Description of the cyanobacterial genus Desmonostoc gen. nov. including D. muscorum comb. nov. as a distinct, phylogenetically coherent taxon related to the genus Nostoc

On the basis of data presented here and in earlier studies, Desmonostoc gen. nov. is described. The new genus includes the traditional species Nostoc muscorum AGARDH ex BORNET et FLAHAULT 1888, and several other strains previously assigned to the genus Nostoc, which present similar morphology and phylogenetic placement within the Desmonostoc lineage. The Desmonostoc clade is phylogenetically coherent according to 16S rRNA gene sequence analysis performed with four distinct approaches. In all phylogenetic trees, Desmonostoc formed a supported group separated from strains belonging to the related taxa Nostoc, Trichormus, and Mojavia. We also suggest that other clusters hosting strains which for their morphology resemble Nostoc, but are more distant from Nostoc commune cluster than Desmonostoc, should be reclassified into new genera in the future. Strains belonging to Desmonostoc form long vegetative filaments embedded in diffluent mucilaginous envelopes, except for primordial stages they never form a firm periderm, and the filaments are never densely coiled with compact trichomes as found in Nostoc. Both terminal and intercalary heterocytes occur, and mostly elliptical akinetes were differentiated apoheterocytically in long chains. Desmonostoc strains can be usually found in moist or wet meadow, field and forest soils, more rarely in periphyton, but to our knowledge, they are missing or very rare in desert areas. Some of these strains have been found to grow in association with mosses or as symbionts of cycadean plants and of Gunnera sp.