We investigated oceanic dissolved inorganic nitrogen (N) dynamics, focussing on processes removing bio-available N and ultimately affecting primary productivity, in sulfidic hydrothermal vent fluids discharging from the subsurface on the Juan de Fuca Ridge (northeast Pacific Ocean) and in anoxic bottom waters of Saanich Inlet, a British Columbia fjord, using a combination of geochemical and molecular microbial ecology techniques. During episodes of mixing with oxygenated sea-water, both systems can switch from anoxic to oxic conditions. Strong inter-site variations in the concentrations and δ15N of ammonium (NH4+) in high-T fluids suggested different N sources (deep-sea nitrate (NO3-) versus organic sediments) for hydrothermally discharged NH4+. Increase in the isotopic composition of NO3- (δ15N and δ18O), concomitant with decreased [NO3-], indicated NO3- assimilation or denitrification in the subsurface. NO3- isotope anomalies, i.e. deviations from the 15N:18O isotopic enrichment of 1:1 in marine environments, were observed and confirmed the occurrence of NO3- regeneration in vent fluids. Denitrification was the dominant N-loss pathway, suggesting that bacterial denitrification out-competes anaerobic NH4+ oxidation (anammox) in diffuse hydrothermal vent waters. The diversity of denitrifying bacteria encoding the nirS-form of nitrite reductase was low in vent fluids. Quantitative polymerase chain reaction (qPCR) analysis revealed that denitrifiers accounted for up to 38% (nirk-encoding γ-proteobacteria of the SUP05 cluster) and 8% (nirS-encoding bacteria) of the total bacterial abundance. Furthermore, nirS gene operational taxonomic units from two vent fields clustered into different groups in the phylogenetic tree, suggesting a link between denitrifying bacterial community membership and small-scale geographic isolation and/or fluid physico-chemical properties. Significant correlations existed between fixed N-loss rates and in-situ dissolved inorganic N deficits in the fluids. Based on our rate measurements, and on published data on hydrothermal fluid fluxes and residence times, we estimated that up to ~10 Tg N yr−1 could be removed globally in the subsurface biosphere. In Saanich Inlet, a gradual increase in both the δ15N and δ18O of NO3- associated with a decrease in [NO3-] and an increase in biological excess N2, was observed after bottom water renewal events in fall 2008, following NO3- consumption by denitrifiers in an expanding suboxic zone. N-to-O negative NO3- isotope anomalies were observed in surface and bottom waters, confirming the occurrence of NO3- regeneration and/or external NO3- input. Closed and open-system-model derived NO3- isotope effects in anoxic bottom waters were lower (as low as ~11‰) than the ~25‰ for water column denitrification reported in other studies, suggesting that ~50% of the denitrification could occur, with a highly suppressed isotope effect, in the sediments of the Inlet. / Graduate