The ocean is a major source of atmospheric nitrous oxide (N₂O), and oxygen (O₂)-depleted marine waters are hotspots of N₂O accumulation and emission. In such systems, N₂O is both a product of and a substrate for microbial metabolism, but we lack understanding of the pathways involved and their environmental controls. In the seasonally anoxic Saanich Inlet, British Columbia, we explored N₂O consumption in incubations of water with ¹⁵N-labelled N₂O, and examined the kinetics of the process by manipulation of N₂O, O₂, and hydrogen sulfide (H₂S) concentrations. N₂O consumption was not detectable in oxycline waters but increased steeply below the oxic-anoxic interface along with the accumulation of H₂S. Consistent with this distribution, the process was highly sensitive to O₂, with 50% inhibition at ~150 nM O₂ added. N₂O consumption exhibited Michaelis-Menten kinetics with respect to N₂O with apparent K_m values of 46 – 92 nM, and was stimulated by low amounts of H₂S (≤ 5 µM) while higher H₂S concentrations were inhibitory, showing higher sensitivity closer to the oxic-anoxic interface than deeper in the anoxic zone. Our results imply that N₂O consumption is to a wide extent coupled to the oxidation of H₂S and that the N₂O-consuming community is adapted to life near the oxic-anoxic interface, where opposing gradients of N₂O and H₂S intersect. Still, organisms deeper in the sulfidic zone retain a high capacity for N₂O consumption, that may potentially be exploited during mixing events. The anoxic waters thus act as a sink for N₂O despite the inhibitory effect of H₂S.

Primary Presenter: Laura Bristow, Univeristy of Gothenburg (laura.bristow@gu.se)

Authors:

Laura Bristow, University of Gothenburg  (laura.bristow@gu.se)

Sheryl Murdock, Bermuda Institute of Ocean Sciences, Bermuda  (sherylmurdock@gmail.com)

Jennifer Crandall, Microbiology & Cell Biology Department, Montana State University, USA  (jennifer.crandall1@student.montana.edu)

Brett Jameson, School of Earth and Ocean Sciences, University of Victoria, Canada  (bjameson@uvic.ca)

Damian Grundle, Bermuda Institute of Ocean Sciences, Bermuda  (damian.grundle@bios.edu)

Bo Thamdrup, Department of Biology, University of Southern Denmark, Denmark  (bot@biology.sdu.dk)

Frank Stewart, Microbiology & Cell Biology Department, Montana State University, USA  (frank.stewart@montana.edu)