Mechanistic understanding of nitrate reduction as the dominant production pathway of nitrous oxide in marine oxygen minimum zones.

Abstract

Nitrous oxide (N₂O), a potent greenhouse gas and ozone-depleting agent, is produced intensely in oxygen minimum zones (OMZs) predominantly through nitrate reduction <math xmlns="http://www.w3.org/1998/Math/MathML"> <mfenced> <mrow> <msubsup><mrow><mi>NO</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>-</mo></mrow> </msubsup> <mo>→</mo> <msub><mrow><mi>N</mi></mrow> <mrow><mn>2</mn></mrow> </msub> <mi>O</mi></mrow> </mfenced> </math> . However, mechanisms and controls of this pathway remain unclear. Here, we investigate the microbial ecology governing this pathway using experiments and an ecosystem model. We experimentally confirm a critical hypothesis: most <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>NO</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>-</mo></mrow> </msubsup> <mo>→</mo> <msub><mrow><mi>N</mi></mrow> <mrow><mn>2</mn></mrow> </msub> <mi>O</mi></math> denitrifiers do not utilize extracellular nitrite, an intermediate of the pathway. Model results demonstrate that the <math xmlns="http://www.w3.org/1998/Math/MathML"> <msubsup><mrow><mi>NO</mi></mrow> <mrow><mn>3</mn></mrow> <mrow><mo>-</mo></mrow> </msubsup> <mo>→</mo> <msub><mrow><mi>N</mi></mrow> <mrow><mn>2</mn></mrow> </msub> <mi>O</mi></math> pathway is compatible with oxygen, and that its response to oxygen is heterogeneous because it is governed by niche partitioning of distinct microbial types and thus may not follow a smooth curve. Lastly, experiments demonstrate that this pathway is sensitive to the type of organic matter, its electron acceptor, in addition to organic matter availability. These findings advance our mechanistic understanding of the primary N₂O production pathway, necessary for predictions of marine N₂O emissions.