Nitrous oxide cycling in oxygen-depleted marine waters: pathways, controls and dynamics

The ocean is a major source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas and ozone destroyer, and oxygen (O2)-depleted marine waters are hotspots of N2O accumulation and emission.

N2O is a product of microbial nitrogen (N) transformations, but we lack a clear understanding of which microbes and bio(geo)chemical mechanisms are involved, and how these are controlled by environmental factors.

Filling this knowledge gap is essential, as O2-depleted regions are expanding and increasing fluxes of reactive N from land to sea stimulate marine N cycling, which may further enhance N2O accumulation.We propose an experimental investigation of N2O dynamics in the O2-depleted waters of the eastern tropical North Pacific and central Baltic as model systems, to elucidate pathways, quantify rates and identify environmental controls of N2O production and consumption, and identify the key microbes involved and their metabolic capabilities.

Recent advancements allow us to address these aims; the development of a dual isotope labeling technique for separating pathways of production, highly sensitive optical O2 sensing techniques, and an in situ incubation system which avoids the otherwise inevitable O2 contamination associated with sample recovery.

The resulting conceptual framework and kinetic parameters will provide valuable input for mechanistic predictive modelling of oceanic N2O emissions in a changing world and also guide marine management aimed at reducing N2O emissions.