Morphological changes of Microcystis aeruginosa colonies in culture

We investigated the changes in the morphology, genetics and photosynthetic characteristics of Microcystis aeruginosa colonies during two months of cultivation. The colonies were collected in Lake Taihu, kept under 25°C on a 12h:12h light/dark cycle at a light density of about 45 μmol m^-2 s^-1. M. novacekii-type colonies were detected on the 10^th day and their percentage of the population tended to increase until the 60^th day. With M. novacekii-like colonies increased, the proportion of M. aeruginosa colonies decreased and reached almost zero by the end of the experiment. M. novacekii-like colony size (D₅₀; this value indicates that particles below D₅₀ accounts for 50%) was greater than 500 μm when these colonies first appeared and was similar to that of M. aeruginosa in the experiment. No differences in cell size were found between these two Microcystis morphotypes either collected from Lake Taihu or cultured in the laboratory. Through molecular tools (16S rDNA, 16S-23S ITS and cpcBA-IGS), there were strong evidences to claim that the original M. aeruginosa colonies isolated from Lake Taihu and the later M. novacekii-like colonies in our cultures were the same species. We suggest that M. aeruginosa colonies consistently changed their colonial morphology to that of a typical M. novacekii.