If life exists on Mars, it likely resides in a deep subsurface environment. The Martian rocky subsurface is a primary target for the search for extraterrestrial life because it offers a stable habitat for microbial life. On Earth, the continental subsurface is a key astrobiological analog for understanding what properties confer habitability to rocky subsurface settings, e.g. potential sources of energy and carbon, and the distribution and diversity of microbial life that may be possible on Mars. Continental subsurface settings are inhabited by both planktonic microbes that float or swim in fluids and biofilm-forming microbes that attach to rock surfaces. Our current understanding of the diversity and biomass of biofilm communities is extremely limited due to the inherently challenging nature of accessing rock-hosted microbial life kilometers deep into Earth’s crust. Here we characterize microbial life in a Mars rocky subsurface analog site: the Deep Mine Microbial Observatory (DeMMO). This research offers insight into rocky subsurface habitability with a focus on biofilms. We explore a central question throughout this research: what role does host rock mineralogy play in subsurface habitability? We use a variety of techniques to address this question, including genetic surveys, tracking fracture fluid chemistry over time, thermodynamic modeling of microbial metabolisms, in situ cultivation experiments, lab-based cultivation experiments, and high resolution scanning electron microscopy and X-ray energy dispersive spectroscopy. We interrogate minerals as ecological drivers of biofilm diversity and biomass using in situ cultivation experiments with minerals representative of the host rock and interpret our results in the context of thermodynamic models of microbial metabolisms. We build upon our findings from these experiments using in situ cultivation experiments with native rock to explore the spatial dependence of biofilms on mineral distributions and probe for hotspots of biomass on energy-rich minerals. We investigate how passive processes like sulfide scavenging may contribute to subsurface habitability using growth experiments with minerals with a representative member of deep subsurface biofilms. Finally, we explore the potential for iron-fueled life among planktonic members of the DeMMO communities using metagenomics. We find that minerals are an important control on biofilm diversity and biomass in the continental subsurface. Biofilms aggregate around energy-yielding minerals on rock surfaces, and biofilm communities are enriched in putative mineral-metabolizing taxa relative to the respective planktonic communities. Minerals can contribute to subsurface habitability by providing a source of energy that promotes locally elevated biofilm biomass and may also promote sulfur cycling through passive processes like sulfide scavenging. Biofilms likely represent the majority of life in the continental subsurface and play an important role in biogeochemical cycles. Specifically, iron cycling is likely an important process that connects and sustains biofilm and planktonic communities. Iron cycling is probably a pervasive process in deep continental settings where iron is ubiquitous. Deep subsurface exploration on Mars may be possible in the future, and this work suggests that specific attention should be placed on metal oxide-rich rock fracture surfaces where fluids have been or are currently present.

Creator

• Casar, Caitlin Page

DOI

• https://doi.org/10.21985/n2-5t0p-jw07

Subject

• Microbiology
• Geobiology
• Geochemistry

Language

• en

Alternate Identifier

• etdadmin_upload_808363
• http://dissertations.umi.com/northwestern:15508

Keyword

• Geochemistry
• Deep Subsurface
• Continental Subsurface
• Biofilms
• Geobiology
• Astrobiology

Date created

• 2021-01-01

Resource type

• Dissertation

Rights statement

• In Copyright