Year of Award
Master of Science (MS)
Department or School/College
Nancy W. Hinman
W. Payton Gardner, Scott R. Miller
reactive oxygen species, biosignature, iron, photochemistry, hot springs, Yellowstone National Park
University of Montana
Life originated on early Earth, despite harsh, highly reducing conditions. Life may have also emerged on early Mars, when conditions on the two planets were similar (i.e. before atmosphere loss and desiccation). NASA’s 2020 Mars rover mission aims to identify biosignatures (i.e. evidence of life) in early Martian deposits. Potential exploration sites include extinct hydrothermal springs, due to their high habitability and preservation potential. This study aims to better understand biosignature preservation in hydrothermal systems analogous to those on early Mars (i.e. reducing and Fe-rich).
Reactive oxygen species (ROS) are highly reactive molecules formed primarily by photochemical reactions. ROS are widespread, shape aquatic redox chemistry, and control biogeochemical cycles with redox-sensitive elements (Fe, S, O, and C). Of interest to this study, ROS can oxidize Fe2+ to Fe3+, which can adsorb or bind to negatively charged cell membranes. Rapid Fe3+-binding (i.e. entombment) can preserve complex organic molecules, or biomarkers. Recent studies have found that entombment by Fe3+, specifically, is key in biomarker preservation. In reducing systems, ROS are the primary oxidants and, thereby, determine Fe-oxidation rates and preservation potential. ROS formation is typically controlled by photo-reactions with dissolved organic carbon. However, Fe redox reactions more likely control ROS formation in these Fe-rich systems. Field and laboratory experiments were conducted at YSNP in relevant water compositions to better understand controls on ROS cycling.
In-situ H2O2 cycles observed in these hydrothermal waters were comparable to other higher-temperature systems. Reactions with reduced metals from hydrothermal source waters were responsible for constant, “baseline” ROS production. Reaction rates varied based on particle size (particulate or soluble matter) and water composition. Fe speciation (photochemical reactivity), concentration, and solubility further determined ROS formation and decay rates. Specifically, photochemically active metal species enhanced both ROS formation and decay rates, depending on incident UV irradiance, and rates increased along with Fe concentration and solubility (i.e. acidic conditions). Low O2 conditions slowed H2O2 decay, allowing H2O2 to accumulate. Hydrothermal reactants appear to produce more H2O2 per O2- molecule compared to other water compositions. Findings can improve our understanding of ROS as they relate to Fe entombment and biomarker formation.
Mave, Megan A., "Photochemical Cycling of Reactive Oxygen Species in Hydrothermal Systems: Impacts on Biosignature Preservation" (2018). Graduate Student Theses, Dissertations, & Professional Papers. 11275.
© Copyright 2018 Megan A. Mave