Bacterial abundance and diversity in 64-74 Ma subseafloor igneous basement from the Louisville Seamount Chain

The aquifer in the 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 the young basement (<10 Ma), where the basaltic crust is still porous and fluid flow through it is active. Here, we test the hypothesis that microbial life exists in subseafloor basement >65 Ma using samples collected from the Louisville Seamount Chain via seafloor drilling. Cell biomass was heterogeneous in nature, ranging from below detection to ~10⁴ cells cm^-3. Bacterial 16S rRNA genes from core samples and enrichment incubations are dominated by lineages putatively carrying out nitrogen, sulfur, and metal redox processes and hydrocarbon oxidation. Taken together, the data indicate that microbial life is indeed present in subseafloor igneous basement >65 Ma, which significantly expands the range of the subseafloor biosphere where microbial life is known to exist.