The conventional view of the aquatic nitrogen (N) cycle is centered around organic nitrogen (R-NH₂) remineralization to ammonium. Remineralized ammonium is subsequently oxidized (via nitrification) to inorganic NO_x providing the oxidized nitrogen needed to support nitrogen removal by denitrification and/or anammox. However, organic N can also exist in an oxidized state (R-NO_x), whereby its remineralization can directly generate inorganic NO_x – bypassing the nitrification branch of the nitrogen cycle. While turnover of organic NO_x is thought to be widespread in nature, the extent to which this oxidized organic N pool is utilized by microorganisms involved in the nitrogen cycle remains unconstrained. We report that denitrifying microbes in freshwater sediments are capable of converting organic NO_x in the form of simple nitro-substituted alkanes to N₂ gas, contributing to the removal of fixed N. Our estimates based on the turnover of laboratory-synthesized ¹⁵N-labelled organic NO_x indicated that organic NO_x driven denitrification (ONDD) can match rates of canonical denitrification, fueled by the sediment nitrate flux. In the studied sediments, uncultured microbes within the order Burkholderiales likely contributed to ONDD. These bacteria carry not only the conventional denitrification enzymes, but also the nitronate monooxygenase (nmo) – a key enzyme that catalyzes the release of nitrate/nitrite from organic NO_x using oxygen. We propose that organic NO_x and its transformation represent a new facet of the lacustrine nitrogen cycle.

Tutorial/Invited: Invited

Primary Presenter: Cameron Callbeck, University of Basel (cameron.callbeck@unibas.ch)

Authors:

Cameron Callbeck, University of Basel  (cameron.callbeck@unibas.ch)

Dimitri Meier, ETH Zurich  (dimitri.meier@usys.ethz.ch)

Alessandra Mazzoli, University of Basel  (alessandra.mazzoli@unibas.ch)

Tim Paulus, University of Basel  (tim.paulus@unibas.ch)

Kathrin Baumann, Eawag  (kathrin.baumann@eawag.ch)

Patrick Kathriner, Eawag  (Patrick.Kathriner@eawag.ch)

David Janssen, Eawag  (David.Janssen@eawag.ch)

Helmut Bürgmann, Eawag  (helmut.buergmann@eawag.ch)

Carsten Schubert, Eawag  (Carsten.Schubert@eawag.ch)

Moritz Lehmann, University of Basel  (moritz.lehmann@unibas.ch)