Distinct community composition and abundance of ammonia oxidizers in the high Arctic fjord sediments of Svalbard

Abstract

Though the process and players of nitrification are important in the ecology and biogeochemistry of Arctic fjords; there are very limited studies available on the ecology of ammonia oxidizers. Hence, we studied the abundance, diversity, and niche separation of ammonia oxidizers in the surface sediment of two high Arctic fjords (Kongsfjorden and Krossfjorden) to explore their ecology in the Arctic system. Surface sediment samples were collected from eight stations (four stations from each fjord). The diversity and abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) were analyzed using ammonia monooxygenase A (amoA) gene from the sediment metagenome. Environmental variables including bottom temperature and salinity, and sediment characteristics such as total carbon (TC), total nitrogen (TN), total sulfur (TS), and total organic carbon (TOC), were measured. AOA were more abundant than AOB in most of the samples and AOA/AOB ratios ranged from 0.63 to 17.11. AOA were dominated by Nitrosopumilus lineage (67.5%), followed by Nitrososphaera lineage (32.2%). Most of the AOB were affiliated with Nitrosospira lineage (82.2%), and comparatively lesser contribution (18.2%) was observed for Nitrosomonas. We could not observe comammox bacteria in our samples. The distribution of ammonia oxidizers had a significant influence with the total nitrogen and total organic carbon content. Both AOA and AOB showed spatial variations in community composition within the fjords and between the fjords with a domination of freshwater/soil communities in the inner fjord stations. Freshwater and soil influx from glacial melt had a significant influence in controlling the environmental variables in the fjords which in turn regulated the distribution and niche selection of ammonia oxidizers.