Variability of Chroococcus (Cyanobacteria) morphospecies with regard to phylogenetic relationships

Recent advances in the taxonomy of cyanobacteria due to the utilization of molecular methods (such as 16S rRNA analysis) require a comparison of genetically identified items with their morphological expression. Morphological variability of seven monoclonal populations of the planktonic coccoid genus Chroococcus Nageli, isolated mostly from reservoirs, fishponds, and littoral substrata from localities in the Czech and Slovak Republics, were studied experimentally under various combinations of nutrient concentration, light intensity, temperature, and water movement. Two cultivation media (WH-WC and BG11) commonly used in algal collections were applied in a liquid state. Of these, the WH medium was found to be more convenient for planktonic forms. Impacts of combined temperature and light gradients, concentration of P-PO4, and a stable versus shaken medium were found to stress different morphological modifications as a consequence of varied growth intensity and media convenience. Cell width was chosen as the parameter for testing changes in morphology; formation of mucilage and packets of cells were also taken into account. According to 16S rRNA gene analysis, the sequences of 10 strains (including seven studied in the experiments), which were assigned to the genus Chroococcus in the Culture Collection CCALA TA (TM) eboA (http://www.cas.ccala.cz) formed four distinct phylogenetic groups. While two of them showed no affiliation to the genus Chroococcus, two other groups proved the polyphyletic character of the genus. Apart from the group of typical species of the genus Chroococcus, a group of planktonic species could be distinguished, i.e., Chroococcus limneticus Lemmermann 1898 (Limnococcus), and was established as a new genus after recombination.