Phylogenetic examination of the cyanobacterial genera Geitlerinema and Limnothrix (Pseudanabaenaceae) using 16S rDNA gene sequence data
Perkerson III, Ralph B.; Perkerson, Emilie A.; Casamatta, Dale A.
Geitlerinema is a widely distributed genus of the Pseudanabaenaceae (Cyanobacteria), commonly found in a variety of aquatic habitats and characterized by highly motile, gliding trichomes under 4 μm in width and an obligate absence of a sheath. A sister genus, Limnothrix, is characterized by solitary, unsheathed trichomes with obligate gas vacuoles and a planktic life cycle. To evaluate the phylogenetic relationships within these genera, we employed five strains from culture collections, combined with those sequences available through GenBank, using 16S rRNA gene sequence data. Sequence analysis revealed the presence of four distinct operational taxonomic units (OTU), whose members were not clearly discernable by microscopic investigation. Two of these highly supported OTUs correspond to Limnothrix and Geitlerinema sensu stricto and contain strains that morphologically correspond to the types. The third OTU contains a mixture of strains with morphology matching that of Limnothrix and Geitlerinema, while the fourth OTU contains Geitlerinema strains isolated solely from marine habitats. Further, we found that folding patterns and presence of a single tRNA transcript in the 16S-23S internal transcribed spacer (ITS) region may be useful as a phylogenetic marker. Combined with cell ultrastructural (type of cell division, thylakoid arrangements) data, 16S rRNA gene (16S rDNA) sequences, and ecological data, we verify that Geitlerinema and Limnothrix as currently circumscribed are polyphyletic.