Our understanding of the global marine xed nitrogen budget has undergone rapid growth, and as a result there is debate as to whether or not it is balanced. The Arctic plays a disproportionately large role in the sink terms of this budget. This paper works to understand the role of the Canada Basin in the nitrogen cycle. We utilize two tracers of denitri cation: N2/Ar, a dissolved gas tracer, and N*, a nutrient ratio tracer. We aim to quantify the current state of nitrogen cycling in the Canada Basin, and determine its role in the global cycle. Our paired tracer method provides support for shelf denitri cation rates while providing an estimate of ventilation in the same water mass, and provides an estimate for deep benthic denitri cation rates. We observe a disconnect between N2/Ar and N* in the Paci c Upper Halocline Layer (PUHL), wherein the excess N2/Ar we expect from N* is nearly 250% larger than the excess we observe. Our calculations suggest that an approximate steady state between benthic denitri cation and gas exchange on the Chukchi shelf maintains this disconnect. Our measurements of the PUHL support the shelf denitri cation rates reported from direct measurements, and can predict wind speeds required for ventilation within a factor of two. A 1D diffusion model of the old deep waters of the Canada Basin supports benthic denitri cation rates of 0.095-0.15 Tg N y-1. Benthic denitri cation rates determined from the model are on the low end of rates in other deep basins. Our results suggest additional measurements of these tracers in the Canada Basin and surrounding areas would help to constrain both the physical and biological processes controlling nitrogen cycling. / Graduate / 0425 / jen.l.reeve@gmail.com

Nitrogen fixation is an ecologically important microbial process that can contribute bioavailable combined N to habitats low in N. Shipworms, or wood-boring bivalves, host N2-fixing and cellulolytic symbiotic bacteria in gill bacteriocytes, which have been implicated as a necessary adaptation to an N-poor C-rich (wooden) diet. Shipworm symbionts are known to fix N within the gill habitat and newly fixed N is subsequently incorporated into non-symbiont containing host tissue. The presence of N2-fixation in gill bacteriocytes presents a conundrum because N2-fixation is tightly regulated by oxygen in most other diazotrophic microbes. Also, the direct evidence of new N being incorporated into the host tissue indicates that there are potentially complex nutrient cycles in this symbiosis, which have not been investigated. We used the cultivated symbiont Teredinibacter turnerae, which has been isolated from many shipworm species, as a model organism to elucidate controls on N2-fixation and N release in the shipworm symbiosis. Our results indicate that headspace oxygen concentration does not control biomass specific N2-fixation and respiration activity in T. turnerae, but it does influence the magnitude of the growth rate and timing of culture growth. Also, we examined the controls of oxygen on inorganic nutrient uptake rates, and documented a small amount of dissolved inorganic nitrogen release. While the N budget is only partially balanced, we provide indirect evidence for the allocation of fixed N to the excretion of exopolymeric substances and dissolved organic nitrogen; future studies that measure these additional N sinks are necessary to close the N budget. Although there are limitations of using pure cultures to investigate a complex symbiotic system, this study provides direct experimental evidence that T. turnerae has adaptations that are conducive to N2-fixation in gill bacteriocytes.