Processes and microorganisms driving nitrous oxide production in the Benguela Upwelling System

Abstract

Upwelling systems and their associated oxygen deficient zones (ODZs) are hotspots of nitrous oxide (N₂O) production in the ocean. The Benguela Upwelling System (BUS) is a highly productive region and an important, yet variable source of N₂O to the atmosphere. This study examined underlying processes and microbial key players governing N₂O production in the BUS during the austral winter. ¹⁵N-tracer incubation experiments were conducted to track N₂O production from NH₄⁺ oxidation and denitrification. N₂O production and consumption mechanisms over a longer temporal scale were determined through natural-abundance isotope analyses. Metagenomics and 16S rRNA gene amplicon sequencing were used to identify potential key prokaryotes driving N₂O production. Our results showed that, compared with permanent ODZs, the BUS is characterized by a higher oxidative and a lower reductive N₂O production, both of which exhibit substantial spatial variability. N₂O production peaked in low-oxygen (O₂) waters, with nearly equal contributions of oxidative and reductive processes, suggesting their co-occurrence across an O₂ concentration range broader than previously thought. However, the observed N₂O isotope signatures implied a legacy of recent and extensive N₂O reduction to N₂. Metagenomic and 16S rRNA gene data identified denitrifiers belonging to Thioglobaceae and the archaeal ammonia-oxidizers Nitrosopumilaceae among the potential key drivers of N₂O production. Our study provides a comprehensive picture of N₂O production in the BUS, revealing significant variability in the N-cycling regime and underlying N₂O production mechanisms, and demonstrating the value of combining direct rate measurements with more integrative approaches, such as molecular omics and natural-abundance stable isotope tracers.