Removal of Microcystis blooms using enhanced colony formation and buoyancy by controlling extracellular polysaccharides and cation concentrations

This study attempted to promote the colony formation and enlarge the colony size of wild Microcystis strain by adjusting extracellular polysaccharides and cationic ion concentrations in culture medium, and then, using the enhanced floating velocity (Stokes' law), wild Microcystis at the water surface was removed. In order to examine the difference between extracellular polysaccharides mixture and tightly cell-bound extracellular polysaccharides used in this study, the component composition and physical properties were also examined. The results showed that tightly cell-bound extracellular polysaccharides contained the greater amounts of cations (Ca²⁺ and Mg²⁺), phosphorus, carboxy group and monosaccharides than extracellular polysaccharides mixture. There were greater amounts of carboxy groups and mannose residue in tightly cell-bound extracellular polysaccharides than extracellular polysaccharides mixture, which would contribute to the stronger cell–cell adhesion of wild Microcystis to make colony formation and expand colony size. The adjustment of extracellular polysaccharides and cationic ion concentrations was effective method to promote the colony formation and enlarge the colony size of wild Microcystis, and that the control of Microcystis buoyancy by enhancing the colony size would be one of the options for the suppression of Microcystis blooms from the viewpoint of cost-effective, low-energy, environmentally-friendly, and non-destructive of Microcystis cells.