Nitrifier Diversity and Distribution Controls in the Chukchi Sea Shelf sediments

In the Arctic, the Chukchi Sea is a region with high rates of primary production. Seasonally, this phytoplankton productivity is light-limited in the spring, due to high ice cover and low sun angles, while in the summer, nitrogen (N) availability is the primary limiting factor. During the fall and winter, the combination of organic matter remineralization and convective mixing due to ice formation resets the water column N inventory. Nitrification, the sole biological oxidative process connecting the reduced and oxidized inorganic N pools, ultimately generates nitrate (NO₃^-), the most abundant form of reduced N in these waters, from ammonium (NH₄⁺). The first and rate-limiting step of nitrification, ammonia oxidation, is carried out by ammonia-oxidizing archaea (AOA) and NH₄⁺-oxidizing bacteria (AOB), both of which contain amoA genes encoding the a-subunit of ammonia monooxygenase. We hypothesized that a combination of environmental factors (organic carbon, pH, salinity, and NH₄⁺concentration) were responsible for differences in abundance and distribution of ammonia-oxidizing microorganisms throughout the Chukchi Sea sediments, a site of nitrification and thus a source of NO₃^-to phytoplankton. In this study, we used the amoA gene to examine how the AOA and AOB populations were distributed across the Chukchi Shelf with respect to water column and sediment characteristics. By quantifying and determining relatedness, we found that AOB was not only more abundant in the Chukchi Sea sediments but also more diverse. Together this data suggest that a single type of AOA dominated the AOA population in the Chukchi Sea sediments, while multiple AOB types contributed to nitrification in the Chukchi Sea sediments.