Abstract
Abstract. Little is known about fixed nitrogen (N) transformation and elimination at diffuse hydrothermal vents where anoxic fluids are mixed with oxygenated crustal seawater prior to discharge. Oceanic N sinks that remove bio-available N ultimately affect chemosynthetic primary productivity in these ecosystems. Using 15N paired isotope techniques, we determined potential rates of fixed N loss pathways (denitrification, anammox) and dissimilatory nitrate reduction to ammonium (DNRA) in sulfidic hydrothermal vent fluids discharging from the subsurface at several sites at Axial Volcano and the Endeavour Segment on the Juan de Fuca Ridge. We also measured physico-chemical parameters (i.e., temperature, pH, nutrients, H2S and N2O concentrations) as well as the biodiversity and abundance of chemolithoautotrophic nitrate-reducing, sulfur-oxidizing γ-proteobacteria (SUP05 cluster) using sequence analysis of amplified small subunit ribosomal RNA (16S rRNA) genes in combination with taxon-specific quantitative polymerase chain reaction (qPCR) assays. Denitrification was the dominant N loss pathway in the subsurface biosphere of the Juan de Fuca Ridge, with rates of up to ~1000 nmol N l−1 day−1. In comparison, anammox rates were always < 5 nmol N l−1 day−1 and below the detection limit at most of the sites. DNRA rates were up to ~150 nmol N l−1 day−1. These results suggest that bacterial denitrification out-competes anammox in sulfidic hydrothermal vent waters. Taxon-specific qPCR revealed that γ-proteobacteria of the SUP05 cluster sometimes dominated the microbial community (SUP05/total bacteria up to 38%). Significant correlations were found between fixed N loss (i.e., denitrification, anammox) rates and in situ nitrate and dissolved inorganic nitrogen (DIN) deficits in the fluids, indicating that DIN availability may ultimately regulate N loss in the subsurface. 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 globally be removed in the subsurface biosphere of hydrothermal vents systems, thus, representing a small fraction of the total marine N loss (~275 to > 400 Tg N yr−1).
Highlights
Nitrogen is an essential macronutrient for all organisms, and oceanic N sinks that remove biologically available N via denitrification and anaerobic ammonium (NH+4 ) oxidation limit marine primary productivity
We examined the relative importance of rates of denitrification, anammox and dissimilative NO−3 reduction to NH+4 (DNRA) in relation to specific microbial agents in diffuse hydrothermal vent fluids of the Juan de Fuca Ridge, using a combination of 15N paired isotope labelling, 16S rRNA gene clone library sequencing and quantitative polymerase chain reaction methods
In this study we report, for the first time, potential denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) rates in diffuse hydrothermal vent fluids of the Juan de Fuca ridge
Summary
Nitrogen is an essential macronutrient for all organisms, and oceanic N sinks that remove biologically available N via denitrification and anaerobic ammonium (NH+4 ) oxidation (anammox) limit marine primary productivity. Anammox, i.e., the conversion of NH+4 and nitrite (NO−2 ) to N2 gas by autotrophic anaerobic bacteria, has been reported to account for a significant part of the N loss in oceanic anoxic zones (Dalsgaard et al, 2003; Kuypers et al, 2003; 2005; Lam et al, 2009; Jensen et al, 2011). While studies of N cycling in marine ecosystems have concentrated on oxygen-deficient waters in coastal and open ocean oxygen minimum zones and sediments (see references above), little is known about metabolic processes and bacterially-mediated N-cycle dynamics that occur in the extensive subsurface biosphere of hydrothermal vent systems
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