Phylogenetic diversity of marine Synechococcus spp. in the Sea of Okhotsk

The phylogenetic composition of marine Synechococcus spp. populations from 5 stations, 4 in the Sea of Okhotsk and 1 in the western subarctic Pacific, representing a wide range of hydrographic and ecological conditions, was examined using internal transcribed spacer (ITS) clone libraries and phylogenetic analysis. Synechococcus spp. at Stn G9, an estuarine station near the Amur River, belonged to Subcluster 5.2 phylotypes, while the other 4 stations were composed exclusively of Subcluster 5.1 Synechococcus spp. and dominated by phylotypes of Clades I and IV; this is in agreement with the general global distribution pattern of marine Synechococcus spp. lineages. Additionally, novel phylotypes that are not clustered to any existing clades were found, and sub-clades that exclusively contain phylotypes from the Sea of Okhotsk were also formed within respective clades, demonstrating the occurrence of unique Synechococcus spp. assemblages in the high-latitude marginal sea. Furthermore, distinct Synechococcus spp. populations were found at different stations with different hydrographic conditions, suggesting that the spatial variations in the phylogenetic composition of Synechococcus spp. were strongly affected by the physical-chemical parameters of the seawater. Overall, our study adds significant new insights into the diversity of Synechococcus spp. populations in the subpolar marginal sea and supports the conclusion drawn by Zwirglmaier et al. (2008; Environ Microbiol 10: 147-161) that Synechococcus spp. phylogeography is driven by environmental conditions rather than by geographical proximity.