Nitrogen is an essential nutrient for all organisms on earth. Primary production in most of the surface ocean is limited by inorganic fixed nitrogen. Diverse groups of microbes transform nitrogen between fixed and gaseous forms, including the greenhouse gas and ozone-depleting agent, N2O. Oxygen concentration is a key variable in determining the fixed nitrogen budget because the loss of fixed nitrogen can only occur under low oxygen conditions, while the retention of nitrogen usually requires oxygen. Marine oxygen minimum zones (OMZs) are unique regions with a full spectrum of oxygen conditions, which allows the retention and the loss of fixed nitrogen to occur within the water column of one OMZ. Despite the small volume of OMZs, they contribute up to 30% of oceanic fixed nitrogen loss. OMZs are predicted to expand in response to climate change. Thus, quantitative information on the responses of nitrogen cycling processes to changes, especially oxygen conditions, in OMZs, is needed to accurately predict feedbacks of the marine nitrogen cycle on the global climate.This dissertation focuses on investigation of nitrogen cycling processes occurring across natural oxygen gradients in OMZs using a combination of chemical, microbiological, and modeling methods. First, the major biological sink of N2O was assumed to occur only under anoxic conditions. This dissertation showed, however, that N2O consuming microbes were present and transcriptionally active in oxygenated seawater. These microbes initiated N2O consumption rapidly after switching to anoxic conditions, and their consumption rates were much higher than those from anoxic seawaters. Second, nitrite oxidation to nitrate, which retains fixed nitrogen, is thought to be restricted to oxic environments. Surprisingly, we found two novel nitrite-oxidizing bacteria from anoxic seawaters. We also presented the first direct experimental evidence of anaerobic nitrite oxidation in samples collected from anoxic waters. Including