Water-mass origin and transformation shapes seawater microbiomes in the northeastern Barents Sea and the St. Anna Trough

The St. Anna Trough region is a key Eurasian Arctic gateway connecting the northeastern Barents Sea, northern Kara Sea, and central Arctic Ocean. The region is characterized by complex hydrodynamics, with multiple water masses of local and Atlantic origin interacting and undergoing transformation. In this study, we investigated whether seawater microbiome composition correlates with water-mass origin and can be used to help delineate water masses. We collected 115 seawater samples representing major regional water masses - Arctic surface water, meltwater, cold dense water, Barents Sea branch water, the recently discovered Keldysh Current water, and mixed waters - and analyzed them using 16S rRNA gene amplicon sequencing. We found that microbiome composition closely followed water mass structure, which, together with thermohaline properties and depth, explained the majority (61.2%) of microbiome composition variation. Distinct microbial patterns were associated with Atlantic and Arctic water masses, except for the Arctic surface water, which lacked a distinct microbiome and resembled those of meltwater or the Keldysh Current microbiomes. Differential abundance and network analyses revealed characteristic bacterial markers: SAR11 (Clades Ia and IV), Roseibacillus, SV1-3 (Microbacteriaceae), SAR86 clade, and AEGEAN-169 marine group in the Keldysh Current; Halieaceae, Planktomarina, NS5 marine group, and Sulfitobacter in meltwater; SUP05 cluster, SAR86, SAR92, and SAR324 clade in cold dense water; Nitrincolaceae and Amylibacter in Barents Sea branch water; Flavobacteriaceae and Chitinophagales in mixed waters. These differences were driven primarily by shifts in relative abundance of widespread Arctic bacterial taxa rather than by differential prevalence of bacterial groups. Our results demonstrate that seawater microbiomes can serve as sensitive biological tracers of water mass origin and transformation in the Eurasian Arctic. Furthermore, the distinct Keldysh Current microbiome supports its recognition as a novel hydrographic feature and indicates its role as a biological corridor transporting Atlantic-associated biota eastward toward the Kara Sea. These findings suggest that "Atlantification" of Eurasian Arctic includes a microbial component, highlighting marine microbiomes as indicators of ongoing ecological changes in the Arctic Ocean.