially separated. A diverse community of toxic phytoplankton, including dinoflagellates and Pseudonitzschia, are observed throughout the year.
The Role of ELMO5 in Arabidopsis thaliana Cell Adhesion
Date: 2022-01-01
Creator: Isabel Kristina Ball
Access: Open access
- Plant cell growth and development relies on proper cellular adhesion. As the extracellular matrix serves as the area of connection between two cells, its synthesis and maintenance are essential for cellular adhesion. The middle lamella region, the layer of the extracellular matrix between two adjacent cell walls, is diffuse with the polysaccharide pectin due to its delivery by Golgi vesicles early during cell division. A Ruthenium Red screen for cellular adhesion mutants identified the family of 5 ELMO proteins that are critical for proper cellular adhesion. To further our understanding of plant cellular adhesion and pathways of pectin synthesis and modification, this work investigates ELMO5. Plants homozygous for a T-DNA insertion in ELMO5 and a new deletion mutant allele generated using CRSPR do not have a cellular adhesion phenotype, suggesting it is either not critical for cellular adhesion or is redundant with another gene. Redundancy within the ELMO family is identified through the analysis of double mutants of elmo5 and each of the other four elmo genes. Both elmo1-/- elmo5-/-and elmo4-/- elmo5-/-mutants have a visibly worse cellular adhesion defect phenotype, suggesting partial redundancy through the ELMO family. The mutants are also rescued by growth on agar, pointing to the importance of turgor pressure and osmotic potential in modulating cellular adhesion. Both ELMO4 and ELMO5 were found to localize to the Golgi using a GFP fusion, consistent with a role for ELMOs as scaffold for pectin biosynthesis.
The role of modulation on the pyloric neurons and the neuromuscular junction in a pattern generator-effector system
Date: 2023-01-01
Creator: Jackie Seddon
Access: Open access
- Neuromodulation, the process of altering the electrical outputs of a neuron or neural circuit, allows an organism to control its physiological processes to meet the needs of both its internal and external environments. Previous work shows that the pyloric pattern of the kelp crab (Pugettia producta) stomatogastric nervous system (STNS) neurons responded to fewer neuromodulators than the Jonah crab (Cancer borealis). Since the kelp crab diet primarily eats kelp, it is possible that the movements of the foregut that control digestion may require less flexibility in functional output compared to an opportunistic feeder. To determine whether a reduced flexibility is correlated with diet, this study compared the modulatory responses in Pugettia to two other species of majoid crabs: Chionoecetes opilio and Libinia emarginata, which are both opportunistic feeders. Pooled data for this study found that Libinia and Chionoecetes responded to all twelve modulators tested. When considering the effect of modulators on stomatogastric ganglion (STG) motor outputs, we must consider whether these modulators also alter the excitatory junction potentials (EJPs) at the neuromuscular junction (NMJ), and whether there are differences in responses across species. To test this, the dorsal gastric nerve (dgn) was stimulated while recording intracellularly from the muscle fibers of the associated gm4 muscles. The NMJ of the gm4 in Cancer borealis did not appear to be broadly modulated, as only RPCH and CabTRP showed increases in amplitude, and RPCH decreased facilitation at 5 Hz.
Eroding the Bedrock: The Future of Public Administration Without Chevron Deference
Date: 2024-01-01
Creator: Rose Keller
Access: Open access
- When Congress passes a bill, it produces words on a page. Who decides what those words mean? Historically, the onus of a statute’s interpretation has rested with the federal agencies charged with its implementation. The Chevron doctrine, a two-step standard that affords federal agencies significant latitude in interpreting their own enabling legislation, has been the applicable deference regime in statutory interpretation cases since 1984. Contrary to this tradition, recent Supreme Court jurisprudence has reasserted the primacy of the judiciary in statutory interpretation cases, often ignoring Chevron entirely. In 2023, the Court granted certiorari to Loper Bright Enterprises v. Raimondo, a case that explicitly asked the Court to consider overruling Chevron vs. NRDC, a foundational case in the field of administrative law. This thesis explores the implications of eroding deference to federal agencies through a case study of the Food and Drug Administration and two of its responsibilities. Ultimately, there are potentially negative consequences to limiting agency jurisdiction via eviscerating judicial deference that counsel a more discerning approach.
An AGN as a Counterpart for Neutrino Event IceCube-220303A
Date: 2024-01-01
Creator: Nur Schettino
Access: Open access
- Cosmic rays have been detected for over a century. While some sources have been confirmed, they cannot explain the high energy of the particles (> 10^15 eV), so it remains unclear where and how they are accelerated to extreme energies. The study of astrophysical high-energy neutrinos may help solve the puzzle. These neutrinos are produced by cosmic rays interacting with other charged particles or photons. Moreover, while cosmic rays do not reveal their sources of origin because they can be deflected by magnetic fields, cosmic neutrinos detected by the IceCube Observatory can be traced back to their sources of origin. We will consider an active galactic nucleus (AGN) as a candidate source for a high-energy neutrino.This thesis examines the AGN WISEA J175051.31+105645.3 as a potential source for IceCube-220303A, a high-energy neutrino with a 78% probability of being astrophysical in origin. Using follow-up NuSTAR and Swift/XRT observations, WISEA J175051.31+105645.3 was the only viable source we found in IceCube-220303A’s uncertainty region. We used follow-up X-ray data to construct a multi-wavelength spectral energy distribution (SED) through which we calculated the AGN’s neutrino energy flux. This calculation yields the number of neutrinos we would expect to detect from the AGN in a given time period. We used this number to calculate the probability that IceCube-220303A was emitted by WISEA J175051.31+105645.3. Finding a statistically significant link between IceCube-220303A and WISEA J175051.31+105645.3 may help us better understand what processes can accelerate particles like cosmic rays to extreme energies and learn more about AGN.
Modulation of the stretch feedback pathway in the cardiac neuromuscular system of the American lobster, Homarus americanus
Date: 2024-01-01
Creator: Karin van Hassel
Access: Open access
- The cardiac ganglion (CG) is a central pattern generator, a neural network that, when activated, produces patterned motor outputs such as breathing and walking. The CG induces the heart contractions of the American lobster, Homarus americanus, making the lobster heart neurogenic. In the American lobster, the CG is made up of nine neurons: four premotor pacemaker neurons that send signals to five motor neurons, causing bursts of action potentials from the motor neurons. These bursts cause cardiac muscle contractions that vary in strength based on the burst duration, frequency, and pattern. The activity of the CG is modulated by feedback pathways and neuromodulators, allowing for flexibility in the CG’s motor output and appropriate responses to changes in the animal’s environment. Two feedback pathways modulate the CG motor output, the excitatory cardiac muscle stretch and inhibitory nitric oxide feedback pathways. Despite our knowledge of the modulation of the CG by feedback pathways and neuromodulators separately, little is known about how neuromodulators influence the sensory feedback response to cardiac muscle stretch. I found one neuromodulator to modulate each phase of the stretch response differently, one neuromodulator to generally not affect the stretch response, and three neuromodulators to suppress the stretch response. These results suggest neuromodulators can act to produce flexibility in a CPG’s motor output, allowing the system to respond appropriately to changes in an organism’s environment, and allow for variation in CPG responses to different stimuli.
Activation of Hydrogen by Sterically Modulated Coinage Metal Catalysts via Mutual Quenching of Hard/Soft Acid/Base Mismatches
Date: 2024-01-01
Creator: Zach Leibowitz
Access: Open access
- To mitigate the devastating environmental impacts of climate change in the coming decades, it is imperative that we replace the use of fossil fuels with renewable energy sources such as wind, solar, and hydroelectric. As these renewable energy sources are inherently intermittent, there exists a need for sustainable mechanisms to store renewable energy for later use. While the direct use of dihydrogen (H2) as a combustible fuel would allow for energy storage without the harmful release of carbon dioxide (CO2) upon combustion, the practicality of H2 as a synthetic fuel is limited by its low volumetric energy density. Combining sustainable H2 production (e.g. electrolysis using energy from renewable sources) with subsequent carbon fixation (e.g. the hydrogenation of CO2) represents a promising pathway to the sustainable production of high-density synthetic fuels. We hypothesize that such a process could be catalyzed by an IPr**-supported catalyst containing a hard/soft acid/base (HSAB) mismatch, with a polarizable coinage metal acting as a soft acid. As such, the aim of our project is the construction of a catalogue of IPr**-supported copper, silver, and gold catalysts that we anticipate will facilitate the heterolysis of dihydrogen and subsequent hydrogenation of CO2. In the present paper, we report the synthesis and characterization of an IPr**-silver complex which will serve as a precursor to many of our proposed HSAB mismatch catalysts and discuss next steps as we construct our catalogue of catalysts.
Democracy Promotion in U.S. Counterinsurgency: Tracing Post-War Security Sector Reconstruction in El Salvador and Iraq
Date: 2020-01-01
Creator: Emma Redington Lawry
Access: Open access
- Throughout the 21st century, certain facets of the democratic peace theory have informed American foreign policy, as policymakers credit democracy promotion with long-term stability and peace. In contrast, many political scientists have documented the often destabilizing and violent effects of democratization, particularly in underdeveloped states. How can we reconcile these tensions, and in what ways do they affect American foreign policy abroad? Under the lens of just war theory, or the doctrine of military ethics detailing the conditions under which it is morally acceptable to go to war, wage war and restore peace after war, this paper seeks to examine security sector reconstruction in post-counterinsurgency eras. In doing so, my analysis documents the effects of electoral processes on security and underscores the many difficulties of post-war rebuilding processes. In understanding these difficulties, I attempt to extract crucial lessons from the “best case” scenario of El Salvador and the “worst case” scenario of Iraq, both of which illuminate the fundamental tension between democratization and stability.
Solvent Effect on Excited State Proton Transfer Mechanism of 8-Amino-2-Naphthol
Date: 2021-01-01
Creator: Gabrielle Vandendries
Access: Open access
- Photoacids, compounds that undergo excited state proton transfer (ESPT), have been utilized in different solar energy and lithographic applications.1, 2 The addition of functional groups and solvent can both change the ESPT mechanism of photoacids. In this study, the effect of solvent on the ESPT mechanism was explored using a model diprotic photoacid, 8-amino-2-naphthol (8N2OH). The photochemistry of 8N2OH in water and common nonaqueous solvents, acetonitrile, tetrahydrofuran (THF), and methanol, were studied using UV/Vis absorption, steady-state emission, and time-correlated single photon counting (TCPSC) emission spectroscopy. The results were analyzed using the Kamlet-Taft parameters. It was found that the ESPT mechanism of the cation in water is different from the mechanism in acetonitrile and THF. In water the excited cation forms the zwitterion, i.e. the OH site undergoes ESPT, while in acetonitrile and THF, the excited cation forms the neutral species, i.e. the NH3+ site undergoes ESPT. No ESPT was observed for 8N2OH in methanol. The effect of solvent mixtures on photoacidity was also investigated using acetonitrile and water mixtures. The solvent effects were more subtle; the time-resolved emission measurements showed the greatest stabilization of the excited neutral 8N2OH species at 20/80% acetonitrile-water mixtures. Finally, the ability to extend the solvent studies to ionic liquids, 1-ethyl-3-methylimidazolium (Im) trifluromethanesulfonate (OTF), was demonstrated. The combined studies reveal that solvent plays a large role in determining the ESPT mechanism and stabilization of 8N2OH.
A Foray into the Camp: Human and Ecological Liberation in Contemporary Queer Conversion Therapy Literature
Date: 2021-01-01
Creator: Mitchel Jurasek
Access: Open access
- Through the analysis of two contemporary conversion therapy novels in North America, this project explores the intersections of biopolitics (specifically camp theory), queer theory, ecocriticism, and YA literature. Emily Danforth’s The Miseducation of Cameron Post and Nick White’s How to Survive a Summer are paired with scholars such as Gilles Deleuze, Felix Guattari, Joshua Whitehead, Greta Gaard, Hannah Arendt, Giorgio Agamben, Claudio Minca, Catriona Sandilands, Luce Irigaray, and Michael Marder to create a complex and intricate understanding of how ecologies impact queer youths’ experience in conversion therapy camps. The effect of such an intersectional and ecological understanding of queer becomings creates a foundation for further discovery and offers examples for current and future people to find mutual liberation with the ecologies we exist in.