Doctor of Philosophy

dissertationThis dissertation presents experimental and computational studies of individual nanobubbles electrochemically generated at platinum nanoelectrodes. Chapter 1 provides an overview of the physics governing bubble dynamics and a brief summary of the literature regarding nanobubbles. Chapter 2 describes a fast scan voltammetric method for measurement of nanobubble dissolution rates. After a nanobubble is nucleated from gas generated via an electrode reaction, the electrode potential is rapidly stepped to a value where the bubble is unstable and begins to dissolve. The electrode potential is immediately scanned back to values where the bubble was initially stable. Depending on the rate of this second voltammetric scan, the initial bubble may or may not have time to dissolve. The fastest scan rate at which the bubble dissolves is used to determine the bubble’s lifetime. The results indicate that dissolution of a H2 or N2 nanobubble is, in part, limited by the transfer of molecules across the gas/water interface. Chapter 3 presents electrochemical measurements of the dissolved gas concentration, at the instant prior to nucleation of a nanobubble of H2, N2, or O2 at a Pt nanodisk electrode. The results are analyzed using classical thermodynamic relationships to provide an estimate of the size of the critical gas nucleus that grows into a stable bubble. This critical nucleus size is independent of the radius of the Pt nanodisk employed and weakly dependent on the nature of the gas. Chapter 4 reports electrochemical measurements of Laplace pressures within single H2 bubbles between 7 and 200 nm radius (corresponding, respectively, to between 200 and 7 atmospheres). The current, associated with H2 gas generation, supporting a steady-state nanobubble is modulated by application of external pressure. The slope of the current-pressure response allows extrapolation of the bubble’s curvature-dependent internal pressure. The results demonstrate a linear relationship between a bubble’s Laplace pressure and its reciprocal radius, verifying the classical thermodynamic description of H2 nanobubbles as small as ~10 nm. Chapter 5 summarizes these results and places them in the context of current research. Future directions for further studies are suggested

Abiotic redox reactions in hydrothermal mixing zones: decreased energy availability for the subsurface biosphere

© The Author(s), 202. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in McDermott, J. M., Sylva, S. P., Ono, S., German, C. R., & Seewald, J. S. Abiotic redox reactions in hydrothermal mixing zones: decreased energy availability for the subsurface biosphere. Proceedings of the National Academy of Sciences of the United States of America, 117(34), (2020): 20453-20461, doi:10.1073/pnas.2003108117.Subseafloor mixing of high-temperature hot-spring fluids with cold seawater creates intermediate-temperature diffuse fluids that are replete with potential chemical energy. This energy can be harnessed by a chemosynthetic biosphere that permeates hydrothermal regions on Earth. Shifts in the abundance of redox-reactive species in diffuse fluids are often interpreted to reflect the direct influence of subseafloor microbial activity on fluid geochemical budgets. Here, we examine hydrothermal fluids venting at 44 to 149 °C at the Piccard hydrothermal field that span the canonical 122 °C limit to life, and thus provide a rare opportunity to study the transition between habitable and uninhabitable environments. In contrast with previous studies, we show that hydrocarbons are contributed by biomass pyrolysis, while abiotic sulfate (SO42−) reduction produces large depletions in H2. The latter process consumes energy that could otherwise support key metabolic strategies employed by the subseafloor biosphere. Available Gibbs free energy is reduced by 71 to 86% across the habitable temperature range for both hydrogenotrophic SO42− reduction to hydrogen sulfide (H2S) and carbon dioxide (CO2) reduction to methane (CH4). The abiotic H2 sink we identify has implications for the productivity of subseafloor microbial ecosystems and is an important process to consider within models of H2 production and consumption in young oceanic crust.Financial support was provided by the National Aeronautics and Space Administration (NASA) Astrobiology program (Awards NNX09AB75G and 80NSSC19K1427 to C.R.G. and J.S.S.) and the NSF (Award OCE-1061863 to C.R.G. and J.S.S.). Ship and vehicle time for cruise FK008 was provided by the Schmidt Ocean Institute. We thank the ROV Jason II and HROV Nereus groups, and the captain, officers, and crew of R/V Atlantis (AT18-16) and R/V Falkor (FK008) for their dedication to skillful operations at sea. We thank our scientific colleagues from both cruises, as well as Meg Tivey, Frieder Klein, and Scott Wankel for insightful discussions. We are grateful to the editor and two anonymous reviewers for providing helpful comments and suggestions

The nucleation rate of single O2 nanobubble at Pt nanoelectrodes

Nanobubble nucleation is a problem that affects efficiency in electrocatalytic reactions since those bubbles can block the surface of the catalytic sites. In this article, we focus on the nucleation rate of O2 nanobubbles resulting from the electrooxidation of H2O2 at Pt disk nanoelectrodes. Bubbles form almost instantaneously when a critical peak current, inbp, is applied, but for lower currents, bubble nucleation is a stochastic process in which the nucleation (induction) time, tind, dramatically decreases as the applied current approaches inbp, a consequence of the local supersaturation level, ζ, increasing at high currents. Here, by applying different currents below inbp, nanobubbles take some time to nucleate and block the surface of the Pt electrode at which the reaction occurs, providing a means to measure the stochastic tind. We study in detail the different conditions in which nanobubbles appear, concluding that the electrode surface needs to be preconditioned to achieve reproducible results. We also measure the activation energy for bubble nucleation, Ea, which varies in the range from (6 to 30)kT, and assuming a spherically cap-shaped nanobubble nucleus, we determine the footprint diameter L = 8–15 nm, the contact angle to the electrode surface θ = 135–155°, and the number of O2 molecules contained in the nucleus (50 to 900 molecules)

Subseafloor microbial communities in hydrogen-rich vent fluids from hydrothermal systems along the Mid-Cayman Rise

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Environmental Microbiology 18 (2016): 1970–1987, doi:10.1111/1462-2920.13173.Warm fluids emanating from hydrothermal vents can be used as windows into the rocky subseafloor habitat and its resident microbial community. Two new vent systems on the Mid-Cayman Rise each exhibits novel geologic settings and distinctively hydrogen-rich vent fluid compositions. We have determined and compared the chemistry, potential energy yielding reactions, abundance, community composition, diversity, and function of microbes in venting fluids from both sites: Piccard, the world's deepest vent site, hosted in mafic rocks; and Von Damm, an adjacent, ultramafic-influenced system. Von Damm hosted a wider diversity of lineages and metabolisms in comparison to Piccard, consistent with thermodynamic models that predict more numerous energy sources at ultramafic systems. There was little overlap in the phylotypes found at each site, although similar and dominant hydrogen-utilizing genera were present at both. Despite the differences in community structure, depth, geology, and fluid chemistry, energetic modelling and metagenomic analysis indicate near functional equivalence between Von Damm and Piccard, likely driven by the high hydrogen concentrations and elevated temperatures at both sites. Results are compared with hydrothermal sites worldwide to provide a global perspective on the distinctiveness of these newly discovered sites and the interplay among rocks, fluid composition and life in the subseafloor.National Aeronautics and Space Administration Grant Number: NNX09AB756; Alfred P. Sloan Foundation; NSF Grant Number: OCE10618

HIF-1–dependent repression of equilibrative nucleoside transporter (ENT) in hypoxia

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia

Extreme Fermi surface smearing in a maximally disordered concentrated solid solution

We show that the Fermi surface can survive the presence of extreme compositional disorder in the equiatomic alloy Ni0.25Fe0.25Co0.25Cr0.25. Our high-resolution Compton scattering experiments reveal a Fermi surface which is smeared across a significant fraction of the Brillouin zone (up to 40% of 2π/a). The extent of this smearing and its variation on and between different sheets of the Fermi surface have been determined, and estimates of the electron mean free path and residual resistivity have been made by connecting this smearing with the coherence length of the quasiparticle states

LSST: from Science Drivers to Reference Design and Anticipated Data Products

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures available from https://www.lsst.org/overvie

Global Spatial Risk Assessment of Sharks Under the Footprint of Fisheries

Effective ocean management and conservation of highly migratory species depends on resolving overlap between animal movements and distributions and fishing effort. Yet, this information is lacking at a global scale. Here we show, using a big-data approach combining satellite-tracked movements of pelagic sharks and global fishing fleets, that 24% of the mean monthly space used by sharks falls under the footprint of pelagic longline fisheries. Space use hotspots of commercially valuable sharks and of internationally protected species had the highest overlap with longlines (up to 76% and 64%, respectively) and were also associated with significant increases in fishing effort. We conclude that pelagic sharks have limited spatial refuge from current levels of high-seas fishing effort. Results demonstrate an urgent need for conservation and management measures at high-seas shark hotspots and highlight the potential of simultaneous satellite surveillance of megafauna and fishers as a tool for near-real time, dynamic management

Surviving Sepsis Campaign: International guidelines for management of severe sepsis and septic shock: 2008

SCOPUS: ar.jinfo:eu-repo/semantics/publishe