Collaborative Research: Nitrogen cycling under pressure: Measuring N-loss and N-recycling across redox gradients at deep-sea hydrothermal vents

This collaborative research project will explore how deep-sea hydrothermal vent ecosystems use the important nutrient element nitrogen, and how these environments influence the broader deep-ocean nitrogen cycle. Despite the intense biological activity at hydrothermal vents, their role in deep-ocean elemental cycling remains poorly understood. A key challenge is that previous studies of nitrogen cycling in these systems have been conducted under atmospheric pressure, whereas hydrothermal vent organisms live under extreme high-pressure conditions. This discrepancy raises concerns about how pressure changes might have affected previous nitrogen cycling experiments conducted on samples from these systems. To address this, this project will employ specialized equipment to conduct nitrogen cycling experiments at deep-sea pressures. By doing so, the investigators will enhance understanding of hydrothermal vents' contributions to the oceanic nitrogen cycle. Additionally, the project will support two Ph.D. students and a Postdoctoral Scholar who will take on a key leadership role. Leveraging the recent merger between Arizona State University (ASU) and the Bermuda Institute of Ocean Sciences (BIOS), the team will also collaborate with the Director of BIOS Education and Community Engagement to create ship-to-shore virtual field trips for classrooms in the U.S., and develop a lesson plan which will be shared with high school educators. Current understanding of the role that deep-sea hydrothermal vent ecosystems play in nitrogen (N) cycling in the ocean is plagued by inaccuracies arising from microbial N-cycling rate measurements made under atmospheric pressure, previously unmeasured N-cycling pathways, and overlooked contributions from characteristic vent habitats. This project will address these issues using 15N-labeled tracer incubations to measure rates of loss, recycling, and microbial assimilation of bioavailable N across reduction-oxidation (redox) gradients at two distinct seafloor vent habitats. The investigators will perform incubations on low-temperature vent fluids emanating from bare rock and faunal assemblages. They hypothesize that the more gradual redox gradients within faunal assemblages will favor N-recycling processes (i.e., nitrification and dissimilatory nitrate reduction to ammonium) that have not been measured at the seafloor. The team will measure rates of N-cycling processes in parallel under atmospheric and in situ pressures at the Endeavour hydrothermal vents, where previous evidence has indicated a tight coupling between reductive and oxidative processes. Molecular genetic data produced from environmental samples and tracer incubations will be used to identify the relevant microbial taxa involved in the different pathways of N cycling. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.