Description of Pseudocalidococcus azoricus gen. sp. nov. (Thermosynechococcaceae, Cyanobacteria), a Rare but Widely Distributed Coccoid Cyanobacteria

Coccoid cyanobacteria represent an important part of cyanobacterial freshwater diversity, with many studied strains in public databases identified as Synechococcus. This is a diverse genus, both morphologically and ecologically, with a global distribution. However, many of the so-called Synechococcus-like cyanobacteria strains could represent several independent genera that require further studies. In this work, four strains of a Synechococcus-like cyanobacteria isolated from freshwater lakes and terrestrial atmophytic habitats on São Miguel and Flores Islands (Azores archipelago) were studied genetically using the 16S rRNA and 16S–23S rRNA ITS, morphologically with light and transmission electron microscopy, and ecologically. A draft genome was produced from the reference strain by Illumina sequencing, which allowed a more complete phylogenetic study and a deeper taxonomic analysis, revealing a divergent phylogenetic evolution and low ANI and AAI values (69.4% and 66.3%, respectively) to Thermosynechococcus, the closest phylogenetic genus. Although morphologically similar to Synechococcus, the 16S rRNA and genome phylogenetic analysis placed the studied strains in a clade sister to Thermosynechococcus, inside the Thermosynechococcaceae. Thus, Pseudocalidococcus azoricus gen. sp. nov. is described as a new coccoid freshwater genus and species from the Azores archipelago. A detailed comparison with similar morphological taxa is provided, supporting the separation of the new genus. The 16S rRNA with a high genetic similarity to other strains from several continents identified as Synechococcus sp. suggests that the new genus probably has a worldwide distribution. Future studies should be performed to clarify the taxonomic identity of those strains.