Distribution and Phylogenetic Diversity of Synechococcus-like Cyanobacteria in the Late Autumn Picophytoplankton of the Kara Sea: The Role of Atlantic and Riverine Water Masses

Increased Atlantic water transport and river discharge are more pronounced effects of global warming at high latitudes. Both phenomena may lead to changes in the species composition of small-celled algae populations in marine ecosystems, as well as to the emergence of new species. This study investigated the spatial distribution of picocyanobacterial (PC) abundance and the phylogenetic diversity of PC <i>Synechococcus</i> in the Kara Sea. PC abundance varied from 2 to 88 cells mL^-1 and increased with warming temperatures and decreasing salinity caused by river water influence. The contribution of <i>Synechococcus</i> to the total picophytoplankton biomass was low (<16%). The <i>Synechococcus</i> community was characterized at deep taxonomic level using amplicon sequencing targeting the <i>petB</i> gene. Diversity was low, revealing only <i>Synechococcus</i> subcluster 5.1 polar lineages I and IV, and euryhaline subcluster 5.2. <i>Synechococcus</i> subcluster 5.1.I represented on average 97% of the total reads assigned to cyanobacteria. For the first time, the presence of estuarine <i>Synechococcus</i> subcluster 5.2 was documented as far north as 82° N. Modified Atlantic water was the main source of cyanobacteria in the Kara Sea, followed by river discharge. Our study contributes to the understanding of PC sources in the Kara Sea and allows for the further monitoring of PC distribution and evolution.