Metagenome-assembled genomes reveal water-mass-associated distribution ecotypes of marine Archaea in the Western Arctic Ocean

Archaea are important contributors to marine biogeochemical cycling, yet their ecology remains poorly characterized in the Arctic Ocean. The three dominant marine archaeal lineages, Marine Groups I, II, and III (MG-I, MG-II, and MG-III), are generally reported to have distinct, clade-consistent vertical distributions that are attributed to light penetration and temperature. Here, we used shotgun metagenomic sequencing of seawater collected during the 2015 U.S. Arctic GEOTRACES expedition (GN01) to recover archaeal metagenome-assembled genomes (MAGs) from the Western Arctic Ocean and to relate their relative abundances to water mass structure. Temperature, salinity, silicate, and N* distinguished four water mass layers: the summer surface layer, the Polar Mixed Layer, Atlantic Water, and Arctic Ocean basin water. Of 155 MAGs recovered from a metagenomic co-assembly, 17 were archaeal and represented all three marine groups (four MG-I, eleven MG-II, and two MG-III). Abundance-based clustering of samples from two deep central Arctic stations separated microbial communities according to water mass. Mapping MAG relative abundance across several hundred samples revealed surface, mid-depth, and deep distribution ecotypes; members of every marine group displayed more than one ecotype, and ecotype was not consistent with MAG phylogeny. These results indicate that archaeal distributions in the Western Arctic Ocean are more closely tied to water mass stratification than to clade identity, and that previously reported clade-specific depth distributions of marine Archaea may partly reflect the limited resolution of amplicon-based surveys and shallow sampling.