Award: OCE-1657663

Nitrous oxide (N2O) is potent greenhouse gas that is produced by several microbially mediated pathways, and consumed by one microbial reaction, in both marine and terrestrial environments. Both production and consumption of N2O are associated with microbes that live in the absence of oxygen and the enzymes that catalyze the reactions are inhibited by oxygen. N2O consumption, for example, is usually associated with the process of denitrification, in which bacteria reduce nitrate to dinitrogen gas (N2), with N2O reduction as the last step. Denitrification is generally restricted to anoxic environments and the N2O reduction step is very sensitive to oxygen. Thus it was surprising to find the genes that encode those enzymes, and to detect the N2O transformation processes, in surface ocean waters, which are fully oxygenated. In this project, we measured the rates of N2O production and consumption using stable isotope tracer experiments and characterized the N2O consuming assemblage using methods based on DNA sequences. Our findings substantiate the existence of microbes capable of N2O production and consumption in the surface ocean. Both processes are, however, inhibited by oxygen, so the in situ rates are low. We suggest that the potential for N2O consumption in surface waters represents a survival strategy for microbes that might experience temporary anoxia, or might encounter fluxes of N2O from nearby anoxic environments. The genes that encode the N2O consumption reaction are the type usually associated with microbes that do not carry out the full denitrification pathway. I.e., they are probably not capable of converting nitrate to N2, but only perform the last step, conversion of N2O to N2. The scientific and practical implications of these findings are that we must rethink our understanding of what controls the flux of N2O from the ocean to the atmosphere: surface waters could be an important barrier to diffusion from N2O sources in deeper water and sediments. If the microbes doing the consumption are unconventional denitrifiers, for example, capable of N2O respiration but not nitrate or nitrite respiration, that would imply a different set of environmental controls on the process in surface waters compared to anoxic environments. The production of N2O in surface waters is usually attributed to nitrification, the oxidation of ammonia to nitrite and nitrate, which is an aerobic process, generally carried out by microbes that require oxygen. Our experiments indicate that multiple pathways are involved and none of the currently proposed mechanisms fully explain our data. Thus our field data will inform future laboratory experiments with microbial cultures in order to investigate the pathways and mechanisms of N2O production. The project was focused around a month-long research cruise in the ocean offshore of western Mexico, where a large volume of low to zero oxygen water naturally occurs. Its broader impacts include wide participation on the sciences by a range of people and the development of human resources in students. It involved participation by several international partners, several graduate and undergraduate students and postdoctoral scientists. In addition, the PI transmitted videos of cruise activities back to the Princeton campus where they were used in her biological oceanography class to introduce students to research at sea. The videos have since been shared in other outreach activities by other members of the research team. The research has led to publication of ~12 papers so far, with others in preparation and review. It also motivates further research on the modularity of denitrification - the uncoupling of N2O reduction from the rest of the process - and exploration of the microbes responsible for these processes using molecular biological tools. Last Modified: 10/13/2021 Submitted by: Bess B Ward