Collaborative Research: Deciphering the mechanisms of marine nitrous oxide cycling using stable isotopes, molecular markers and in situ rates

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Nitrous oxide (N2O) is a powerful greenhouse gas, and a driver of ozone depletion in the stratosphere. Marine low-oxygen regions are important for N2O cycling and are a large source of N2O to the atmosphere. The Arabian Sea in the northwest Indian Ocean hosts one of the world’s largest low-oxygen zones, but it is less well-studied than other regions. In partnership with other researchers in the U.S. and India, this team will combine geochemical and microbial ecology approaches to study N2O cycling in the Arabian Sea. The project will support one postdoctoral investigator and two graduate students. Several undergraduate students will participate in the laboratory and at sea. One MS student from the Cochin University of Science and Technology (CUSAT), located in Kochi, Kerala, India will also be invited on at least one of the research cruises and participate in a 6-week internship in the Marine Stable Isotope Biogeochemistry Laboratory at the University of South Carolina. The U.S. postdoc and graduate students will participate in a 10-day regional training program in biogeochemistry for early career researchers in India. The project team will conduct public outreach from sea. They will also produce a freely available E-lecture on nitrogen cycling in low-oxygen zones.This project will advance current understanding of the microbiology and biogeochemistry of N2O production and consumption in marine oxygen deficient zones (ODZs) by quantifying rates and identifying major sources in the Arabian Sea using natural abundance stable isotopic and isotopomer data, 15N-labeled incubations, as well as genomic and metatranscriptomic methods. Field work is planned for the summers of 2024 and 2025. The first field activity will be during a cruise of opportunity funded by the Office of Naval Research (ONR) as part of the Arabian Sea Transition Layer (ASTraL): Exchange Across the Air-Sea Interface program. Four ship days supported through this NSF award will enable biogeochemical sampling at ten stations. In 2025, a four-week dedicated research expedition will focus on in-situ incubation rate experiments at two stations. Both ex-situ and novel in-situ 15N-incubations, which maintain natural trace-level O2 concentrations, will be used. Kinetic experiments will also be performed to determine the effect of substrate addition during 15N- labeled incubations. Combination of these approaches will reveal dominant pathways of N2O production and N2O consumption as well as the microorganisms mediating these processes along vertical O2 and productivity gradients in the Arabian Sea ODZ. Findings will allow inferences regarding larger scale forcings and impacts of future climate change.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.