LAUSR

The marine N cycle is driven mainly by microorganisms whose distribution can be shaped by mesoscale eddies. Recently, eddies containing oxygen minimum zones (OMZs) have been recognised as N-loss hotspots, complicating even more the calculations of the marine N budgets. As a contribution to this understanding, we investigated the effect of a cyclonic eddy confined in an oxygen-depleted basin on the distribution of the N-cycling communities. We measured hydrographic properties of the water column, nutrient concentrations, and the abundance of key genes involved in the processes of nitrification (archaeal and bacterial amoA), denitrification (nirS and nirK), dissimilatory nitrate reduction to ammonia (DNRA; nrfA) and anammox (hzo) within the eddy. Our results indicated that the cyclonic circulation affected the distribution of nutrients and the abundance of amoA and nirS genes, whereas oxygen influenced the distribution of nirK, nrfA, and hzo genes. Additionally, the aerobic ammonium oxidation by archaea seems to be an important source of nitrite, which might fuel denitrifier, DNRA, and anammox communities in this basin. These findings along with the high N deficit in the OMZ suggest the existence of an active N cycling that might enhance the N-loss in this oxygen-depleted basin.