Viral characteristics of the warm Atlantic and cold Arctic water masses in the Nordic Seas.

Nordic Seas are the subarctic seas connecting the Arctic Ocean and North Atlantic Ocean with complex water masses, experiencing an abrupt climate change. Though the knowledge of the marine virosphere has been expanded rapidly, the diversity of viruses and their relationships with host cells and water masses in the Nordic Seas remains to be fully revealed. Here we establish the Nordic Seas DNA viromes (NSVs) dataset of 55,315 viral contigs including 1,478 unique viral populations from seven stations influenced by both the warm Atlantic and cold Arctic water masses. Caudovirales dominated in the seven NSVs, especially in the warm Atlantic waters. The major giant nucleocytoplasmic large DNA viruses (NCLDVs) contributed a significant proportion of the classified viral contigs in the NSVs (32.2%), especially in the cold Arctic waters (44.9%). The distribution patterns of Caudovirales and NCLDVs were a reflection of the community structure of their hosts in the corresponding water masses and currents. Latitude, pH, and flow speed were found to be key factors influencing the microbial communities and co-influencing the variation of viral communities. Network analysis illustrated the tight coupling between the variation of viral communities and microbial communities in the Nordic Seas. This study suggests a probable linkage between the viromes, host cells and the surface water masses from both the cool Arctic and warm Atlantic Oceans. Importance: This is a systematic study of Nordic Seas Viromes using metagenomic analysis. The viral diversity, community structure, and their relationships with host cells and the complex water masses from both the cool Arctic and warm Atlantic Oceans were illustrated. The NCLDVs and Caudovirales are proposed as the viral characteristics of the cold Arctic and the warm Atlantic waters, respectively. This study provided an important background for the viromes in the subarctic seas connecting the Arctic Ocean and North Atlantic Ocean, and will shed a light on their responses to the abrupt climate change in the future.