Nitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of the total natural flux to the atmosphere. In oxygen‐deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise in the ETNP were analyzed to examine sources of and controls on N2O cycling across this region. Along the north‐south transect, three distinct biogeochemical regimes were identified: background, core‐ODZ, and high‐N2O stations. Background stations were characterized by smaller variations in N2O concentration and isotopic profiles relative to the other regimes. Core‐ODZ stations were characterized by co‐occurring N2O production and consumption at anoxic depths, indicated by high δ18O‐N2O (>90‰) and low δ15N2Oβ (<−10‰) values, and confirmed by a time‐dependent model, which indicated that N2O production via denitrification was significant and may occur with a nonzero site preference. High‐N2O stations, located at the periphery of a mesoscale eddy, were defined by N2O reaching 126.07 ± 12.6 nM and low oxygen concentrations expanding into near‐surface isopycnals. At these stations, model results indicated significant N2O production from ammonia‐oxidizing archaea and denitrification from nitrate at the N2O maximum within the oxycline, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O production mechanisms to a mesoscale eddy through isotopocule measurements, suggesting the importance of eddies to spatiotemporal variability in N2O cycling and emissions from this region.

Specialized molecules, such as siderophores, are used to access and retain iron in soluble forms by marine microorganisms. These siderophores form part of the ocean dissolved iron-binding ligand pool and are hypothesized to exert a key control on the persistence of iron in hydrothermal environments. To explore this hypothesis, we measured iron, iron-binding ligands, and siderophores from 11 geochemically distinct sites along a 1,700 km section of the Mid-Atlantic Ridge. We found siderophores at all sites and proximity to the vent played an important role in dictating siderophore types and diversity. The notable presence of amphiphilic siderophores may enable microbes to access particulate iron in hydrothermal plumes. The tight coupling between strong ligands and dissolved iron across six distinct hydrothermal environments, combined with the local presence of siderophore producing microbial genera suggests that biological production of siderophores exerts a key control on hydrothermal dissolved iron concentrations.

Nitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of total flux to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes that are controlled by a variety of oceanographic conditions. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise to the ETNP were analyzed to examine heterogeneity in N2O cycling across the region. Along the north-south transect, three distinct biogeochemical regimes were identified: background, core-ODZ, and high-N2O stations. Background stations were characterized by less dynamic N2O cycling. Core-ODZ stations were characterized by co-occurring N2O production and consumption at anoxic depths, indicated by high δ18O (> 90values, and confirmed by a time-dependent model, which indicated that N2O production via denitrification was significant and may occur with a non-zero site preference. High-N2O stations were defined by [N2O] reaching 126.07±12.6 nM, low oxygen concentrations expanding into near-surface isopycnals, and the presence of a mesoscale eddy. At these stations, model results indicated significant N2O production from ammonia-oxidizing archaea and denitrification from nitrate in the near-surface N2O maximum, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O isotopocule measurements to a mesoscale eddy, suggesting the importance of eddies to the spatiotemporal variability in N2O cycling in this region.

Nitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of total flux to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes that are controlled by a variety of oceanographic conditions. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise to the ETNP were analyzed to examine heterogeneity in N2O cycling across the region. Along the north-south transect, three distinct biogeochemical regimes were identified: background, core-ODZ, and high-N2O stations. Background stations were characterized by less dynamic N2O cycling. Core-ODZ stations were characterized by co-occurring N2O production and consumption at anoxic depths, indicated by high δ18O (> 90values, and confirmed by a time-dependent model, which indicated that N2O production via denitrification was significant and may occur with a non-zero site preference. High-N2O stations were defined by [N2O] reaching 126.07±12.6 nM, low oxygen concentrations expanding into near-surface isopycnals, and the presence of a mesoscale eddy. At these stations, model results indicated significant N2O production from ammonia-oxidizing archaea and denitrification from nitrate in the near-surface N2O maximum, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O isotopocule measurements to a mesoscale eddy, suggesting the importance of eddies to the spatiotemporal variability in N2O cycling in this region.