The paper aims to solve some of the confusions in modern interpretations and generic affiliation of two terrestrial cyanobacteria originally described as Plectonema puteale f. edaphicum Elenkin and P. boryanum f. hollerbachianum Elenkin. The first taxon was recombined soon after its validation as Plectonema edaphicum (Elenkin) Vaulina. Many authors recorded this taxon from various soils, sometimes as a dominant in xerophytic algal communities. It was also found in two caves near Haifa. We discuss here the nomenclatural history of the genus Plectonema s. l., with special reference to its above-mentioned representatives, and provide a comparison of their morphological characteristics based on published sources. It is demonstrated that, in the process of the taxonomic revision of the order Oscillatoriales and resulting nomenclatural changes, a discrepancy arose between the “classical” interpretation of these taxa and their interpretation in the Süsswasserflora von Mitteleuropa (Komárek and Anagnostidis, 2005). Based on the analysis of literature data and original research, authors argued that both taxa described by Elenkin are found in nature. These ecologically similar terrestrial cyanobacteria have clearly expressed morphological features, which help in their identification. Phylogenetic analysis using sequences of the 16S rRNA gene of original strains of these taxa made it possible to clarify their taxonomic affiliations. Strains isolated from biological crusts and identified by morphological features as Pseudophormidium hollerbachianum (Oscillatoriophycideae) and Leptolyngbya edaphica (Synechococcophycideae) in phylogenetic reconstruction both clustered in the clade of the Leptolyngbyaceae. According to SSU rRNA phylogeny, the strain Ru-1-5, morphologically identified as P. hollerbachianum, is a representative of the clade Leptolyngbya and is closely related to L. boryana. Therefore, the name Leptolyngbya hollerbachiana (Elenkin) Anagnostidis et Komárek 1988 (instead of P. hollerbachianum) better corresponds to the phylogeny of this phenotype. The sequences of L. edaphica strains isolated from terrestrial habitats joined a well-supported clade of the newly described genus Timaviella Sciuto et Moro (Sciuto et al., 2017). Based on this finding, we propose a new nomenclatural combination: Timaviella edaphica (Elenkin) O.M. Vynogr. et Mikhailyuk comb. nov.
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