Photochemical Cycling of Reactive Oxygen Species in Hydrothermal Systems: Impacts on Biosignature Preservation

Author

Megan A. MaveFollow

Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Nancy W. Hinman

Commitee Members

W. Payton Gardner, Scott R. Miller

Keywords

reactive oxygen species, biosignature, iron, photochemistry, hot springs, Yellowstone National Park

Subject Categories

Geochemistry

Abstract

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.

Recommended Citation

Mave, Megan A., "Photochemical Cycling of Reactive Oxygen Species in Hydrothermal Systems: Impacts on Biosignature Preservation" (2018). Graduate Student Theses, Dissertations, & Professional Papers. 11275.
https://scholarworks.umt.edu/etd/11275