Award: OCE-1745589

The global ocean comprises Earth's largest microbiome, with at least half of the ocean's microbial biomass occurring beneath the ocean floor. In particular, oceanic crust encompasses the largest aquifer on Earth, with a liquid volume equal to approximately 2% of the ocean's volume. It also harbors a substantial reservoir of microbial life that may influence global-scale biogeochemical cycles. This project investigates this largest actively flowing aquifer system on Earth- the fluids circulating through oceanic crust underlying the oceans and sediments. Despite advancing knowledge about life in the deep ocean, the understanding of microorganisms in the rocky oceanic crust and the fluids flowing through it remains rudimentary. This project was focused on understanding the linkages between microbial activity and the cycling of carbon in the cool, subseafloor biosphere. The balance between organic carbon-consuming and organic carbon-producing metabolisms within the crustal biosphere was determined using seafloor observatories put in place by the International Ocean Discovery Program (IODP) drilling program on the western flank of the Mid-Atlantic Ridge, a plate boundary located along the seafloor of the Atlantic Ocean at a depth of 4,500 meters (14,500 feet). On a research expedition, we sampled the crustal fluid samples from the borehole seafloor observatories with the remotely operated vehicle Jason. The samples were brought back to the ship and lab to determine rates of microbial transformations of carbon, as well as determine which microbes are present by sequencing their DNA and RNA. Through these analyses, the team discovered that the subseafloor microbial community is active and poised to eat, despite an environment with low biomass and low-carbon conditions. The microbes beneath the seafloor appear to be quite resourceful, using both carbon dioxide as well as organic carbon at rates similar to those in highly productive deep-sea ecosystems, such as hydrothermal vents. Our work represents a significant cross-disciplinary advance in the understanding of the microbial ecology and geochemistry of the subseafloor biosphere in young oceanic crust and their role in maintaining global deep-sea redox balance. This new knowledge about carbon cycling in the deep ocean also informs decision-making related to emergent industrial uses of the deep sea, including subseafloor carbon sequestration and deep-sea mining. In addition to our scientific goals, we also trained multiple early career researchers, including 3 postdoctoral investigators, all of whom are now in their own faculty positions at both public and private universities. We promoted science, technology, engineering and math literacy among community college and undergraduate students through hand-on research experiences, peer-to-peer mentoring, and professional development opportunities. Two undergraduates were directly involved in our research and are co-authors on published manuscripts. Both graduated from a 4-year college with a STEM degree and are pursuing careers and further studies in STEM. Last Modified: 11/02/2021 Submitted by: Julie A Huber