Igneous oceanic crust contains the largest aquifer on earth (Johnson & Pruis, 2003). The basaltic layer contains ~2300 m2 kg-1 of surface area that supports an extensive subsurface microbial ecosystem (Heberling et al., 2010; Nielsen & Fisk, 2010; Santelli et al., 2008). Biological activities in this ecosystem can impact global carbon cycling and increase productivity in the ocean (Edwards et al., 2011; McCarthy et al., 2010). Previous studies of surface layer rocks and mineral deposits have suggested that mineralogy dictates microbial community structure in the upper oceanic crust (Flores et al., 2011; Sylvan et al., 2013; Toner et al., 2013) and subseafloor planktonic communities have been previously investigated (Cowen et al., 2003; Jungbluth et al., 2014, 2013); however, the role of mineralogy and composition in the attached community that comprises the bulk of the habitable zone in the subseafloor has not been previously determined. Planktonic and mineral-attached microbial communities in aquifers are distinct from one another (Lehman, 2007), and investigating the attached community will lead to a more holistic view of this largest crustal aquifer. As a C-DEBI graduate fellow, I analyzed microbial communities attached to igneous minerals and glasses incubated in IODP Hole 1301A of the Juan de Fuca Ridge. I found that Archaeal communities on olivine (a Fe-bearing silicate mineral common in the crust) were distinct from those on other common minerals and glasses, while bacterial communities were influenced by mineral composition. Using olivine bioreactors in the laboratory, I found that isolated subseafloor microbes increased the dissolution of olivine and promoted mineralization of secondary phases, potentially influencing biogeochemical cycles in the ocean. Finally, preliminary metagenomic analysis of attached communities from the JFR indicate that genes for fermentation, one-carbon metabolism, protein synthesis, aromatic compound metabolism, respiration, and fatty acid metabolism are more abundant in the subseafloor than other subsurface or marine environments. These new insights into the attached subseafloor biosphere, through support from C-DEBI, are helping to propel the field of deep biosphere research into a new era and challenging our view of this extensive subseafloor biosphere.