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Attend a Graduate Defense

By: taylorreeves737   Published 3:50 pm / October 25, 2016

The campus community is invited to attend the following graduate defenses:

Larry Gelb

When: 3 p.m. Wednesday, Nov. 2
Where: Engineering Building, Room 103
Title: Using Landlab, a Fine Scale Biogeography Model, to Measure the Sustainability of Shrublands in a Changing Climate
Program: Master of Science in Hydrologic Sciences

The distribution of vegetation in water-limited ecosystems is a product of complex and nonlinear interactions between climatic forcings (e.g., precipitation, temperature, solar radiation, etc.) and the underlying geomorphic template, which includes topography and soils. Changes in climate, particularly in precipitation and temperature, can dramatically alter the organization of vegetation. Understanding and predicting how the spatial composition of terrestrial vegetation communities will change in these ecosystems is critical to predicting important future landscape changes such as landslides, erosion, fires, and water storage capacity. This study promotes understanding of the relative sensitivity of vegetation types to changes in precipitation and temperature in water-limited environments using a land modeling framework. Specifically, we use the Landlab framework to develop and conduct a suite of numerical experiments that diagnose how changes in precipitation and temperature regimes effect the organization of plant functional types across varying hillslope aspects in a semi-arid, savanna-like ecosystem. To enhance model robustness, and therefore the robustness of any conclusions about changes in vegetation composition to climate change, we took an ensemble approach. Using the Landlab framework in conjunction with this ensemble approach allowed us to rapidly develop an effective model of the relative sensitives of vegetation types and conclude that precipitation is the most important variable with regard to forest conservation.

Jesse Dean

When: 3 p.m. Wednesday, Nov. 9
Where: Engineering Building, Room 103
Title: Using Near-Infrared Photography to Better Study Snow Microstructure and Its Variability Over Time and Space
Program: Master of Science in Geophysics

The methods typically used to study snow stratigraphy, microstructure, and variability are expensive, cumbersome, and often highly subjective. Near-infrared (NIR) photography is a low-cost, portable tool to rapidly collect high-resolution, objective measurements of snow microstructure and variability. To expand its application, an active-source NIR flash was introduced to the traditionally passive-source method. NIR imagery was collected alongside proven snowpit methods such as manual observation, Snow Fork wetness, and Snow Micro-Penetrometer hardness profiles. NIR photography was also deployed in five pits along a 10.6 km transect in Grand Mesa, CO, to track stratigraphy variations in space. The NIR flash was found to improve contrast and lower noise for layer detection using automated statistical processing of the images. NIR photography data complemented traditional methods and was shown to provide unique, insightful observations, especially on stratigraphy and microstructure. NIR photography is demonstrated to be a convenient, valuable method to correlate layer stratigraphy across small and large distances. NIR photography is shown to be a rapid snow stratigraphy technique providing repeatable, unique, and informative insight into the complex and rapidly evolving nature of snowpack stratigraphy, microstructure, and variability.

Nicole Cornell 

When: 1:30 p.m. Friday, Nov. 11
Where: Education Building, Room 112
Title: Molecular Basis of Substrate Recognition in BjaI, an AHL Synthase from Bradyrhizobium japonicum
Program: Master of Science in Chemistry

Resistance to antibiotics has become a major challenge in today’s society for treating bacterial infections. Inhibition of quorum sensing has a potential to be a non-antibiotic based therapeutic that could be used to fight these bacterial infections. Quorum sensing is a cell density dependent, intercellular communication mechanism that bacteria use to synchronize behavior such as virulence, resistance to antibiotics, etc. If this switch from singular to multicellular behavior can be inhibited, the bacteria will be less virulent. One way to accomplish this is by inhibiting the enzymes that are responsible for making the quorum sensing signaling molecules in Gram-negative bacteria – acyl-homoserine lactone (AHL) synthases. Since AHL synthases are mostly uncharacterized, understanding how these enzymes recognize its acyl-substrate is necessary to designing effective quorum sensing inhibitors. The focus of this thesis is to investigate substrate recognition mechanism in BjaI, an acyl-homoserine lactone synthase found in soybean symbiont Bradyrhizobium japonicum. We found that any single point mutation within either of the substrate binding pockets, S-adenosyl-L- methionine and isovaleryl-CoA, were detrimental to enzyme activity. Kinetic constants were measured for the native and other similar nonnative acyl-CoAs as well as their respective alkyl-CoA inhibitors. For too long and too short acyl-CoAs, we found that BjaI rejected nonspecific substrates at the binding step. However, for substrates that are structurally similar to isovaleryl-CoA, BjaI uses a combination of both the binding and catalytic steps to reject the nonspecific substrate. The tools used in this study should open new doors to designing effective quorum sensing inhibitors.

Andrew Gase

When: 3 p.m. Friday, Nov. 11
Where: Engineering Building, Room 103
Title: Near-surface geophysical investigations of pyroclastic deposits from the 18 May 1980 eruption of Mount St Helens: Constraints on geophysical properties and implications for pyroclastic density current self-channelization
Program: Master of Science in Geophysics

I apply geophysical imaging techniques to improve our understanding of the physical properties and geology of shallow, unconsolidated pyroclastic deposits. The thesis is divided into two studies set in the deposits of the 1980-1981 eruptive period at Mount St. Helens, Washington (USA). Vesicularity is a unique textural characteristic that affects the seismic and electromagnetic velocities of pyroclasts. We present the results of coincident multi-fold GPR and active-source seismic surveys to constrain the seismic and electromagnetic velocities of shallow, unconsolidated pyroclastic deposits and invert for deposit porosity and water saturation. Electromagnetic velocities are primarily affected by volumetric water-content. Inverted P-wave velocities from first arrival tomography and S-wave velocities from Rayleigh-wave inversion and inverted intergranular porosities (~0.2-0.4) are consistent with unconsolidated sands. The anomalously high critical porosity of pumice (~0.80) allows pyroclasts maintain higher rigidity despite vesicularity on the order of ~0.7. Breccias correspond to zones of reduced porosity, which is caused by poor-sorting and compositional variation between accidental blocks and the pyroclasts. We suggest that rock-physics experiments further explore the effects of vesicularity on seismic properties.

Within the 18 May 1980 eruption deposits at Mount St. Helens, a GPR survey reveals the most spatially extensive case of erosional scouring by pyroclastic density currents (PDCs) found to date. We trace two broad PDC scour-and-fill features interpreted as channels to stratigraphy in nearby outcrops. Both channels are >200 m wide and at least 500 m in length; estimated eroded volumes are on the order of one million cubic meters. Erosion appears to be promoted by moderate slope angle (5-15°), substrate pore-air retention, pulses of increased flow energy. We interpret that these scours are similar to incipient channels carved by submarine turbidity currents. These finding are the first direct evidence of self-channelization by sustained, waxing and waning PDCs, a phenomena that may increase flow velocity and run-out distance.