The oceanic upper crustal reservoir is a mosaic of environments with different thermal and hydrological characteristics that may form distinct biospheres. The upper 500 meters of oceanic crust is porous, permeable, and the site of extensive circulation of warm sea water—from the axis out to the sea floor that is more than tens of millions of years in age. This upper crustal section represents an enormous reservoir of hydrothermal fluid that has been suggested as a microbial incubator of global proportions. In the axial crustal formation zone at mid-ocean ridges, local environments near high temperature hydrothermal vent fields can harbor unique microbial populations [Summit and Baross, 2001; Huber et al., 2002]. The geographic distribution of these microbial populations is not known, and they may exist only in the immediate proximity of individual vent fields, or they may be more extensively distributed within the near-surface rocks along the entire spreading axis.