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

Abstract

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.

Keywords: Fischerella; Symphyonema; ambigol; heterocytes; polyphasic approach; true-branching.

Figure 1

Photographs and drawing of Symphyonema bifilamentata. (A) One-year-old culture on standard BG11 agar with segregated brownish to orange substance that stained the medium. (B) Photograph of mature thallus-like culture in liquid BG11 with fresh medium. (C) Overview drawing of a mature filament showing bloated basal sections with several lateral branches. (D) Scytonematoid type of false branching of young basal section with necridic cell. (E,F) Development of true T-branching with initial swollen basal cells within young basal section in (E) and trapezoidal cells on each basal cells that extend to lateral branches in (F). (G,H) True Y-branching of basal sections with initial stage in (G) and mature filament in (H–J) show DIC micrographs of mature culture with wider sheaths, multiple branchings and heterocytes.

Figure 2

Morphological details of Symphyonema bifilamentata sp. nov. (A) Overview image showing a mature filament growing on agar with swollen basal section that is tapered at heterocytes and several bilateral branches with heterocytes. Scale bar 50 µm. (B–D) Formation of true T-branching out of young basal sections. € True Y-branching with two elongated cells that unite to a lateral section. (F–G) Necridic cells in young basal sections in (F) and scytonematoid false branching in (G) where both cells surround a necridic cell form lateral filaments that grow out of the sheath. (H) Squeezed globose heterocyte of basal section. (I) Elongated heterocytes of lateral branches. (J) Yellowish apical cells of young lateral branches and (K) indicating yellowish rounded tips of mature lateral branches. Note fine granulation of lateral sections in (I–K) compared to the coarse vacuole-like granulation of basal sections in (B–D) and (H). Scale bar of (B–K) indicates 10 µm.

Figure 3

Comparative phylogenetic trees of 16S rRNA and protein coding gene region rbcLX. (A) Maximum likelihood (ML) tree obtained from 89 aligned 16S rRNA gene sequences with 1508 bp indicating the position of Symphyonema bifilamentata sp. nov. 97.28 rooted to Gloeobacter violacaeus PCC7421. (B) ML tree from 57 aligned sequences of the protein coding gene region rbcLX with 782 bp rooted to Synechocystis. Numbers on the nodes represent ML- and Bayesian bootstrap values respectively, (1000 replicates, each). Bold horizontal lines show lineages supported by at least 75 % of both bootstrap values. Bar represents 0.05 substitutions per nucleotide position. Symphyonema clade is highlighted in green, strains assigned to Fischerella ambigua in dark blue, strains assigned to F. muscicola are marked in light blue and Fischerella strains unassigned to species level are highlighted in purple.

Figure 4

Predicted secondary RNA structures of the main informative helices of the 16S-23S ITS region (D1-D1′ and Box B) of Symphyonema bifilamentata sp. nov. 97.28 in comparison with the closest strains in terms of phylogenetic position and morphology. Color code indicates the probability in percent of the calculated base position.

Figure 5

Representative chemical structures from Symphyonema bifilamentata sp. nov. 97.28 and various strains of Fischerella.