The 16S rRNA gene and cyanobacterial taxonomy; current problems and future prospects

Research into cyanobacterial diversity dates back to more than 150 years. Advancement in modern molecular, ultrastructural, ecophysiological and in vitro culture techniques broadened our understanding in the cyanobacterial diversity. Molecular data, especially 16S rRNA gene sequence provide basic criteria for present day cyanobacterial taxonomy. As more DNA sequence data become available it came into notice that morphology-based taxonomic classification is unreliable and it could not infer evolutionary relationships. Some strains belonging to the previously assembled taxa which were classified based on traditional morphological distinctness appeared phylogenetically unrelated when their 16S rRNA gene was sequenced. Therefore, this editorial note was written with the objective of highlighting the necessity in revising present system of cyanobacterial classification and importance in establishment of universal criteria for future taxonomic proposals for cyanobacteria. A cyanobacterial phylogenetic tree was reconstructed using past and present 16S rRNA sequence assemblages from the database and from our studies. Phylogenetic tree revealed polyphyletic origin of unicellular order Chroococcales and filamentous order Oscillatoriales. Strains in the genera Pseudanabaena from the present study were phylogenetically more distant from rest of the Oscillatorialeans in the database and may have independently diverged from the common ancestor at an early stage in the evolution. On the other hand, two Leptolyngbya strains from the present study clustered with Leptolyngbya accessions from the database, although two strains shared only 89% sequence identity. It appears that those two strains could be distinct species belong to the genera of Leptolyngbya and each may have independent evolutionary history. This hypothesis was supported by distinct morphological characters shown in axenic cultures. Present study highlight the importance in understanding that molecular data alone could only provide insights into genetic variability and phylogenetic relatedness, but could not recognize phenotypic variability and their ecological importance and ongoing diversification of strains etc. Thus, construction of an accurate taxonomic classification system requires a ‘polyphasic’ approach that combines molecular data with phenotypic, biochemical and ecophysiological data. Also it is necessary to revisit all past assemblages of taxa available in the database in order to avoid future taxonomic mislabelling.