Polyphyly of true branching cyanobacteria (Stigonematales)

Cyanobacteria with true branching are classified in Subsection V (formerly order Stigonematales) in the phylum Cyanobacteria. They exhibit a high degree of morphological complexity and are known from particular biotopes. Only a few stigonematalean morphotypes have been cultured, and therefore the high variability of morphotypes found in nature is under-represented in culture. Axenic cultures of Chlorogloeopsis and Fischerella sensu Rippka et al. were, to date, the only representatives of this Subsection in phylogenetic studies. The 16S rDNA sequence analysis data in this report confirm that heterocyst-forming cyanobacteria are a monophyletic group. However, unlike previous studies have suggested, these 16S rDNA data on new Stigonematales strains show that the true branching cyanobacteria are polyphyletic and can be separated into at least two major groups defined by their branching type, the first group being characterized by T-branching and the second group by Y-branching. Cyanobacteria with intercalary heterocysts and either no branching or false-branching also formed separate clusters. In consequence, our phylogenetic data do not correlate with the bacteriological and traditional classifications, which distinguish filamentous heterocystous cyanobacteria with or without true branching (Nostocales/Stigonematales).