Chairperson, Graduate Committee: Eric BoydLindsay, Melody RoseDaniel R. Colman, Maximiliano J. Amenabar, Kirsten E. Fristad, Kristopher M. Fecteau, Randall V. Debes, John R. Spear, Everett L. Shock, Tori M. Hoehler and Eric S. Boyd were co-authors of the article, 'Geological source and biological fate of hydrogen in Yellowstone hot springs' which is contained within this dissertation.Maximiliano J. Amenabar, Kristopher M. Fecteau, R. Vincent Debes II, Maria Clara Fernandes, Kirsten E. Fristad, Huifang Xu, Tori M. Hoehler, Everett L. Shock and Eric S. Boyd were co-authors of the article, 'Subsurface processes influence oxidant availability and chemoautotrophic hydrogen metabolism in Yellowstone hot springs' in the journal 'Geobiology' which is contained within this dissertation.2021-06-092021-06-092019https://scholarworks.montana.edu/handle/1/16193Hydrogen (H 2) connects the geosphere and biosphere in rock-hosted ecosystems and has likely done so since early in Earth's history. High temperature hydrothermal environments, such as hot springs, can be enriched in H 2 and were likely widespread on early Earth. As such, linking the geological processes that supply H 2 to contemporary hot springs and the distribution of extant thermophilic organisms that can utilize H 2 as a component of their energy metabolism can provide insights into the environment types that supported early H 2 dependent life. Using a series of geochemical proxies, I developed a model to describe variable H 2 concentrations in Yellowstone National Park (YNP) hot springs. The model invokes interaction between water and crustal minerals that generates H 2 that can partition into the vapor phase during decompressional boiling of ascending hydrothermal waters. Fractures and faults in bedrock, combined with topographic features such as high elevation, allow for vapor to migrate and concentrate in certain areas of YNP leading to elevated concentrations of H 2. Metagenomes from chemosynthetic communities in YNP springs sourced with vapor-phase gas are enriched in genes coding for enzymes predicted to be involved in H 2-oxidation. A spring in an area of YNP (Smokejumper, SJ3) sourced with vapor-phase gas, that has the highest concentration of H 2 measured in YNP, and that is enriched in hydrogenase encoding genes was chosen to further examine the biological fate of H 2. SJ3 harbors a hyperdiverse community that is supported by mixing of oxidized meteoric fluids and volcanic gases. Transcripts coding for genes involved in H 2 uptake and CO 2 fixation were detected. The processes that control the availability of oxidants and their effect on the activity and abundance of H 2 dependent organisms was also investigated in two paired hot springs. H 2-oxidizing chemoautotrophs utilized different oxidants in the two springs and this underpinned differences in H2 oxidation activity and their identity. Together, these observations indicate that the subsurface geological processes of decompressional boiling and phase separation influence the distribution, identity, and activity of hydrogenotrophs through their combined effects on the availability of H 2 and oxidants.enHot springsHydrogenGeomicrobiologyThermophilic microorganismsDissertationCopyright 2019 by Melody Rose Lindsay