<p>Investigating the presence of microbial communities in the Earth's deep terrestrial subsurface and the metabolic processes taking place in these environments provides insight into the some of the ultimate limits for life on Earth, as well as the potential for microbial life to exist within the subsurface of other planetary bodies. This Master's thesis project utilized phospholipid fatty acid (PLFA) analysis, in combination with carbon isotope analyses (δ¹³C and Δ¹⁴C), to explore the presence and activity of microbial communities living within deep terrestrial subsurface fracture water systems and low permeability, deep sedimentary rocks. Deep fracture water systems, ranging from 0.9 to 3.2 km below land surface, were sampled for microbial communities via deep mine boreholes in the Witwatersrand Basin of South Africa. PLFA concentrations revealed low biomass microbial communities, ranging from 2x10¹ to 5x10⁴ cells per mL and the PLFA profiles contained indicators for environmental stressors, including high temperatures and nutrient deprivation. δ¹³C and Δ¹⁴C analyses of PLFAs and potential carbon sources (dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and methane) identified microbial utilization of methane in some systems and utilization of DIC in others. Evidence for microbial oxidation of methane and chemoautotrophy in these systems is consistent with a self-sustaining deep terrestrial subsurface biosphere that is capable of surviving independent of the photosphere. Viable microbial communities were also identified within deep (334 to 694 m depth) sedimentary rock cores sampled from the Michigan Basin, Canada. PLFA analyses revealed microbial cell densities ranging from 1-3 x 10⁵ cells/mL and identified PLFA indicators for environmental stressors. These results demonstrate the ubiquity of microbial life in the deep terrestrial subsurface and provide insight into microbial carbon sources and cycling in deep microbial systems which may persist in isolation over geologic timescales.</p> / Master of Science (MSc)