An unaccounted for N₂O sink in the surface water of the eastern subtropical South Pacific: Physical versus biological mechanisms

Nitrous oxide (N₂O) is a trace gas affecting atmospheric radiative forcing through its greenhouse effect in the troposphere and destroying the ozone in the stratosphere. The oceans account for one-third of the global atmospheric N₂O emissions, in which they are primarily cycled by nitrification and denitrification, with high N₂O production in the subsurface waters. The surface waters are generally reported to be in equilibrium or slightly supersaturated with respect to the atmosphere. However, surface N₂O sub-saturations have been observed in several regions of the world's oceans, such as off south-central Chile, which is bathed by the Sub-Antarctic Water Mass (SAAW), where N₂O subsaturations as low as 35% have been registered during the austral spring and summer. An analysis of the mechanisms driving such surface N₂O subsaturations (physical or biological) showed that physical mechanisms were not responsible for the high surface N₂O deficit. In contrast, in situ potential experiments in surface waters with ¹⁵N₂O addition showed an active biological N₂O fixation (between 0.43 and 87.34nmol/L/d), with the highest N₂O fixation rates associated with the SAAW (25.25-25.75kg/m³).Additionally, incubation experiments with ¹⁵N₂O in surface water samples from one oceanic station showed high ¹⁵N-POM enrichment (0.44‰) and an inhibition of ¹⁵N-POM enrichment when an additional nitrogen source was added (NO₂^- and NH₄⁺). These results suggest the existence of a mechanism able to use several nitrogen sources, including N₂O. Molecular analyses (16S rRNA gene) from these experiments showed the presence of three major groups of bacteria: Gammaproteobacteria, Flavobacteria and Cyanobacteria, with Synechococcus sp. being the dominant group in the culture. However, the analysis of the nifH gene showed a taxonomic affiliation to the order Stigonematales associated with Mastigocladus sp. and Fischerella sp. and the order Oscillatoriales associated with Trichodesmium sp.Finally, the oceanic region exhibiting surface N₂O subsaturations acts as a sink for atmospheric N₂O, consuming ~11.4 Gg of N₂O in a six-month period. The N₂O levels in the sink are 75% higher than those of the reported N₂O source from the coastal band. The balance between the oceanic region and the coastal band results in a sink region of 4.94 Gg of N₂O during this period.