Previous studies of microbial abundance and geochemistry in deep marine sediments indicate a stimulation of microbial activity near the sediment-basement interface. Furthermore, the few studies thus far conducted in deep oceanic crust reveal an active microbial community in this habitat. Yet, the extent to which microbial communities in bottom sediments and underlying crustal habitats interact is unclear. Are microbial communities in sediments seeded from basement communities, or vice versa? If there is influence from one habitat type on the other, how far does this influence extend, both with vertical distance away from the interface and laterally along the flow path of fluids within basement? We propose to conduct tag pyrosequencing on samples from a spectrum of sediment-basement habitats to identify patterns in microbial community shifts across sediment-basement interfaces. Such comparisons will allow us to evaluate whether microbial groups from one habitat are found in another, and to examine whether members of the ''rare biosphere'' in one habitat come to dominate in another. Analysis of the pyrosequencing data will help us better understand how subsurface microbial communities are established and evolve, in addition to expanding our knowledge of microbial biogeography across sediment-basement interfaces in different oceanic basins.