Award: OCE-1131671

In the past two decades a microbial ecosystem buried deep in the sediments beneath the ocean floor has been discovered, yet we still know extremely little about this hidden biosphere. Most sub-seafloor investigations and ocean drilling expeditions have focused on high productivity regions near continental margins, where organic matter export to the seafloor is high, sustaining an anaerobic subsurface microbial biome. IODP Expeditions 329 (South Pacific Gyre – Microbiology) and 336 (Mid-Atlantic-Ridge/North Pond – Microbiology) were both dedicated to studying subsurface life underlying oligotrophic open ocean regions. Wiebke Ziebis was a shipboard participating scientist of Expedition 329 and a shore-based scientist for Expedition 336. Detailed oxygen measurements during expeditions the site survey cruise to North Pond (Ziebis et al., 2012) and on the drilling expeditions 329 (DÆHondt et al., 2011, DÆHondt et al. subm.) and 336 (Orcutt et al., 2013) revealed that, in contrast to the better-studied ocean margin regions, where oxygen only penetrates a few mm or cm, the seafloor underlying oligotrophic ocean gyres is characterized by extremely deep oxygen penetration (several meters to tens of meters). In the case of the South Pacific Gyre, oxygen penetrates the entire sediment column (> 80 m) and reaches the ocean crust. It was also recently discovered at the North Pond site, which is located on the ridge flank of the Mid-Atlantic-Ridge, that oxygen is diffusing upward from the basaltic basement creating another deep zone just above the crust which is oxic (containing molecular oxygen). The presence of oxygen in deep sediment layers greatly influences the microbiology and biogeochemistry beneath the ocean floor. The discovery that the seafloor beneath ocean gyres is mainly or entirely oxic is an extremely important finding since oligotrophic open ocean regions constitute 48 % of the entire ocean. The discovery of an oxic subsurface is also in stark contrast to what we learned from studies at ocean margins, where oxygen is rapidly consumed in the surface layers and most of the sediment is anoxic. These novel findings have changed our view of the ocean floor. Yet, the microbial communities residing in this oxic deep subsurface are unknown. In fact, we have not identified the members of these deeply buried communities and it is not even clear whether for example bacteria or archaea dominate. It has been suggested, that while archaea dominate in the anoxic sediments of continental margins, bacteria might be more abundant in the oxic seafloor underlying oligotrophic ocean gyres where aerobic respiration prevails. The extremely low nutrient and carbon fluxes within mid-ocean gyres result in the WorldÆs lowest sedimentation rates and lowest abundances of microorganism per volume of sediment. We know that cells are present, yet the extremely low cell numbers render microbiological investigations exceptionally difficult and therefore these deeply buried microorganisms have been pretty elusive to science but seem to have evolved previously unknown metabolic capabilities. Through this project, we conducted experiments with samples obtained on the drilling expeditions to North Pond and the South Pacific gyre from different sediment depths representing different geological times, going back in time to up to 120 years at the South Pacific gyre site. The main goal of this project was to explore the metabolic activities of these deeply buried microbial communities using a suite of radio and stable isotopes in combination with single cell analyses. The main objective was to determine basic metabolic activities. Are these organisms heterotrophs or autotrophs? Meaning, do they depend on the uptake of organic carbon or can they fix carbon from CO2? At the heart of our investigations was the development of a protocol to extract and concentrate intact cells from these deep sub-surface sediments at the end of the incubations and from...