Spatial patterns of nitrogen-transforming processes in the Southern Ocean: Potential role of Dissimilatory Nitrate Reduction (DNR)

The nitrogen cycle is crucial for aquatic systems, as it governs the availability of this essential nutrient and thereby supports the productivity of marine ecosystems. While most research in the oxygenated ocean has thus far focused on nitrogen fixation and nitrification, the potential role of dissimilatory processes remains poorly investigated. The objectives of the present study were to determine the spatial patterns of DNR processes [denitrification and dissimilatory nitrate reduction to ammonium], their associations with trace metals that are cofactors, and to identify the involved microbial taxa. We analyzed metagenomes collected along a latitudinal gradient and diverse water masses in the Indian sector of the Southern Ocean during the South West Indian GEOTRACERS section cruise. Genes involved in DNR dominated over those of other nitrogen-transforming processes across all water masses, with a notable preference for enzymes requiring copper over iron, indicating alternative trace metal adaptation. Metagenome-assembled genomes associated with DNR possessed inorganic carbon fixing pathways suggesting mixotrophic strategies. Our observations show the metagenomic potential of DNR as an alternative or additional source of energy for the microbial community in the carbon limited deep ocean.