Nitrous oxide (N₂O) is a potent greenhouse gas and ozone-depleting agent. The ocean contributes about 20% of the global N₂O emissions to the atmosphere, but how the marine N₂O cycle responds to environmental and climate changes is not well constrained. N₂O is produced and consumed by microbes. N₂O can be produced from nitrification, nitrifier-denitrification and denitrification, but it only has one major consumption pathway, the reduction of N₂O to N₂. However, the consumption of N₂O is much less studied than its production in marine systems. In particular, the lack of kinetics parameters and oxygen sensitivity for marine N₂O consumption hinders the accurate prediction of N₂O cycling in a changing environment.

When we started to perform N₂O consumption experiments, we understood why N₂O consumption is less studied. Measuring N₂O consumption is not trivial, because both the substrate and product of the reaction are gases. Adding N₂O gas and measuring N₂ gas are technically difficult and I feel crazy working all day with a syringe that appears to be completely empty. Handling gases can easily introduce big errors, partly because you cannot visually verify volumes or transfers, or note leaks. To get more reliable results, we used multiple replicate timepoints: 15 incubation bottles (3 bottles per time point * 5 time points) to determine one rate. In total, we sampled, incubated and measured ~3000 bottles to explore N₂O consumption in the current work.

Figure 1 one of the ~3000 bottles with seawater from the Eastern Tropical North Pacific.

We went to the Eastern Tropical North Pacific (ETNP) Ocean in 2018 on R/V Sally Ride to investigate N₂O cycling. The ETNP is one of the three major oxygen minimum zones. Marine oxygen minimum zones, featuring a full spectrum of oxygen conditions (from oxic to anoxic), are regions of intense N₂O production and consumption. 

Of course, science comes with surprises. While N₂O consumption was thought to be restricted to anoxic zones, we had previously found DNA and transcripts of N₂O-consuming microbes in both oxic and anoxic layers of the Eastern Tropical South Pacific oxygen minimum zone (Sun et al., 2017). Here in the ETNP, we for the first time experimentally proved that N₂O-consuming microbes from the oxic layer not only consume N₂O, but do so at a much faster rate than microbes from anoxic zones, if anoxia occurs! This points to the importance of a dynamic region: the fluctuating oxycline of the marine oxygen minimum zone, which will stimulate N₂O consumption and probably also N₂O production.

Figure 2. Last day on R/V Sally Ride before landing Manzanillo, Mexico.

The global implications of this newly found N₂O sink in oxic layers depend on how N₂O consumption in both oxic and anoxic layers responds to environmental conditions. We provide previously lacking information for predicting the marine N₂O budget by experimentally determining kinetic parameters in both oxic and anoxic seawater. We then applied these parameters to a biogeochemical model, which improves the prediction of N₂O concentration profiles. This study is inspired by the discovery of N₂O-consuming microbes in oxic seawater. Thus, understanding microbes in a changing environment can help us understand and better predict biogeochemical cycles.

Check out our paper here: https://www.nature.com/articles/s41396-020-00861-2

I want to thank Professor Bess B Ward for her valuable editorial advice on this post.

Xin Sun did this work when she was a graduate student in Professor Bess B Ward’s lab in the Department of Geosciences at Princeton University. Xin became a Hutchinson Postdoctoral Fellow at the Institute for Biospheric Studies and the Department of Ecology and Evolutionary Biology at Yale University in December 2020.