SRP469142 PRJNA1033664 IODP Expedition 330 enrichments The aquifer in subseafloor igneous basement is a massive, continuous microbial substrate, yet sparingly little is known about life in this habitat. The work to date has focused largely on describing microbial diversity in young basement (<10 Ma) at oceanic spreading regions and ridge flanks, where the basaltic crust is still porous and fluid flow through it is active. While the prevailing belief used to be that fluid flow through older parts of the seafloor was non-existent, recent heat flow models predict that fluid moves through subseafloor basement >65 Ma, and that seamounts can act as mid-plate conduits for fluids into and out of the subsurface aquifer in older crustal settings. Here we test the hypothesis that microbial life exists in subseafloor basement >65Ma using samples collected from the Louisville Seamount Chain via seafloor drilling. Bacterial 16S rRNA genes were amplified for the V4V6 region from enrichment incubations started during Expedition 330 targeting general heterotrophs (with 1% and 10% marine broth media), heterotrophic sulfur oxidizers, and heterotrophic and autotrophic iron reducers. IODP Expedition 330 website https://iodp.tamu.edu/scienceops/expeditions/louisville_seamounts.html