A total of 16 strains phylogenetically placed within the Nostocaceae were found to possess morphological features of the Rivulariaceae and Tolypothrichaceae (tapering trichomes and single false branching, respectively) in addition to their typical Nostocacean features (production of arthrospores in series). These strains formed a strongly supported clade separate from other strains that are phylogenetically and morphologically close. We describe four new species within the genus Roholtiella gen. nov. The four species include three distinguishable morphotypes. Roholtiella mojaviensis and R. edaphica are morphologically distinct from each other and from the other two species, R. fluviatilis and R. bashkiriorum. Roholtiella fluviatilis and R. bashkiriorum are cryptic species with respect to each other. All four species are easily distinguished based on the sequence of the 16S-23S ITS regions, in particular the flanking regions to the conserved Box-B and V3 helices. The species are further established by the elevated p-distance between species that is much reduced among strains within the same species. Calochaete cimrmanii, a recently described tapering species from tropical biomes, is the most likely sister taxon to Roholtiella.

Raphidiopsis and Cylindrospermopsis are planktic, freshwater bloom-forming cyanobacteria of great concern to human health due to the production of potent cyanotoxins. The presence (in Cylindrospermopsis) or absence (in Raphidiopsis) of heterocytes is the traditional character used to distinguish them. This has led to misidentifications and to questions about the validity of the genus Raphidiopsis. We studied two strains of R. mediterranea isolated from Argentinean shallow lakes using a polyphasic approach that included the morphological description of the natural populations and their ultrastructural, physiological and molecular characterisation. Heterocyte differentiation was not observed in the field or in cultures of R. mediterranea submitted to nitrogen deprivation. These results support the occurrence of stable native populations of R. mediterranea without heterocytes, which would not be a part of the Cylindrospermopsis complex life cycle. Based on 16S rRNA, 16S-23S ITS, and cpcBA-IGS sequences, these two genera are virtually identical. Thus, strains of Raphidiopsis and Cylindrospermopsis make up a monophyletic lineage in all phylogenetic reconstructions. Furthermore, the 16S-23S ITS secondary structure provided further evidence that these two genera cannot be separated. The intermixed position in the trees points to several losses of heterocytes during the evolution of these cyanobacteria. We conclude that these two genera should not be regarded as separate and distinct generic units and propose their unification under the name Raphidiopsis, respecting the principle of priority. Accordingly, we revisited and emended the description of Raphidiopsis.

Two populations of Rivularia-like cyanobacteria were isolated from ecologically distinct and biogeographically distant sites. One population was from an unpolluted stream in the Kola Peninsula of Russia, whereas the other was from a wet wall in the Grand Staircase-Escalante National Monument, a desert park-land in Utah. Though both were virtually indistinguishable from Rivularia in field and cultured material, they were both phylogenetically distant from Rivularia and the Rivulariaceae based on both 16S rRNA and rbcLX phylogenies. We here name the new cryptic genus Cyanomargarita gen. nov., with type species C. melechinii sp. nov., and additional species C. calcarea sp. nov. We also name a new family for these taxa, the Cyanomargaritaceae.

The thermal springs of Karlovy Vary (Carlsbad) is the locus classicus of the well-known thermal cyanobacterium Mastigocladus laminosus. In addition to the nominate variety, several other varieties and forms were described based on differences in morphology (true-branching versus non-branching) or ecology (thermal versus non-thermal). The cyanobacterial strain Kaštovský 1996/2, which was provisionally identified as M. laminosus f. nostocoides, was also isolated from this locality and discussed in previous work. Based on both morphological and molecular (SSU) analyses, this strain was found to not belong to Mastigocladus, but rather to an undescribed genus, presumably within the Nostocaceae. This strain was subsequently lost, and absence of type materials prevented the description of the genus. The species was successfully re-isolated in 2012. The new strain is identical in morphology, life cycle, and 16S rRNA sequence to the lost strain 1996/2. It is herein described as Cyanocohniella calida gen. et sp. nov. The genus differs from all other Nostocaceae and Aphanizomenonaceae by the unique combination of these characteristics: 1) thermal-tolerant ecology, 2) life cycle that includes Pseudanabaenaceae-like, Nostoc-like and Chlorogloeopsis-like stages, 3) absence of aerotopes, and 4) phylogenetic placement in the Aphanizomenonaceae. The sister taxa, based on 16S rRNA gene sequence phylogenetic analysis, are Cyanospira and Anabaenopsis, both spiral, planktonic, aerotope-bearing, tropical genera and species clearly distinct from C. calida.

Historically, the genus Calothrix included all noncolonial, tapered, heterocytous filaments within the cyanobacteria. However, recent molecular phylogenies show that "Calothrix" defined in this sense represents five distinct clades. The type species of Calothrix is marine, with solitary basal heterocytes, no akinetes, and distal ends tapering abruptly into short hairs. We examined the morphology and phylogeny of 45 tapering cyanobacteria in the Rivulariaceae, including freshwater and marine representatives of both Calothrix (35 strains) and its sister taxon Rivularia (10 strains). The marine Calothrix fall into two lineages, but we lack the generitype and so cannot identify the clade corresponding to the type species. The freshwater and soil Calothrix fall into the C. parietina clade and are characterized by having a basal heterocyte, no akinetes, and gradual tapering-but not into a long hyaline hair. Macrochaete gen. nov. is a freshwater taxon sister to the Calothrix lineages but clearly separated from Rivularia. The species in this genus differ morphologically from Calothrix by their ability to produce two heteromorphic basal heterocytes and specific secondary structures of the 16S-23S ITS. An additional feature present in most species is the presence of a distal, long hyaline hair, but this character has incomplete penetrance due to its expression only under specific environmental conditions (low phosphate), and in one species appears to be lost. We recognize three species: M. psychrophila (type species) from cold environments (high mountains, Antarctica), M. santannae from wet walls of subtropical South America, and M. lichenoides, a phycobiont of lichens from Europe.