Revision and expansion of the genus Spirirestis (Tolypothrichaceae, Cyanobacteria)

Recent phylogenetic analyses of members of the Tolypothrichaceae (Nostocales, Cyanobacteria) based on 16S rRNA gene sequence data have demonstrated that the soil-inhabiting members of the family belong to a clade separate from the aquatic and subaerial members of the family. The soil-inhabiting species clade includes Spirirestis, a monophyletic taxon originally defined by its tight spiral coiling. Most of the soil-inhabiting species have been identified in the past as belonging either to Hassallia or Tolypothrix, which are subaerial and aquatic taxa, respectively. A comprehensive study of the terrestrial Tolypothrichaceae led us to conclude that all terrestrial Tolypothrichaceae should be included in the genus Spirirestis, even though most of those isolates lack the spiral coiling diagnostic of the genus. Using a polyphasic approach, we recognize seven distinct clades in Spirirestis, which we split into seven species: S. rafaelensis (the generitype), S. californica comb. nov., S. pseudoramosissima comb. nov., S. lignicolor sp. nov., S. williamsae sp. nov., S. hydroterrestris sp. nov., and S. atacamensis sp. nov. Spirirestis rafaelensis and S. californica are represented by multiple isolates, and we postulate that with time and further taxon sampling, some of the strains we included in these two species may be recognized as additional species. As the study of soil cyanobacteria continues, additional species of Spirirestis will likely be discovered and described.