The production and fate of nitrogen species in deep-sea hydrothermal environments

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Applied Ocean Science & Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2019.Nitrogen (N) species in hydrothermal vent fluids serve as both a nutrient and energy source for the chemosynthetic ecosystems surrounding deep-sea vents. While numerous pathways have been identified in which N-species can be produced and consumed in the context of submarine hydrothermal vent systems, their exact nature has been largely limited to interpretation of variations in concentrations. This thesis applies stable isotope approaches to further constrain the sources and fate of N-species in deep-sea vents across a variety of geological settings. First, I discuss isotope fractionation and reaction kinetics during abiotic reduction of nitrate (NO3-) to ammonium (ΣNH4+ = NH3+NH4+) under hydrothermal conditions. Results of lab experiments conducted at high temperatures and pressures revealed a wide degree of N isotope fractionation as affected by temperature, fluid/rock ratio, and pH—all of which exert control over reaction rates. Moreover, a clear pattern in terms of reaction products can be discerned with the reaction producing ΣNH4+ only at high pH, but both ΣNH4+ and N2 at low pH. This challenges previous assumptions that O3 - is always quantitatively converted to NH4+ during submarine hydrothermal circulation. Next, I report measurements of ΣNH4+ concentrations and N isotopic composition (δ15NNH4) from vent fluid samples, together with the largest compilation to date of these measurements made from other studies of deep-sea vent systems for comparison. The importance of different processes at sediment-influenced and unsedimented systems are discussed with a focus on how they ultimately yield observed vent δ15NNH4 values. Notable findings include the role that phase separation might play under some conditions and a description of how an unsedimented site from Mid-Cayman Rise with unexpectedly high NH4+ may be uniquely influenced by N2 reduction to ΣNH4+. Lastly, I explore ΣNH4+ dynamics in the context of low-temperature vent sites at 9°50’N East Pacific Rise to investigate dynamics of microbially-mediated N transformations. Through both measurements of natural samples, as well as isotopic characterization of N species from incubation experiments and model simulations thereof, an exceptionally high variability observed in δ15NNH4 values emphasizes the complexity of these microbe-rich systems. In sum, this thesis highlights the role of microbial processes in low temperature systems, demonstrates a more mechanistic understanding of lesser-understood abiotic N reactions and improves the coverage of available data on deep-sea vent ΣNH4+ measurements.This thesis research was made possible through funding by National Science Foundation (NSF) grants OCE-1537372, OCE-1559198, OCE-1136727, OCE-1061863, OCE-0702677, and OCE-0549829, and WHOI’s Ocean Ventures Fund. Funding for Net Charoenpong was provided by the Royal Thai Government Scholarship, WHOI academic program and NSF-OCE-1537372 grant

N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N-loss was calculated to be only ~13-14‰ (as compared to canonical values of ~20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16-21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2-, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss

Evidence for microbial mediation of subseafloor nitrogen redox processes at Loihi Seamount, Hawaii

© The Author(s), 2016. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 198 (2017): 131-150, doi:10.1016/j.gca.2016.10.029.The role of nitrogen cycling in submarine hydrothermal systems is far less studied than that of other biologically reactive elements such as sulfur and iron. In order to address this knowledge gap, we investigated nitrogen redox processes at Loihi Seamount, Hawaii, using a combination of biogeochemical and isotopic measurements, bioenergetic calculations and analysis of the prokaryotic community composition in venting fluids sampled during four cruises in 2006, 2008, 2009 and 2013. Concentrations of NH4+ were positively correlated to dissolved Si and negatively correlated to NO3-+NO2-, while NO2- was not correlated to NO3-+NO2-, dissolved Si or NH4+. This is indicative of hydrothermal input of NH4+ and biological mediation influencing NO2- concentrations. The stable isotope ratios of NO3- (d15N and d18O) was elevated with respect to background seawater, with d18O values exhibiting larger changes than corresponding d15N values, reflecting the occurrence of both production and reduction of NO3- by an active microbial community. d15N-NH4+ values ranged from 0‰ to +16.7‰, suggesting fractionation during consumption and potentially N-fixation as well. Bioenergetic calculations reveal that several catabolic strategies involving the reduction of NO3- and NO2- coupled to sulfide and iron oxidation could provide energy to microbes in Loihi fluids, while 16S rRNA gene sequencing of Archaea and Bacteria in the fluids reveals groups known to participate in denitrification and N-fixation. Taken together, our data support the hypothesis that microbes are mediating N-based redox processes in venting hydrothermal fluids at Loihi Seamount.This work was supported by the NSF Microbial Observatories program (MCB 0653265), the Gordon and Betty Moore Foundation (GBMF1609), NSF-OCE 0648287, the Center for Dark Energy Biosphere Investigations (C-DEBI) and the NASA Astrobiology Institute — Life Underground (NAI-LU). Sequence data was generated as part of the Alfred P. Sloan Foundation's ICoMM field project and the W. M. Keck Foundation