GEOBIOLOGICAL EXPLORATION OF THE DEEP SUBSEAFLOOR

Drilling provides the only direct access to the deep subseafloor—and to the vast but little-known ecosystems it harbors. Ocean Drilling Program (ODP) Leg 201 in early 2002 to the Eastern Equatorial Pacific and the Peru Margin was the first ocean drilling expedition dedicated to subseafloor microbiology. Together with three recent Integrated Ocean Drilling Program (IODP) expeditions to the Juan de Fuca ridge flank, the South Pacific Gyre, and ‘North Pond’ in the northern Mid-Atlantic, this represents the bulk of our knowledge about arguably the largest biome on Earth. The Center for Dark Energy Biosphere Investigations (C-DEBI) was created to advance our understanding of the deep subseafloor biosphere with research foci on the function and rates (activity) of global biogeochemical processes, the extent of life and degree of biogeographic connectivity, the physicochemical limits of life, and the evolution and survival adaptations of life. At the Juan de Fuca ridge, moderate hydrothermal fluids (<100 ºC) circulate the upper ocean basement rocks, generating steep geochemical gradients for diverse archaeal and bacterial communities. A network of several borehole observatory systems is now in place to operate multidisciplinary cross-hole experiments, including a long-term free-flow perturbation experiment. Geochemical data coupled with preliminary thermodynamic calculations have revealed that the oxidation of hydrogen, methane, and sulfide are the most exergonic metabolisms for anaerobic chemolithotrophs there. First geochemical data from the South Pacific Gyre showed that the sediment is oxic all the way to the basement rock, approximately 20-80 m below the seafloor, with abundant nutrient levels (e.g., nitrate, phosphate) throughout. The sediments feature extremely low metabolic activity, driven in part by radiolytically generated hydrogen, and corresponding microbial cell counts are several orders of magnitude lower than observed elsewhere in marine sediments. At North Pond, the primary questions to be addressed in the shallow sediment pond and underlying basement include the nature of microbial communities in young ridge flanks, their role in ocean crust alteration, and the sources of these deep-seated microbial communities.