Thaumarchaeota formerly known as Marine Group I Crenarchaeota are highly abundant in the worlds oceans, making up ~20% of the total prokaryotic population. Members of the Thaumarchaeota are capable of oxidizing ammonia using the ammonia monooxygenase enzyme (amoA), which is the first step in nitrification and a key process in the global nitrogen cycle. They are most abundant in deeper, colder waters with appearances in surface waters generally limited to higher latitudes and polar oceans in winter. Reasons for this distribution have been postulated, but no definitive explanation has been found to date. A hypothesis tested with this work is that reactive oxygen species (ROS), particularly hydrogen peroxide (H2O2), play a role in exclusion of Thaumarchaeota from surface waters.This dissertation examines the spatial distribution of Thaumarchaeota in coastal and open-ocean, polar and temperate marine environments, where correlations with increased depth and decreased oxygen were common regardless of sample site. We also investigated the temporal distribution of Thaumarchaeota on Sapelo Island, Georgia, where annual spikes in abundance correlated to summer conditions (increased temperature; decreased pH, oxygen). The potential for Thaumarchaeota to use urea as an alternate substrate for ammonia oxidation was also investigated; our findings suggest that this is not a widespread attribute and is most likely due to removal of amine groups that are subsequently oxidized. Additionally, we found that nitrification is inhibited with increased [H2O2] in open ocean samples, with the most sensitive populations coming from the Southern Ocean. Populations from Sapelo Island, Georgia, were not as sensitive, but these microbial communities encounter high daily H2O2 concentrations. In conclusion, we have found that Thaumarchaeota distributions correlate to a variety of environmental factors and it is unlikely that any single one can be used to predict dynamics of the entire group. However, evidence from this work indicates that clades of Thaumarchaeota could be differentially affected by certain conditions, justifying the separation of this group into ecotypes for future studies. We have shown that direct oxidation of urea by Thaumarchaeota is unlikely, and that ROS can inhibit ammonia oxidation. This may explain why Thaumarchaeota are typically absent from surface waters.