Contrasting Marine Microbial Communities of the Fram Strait with the First Confirmed Record of Cyanobacteria Prochlorococcus marinus in the Arctic Region

SIMPLE SUMMARY: Recently observed rapid climate change in the Arctic region affects the ecology of all organisms; however, little attention has been paid to the impact on microbial communities and large-scale microbial processes in the Arctic. Therefore, we analyzed the microbiome collected from the Greenland and Spitsbergen shelves, on the opposite sides of the Fram Strait, which is the main gateway of Atlantic water to the Arctic Ocean. We found that salinity was the most important factor shaping the microbial communities, which were also stratified by depth. Interestingly, for the very first time, we identified the cyanobacteria Prochlorococcus marinus in polar waters (75–77° N), whose distribution in oceans had been previously described only in temperate, subtropical, and tropical waters, between 40° N and 40° S. We believe that our results revolutionize the knowledge about the distribution of P. marinus in the oceans, which northward shift could have been connected with the process of Atlantification of the Arctic, which involves intensified transport of Atlantic water masses through the Fram Strait towards the Arctic Ocean. Considering the accelerated rate of climate warming in the Arctic, our results indicated that the microbiome community can be viewed as an amplifier of global change and that the Atlantification process is in progress. ABSTRACT: The seawater microbiome is crucial in marine ecosystems because of its role in food chains and biogeochemical cycles; thus, we studied the composition of the pelagic marine microbiome collected in the upper 50 m on the opposite sides of Fram Strait: Spitsbergen and Greenland shelves. We found out that it differed significantly, with salinity being the main environmental variable responsible for these differences. The Spitsbergen shelf was dominated by Atlantic Waters, with a rather homogenous water column in terms of salinity and temperature down to 300 m; hence, the marine microbial community was also homogenous at all sampled depths (0, 25, 50 m). On the contrary, stations on the Greenland shelf were exposed to different water masses of both Arctic and Atlantic origin, which resulted in a more diverse microbial community there. Unexpectedly, for the very first time, we identified cyanobacterium Prochlorococcus marinus in Arctic waters (Spitsbergen shelf, 75–77° N). Till now, the distribution of this cyanobacteria in oceans has been described only between 40° N and 40° S. Considering the accelerated rate of climate warming in the Arctic, our results indicated that the seawater microbiome can be viewed as an amplifier of global change and that the Atlantification is in progress.