Enhancement of Floating Velocity of Wild Microcystis by Manipulating Colony Morphology and Cell Density

The colony size expansion and the reduction of cell density of wild Microcystis were attempted to accelerate its buoyancy. Culture experiments of Microcystis exhibited that the colony size expansion was observed after 24 h preculture with higher Ca2+ concentration (1,000 mg L−1) in medium, while the preculture with the addition of tightly-bound extracellular polysaccharides (TB-EPS) extracted from cyanobacterial blooms into the same medium did not largely expand the colony size compared to the Ca2+ added medium. Surface analysis of TB-EPS indicated the presence of negatively charged carboxy groups, and the added TB-EPS and the EPS originally surrounding Microcystis would be repulsed, causing prevention of colony size expansion. The floating velocity of Microcystis precultured with light exposure for 24 h was 8.19 × 10−3 cm s−1 at Ca2+ = 1,000 mg L−1, which was 1.4 times higher than that in the control. Furthermore, in case of the preculture without light exposure, the velocity had a remarkable acceleration (5.49 × 10−2 cm s−1), which was ca. 9.1 and 5.7 times greater than the control with and without light exposure, respectively (p < 0.05). These results suggest that the reduction of cell density of Microcystis by respiration in dark would further enhance the buoyancy of Microcystis.