Poster Abstract Title: 
Denitrication as the dominant N-elimination process in hydrothermal vents of the Juan de Fuca Ridge, North-East Pacific Ocean
Authors and their affiliations: 
Annie Bourbonnais(a), Moritz F. Lehmann(b), David A. Butterfield(c,d), Marcel M. M. Kuypers(e), Gaute Lavik(e) and S. Kim Juniper(a) (a)School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada, V8P 5C2 (b)Institute for Environmental Geoscience, Basel University, CH-4056, Basel, Switzerland (c)Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, USA, 98105-5672 (d)Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA, 98115 (e)Max Planck Institute for Marine Microbiology (MPI), Department of Biogeochemistry, Celsiusstrasse 1, 28359 Bremen, Germany

Nitrogen (N) is an essential macronutrient for all organisms. Therefore, oceanic N sinks, removing bio-available (or fixed) N, ultimately affect primary productivity. The relative importance of the two main N-elimination pathways, i.e. denitrification and anaerobic ammonium oxidation (anammox) in different oceanic environments and in the global ocean is still a matter of debate. Little is known about metabolic processes and bacterially-mediated N-cycle dynamics occurring in the subsurface biosphere of hydrothermal vent systems. Rates of major N-elimination processes have never directly been quantified in diffuse vent fluids. In this study, we measured rates of major fixed N-elimination pathways (denitrification, anammox) and dissimilative nitrate reduction to ammonium (DNRA) in hydrothermal vent fluids at 7 different sites on the Juan de Fuca Ridge using 15N-label incubations. We also measured the isotopic composition of dissolved inorganic nitrogen (DIN) (nitrate and ammonium) and N2O concentrations, an intermediate product of denitrification. All samples were collected during a cruise in the Northeast Pacific Ocean onboard the R/V Atlantis in June 2009. Elevated nitrate δ15N and δ18O in the high-Mg2+, low-T vent fluids associated with a [NO3-] decrease is indicative of dissimilatory (denitrification) or assimilatory (nitrate uptake) nitrate consumption. Denitrification rates in hydrothermal vent fluids at Axial Volcano and Endeavour Segment are high and variable between sites, ranging from ~30 (Fairy Castle, Main Endeavour Field) to 1040 (Hermosa, Axial Volcano) nM/day. [N2O] ranged between ~0-430 nM, with no observable correlation with denitrification rates. Anammox rates are below ~5 nM/day at 3 sites and not detectable at all other sites. DNRA rates are ranging from ~0 to 200 nM/day. These results suggest that denitrification is by far the dominant N sink in hydrothermal vent fluids of the Juan de Fuca Ridge. In agreement with previous studies, our results suggest that, at least in sulfidic waters, nitrate reduction by bacteria (possibly associated with sulphide oxidation) can out-compete anammox bacteria. Future microbiological work will evaluate the identity and abundance of the denitrifying bacteria present in vent fluids.

Contributions to Integration and Synthesis: 
As mentioned above, Nitrogen (N) is an essential macronutrient for all living organisms. This work represents the first rate measurements of N-cycle processes in diffuse hydrothermal vent fluids. This research is crucial for integration and synthesis at the R2K meeting because it allows to better understand the fluxes or N in the subsurface biosphere of hydrothermal vent systems, with important implications for chemosynthetic primary productivity.