Phylogenetic diversity of non-marine picocyanobacteria

The most updated picture of the 16S rRNA phylogenetic tree of picocyanobacteria, including marine and freshwater strains, confirms the indication that Synechococcus 5.3 is not included in Synechococcus 5.1 but is its sister clade. The non-marine picocyanobacteria are considered as closely related to Cyanobium strains and proposed as monophyletic "Cyanobium-like" lineage. Nevertheless the relationships of these lineages to one another remain ambiguous. From this comes the need to enlarge the database with new sequences of isolated Synechococcus strains from a wider range of locations and environments to obtain a more realistic view of Synechococcus genus dispersal and evolution. In this study we aimed to resolve better the Synechococcus phylogenetic tree enlarging the sequence data set of non-marine picocyanobacteria including strains isolated from "extreme" lakes. Therefore we selected four high altitude athalassohaline maar crater-lakes in Mexico as a source for "non-marine halotolerant" Synechococcus, five glacial ultraoligotrophic North Patagonian lakes as extreme ecosystems, and six Italian lakes of glacial, volcanic and morenic origin, with different trophic conditions. Our objective was to create a new phylogenetic tree of non-marine Synechococcus upgraded adding 43 new sequences derived from isolated picocyanobacteria and to refine the taxonomic relationships between non-marine Synechococcus clades and Synechococcus 5.2 and 5.3. The new sequences fall into a number of both novel and previously described clades within the phylogenetic tree of 16S rRNA gene. The new cluster of Lake Nahuel Huapi (North Patagonia) forms a sister clade to the subalpine cluster II and the marine Synechococcus sub-cluster 5.2. Our finding of the novel clade of "Halotolerants" close to the marine sub-cluster 5.3 (Synechococcus RCC307) constitutes an important demonstration that euryhaline and marine strains affiliate closely. The intriguing results obtained shed new light on the importance of the non-marine halotolerants in the phylogenesis of picocyanobacteria.