Progress in phylogenomic analysis has led to a considerable re-evaluation of former cyanobacterial system, with many
new taxa being established at different nomenclatural levels. The family Geminocystaceae is among cyanobacterial taxa
recently described on the basis of polyphasic approach. Within this family, there are six genera: Geminocystis, Cyanobacterium, Geminobacterium, Annamia, Picocyanobacterium, and Microcrocis. The genus Geminocystis previously encompassed two species: G. herdmanii and G. papuanica. Herein, a new species G. urbisnovae was proposed under the provision of the International Code of Nomenclature for algae, fungi, and plants (ICN). Polyphasic analysis was performed
for five strains from the CALU culture collection (St. Petersburg State University, Russian Federation), and they were assigned to the genus Geminocystis in accordance with high 16S rRNA gene similarity to existing species, as well as because
of proximity to these species on the phylogenetic trees reconstructed with RaxML and Bayes methods. Plausibility of their
assignment to a separate species of the genus Geminocystis was substantiated with smaller cell size; stenohaline freshwater ecotype; capability to complementary chromatic adaptation of second type (CA2); distinct 16S rRNA gene clustering; sequences and folding of D1−D1′ and B box domains of the 16S−23S internal transcribed spacer region. The second
objective pursued by this communication was to provide a survey of the family Geminocystaceae. The overall assessment
was that, despite attention of many researchers, this cyanobacterial family has been understudied and, especially in the
case of the crucially important genus Cyanobacterium, taxonomically problematic.