Several populations of a non-tapering and tapering, fasciculated, single and geminate false branching heterocytous cyanobacterium were collected from rocky shores in the Pacific Ocean and Gulf of Mexico. The populations were provisionally placed in Brasilonema based on morphology, but upon sequencing of both environmental and culture material it was discovered that the populations/cultures belonged to the Rivulariaceae, in a marine subclade of the family containing Kyrtuthrix huatulcensis. In culture, the taxon exhibited tapering in isopolar filaments, providing further evidence that it was a member of the rivulariacean clade. Based on molecular data for other cyanobacteria within the rivulariacean clade, we identified at least three more species morphologically distinguishable from the Brasilonema-like material, all of which show more pronounced tapering. These cyanobacteria include not only tropical marine strains, but also a strain isolated from the English coastline in the Atlantic Ocean. We propose a new genus and four species for members of this distinctive clade, Nunduva fasciculata gen. nov., sp. nov., N. kania sp. nov., N. biania sp. nov., and N. britannica sp. nov. Other strains that others and we have isolated are sister to Nunduva and may eventually be placed within this genus, but at present, we consider the evidence for inclusion in Nunduva to be insufficient.

A population of Desertifilum (Cyanobacteria, Oscillatoriales) from an oligotrophic desertic biotope was isolated and characterized using a polyphasic approach including molecular, morphological, and ecological information. The population was initially assumed to be a new species based on ecological and biogeographic separation from other existing species, however, phylogenetic analyses based on sequences of the 16S rRNA gene and 16S–23S ITS region, placed this strain clearly within the type species, Desertifilum tharense. Comparative analysis of morphology, 16S rRNA gene similarity, 16S–23S ITS secondary structure, and percent dissimilarity of the ITS regions for all characterized strains supports placing the six Desertifilum strains (designated as PD2001/TDC17, UAM‐C/S02, CHAB7200, NapGTcm17, IPPAS B‐1220, and PMC 872.14) into D. tharense. The recognition of Desertifilum salkalinema and Desertifilum dzianense is not supported, although our analysis does support continued recognition of Desertifilum fontinale. Pragmatic criteria for recognition of closely related species are proposed based on this study and others, and more rigorous review of future taxonomic papers is recommended.

Benthic marine heterocytous cyanoprokaryotes of Mexico´s tropical coast are being recognized as an important and conspicuous component of the supralittoral and intertidal zones usually described as an extreme and low diversity biotope. Although Kyrtuthrix has been reported from different coasts worldwide, its complex morphology has led to differing taxonomic interpretations and positioning. Ten marine supra and intertidal populations of Kyrtuthrix were analyzed using a detailed morphological approach, complemented with ecological and geographical information as well as DNA sequence data of the 16S rRNA gene and associated 16S-23S ITS. Kyrtuthrix huatulcensis is described as a new species, different from K. dalmatica Ercegovic and K. maculans (Gomont) Umezaki based primarily on morphological data. Our material has smaller dimensions in thalli, filaments, trichomes and cells, and possesses differences in qualitative characters as well. Our material is epilithic as in K. maculans but not endolithic as in K. dalmatica. The analysis of molecular data, mainly 16S rRNA gene sequence, show that this genus is within the Rivulariaceae clade, and outside of the Scytonemataceae clade. ITS secondary structure comparison with some of the closer representatives of Rivulariaceae shows considerable differences with these related genera. Based on the evidence gathered in this study, inclusion of Kyrtuthrix within the Rivulariaceae, rather than the Scytonemataceae, is fully supported.

Two untapered, heterocytous species were observed and collected from the intertidal and supratidal zones of the Mexican coastline of the Pacific Ocean near Oaxaca and from the Gulf of Mexico. These populations were highly similar in morphology to the freshwater taxon Petalonema incrustans in the Scytonemataceae. However, 16S rRNA sequence data and phylogenetic analysis indicated that they were sister taxa to the epiphyllic, Brazilian species Phyllonema aveceniicola in the Rivulariaceae, described from culture material. While genetic identity between the two new species was high, they differed significantly in morphology, 16S rRNA gene sequence identity, and sequence and structure of the 16S-23S ITS region. Their morphology differed markedly from the generitype of the previously monotypic Phyllonema, which has tapered, heteropolar, single-false branched trichomes with very thin or absent sheath. The two new species, Phyllonema ansata and Phyllonema tangolundensis, described from both culture and environmental material, have untapered, isopolar, geminately false branched trichomes with thick, lamellated sheaths, differences so significant that the species would not be placed in Phyllonema without molecular corroboration. The morphological differences are so significant that a formal emendation of the genus is required. These taxa provide a challenge to algal taxonomy because the morphological differences are such that one would logically conclude that they represent different genera, but the phylogenetic evidence for including them all in the same genus is conclusive. This conclusion is counter to the current trend in algal taxonomy in which taxa with minor morphological differences have been repeatedly placed in separate genera based primarily upon DNA sequence evidence.

A number of heterocytous, mat-forming, tapering cyanobacteria in Rivulariaceae have recently been observed in both the Atlantic and Pacific coasts in the rocky intertidal and supratidal zones. These belong to the genera Nunduva, Kyrtuthrix, and Phyllonema and have been the subject of several recent studies. Herein, two new species of Nunduva (N. komarkovae and N. sanagustinensis) and two new species of Kyrtuthrix (K. munecosensis and K. totonaca) are characterized and described from the coasts of Mexico. Genetic separation based on the 16S-23S ITS region was pronounced (>10% in all comparisons). Morphological differences between all existing species in these two genera were also observed, but the group is morphologically complex, and these taxa are considered pseudocryptic. Nunduva and Kyrtuthrix remain morphologically and phylogenetically separate even with the addition of new species. However, how long will this remain the case? Many new genera and species of cyanobacteria have recently been described. Will the taxonomy of cyanobacteria eventually become saturated? Will we start to see multiple populations for the same cryptic species, or will future taxonomists collapse multiple species into fewer species, or multiple genera into single genera. The description of even more Nunduva and Kyrtuthrix species causes us to pause and evaluate the future of cyanobacterial taxonomy. These same questions are faced by algal taxonomists studying other phyla, and the resolution may ultimately be similar.