Nitrogen and Oxygen Isotope Effects of Nitrite Oxidation in Various Species of Nitrite-oxidizing Bacteria

Abstract

Bioavailable nitrogen is a limiting nutrient in many parts of the ocean. Nitrite oxidation produces biologically available nitrate, which in turn can help regulate primary production and the absorption of carbon dioxide from the atmosphere. Despite the importance, the global mass and nitrogen isotope budgets are poorly constrained. This thesis uses dual isotope tracers of nitrogen and oxygen in nitrite to investigate bacterial nitrite oxidation isotope systematics which may be used to improve future oceanic models requiring isotope effect data. Nitrogen and oxygen isotope effects of nitrite oxidation in continuous culture from Nitrococcus mobilis, Nitrospira marina and Nitrobacter winogradskyi were recorded. Nitrogen isotope effect values ranged from 31‰ to -10‰, whereas oxygen isotope values ranged from -12‰ to 1‰. Low oxidation rates seen in N. marina and N. winogradksyi were associated with increased inverse nitrogen isotopic fractionation. Variable oxidation rates observed here may be due to differences in NXR enzyme active sites and altered demands for oxygen. Nitrogen and oxygen nitrite isotope effects for Nitrospina gracilis in batch culture are also reported (15εNXR of -11‰ and 18εNXR of -2‰). N. gracilis is of particular importance to the study of nitrogen cycling in oxygen deficient zones due to its prevalence and flexible metabolic pathways of nitrite oxidation and nitrate reduction. This thesis contributes to the field of oceanic isotope research by helping to disentangle individual isotope effects for nitrite oxidation in various species of nitrite oxidizing bacteria and shows how oxidation rate can affect isotopic fractionation.