Symphyonema bifilamentata sp. nov., the Right Fischerella ambigua 108b: Half a Decade of Research on Taxonomy and Bioactive Compounds in New Light

Since 1965 a cyanobacterial strain termed ‘Fischerella ambigua 108b’ was the object of several studies investigating its potential as a resource for new bioactive compounds in several European institutes. Over decades these investigations uncovered several unique small molecules and their respective biosynthetic pathways, including the polychlorinated triphenyls of the ambigol family and the tjipanazoles. However, the true taxonomic character of the producing strain remained concealed until now. Applying a polyphasic approach considering the phylogenetic position based on the 16S rRNA and the protein coding gene rbcLX, secondary structures and morphological features, we present the strain ‘Fischerella ambigua 108b’ as Symphyonema bifilamentata sp. nov. 97.28. Although there is the type species (holotype) S. sinense C.-C. Jao 1944 there is no authentic living strain or material for genetic analyses for the genus Symphyonema available. Thus we suggest and provide an epitypification of S. bifilamentata sp. nov. 97.28 as a valid reference for the genus Symphyonema. Its affiliation to the family Symphyonemataceae sheds not only new light on this rare taxon but also on the classes of bioactive metabolites of these heterocytous and true-branching cyanobacteria which we report here. We show conclusively that the literature on the isolation of bioactive products from this organism provides further support for a clear distinction between the secondary metabolism of Symphyonema bifilamentata sp. nov. 97.28 compared to related and other taxa, pointing to the assignment of this organism into a separate genus.

Polyphasic characterization of Stigonema dinghuense, sp. nov. (Cyanophyceae, Nostocophycidae, Stigonemaceae), from Dinghu Mountain, south China

Polyphasic taxonomic studies have been largely conducted at genus/species levels in cyanobacteria, leading to the description of numerous cyanobacterial genera and species. The present study describes a new cyanobacterial species Stigonema dinghuense, sp. nov. Song et Li based on the combination of morphological, genetic, and ecological evidences. This new species was collected from a wet rocky wall in Dinghu Mountain, Guangdong Province, China. Stigonema dinghuense was morphologically similar to Stigonema mamillosum, the type species of the genus, however, Stigonema dinghuense grew on the rocky wall in subtropical region and the type species Stigonema mamillosum inhabited in subfrigid and frigid zones, reflecting a relatively large difference in ecology. 16S rRNA sequences based phylogeny revealed that all the Stigonema species/strains reported so far, including S. dinghuense, grouped into a monophyletic cluster. Phylogenetic analyses based on 16S rRNA, nifH and rbcLX gene sequences yielded the congruent support for the monophyly of heterocytous cyanobacteria. The true branching cyanobacteria (formerly Stigonematales) were separated into four major groups, three of which intermixed with Scytonemataceae.

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).